spring-framework/src/docs/asciidoc/web/websocket.adoc

[[websocket]]
= WebSockets
:doc-spring-security: {doc-root}/spring-security/site/docs/current/reference
[.small]#<<web-reactive.adoc#webflux-websocket,Same in Spring WebFlux>>#

This part of the reference documentation covers support for Servlet stack, WebSocket
messaging that includes raw WebSocket interactions, WebSocket emulation via SockJS, and
pub-sub messaging via STOMP as a sub-protocol over WebSocket.


include::websocket-intro.adoc[leveloffset=+1]



[[websocket-server]]
== WebSocket API
[.small]#<<web-reactive.adoc#webflux-websocket-server,Same in Spring WebFlux>>#

The Spring Framework provides a WebSocket API that can be used to write client and
server side applications that handle WebSocket messages.



[[websocket-server-handler]]
=== WebSocketHandler
[.small]#<<web-reactive.adoc#webflux-websocket-server-handler,Same in Spring WebFlux>>#

Creating a WebSocket server is as simple as implementing `WebSocketHandler` or more
likely extending either `TextWebSocketHandler` or `BinaryWebSocketHandler`:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	import org.springframework.web.socket.WebSocketHandler;
	import org.springframework.web.socket.WebSocketSession;
	import org.springframework.web.socket.TextMessage;

	public class MyHandler extends TextWebSocketHandler {

		@Override
		public void handleTextMessage(WebSocketSession session, TextMessage message) {
			// ...
		}

	}
----

There is dedicated WebSocket Java-config and XML namespace support for mapping the above
WebSocket handler to a specific URL:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	import org.springframework.web.socket.config.annotation.EnableWebSocket;
	import org.springframework.web.socket.config.annotation.WebSocketConfigurer;
	import org.springframework.web.socket.config.annotation.WebSocketHandlerRegistry;

	@Configuration
	@EnableWebSocket
	public class WebSocketConfig implements WebSocketConfigurer {

		@Override
		public void registerWebSocketHandlers(WebSocketHandlerRegistry registry) {
			registry.addHandler(myHandler(), "/myHandler");
		}

		@Bean
		public WebSocketHandler myHandler() {
			return new MyHandler();
		}

	}
----

XML configuration equivalent:

[source,xml,indent=0]
[subs="verbatim,quotes,attributes"]
----
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns:websocket="http://www.springframework.org/schema/websocket"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/websocket
			http://www.springframework.org/schema/websocket/spring-websocket.xsd">

		<websocket:handlers>
			<websocket:mapping path="/myHandler" handler="myHandler"/>
		</websocket:handlers>

		<bean id="myHandler" class="org.springframework.samples.MyHandler"/>

	</beans>
----

The above is for use in Spring MVC applications and should be included in the
configuration of a <<mvc-servlet,DispatcherServlet>>. However, Spring's WebSocket
support does not depend on Spring MVC. It is relatively simple to integrate a `WebSocketHandler`
into other HTTP serving environments with the help of
{api-spring-framework}/web/socket/server/support/WebSocketHttpRequestHandler.html[WebSocketHttpRequestHandler].



[[websocket-server-handshake]]
=== WebSocket Handshake
[.small]#<<web-reactive.adoc#webflux-websocket-server-handshake,Same in Spring WebFlux>>#

The easiest way to customize the initial HTTP WebSocket handshake request is through
a `HandshakeInterceptor`, which exposes "before" and "after" the handshake methods.
Such an interceptor can be used to preclude the handshake or to make any attributes
available to the `WebSocketSession`. For example, there is a built-in interceptor
for passing HTTP session attributes to the WebSocket session:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebSocket
	public class WebSocketConfig implements WebSocketConfigurer {

		@Override
		public void registerWebSocketHandlers(WebSocketHandlerRegistry registry) {
			registry.addHandler(new MyHandler(), "/myHandler")
				.addInterceptors(new HttpSessionHandshakeInterceptor());
		}

	}
----

And the XML configuration equivalent:

[source,xml,indent=0]
[subs="verbatim,quotes,attributes"]
----
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns:websocket="http://www.springframework.org/schema/websocket"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/websocket
			http://www.springframework.org/schema/websocket/spring-websocket.xsd">

		<websocket:handlers>
			<websocket:mapping path="/myHandler" handler="myHandler"/>
			<websocket:handshake-interceptors>
				<bean class="org.springframework.web.socket.server.support.HttpSessionHandshakeInterceptor"/>
			</websocket:handshake-interceptors>
		</websocket:handlers>

		<bean id="myHandler" class="org.springframework.samples.MyHandler"/>

	</beans>
----

A more advanced option is to extend the `DefaultHandshakeHandler` that performs
the steps of the WebSocket handshake, including validating the client origin,
negotiating a sub-protocol, and others. An application may also need to use this
option if it needs to configure a custom `RequestUpgradeStrategy` in order to
adapt to a WebSocket server engine and version that is not yet supported
(also see <<websocket-server-deployment>> for more on this subject).
Both the Java-config and XML namespace make it possible to configure a custom
`HandshakeHandler`.


[TIP]
====
Spring provides a `WebSocketHandlerDecorator` base class that can be used to decorate
a `WebSocketHandler` with additional behavior. Logging and exception handling
implementations are provided and added by default when using the WebSocket Java-config
or XML namespace. The `ExceptionWebSocketHandlerDecorator` catches all uncaught
exceptions arising from any WebSocketHandler method and closes the WebSocket
session with status `1011` that indicates a server error.
====



[[websocket-server-deployment]]
=== Deployment

The Spring WebSocket API is easy to integrate into a Spring MVC application where
the `DispatcherServlet` serves both HTTP WebSocket handshake as well as other
HTTP requests. It is also easy to integrate into other HTTP processing scenarios
by invoking `WebSocketHttpRequestHandler`. This is convenient and easy to
understand. However, special considerations apply with regards to JSR-356 runtimes.

The Java WebSocket API (JSR-356) provides two deployment mechanisms. The first
involves a Servlet container classpath scan (Servlet 3 feature) at startup; and
the other is a registration API to use at Servlet container initialization.
Neither of these mechanism makes it possible to use a single "front controller"
for all HTTP processing -- including WebSocket handshake and all other HTTP
requests -- such as Spring MVC's `DispatcherServlet`.

This is a significant limitation of JSR-356 that Spring's WebSocket support addresses
server-specific ``RequestUpgradeStrategy``'s even when running in a JSR-356 runtime.
Such strategies currently exist for Tomcat, Jetty, GlassFish, WebLogic, WebSphere, and
Undertow (and WildFly).



[NOTE]
====
A request to overcome the above limitation in the Java WebSocket API has been
created and can be followed at
https://github.com/eclipse-ee4j/websocket-api/issues/211[WEBSOCKET_SPEC-211].
Tomcat, Undertow and WebSphere provide their own API alternatives that
makes it possible to this, and it's also possible with Jetty. We are hopeful
that more servers will follow do the same.
====

A secondary consideration is that Servlet containers with JSR-356 support are expected
to perform a `ServletContainerInitializer` (SCI) scan that can slow down application
startup, in some cases dramatically. If a significant impact is observed after an
upgrade to a Servlet container version with JSR-356 support, it should
be possible to selectively enable or disable web fragments (and SCI scanning)
through the use of the `<absolute-ordering />` element in `web.xml`:

[source,xml,indent=0]
[subs="verbatim,quotes,attributes"]
----
	<web-app xmlns="http://java.sun.com/xml/ns/javaee"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xsi:schemaLocation="
			http://java.sun.com/xml/ns/javaee
			http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
		version="3.0">

		<absolute-ordering/>

	</web-app>
----

You can then selectively enable web fragments by name, such as Spring's own
`SpringServletContainerInitializer` that provides support for the Servlet 3
Java initialization API, if required:

[source,xml,indent=0]
[subs="verbatim,quotes,attributes"]
----
	<web-app xmlns="http://java.sun.com/xml/ns/javaee"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xsi:schemaLocation="
			http://java.sun.com/xml/ns/javaee
			http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
		version="3.0">

		<absolute-ordering>
			<name>spring_web</name>
		</absolute-ordering>

	</web-app>
----



[[websocket-server-runtime-configuration]]
=== Server config
[.small]#<<web-reactive.adoc#webflux-websocket-server-config,Same in Spring WebFlux>>#

Each underlying WebSocket engine exposes configuration properties that control
runtime characteristics such as the size of message buffer sizes, idle timeout,
and others.

For Tomcat, WildFly, and GlassFish add a `ServletServerContainerFactoryBean` to your
WebSocket Java config:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebSocket
	public class WebSocketConfig implements WebSocketConfigurer {

		@Bean
		public ServletServerContainerFactoryBean createWebSocketContainer() {
			ServletServerContainerFactoryBean container = new ServletServerContainerFactoryBean();
			container.setMaxTextMessageBufferSize(8192);
			container.setMaxBinaryMessageBufferSize(8192);
			return container;
		}

	}
----

or WebSocket XML namespace:

[source,xml,indent=0]
[subs="verbatim,quotes,attributes"]
----
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns:websocket="http://www.springframework.org/schema/websocket"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/websocket
			http://www.springframework.org/schema/websocket/spring-websocket.xsd">

		<bean class="org.springframework...ServletServerContainerFactoryBean">
			<property name="maxTextMessageBufferSize" value="8192"/>
			<property name="maxBinaryMessageBufferSize" value="8192"/>
		</bean>

	</beans>
----

[NOTE]
====
For client side WebSocket configuration, you should use `WebSocketContainerFactoryBean`
(XML) or `ContainerProvider.getWebSocketContainer()` (Java config).
====

For Jetty, you'll need to supply a pre-configured Jetty `WebSocketServerFactory` and plug
that into Spring's `DefaultHandshakeHandler` through your WebSocket Java config:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebSocket
	public class WebSocketConfig implements WebSocketConfigurer {

		@Override
		public void registerWebSocketHandlers(WebSocketHandlerRegistry registry) {
			registry.addHandler(echoWebSocketHandler(),
				"/echo").setHandshakeHandler(handshakeHandler());
		}

		@Bean
		public DefaultHandshakeHandler handshakeHandler() {

			WebSocketPolicy policy = new WebSocketPolicy(WebSocketBehavior.SERVER);
			policy.setInputBufferSize(8192);
			policy.setIdleTimeout(600000);

			return new DefaultHandshakeHandler(
					new JettyRequestUpgradeStrategy(new WebSocketServerFactory(policy)));
		}

	}
----

or WebSocket XML namespace:

[source,xml,indent=0]
[subs="verbatim,quotes,attributes"]
----
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns:websocket="http://www.springframework.org/schema/websocket"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/websocket
			http://www.springframework.org/schema/websocket/spring-websocket.xsd">

		<websocket:handlers>
			<websocket:mapping path="/echo" handler="echoHandler"/>
			<websocket:handshake-handler ref="handshakeHandler"/>
		</websocket:handlers>

		<bean id="handshakeHandler" class="org.springframework...DefaultHandshakeHandler">
			<constructor-arg ref="upgradeStrategy"/>
		</bean>

		<bean id="upgradeStrategy" class="org.springframework...JettyRequestUpgradeStrategy">
			<constructor-arg ref="serverFactory"/>
		</bean>

		<bean id="serverFactory" class="org.eclipse.jetty...WebSocketServerFactory">
			<constructor-arg>
				<bean class="org.eclipse.jetty...WebSocketPolicy">
					<constructor-arg value="SERVER"/>
					<property name="inputBufferSize" value="8092"/>
					<property name="idleTimeout" value="600000"/>
				</bean>
			</constructor-arg>
		</bean>

	</beans>
----



[[websocket-server-allowed-origins]]
=== Allowed origins
[.small]#<<web-reactive.adoc#webflux-websocket-server-cors,Same in Spring WebFlux>>#

As of Spring Framework 4.1.5, the default behavior for WebSocket and SockJS is to accept
only _same origin_ requests. It is also possible to allow _all_ or a specified list of origins.
This check is mostly designed for browser clients. There is nothing preventing other types
of clients from modifying the `Origin` header value (see
https://tools.ietf.org/html/rfc6454[RFC 6454: The Web Origin Concept] for more details).

The 3 possible behaviors are:

 * Allow only same origin requests (default): in this mode, when SockJS is enabled, the
   Iframe HTTP response header `X-Frame-Options` is set to `SAMEORIGIN`, and JSONP
   transport is disabled since it does not allow to check the origin of a request.
   As a consequence, IE6 and IE7 are not supported when this mode is enabled.
 * Allow a specified list of origins: each provided _allowed origin_ must start with `http://`
   or `https://`. In this mode, when SockJS is enabled, IFrame transport is disabled.
   As a consequence, IE6 through IE9 are not supported when this
   mode is enabled.
 * Allow all origins: to enable this mode, you should provide `{asterisk}` as the allowed origin
   value. In this mode, all transports are available.

WebSocket and SockJS allowed origins can be configured as shown bellow:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	import org.springframework.web.socket.config.annotation.EnableWebSocket;
	import org.springframework.web.socket.config.annotation.WebSocketConfigurer;
	import org.springframework.web.socket.config.annotation.WebSocketHandlerRegistry;

	@Configuration
	@EnableWebSocket
	public class WebSocketConfig implements WebSocketConfigurer {

		@Override
		public void registerWebSocketHandlers(WebSocketHandlerRegistry registry) {
			registry.addHandler(myHandler(), "/myHandler").setAllowedOrigins("http://mydomain.com");
		}

		@Bean
		public WebSocketHandler myHandler() {
			return new MyHandler();
		}

	}
----

XML configuration equivalent:

[source,xml,indent=0]
[subs="verbatim,quotes,attributes"]
----
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns:websocket="http://www.springframework.org/schema/websocket"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/websocket
			http://www.springframework.org/schema/websocket/spring-websocket.xsd">

		<websocket:handlers allowed-origins="http://mydomain.com">
			<websocket:mapping path="/myHandler" handler="myHandler" />
		</websocket:handlers>

		<bean id="myHandler" class="org.springframework.samples.MyHandler"/>

	</beans>
----




[[websocket-fallback]]
== SockJS Fallback

Over the public Internet, restrictive proxies outside your control may preclude WebSocket
interactions either because they are not configured to pass on the `Upgrade` header or
because they close long lived connections that appear idle.

The solution to this problem is WebSocket emulation, i.e. attempting to use WebSocket
first and then falling back on HTTP-based techniques that emulate a WebSocket
interaction and expose the same application-level API.

On the Servlet stack the Spring Framework provides both server (and also client) support
for the SockJS protocol.


[[websocket-fallback-sockjs-overview]]
=== Overview

The goal of SockJS is to let applications use a WebSocket API but fall back to
non-WebSocket alternatives when necessary at runtime, i.e. without the need to
change application code.

SockJS consists of:

* The https://github.com/sockjs/sockjs-protocol[SockJS protocol]
defined in the form of executable
http://sockjs.github.io/sockjs-protocol/sockjs-protocol-0.3.3.html[narrated tests].
* The https://github.com/sockjs/sockjs-client/[SockJS JavaScript client] - a client library for use in browsers.
* SockJS server implementations including one in the Spring Framework `spring-websocket` module.
* As of 4.1 `spring-websocket` also provides a SockJS Java client.

SockJS is designed for use in browsers. It goes to great lengths
to support a wide range of browser versions using a variety of techniques.
For the full list of SockJS transport types and browsers see the
https://github.com/sockjs/sockjs-client/[SockJS client] page. Transports
fall in 3 general categories: WebSocket, HTTP Streaming, and HTTP Long Polling.
For an overview of these categories see
https://spring.io/blog/2012/05/08/spring-mvc-3-2-preview-techniques-for-real-time-updates/[this blog post].

The SockJS client begins by sending `"GET /info"` to
obtain basic information from the server. After that it must decide what transport
to use. If possible WebSocket is used. If not, in most browsers
there is at least one HTTP streaming option and if not then HTTP (long)
polling is used.

All transport requests have the following URL structure:
----
http://host:port/myApp/myEndpoint/{server-id}/{session-id}/{transport}
----

* `{server-id}` - useful for routing requests in a cluster but not used otherwise.
* `{session-id}` - correlates HTTP requests belonging to a SockJS session.
* `{transport}` - indicates the transport type, e.g. "websocket", "xhr-streaming", etc.

The WebSocket transport needs only a single HTTP request to do the WebSocket handshake.
All messages thereafter are exchanged on that socket.

HTTP transports require more requests. Ajax/XHR streaming for example relies on
one long-running request for server-to-client messages and additional HTTP POST
requests for client-to-server messages. Long polling is similar except it
ends the current request after each server-to-client send.

SockJS adds minimal message framing. For example the server sends the letter +o+
("open" frame) initially, messages are sent as +a["message1","message2"]+
(JSON-encoded array), the letter +h+ ("heartbeat" frame) if no messages flow
for 25 seconds by default, and the letter +c+ ("close" frame) to close the session.

To learn more, run an example in a browser and watch the HTTP requests.
The SockJS client allows fixing the list of transports so it is possible to
see each transport one at a time. The SockJS client also provides a debug flag
which enables helpful messages in the browser console. On the server side enable
`TRACE` logging for `org.springframework.web.socket`.
For even more detail refer to the SockJS protocol
http://sockjs.github.io/sockjs-protocol/sockjs-protocol-0.3.3.html[narrated test].



[[websocket-fallback-sockjs-enable]]
=== Enable SockJS

SockJS is easy to enable through Java configuration:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebSocket
	public class WebSocketConfig implements WebSocketConfigurer {

		@Override
		public void registerWebSocketHandlers(WebSocketHandlerRegistry registry) {
			registry.addHandler(myHandler(), "/myHandler").withSockJS();
		}

		@Bean
		public WebSocketHandler myHandler() {
			return new MyHandler();
		}

	}
----

and the XML configuration equivalent:

[source,xml,indent=0]
[subs="verbatim,quotes,attributes"]
----
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns:websocket="http://www.springframework.org/schema/websocket"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/websocket
			http://www.springframework.org/schema/websocket/spring-websocket.xsd">

		<websocket:handlers>
			<websocket:mapping path="/myHandler" handler="myHandler"/>
			<websocket:sockjs/>
		</websocket:handlers>

		<bean id="myHandler" class="org.springframework.samples.MyHandler"/>

	</beans>
----

The above is for use in Spring MVC applications and should be included in the
configuration of a <<mvc-servlet,DispatcherServlet>>. However, Spring's WebSocket
and SockJS support does not depend on Spring MVC. It is relatively simple to
integrate into other HTTP serving environments with the help of
{api-spring-framework}/web/socket/sockjs/support/SockJsHttpRequestHandler.html[SockJsHttpRequestHandler].

On the browser side, applications can use the
https://github.com/sockjs/sockjs-client/[sockjs-client] (version 1.0.x) that
emulates the W3C WebSocket API and communicates with the server to select the best
transport option depending on the browser it's running in. Review the
https://github.com/sockjs/sockjs-client/[sockjs-client] page and the list of
transport types supported by browser. The client also provides several
configuration options, for example, to specify which transports to include.



[[websocket-fallback-xhr-vs-iframe]]
=== IE 8, 9

Internet Explorer 8 and 9 are and will remain common for some time. They are
a key reason for having SockJS. This section covers important
considerations about running in those browsers.

The SockJS client supports Ajax/XHR streaming in IE 8 and 9 via Microsoft's
http://blogs.msdn.com/b/ieinternals/archive/2010/05/13/xdomainrequest-restrictions-limitations-and-workarounds.aspx[XDomainRequest].
That works across domains but does not support sending cookies.
Cookies are very often essential for Java applications.
However since the SockJS client can be used with many server
types (not just Java ones), it needs to know whether cookies matter.
If so the SockJS client prefers Ajax/XHR for streaming or otherwise it
relies on a iframe-based technique.

The very first `"/info"` request from the SockJS client is a request for
information that can influence the client's choice of transports.
One of those details is whether the server application relies on cookies,
e.g. for authentication purposes or clustering with sticky sessions.
Spring's SockJS support includes a property called `sessionCookieNeeded`.
It is enabled by default since most Java applications rely on the `JSESSIONID`
cookie. If your application does not need it, you can turn off this option
and the SockJS client should choose `xdr-streaming` in IE 8 and 9.

If you do use an iframe-based transport, and in any case, it is good to know
that browsers can be instructed to block the use of IFrames on a given page by
setting the HTTP response header `X-Frame-Options` to `DENY`,
`SAMEORIGIN`, or `ALLOW-FROM <origin>`. This is used to prevent
https://www.owasp.org/index.php/Clickjacking[clickjacking].

[NOTE]
====
Spring Security 3.2+ provides support for setting `X-Frame-Options` on every
response. By default the Spring Security Java config sets it to `DENY`.
In 3.2 the Spring Security XML namespace does not set that header by default
but may be configured to do so, and in the future it may set it by default.

See {doc-spring-security}/htmlsingle/#headers[Section 7.1. "Default Security Headers"]
of the Spring Security documentation for details on how to configure the
setting of the `X-Frame-Options` header. You may also check or watch
https://jira.spring.io/browse/SEC-2501[SEC-2501] for additional background.
====

If your application adds the `X-Frame-Options` response header (as it should!)
and relies on an iframe-based transport, you will need to set the header value to
`SAMEORIGIN` or `ALLOW-FROM <origin>`. Along with that the Spring SockJS
support also needs to know the location of the SockJS client because it is loaded
from the iframe. By default the iframe is set to download the SockJS client
from a CDN location. It is a good idea to configure this option to
a URL from the same origin as the application.

In Java config this can be done as shown below. The XML namespace provides a
similar option via the `<websocket:sockjs>` element:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebSocket
	public class WebSocketConfig implements WebSocketConfigurer {

		@Override
		public void registerStompEndpoints(StompEndpointRegistry registry) {
			registry.addEndpoint("/portfolio").withSockJS()
					.setClientLibraryUrl("http://localhost:8080/myapp/js/sockjs-client.js");
		}

		// ...

	}
----

[NOTE]
====
During initial development, do enable the SockJS client `devel` mode that prevents
the browser from caching SockJS requests (like the iframe) that would otherwise
be cached. For details on how to enable it see the
https://github.com/sockjs/sockjs-client/[SockJS client] page.
====



[[websocket-fallback-sockjs-heartbeat]]
=== Heartbeats

The SockJS protocol requires servers to send heartbeat messages to preclude proxies
from concluding a connection is hung. The Spring SockJS configuration has a property
called `heartbeatTime` that can be used to customize the frequency. By default a
heartbeat is sent after 25 seconds assuming no other messages were sent on that
connection. This 25 seconds value is in line with the following
http://tools.ietf.org/html/rfc6202[IETF recommendation] for public Internet applications.

[NOTE]
====
When using STOMP over WebSocket/SockJS, if the STOMP client and server negotiate
heartbeats to be exchanged, the SockJS heartbeats are disabled.
====

The Spring SockJS support also allows configuring the `TaskScheduler` to use
for scheduling heartbeats tasks. The task scheduler is backed by a thread pool
with default settings based on the number of available processors. Applications
should consider customizing the settings according to their specific needs.



[[websocket-fallback-sockjs-servlet3-async]]
=== Client disconnects

HTTP streaming and HTTP long polling SockJS transports require a connection to remain
open longer than usual. For an overview of these techniques see
https://spring.io/blog/2012/05/08/spring-mvc-3-2-preview-techniques-for-real-time-updates/[this blog post].

In Servlet containers this is done through Servlet 3 async support that
allows exiting the Servlet container thread processing a request and continuing
to write to the response from another thread.

A specific issue is that the Servlet API does not provide notifications for a client
that has gone away, see https://java.net/jira/browse/SERVLET_SPEC-44[SERVLET_SPEC-44].
However, Servlet containers raise an exception on subsequent attempts to write
to the response. Since Spring's SockJS Service supports sever-sent heartbeats (every
25 seconds by default), that means a client disconnect is usually detected within that
time period or earlier if messages are sent more frequently.

[NOTE]
====
As a result network IO failures may occur simply because a client has disconnected, which
can fill the log with unnecessary stack traces. Spring makes a best effort to identify
such network failures that represent client disconnects (specific to each server) and log
a minimal message using the dedicated log category `DISCONNECTED_CLIENT_LOG_CATEGORY`
defined in `AbstractSockJsSession`. If you need to see the stack traces, set that
log category to TRACE.
====



[[websocket-fallback-cors]]
=== SockJS and CORS

If you allow cross-origin requests (see <<websocket-server-allowed-origins>>), the SockJS protocol
uses CORS for cross-domain support in the XHR streaming and polling transports. Therefore
CORS headers are added automatically unless the presence of CORS headers in the response
is detected. So if an application is already configured to provide CORS support, e.g.
through a Servlet Filter, Spring's SockJsService will skip this part.

It is also possible to disable the addition of these CORS headers via the
`suppressCors` property in Spring's SockJsService.

The following is the list of headers and values expected by SockJS:

* `"Access-Control-Allow-Origin"` - initialized from the value of the "Origin" request header.
* `"Access-Control-Allow-Credentials"` - always set to `true`.
* `"Access-Control-Request-Headers"` - initialized from values from the equivalent request header.
* `"Access-Control-Allow-Methods"` - the HTTP methods a transport supports (see `TransportType` enum).
* `"Access-Control-Max-Age"` - set to 31536000 (1 year).

For the exact implementation see `addCorsHeaders` in `AbstractSockJsService` as well
as the `TransportType` enum in the source code.

Alternatively if the CORS configuration allows it consider excluding URLs with the
SockJS endpoint prefix thus letting Spring's `SockJsService` handle it.



[[websocket-fallback-sockjs-client]]
=== SockJsClient

A SockJS Java client is provided in order to connect to remote SockJS endpoints without
using a browser. This can be especially useful when there is a need for bidirectional
communication between 2 servers over a public network, i.e. where network proxies may
preclude the use of the WebSocket protocol. A SockJS Java client is also very useful
for testing purposes, for example to simulate a large number of concurrent users.

The SockJS Java client supports the "websocket", "xhr-streaming", and "xhr-polling"
transports. The remaining ones only make sense for use in a browser.

The `WebSocketTransport` can be configured with:

* `StandardWebSocketClient` in a JSR-356 runtime
* `JettyWebSocketClient` using the Jetty 9+ native WebSocket API
* Any implementation of Spring's `WebSocketClient`

An `XhrTransport` by definition supports both "xhr-streaming" and "xhr-polling" since
from a client perspective there is no difference other than in the URL used to connect
to the server. At present there are two implementations:

* `RestTemplateXhrTransport` uses Spring's `RestTemplate` for HTTP requests.
* `JettyXhrTransport` uses Jetty's `HttpClient` for HTTP requests.

The example below shows how to create a SockJS client and connect to a SockJS endpoint:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
  List<Transport> transports = new ArrayList<>(2);
  transports.add(new WebSocketTransport(new StandardWebSocketClient()));
  transports.add(new RestTemplateXhrTransport());

  SockJsClient sockJsClient = new SockJsClient(transports);
  sockJsClient.doHandshake(new MyWebSocketHandler(), "ws://example.com:8080/sockjs");
----

[NOTE]
====
SockJS uses JSON formatted arrays for messages. By default Jackson 2 is used and needs
to be on the classpath. Alternatively you can configure a custom implementation of
`SockJsMessageCodec` and configure it on the `SockJsClient`.
====

To use the SockJsClient for simulating a large number of concurrent users you will
need to configure the underlying HTTP client (for XHR transports) to allow a sufficient
number of connections and threads. For example with Jetty:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
HttpClient jettyHttpClient = new HttpClient();
jettyHttpClient.setMaxConnectionsPerDestination(1000);
jettyHttpClient.setExecutor(new QueuedThreadPool(1000));
----

Consider also customizing these server-side SockJS related properties (see Javadoc for details):

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	public class WebSocketConfig extends WebSocketMessageBrokerConfigurationSupport {

		@Override
		public void registerStompEndpoints(StompEndpointRegistry registry) {
			registry.addEndpoint("/sockjs").withSockJS()
				.setStreamBytesLimit(512 * 1024)
				.setHttpMessageCacheSize(1000)
				.setDisconnectDelay(30 * 1000);
		}

		// ...
	}
----




[[websocket-stomp]]
== STOMP

The WebSocket protocol defines two types of messages, text and binary, but their
content is undefined. The defines a mechanism for client and server to negotiate a
sub-protocol -- i.e. a higher level messaging protocol, to use on top of WebSocket to
define what kind of messages each can send, what is the format and content for each
message, and so on. The use of a sub-protocol is optional but either way client and
server will need to agree on some protocol that defines message content.



[[websocket-stomp-overview]]
=== Overview

http://stomp.github.io/stomp-specification-1.2.html#Abstract[STOMP] is a simple,
text-oriented messaging protocol that was originally created for scripting languages
such as Ruby, Python, and Perl to connect to enterprise message brokers. It is
designed to address a minimal subset of commonly used messaging patterns. STOMP can be
used over any reliable, 2-way streaming network protocol such as TCP and WebSocket.
Although STOMP is a text-oriented protocol, message payloads can be
either text or binary.

STOMP is a frame based protocol whose frames are modeled on HTTP. The structure
of a STOMP frame:

----
COMMAND
header1:value1
header2:value2

Body^@
----

Clients can use the +SEND+ or +SUBSCRIBE+ commands to send or subscribe for
messages along with a +"destination"+ header that describes what the
message is about and who should receive it. This enables a simple
publish-subscribe mechanism that can be used to send messages through the broker
to other connected clients or to send messages to the server to request that
some work be performed.

When using Spring's STOMP support, the Spring WebSocket application acts
as the STOMP broker to clients. Messages are routed to `@Controller` message-handling
methods or to a simple, in-memory broker that keeps track of subscriptions and
broadcasts messages to subscribed users. You can also configure Spring to work
with a dedicated STOMP broker (e.g. RabbitMQ, ActiveMQ, etc) for the actual
broadcasting of messages. In that case Spring maintains
TCP connections to the broker, relays messages to it, and also passes messages
from it down to connected WebSocket clients. Thus Spring web applications can
rely on unified HTTP-based security, common validation, and a familiar programming
model message-handling work.

Here is an example of a client subscribing to receive stock quotes which
the server may emit periodically e.g. via a scheduled task sending messages
through a `SimpMessagingTemplate` to the broker:

----
SUBSCRIBE
id:sub-1
destination:/topic/price.stock.*

^@
----

Here is an example of a client sending a trade request, which the server
may handle through an `@MessageMapping` method and later on, after the execution,
broadcast a trade confirmation message and details down to the client:

----
SEND
destination:/queue/trade
content-type:application/json
content-length:44

{"action":"BUY","ticker":"MMM","shares",44}^@
----

The meaning of a destination is intentionally left opaque in the STOMP spec. It can
be any string, and it's entirely up to STOMP servers to define the semantics and
the syntax of the destinations that they support. It is very common, however, for
destinations to be path-like strings where `"/topic/.."` implies publish-subscribe
(__one-to-many__) and `"/queue/"` implies point-to-point (__one-to-one__) message
exchanges.

STOMP servers can use the +MESSAGE+ command to broadcast messages to all subscribers.
Here is an example of a server sending a stock quote to a subscribed client:

----
MESSAGE
message-id:nxahklf6-1
subscription:sub-1
destination:/topic/price.stock.MMM

{"ticker":"MMM","price":129.45}^@
----

It is important to know that a server cannot send unsolicited messages. All messages
from a server must be in response to a specific client subscription, and the
+"subscription-id"+ header of the server message must match the +"id"+ header of the
client subscription.

The above overview is intended to provide the most basic understanding of the
STOMP protocol. It is recommended to review the protocol
http://stomp.github.io/stomp-specification-1.2.html[specification] in full.



[[websocket-stomp-benefits]]
=== Benefits

Use of STOMP as a sub-protocol enables the Spring Framework and Spring Security to
provide a richer programming model vs using raw WebSockets. The same point can be
made about how HTTP vs raw TCP and how it enables Spring MVC and other web frameworks
to provide rich functionality. The following is a list of benefits:

* No need to invent a custom messaging protocol and message format.
* STOMP clients are available including a <<websocket-stomp-client,Java client>>
in the Spring Framework.
* Message brokers such as RabbitMQ, ActiveMQ, and others can be used (optionally) to
manage subscriptions and broadcast messages.
* Application logic can be organized in any number of ``@Controller``'s and messages
routed to them based on the STOMP destination header vs handling raw WebSocket messages
with a single `WebSocketHandler` for a given connection.
* Use Spring Security to secure messages based on STOMP destinations and message types.



[[websocket-stomp-enable]]
=== Enable STOMP

STOMP over WebSocket support is available in the `spring-messaging` and the
`spring-websocket` modules. Once you have those dependencies, you can expose a STOMP
endpoints, over WebSocket with <<websocket-fallback>>, as shown below:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	import org.springframework.web.socket.config.annotation.EnableWebSocketMessageBroker;
	import org.springframework.web.socket.config.annotation.StompEndpointRegistry;

	@Configuration
	@EnableWebSocketMessageBroker
	public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {

		@Override
		public void registerStompEndpoints(StompEndpointRegistry registry) {
			registry.addEndpoint("/portfolio").withSockJS();  // <1>
		}

		@Override
		public void configureMessageBroker(MessageBrokerRegistry config) {
			config.setApplicationDestinationPrefixes("/app"); // <2>
			config.enableSimpleBroker("/topic", "/queue"); // <3>
		}
	}
----

<1> `"/portfolio"` is the HTTP URL for the endpoint to which a WebSocket (or SockJS)
client will need to connect to for the WebSocket handshake.
<2> STOMP messages whose destination header begins with `"/app"` are routed to
`@MessageMapping` methods in `@Controller` classes.
<3> Use the built-in, message broker for subscriptions and broadcasting;
Route messages whose destination header begins with "/topic" or "/queue" to the broker.

The same configuration in XML:

[source,xml,indent=0]
[subs="verbatim,quotes,attributes"]
----
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns:websocket="http://www.springframework.org/schema/websocket"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/websocket
			http://www.springframework.org/schema/websocket/spring-websocket.xsd">

		<websocket:message-broker application-destination-prefix="/app">
			<websocket:stomp-endpoint path="/portfolio">
				<websocket:sockjs/>
			</websocket:stomp-endpoint>
			<websocket:simple-broker prefix="/topic, /queue"/>
		</websocket:message-broker>

	</beans>
----

[NOTE]
====
For the built-in, simple broker the "/topic" and "/queue" prefixes do not have any special
meaning. They're merely a convention to differentiate between pub-sub vs point-to-point
messaging (i.e. many subscribers vs one consumer). When using an external broker, please
check the STOMP page of the broker to understand what kind of STOMP destinations and
prefixes it supports.
====

To connect from a browser, for SockJS you can use the
https://github.com/sockjs/sockjs-client[sockjs-client]. For STOMP many applications have
used the https://github.com/jmesnil/stomp-websocket[jmesnil/stomp-websocket] library
(also known as stomp.js) which is feature complete and has been used in production for
years but is no longer maintained. At present the
https://github.com/JSteunou/webstomp-client[JSteunou/webstomp-client] is the most
actively maintained and evolving successor of that library and the example code below
is based on it:

[source,javascript,indent=0]
[subs="verbatim,quotes"]
----
	var socket = new SockJS("/spring-websocket-portfolio/portfolio");
	var stompClient = webstomp.over(socket);

	stompClient.connect({}, function(frame) {
	}
----

Or if connecting via WebSocket (without SockJS):

[source,javascript,indent=0]
[subs="verbatim,quotes"]
----
	var socket = new WebSocket("/spring-websocket-portfolio/portfolio");
	var stompClient = Stomp.over(socket);

	stompClient.connect({}, function(frame) {
	}
----

Note that the `stompClient` above does not need to specify `login` and `passcode` headers.
Even if it did, they would be ignored, or rather overridden, on the server side. See the
sections <<websocket-stomp-handle-broker-relay-configure>> and
<<websocket-stomp-authentication>> for more information on authentication.

For a more example code see:

* https://spring.io/guides/gs/messaging-stomp-websocket/[Using WebSocket to build an
interactive web application] getting started guide.
* https://github.com/rstoyanchev/spring-websocket-portfolio[Stock Portfolio] sample
application.



[[websocket-stomp-message-flow]]
=== Flow of Messages

Once a STOMP endpoint is exposed, the Spring application becomes a STOMP broker for
connected clients. This section describes the flow of messages on the server side.

The `spring-messaging` module contains foundational support for messaging applications
that originated in https://spring.io/spring-integration[Spring Integration] and was
later extracted and incorporated into the Spring Framework for broader use across many
https://spring.io/projects[Spring projects] and application scenarios.
Below is a list of a few of the available messaging abstractions:

* {api-spring-framework}/messaging/Message.html[Message] --
simple representation for a message including headers and payload.
* {api-spring-framework}/messaging/MessageHandler.html[MessageHandler] --
contract for handling a message.
* {api-spring-framework}/messaging/MessageChannel.html[MessageChannel] --
contract for sending a message that enables loose coupling between producers and consumers.
* {api-spring-framework}/messaging/SubscribableChannel.html[SubscribableChannel] --
`MessageChannel` with `MessageHandler` subscribers.
* {api-spring-framework}/messaging/support/ExecutorSubscribableChannel.html[ExecutorSubscribableChannel] --
`SubscribableChannel` that uses an `Executor` for delivering messages.

Both the Java config (i.e. `@EnableWebSocketMessageBroker`) and the XML namespace config
(i.e. `<websocket:message-broker>`) use the above components to assemble a message
workflow. The diagram below shows the components used when the simple, built-in message
broker is enabled:

image::images/message-flow-simple-broker.png[]

There are 3 message channels in the above diagram:

* `"clientInboundChannel"` -- for passing messages received from WebSocket clients.
* `"clientOutboundChannel"` -- for sending server messages to WebSocket clients.
* `"brokerChannel"` -- for sending messages to the message broker from within
server-side, application code.

The next diagram shows the components used when an external broker (e.g. RabbitMQ)
is configured for managing subscriptions and broadcasting messages:

image::images/message-flow-broker-relay.png[]

The main difference in the above diagram is the use of the "broker relay" for passing
messages up to the external STOMP broker over TCP, and for passing messages down from the
broker to subscribed clients.

When messages are received from a WebSocket connectin, they're decoded to STOMP frames,
then turned into a Spring `Message` representation, and sent to the
`"clientInboundChannel"` for further processing. For example STOMP messages whose
destination header starts with `"/app"` may be routed to `@MessageMapping` methods in
annotated controllers, while `"/topic"` and `"/queue"` messages may be routed directly
to the message broker.

An annotated `@Controller` handling a STOMP message from a client may send a message to
the message broker through the `"brokerChannel"`, and the broker will broadcast the
message to matching subscribers through the `"clientOutboundChannel"`. The same
controller can also do the same in response to HTTP requests, so a client may perform an
HTTP POST and then an `@PostMapping` method can send a message to the message broker
to broadcast to subscribed clients.

Let's trace the flow through a simple example. Given the following server setup:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebSocketMessageBroker
	public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {

		@Override
		public void registerStompEndpoints(StompEndpointRegistry registry) {
			registry.addEndpoint("/portfolio");
		}

		@Override
		public void configureMessageBroker(MessageBrokerRegistry registry) {
			registry.setApplicationDestinationPrefixes("/app");
			registry.enableSimpleBroker("/topic");
		}

	}

	@Controller
	public class GreetingController {

		@MessageMapping("/greeting") {
		public String handle(String greeting) {
			return "[" + getTimestamp() + ": " + greeting;
		}

	}

----

. Client connects to `"http://localhost:8080/portfolio"` and once a WebSocket connection
is established, STOMP frames begin to flow on it.
. Client sends SUBSCRIBE frame with destination header `"/topic/greeting"`. Once received
and decoded, the message is sent to the `"clientInboundChannel"`, then routed to the
message broker which stores the client subscription.
. Client sends SEND frame to `"/app/greeting"`. The `"/app"` prefix helps to route it to
annotated controllers. After the `"/app"` prefix is stripped, the remaining `"/greeting"`
part of the destination is mapped to the `@MessageMapping` method in `GreetingController`.
. The value returned from `GreetingController` is turned into a Spring `Message` with
a payload based on the return value and a default destination header of
`"/topic/greeting"` (derived from the input destination with `"/app"` replaced by
`"/topic"`). The resulting message is sent to the "brokerChannel" and handled
by the message broker.
. The message broker finds all matching subscribers, and sends a MESSAGE frame to each
through the `"clientOutboundChannel"` from where messages are encoded as STOMP frames
and sent on the WebSocket connection.

The next section provides more details on annotated methods including the
kinds of arguments and return values supported.



[[websocket-stomp-handle-annotations]]
=== Annotated Controllers

Applications can use annotated `@Controller` classes to handle messages from clients.
Such classes can declare `@MessageMapping`, `@SubscribeMapping`, and `@ExceptionHandler`
methods as described next.


[[websocket-stomp-message-mapping]]
==== `@MessageMapping`

`@MessageMapping` can be used to annotate methods to route messages based on their
destination. It is supported at the method level as well as at the type level. At type
level `@MessageMapping` is used to express shared mappings across all methods in a
controller.

The mapping values are Ant-style path patterns by default, e.g. "/foo*", "/foo/**"
including support for template variables, e.g. "/foo/{id}", that can be referenced via
`@DestinationVariable` method arguments. Applications can also switch to a dot-separated
destination convention for mappings, as explained in <<websocket-stomp-destination-separator>>.

*Supported Method Arguments*

[cols="1,2", options="header"]
|===
| Method argument | Description

| `Message`
| For access to the complete message.

| `MessageHeaders`
| For access to the headers within the `Message`.

| `MessageHeaderAccessor`, `SimpMessageHeaderAccessor`, `StompHeaderAccessor`
| For access to the headers via typed accessor methods.

| `@Payload`
| For access to the payload of the message, converted (e.g. from JSON) via a configured
`MessageConverter`.

The presence of this annotation is not required since it is assumed by default if no
other argument is matched.

Payload arguments may be annotated with `@javax.validation.Valid` or Spring's `@Validated`
in order to be automatically validated.

| `@Header`
| For access to a specific header value along with type conversion using an
`org.springframework.core.convert.converter.Converter` if necessary.

| `@Headers`
| For access to all headers in the message. This argument must be assignable to
`java.util.Map`.

| `@DestinationVariable`
| For access to template variables extracted from the message destination.
Values will be converted to the declared method argument type as necessary.

| `java.security.Principal`
| Reflects the user logged in at the time of the WebSocket HTTP handshake.

|===

*Return Values*

By default, the return value from an `@MessageMapping` method is serialized to a payload
through a matching `MessageConverter`, and sent as a `Message` to the `"brokerChannel"`
from where it is broadcast to subscribers. The destination of the outbound message is the
same as that of the inbound message but prefixed with `"/topic"`.

The `@SendTo` and `@SendToUser` annotations can be used to customize the destination of
the output message. `@SendTo` is used to simply customize target destination, or to
specify multiple destinations. `@SendToUser` is used to direct the output message only
to the user associated with the input message, see <<websocket-stomp-user-destination>>.

`@SendTo` and `@SendToUser` may both be used at the same time on the same method, and both
are supported at the class level in which case they act as a default for methods in the
class. However keep in mind that _any_ method-level `@SendTo` or `@SendToUser` annotations
override _any_ such annotations at the class level.

Messages may be handled asynchronously and a `@MessageMapping` method may return
`ListenableFuture`, `CompletableFuture`, or `CompletionStage`.

Note that `@SendTo` and `@SendToUser` are merely a convenience that amounts to using the
`SimpMessagingTemplate` to send messages. If necessary, for more advanced scenarios,
`@MessageMapping` methods can fall back on using the `SimpMessagingTemplate` directly.
This can be done instead of, or possibly in addition to returning a value.
See <<websocket-stomp-handle-send>>.


[[websocket-stomp-subscribe-mapping]]
==== `@SubscribeMapping`

`@SubscribeMapping` is similar to `@MessageMapping` but narrows the mapping to
subscription messages only. It supports the same
<<websocket-stomp-message-mapping,method arguments>> as `@MessageMapping` does. However
for the return value, by default a message is sent directly to the client via
"clientOutboundChannel" in response to the subscription, and not to the broker via
"brokerChannel" as a broadcast to matching subscriptions. Adding `@SendTo` or
`@SendToUser` overrides this behavior and sends to the broker instead.

When is this useful? Let's assume the broker is mapped to "/topic" and "/queue" while
application controllers are mapped to "/app". In this setup, the broker *stores* all
subscriptions to "/topic" and "/queue" that are intended for *repeated* broadcasts, and
there is no need for the application to get involved. A client could also also subscribe to
some "/app" destination and a controller could return a value in response to that
subscription without involving the broker, effectively a *one-off*, *request-reply* exchange,
without storing or using the subscription again. One case for this is populating a UI
with initial data on startup.

When is this not useful? Do not try to map broker and controllers to the same destination
prefix unless you want both to process messages, including subscriptions, independently
for some reason. Inbound messages are handled in parallel. There are no guarantees whether
broker or controller will process a given message first. If the goal is to be notified
when a subscription is stored and ready for broadcasts, then a client should ask for a
receipt if the server supports it (simple broker does not). For example with the Java
<<websocket-stomp-client>>:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Autowired
	private TaskScheduler messageBrokerTaskScheduler;

	// During initialization..
	stompClient.setTaskScheduler(this.messageBrokerTaskScheduler);

	// When subscribing..
	StompHeaders headers = new StompHeaders();
	headers.setDestination("/topic/...");
	headers.setReceipt("r1");
	FrameHandler handler = ...;
	stompSession.subscribe(headers, handler).addReceiptTask(() -> {
		// Subscription ready...
	});
----

A server side option is <<websocket-stomp-interceptors,to register>> an
`ExecutorChannelInterceptor` on the `brokerChannel` and implement the `afterMessageHandled`
method that is invoked after messages, including subscriptions, have been handled.


[[websocket-stomp-exception-handler]]
==== `@MessageExceptionHandler`

An application can use `@MessageExceptionHandler` methods to handle exceptions from
`@MessageMapping` methods. Exceptions of interest can be declared in the annotation
itself, or through a method argument if you want to get access to the exception instance:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Controller
	public class MyController {

		// ...

		@MessageExceptionHandler
		public ApplicationError handleException(MyException exception) {
			// ...
			return appError;
		}
	}
----

`@MessageExceptionHandler` methods support flexible method signatures and support the same
method argument types and return values as <<websocket-stomp-message-mapping>> methods.

Typically `@MessageExceptionHandler` methods apply within the `@Controller` class (or
class hierarchy) they are declared in. If you want such methods to apply more globally,
across controllers, you can declare them in a class marked with `@ControllerAdvice`.
This is comparable to <<web.adoc#mvc-ann-controller-advice,similar support>> in Spring MVC.



[[websocket-stomp-handle-send]]
=== Send Messages

What if you want to send messages to connected clients from any part of the
application? Any application component can send messages to the `"brokerChannel"`.
The easiest way to do that is to have a `SimpMessagingTemplate` injected, and
use it to send messages. Typically it should be easy to have it injected by
type, for example:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Controller
	public class GreetingController {

		private SimpMessagingTemplate template;

		@Autowired
		public GreetingController(SimpMessagingTemplate template) {
			this.template = template;
		}

		@RequestMapping(path="/greetings", method=POST)
		public void greet(String greeting) {
			String text = "[" + getTimestamp() + "]:" + greeting;
			this.template.convertAndSend("/topic/greetings", text);
		}

	}
----

But it can also be qualified by its name "brokerMessagingTemplate" if another
bean of the same type exists.



[[websocket-stomp-handle-simple-broker]]
=== Simple Broker

The built-in, simple message broker handles subscription requests from clients,
stores them in memory, and broadcasts messages to connected clients with matching
destinations. The broker supports path-like destinations, including subscriptions
to Ant-style destination patterns.

[NOTE]
====
Applications can also use dot-separated destinations (vs slash).
See <<websocket-stomp-destination-separator>>.
====

If configured with a task scheduler, the simple broker supports
https://stomp.github.io/stomp-specification-1.2.html#Heart-beating[STOMP heartbeats].
For that you can declare your own scheduler, or use the one that's automatically
declared and used internally:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
@Configuration
@EnableWebSocketMessageBroker
public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {

	private TaskScheduler messageBrokerTaskScheduler;

	@Autowired
	public void setMessageBrokerTaskScheduler(TaskScheduler taskScheduler) {
		this.messageBrokerTaskScheduler = taskScheduler;
	}

	@Override
	public void configureMessageBroker(MessageBrokerRegistry registry) {

		registry.enableSimpleBroker("/queue/", "/topic/")
				.setHeartbeatValue(new long[] {10000, 20000})
				.setTaskScheduler(this.messageBrokerTaskScheduler);

		// ...
	}
}
----



[[websocket-stomp-handle-broker-relay]]
=== External Broker

The simple broker is great for getting started but supports only a subset of
STOMP commands (e.g. no acks, receipts, etc.), relies on a simple message
sending loop, and is not suitable for clustering. As an alternative, applications
can upgrade to using a full-featured message broker.

Check the STOMP documentation for your message broker of choice (e.g.
http://www.rabbitmq.com/stomp.html[RabbitMQ],
http://activemq.apache.org/stomp.html[ActiveMQ], etc.), install the broker,
and run it with STOMP support enabled. Then enable the STOMP broker relay in the
Spring configuration instead of the simple broker.

Below is example configuration that enables a full-featured broker:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebSocketMessageBroker
	public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {

		@Override
		public void registerStompEndpoints(StompEndpointRegistry registry) {
			registry.addEndpoint("/portfolio").withSockJS();
		}

		@Override
		public void configureMessageBroker(MessageBrokerRegistry registry) {
			registry.enableStompBrokerRelay("/topic", "/queue");
			registry.setApplicationDestinationPrefixes("/app");
		}

	}
----

XML configuration equivalent:

[source,xml,indent=0]
[subs="verbatim,quotes,attributes"]
----
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns:websocket="http://www.springframework.org/schema/websocket"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/websocket
			http://www.springframework.org/schema/websocket/spring-websocket.xsd">

		<websocket:message-broker application-destination-prefix="/app">
			<websocket:stomp-endpoint path="/portfolio" />
				<websocket:sockjs/>
			</websocket:stomp-endpoint>
			<websocket:stomp-broker-relay prefix="/topic,/queue" />
		</websocket:message-broker>

	</beans>
----

The "STOMP broker relay" in the above configuration is a Spring
{api-spring-framework}/messaging/MessageHandler.html[MessageHandler]
that handles messages by forwarding them to an external message broker.
To do so it establishes TCP connections to the broker, forwards all
messages to it, and then forwards all messages received
from the broker to clients through their WebSocket sessions. Essentially
it acts as a "relay" that forwards messages in both directions.

[NOTE]
====
Please add `io.projectreactor.netty:reactor-netty` and `io.netty:netty-all`
dependencies to your project for TCP connection management.
====

Furthermore, application components (e.g. HTTP request handling methods,
business services, etc.) can also send messages to the broker relay, as described
in <<websocket-stomp-handle-send>>, in order to broadcast messages to
subscribed WebSocket clients.

In effect, the broker relay enables robust and scalable message broadcasting.



[[websocket-stomp-handle-broker-relay-configure]]
=== Connect to Broker

A STOMP broker relay maintains a single "system" TCP connection to the broker.
This connection is used for messages originating from the server-side application
only, not for receiving messages. You can configure the STOMP credentials
for this connection, i.e. the STOMP frame `login` and `passcode` headers. This
is exposed in both the XML namespace and the Java config as the
``systemLogin``/``systemPasscode`` properties with default values ``guest``/``guest``.

The STOMP broker relay also creates a separate TCP connection for every connected
WebSocket client. You can configure the STOMP credentials to use for all TCP
connections created on behalf of clients. This is exposed in both the XML namespace
and the Java config as the ``clientLogin``/``clientPasscode`` properties with default
values ``guest``/``guest``.

[NOTE]
====
The STOMP broker relay always sets the `login` and `passcode` headers on every `CONNECT`
frame that it forwards to the broker on behalf of clients. Therefore WebSocket clients
need not set those headers; they will be ignored. As the <<websocket-stomp-authentication>>
section explains, instead WebSocket clients should rely on HTTP authentication to protect
the WebSocket endpoint and establish the client identity.
====

The STOMP broker relay also sends and receives heartbeats to and from the message
broker over the "system" TCP connection. You can configure the intervals for sending
and receiving heartbeats (10 seconds each by default). If connectivity to the broker
is lost, the broker relay will continue to try to reconnect, every 5 seconds,
until it succeeds.

Any Spring bean can implement `ApplicationListener<BrokerAvailabilityEvent>` in order
to receive notifications when the "system" connection to the broker is lost and
re-established. For example a Stock Quote service broadcasting stock quotes can
stop trying to send messages when there is no active "system" connection.

By default, the STOMP broker relay always connects, and reconnects as needed if
connectivity is lost, to the same host and port. If you wish to supply multiple addresses,
on each attempt to connect, you can configure a supplier of addresses, instead of a
fixed host and port. For example:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
@Configuration
@EnableWebSocketMessageBroker
public class WebSocketConfig extends AbstractWebSocketMessageBrokerConfigurer {

	// ...

	@Override
	public void configureMessageBroker(MessageBrokerRegistry registry) {
		registry.enableStompBrokerRelay("/queue/", "/topic/").setTcpClient(createTcpClient());
		registry.setApplicationDestinationPrefixes("/app");
	}

	private ReactorNettyTcpClient<byte[]> createTcpClient() {

		Consumer<ClientOptions.Builder<?>> builderConsumer = builder -> {
			builder.connectAddress(()-> {
				// Select address to connect to ...
			});
		};

		return new ReactorNettyTcpClient<>(builderConsumer, new StompReactorNettyCodec());
	}
}
----

The STOMP broker relay can also be configured with a `virtualHost` property.
The value of this property will be set as the `host` header of every `CONNECT` frame
and may be useful for example in a cloud environment where the actual host to which
the TCP connection is established is different from the host providing the
cloud-based STOMP service.



[[websocket-stomp-destination-separator]]
=== Dot as Separator

When messages are routed to `@MessageMapping` methods, they're matched with
`AntPathMatcher` and by default patterns are expected to use slash "/" as separator.
This is a good convention in a web applications and similar to HTTP URLs. However if
you are more used to messaging conventions, you can switch to using dot "." as separator.

In Java config:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebSocketMessageBroker
	public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {

		// ...

		@Override
		public void configureMessageBroker(MessageBrokerRegistry registry) {
			registry.setPathMatcher(**new AntPathMatcher("."));**
			registry.enableStompBrokerRelay("/queue", "/topic");
			registry.setApplicationDestinationPrefixes("/app");
		}
	}
----

In XML:

[source,xml,indent=0]
[subs="verbatim,quotes,attributes"]
----
	<beans xmlns="http://www.springframework.org/schema/beans"
			xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
			xmlns:websocket="http://www.springframework.org/schema/websocket"
			xsi:schemaLocation="
					http://www.springframework.org/schema/beans
					http://www.springframework.org/schema/beans/spring-beans.xsd
					http://www.springframework.org/schema/websocket
					http://www.springframework.org/schema/websocket/spring-websocket.xsd">

		<websocket:message-broker application-destination-prefix="/app" path-matcher="**pathMatcher**">
			<websocket:stomp-endpoint path="/stomp"/>
			<websocket:stomp-broker-relay prefix="/topic,/queue" />
		</websocket:message-broker>

		**
		<bean id="pathMatcher" class="org.springframework.util.AntPathMatcher">
			<constructor-arg index="0" value="."/>
		</bean>
		**

	</beans>
----

After that a controller may use dot "." as separator in `@MessageMapping` methods:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Controller
	@MessageMapping("foo")
	public class FooController {

		@MessageMapping("bar.{baz}")
		public void handleBaz(@DestinationVariable String baz) {
			// ...
		}
	}
----

The client can now send a message to `"/app/foo.bar.baz123"`.

In the example above we did not change the prefixes on the "broker relay" because those
depend entirely on the external message broker. Check the STOMP documentation pages of
the broker you're using to see what conventions it supports for the destination header.

The "simple broker" on the other hand does rely on the configured `PathMatcher` so if
you switch the separator that will also apply to the broker and the way matches
destinations from a message to patterns in subscriptions.



[[websocket-stomp-authentication]]
=== Authentication

Every STOMP over WebSocket messaging session begins with an HTTP request --
that can be a request to upgrade to WebSockets (i.e. a WebSocket handshake)
or in the case of SockJS fallbacks a series of SockJS HTTP transport requests.

Web applications already have authentication and authorization in place to
secure HTTP requests. Typically a user is authenticated via Spring Security
using some mechanism such as a login page, HTTP basic authentication, or other.
The security context for the authenticated user is saved in the HTTP session
and is associated with subsequent requests in the same cookie-based session.

Therefore for a WebSocket handshake, or for SockJS HTTP transport requests,
typically there will already be an authenticated user accessible via
`HttpServletRequest#getUserPrincipal()`. Spring automatically associates that user
with a WebSocket or SockJS session created for them and subsequently with all
STOMP messages transported over that session through a user header.

In short there is nothing special a typical web application needs to do above
and beyond what it already does for security. The user is authenticated at
the HTTP request level with a security context maintained through a cookie-based
HTTP session which is then associated with WebSocket or SockJS sessions created
for that user and results in a user header stamped on every `Message` flowing
through the application.

Note that the STOMP protocol does have a "login" and "passcode" headers
on the `CONNECT` frame. Those were originally designed for and are still needed
for example for STOMP over TCP. However for STOMP over WebSocket by default
Spring ignores authorization headers at the STOMP protocol level and assumes
the user is already authenticated at the HTTP transport level and expects that
the WebSocket or SockJS session contain the authenticated user.

[NOTE]
====
Spring Security provides
https://docs.spring.io/spring-security/site/docs/current/reference/htmlsingle/#websocket[WebSocket sub-protocol authorization]
that uses a `ChannelInterceptor` to authorize messages based on the user header in them.
Also Spring Session provides a
http://docs.spring.io/spring-session/docs/current/reference/html5/#websocket[WebSocket integration]
that ensures the user HTTP session does not expire when the WebSocket session is still active.
====



[[websocket-stomp-authentication-token-based]]
=== Token Authentication

https://github.com/spring-projects/spring-security-oauth[Spring Security OAuth]
provides support for token based security including JSON Web Token (JWT).
This can be used as the authentication mechanism in Web applications
including STOMP over WebSocket interactions just as described in the previous
section, i.e. maintaining identity through a cookie-based session.

At the same time cookie-based sessions are not always the best fit for example
in applications that don't wish to maintain a server-side session at all or in
mobile applications where it's common to use headers for authentication.

The https://tools.ietf.org/html/rfc6455#section-10.5[WebSocket protocol RFC 6455]
"doesn't prescribe any particular way that servers can authenticate clients during
the WebSocket handshake." In practice however browser clients can only use standard
authentication headers (i.e. basic HTTP authentication) or cookies and cannot for example
provide custom headers. Likewise the SockJS JavaScript client does not provide
a way to send HTTP headers with SockJS transport requests, see
https://github.com/sockjs/sockjs-client/issues/196[sockjs-client issue 196].
Instead it does allow sending query parameters that can be used to send a token
but that has its own drawbacks, for example as the token may be inadvertently
logged with the URL in server logs.

[NOTE]
====
The above limitations are for browser-based clients and do not apply to the
Spring Java-based STOMP client which does support sending headers with both
WebSocket and SockJS requests.
====

Therefore applications that wish to avoid the use of cookies may not have any good
alternatives for authentication at the HTTP protocol level. Instead of using cookies
they may prefer to authenticate with headers at the STOMP messaging protocol level
There are 2 simple steps to doing that:

1. Use the STOMP client to pass authentication header(s) at connect time.
2. Process the authentication header(s) with a `ChannelInterceptor`.

Below is the example server-side configuration to register a custom authentication
interceptor. Note that an interceptor only needs to authenticate and set
the user header on the CONNECT `Message`. Spring will note and save the authenticated
user and associate it with subsequent STOMP messages on the same session:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebSocketMessageBroker
	public class MyConfig implements WebSocketMessageBrokerConfigurer {

		@Override
		public void configureClientInboundChannel(ChannelRegistration registration) {
			registration.setInterceptors(new ChannelInterceptorAdapter() {
				@Override
				public Message<?> preSend(Message<?> message, MessageChannel channel) {
					StompHeaderAccessor accessor =
							MessageHeaderAccessor.getAccessor(message, StompHeaderAccessor.class);
					if (StompCommand.CONNECT.equals(accessor.getCommand())) {
						Authentication user = ... ; // access authentication header(s)
						accessor.setUser(user);
					}
					return message;
				}
			});
		}
	}
----

Also note that when using Spring Security's authorization for messages, at present
you will need to ensure that the authentication `ChannelInterceptor` config is ordered
ahead of Spring Security's. This is best done by declaring the custom interceptor in
its own implementation of `WebSocketMessageBrokerConfigurer` marked with
`@Order(Ordered.HIGHEST_PRECEDENCE + 99)`.



[[websocket-stomp-user-destination]]
=== User Destinations

An application can send messages targeting a specific user, and Spring's STOMP support
recognizes destinations prefixed with `"/user/"` for this purpose.
For example, a client might subscribe to the destination `"/user/queue/position-updates"`.
This destination will be handled by the `UserDestinationMessageHandler` and
transformed into a destination unique to the user session,
e.g. `"/queue/position-updates-user123"`. This provides the convenience of subscribing
to a generically named destination while at the same time ensuring no collisions
with other users subscribing to the same destination so that each user can receive
unique stock position updates.

On the sending side messages can be sent to a destination such as
`"/user/{username}/queue/position-updates"`, which in turn will be translated
by the `UserDestinationMessageHandler` into one or more destinations, one for each
session associated with the user. This allows any component within the application to
send messages targeting a specific user without necessarily knowing anything more
than their name and the generic destination. This is also supported through an
annotation as well as a messaging template.

For example, a message-handling method can send messages to the user associated with
the message being handled through the `@SendToUser` annotation (also supported on
the class-level to share a common destination):

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Controller
	public class PortfolioController {

		@MessageMapping("/trade")
		@SendToUser("/queue/position-updates")
		public TradeResult executeTrade(Trade trade, Principal principal) {
			// ...
			return tradeResult;
		}
	}
----

If the user has more than one session, by default all of the sessions subscribed
to the given destination are targeted. However sometimes, it may be necessary to
target only the session that sent the message being handled. This can be done by
setting the `broadcast` attribute to false, for example:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Controller
	public class MyController {

		@MessageMapping("/action")
		public void handleAction() throws Exception{
			// raise MyBusinessException here
		}

		@MessageExceptionHandler
		@SendToUser(destinations="/queue/errors", broadcast=false)
		public ApplicationError handleException(MyBusinessException exception) {
			// ...
			return appError;
		}
	}
----

[NOTE]
====
While user destinations generally imply an authenticated user, it isn't required
strictly. A WebSocket session that is not associated with an authenticated user
can subscribe to a user destination. In such cases the `@SendToUser` annotation
will behave exactly the same as with `broadcast=false`, i.e. targeting only the
session that sent the message being handled.
====

It is also possible to send a message to user destinations from any application
component by injecting the `SimpMessagingTemplate` created by the Java config or
XML namespace, for example (the bean name is `"brokerMessagingTemplate"` if required
for qualification with `@Qualifier`):

[source,java,indent=0]
[subs="verbatim,quotes"]
----
@Service
public class TradeServiceImpl implements TradeService {

	private final SimpMessagingTemplate messagingTemplate;

	@Autowired
	public TradeServiceImpl(SimpMessagingTemplate messagingTemplate) {
		this.messagingTemplate = messagingTemplate;
	}

	// ...

	public void afterTradeExecuted(Trade trade) {
		this.messagingTemplate.convertAndSendToUser(
				trade.getUserName(), "/queue/position-updates", trade.getResult());
	}
}
----

[NOTE]
====
When using user destinations with an external message broker, check the broker
documentation on how to manage inactive queues, so that when the user session is
over, all unique user queues are removed. For example, RabbitMQ creates auto-delete
queues when destinations like `/exchange/amq.direct/position-updates` are used.
So in that case the client could subscribe to `/user/exchange/amq.direct/position-updates`.
Similarly, ActiveMQ has
http://activemq.apache.org/delete-inactive-destinations.html[configuration options]
for purging inactive destinations.
====

In a multi-application server scenario a user destination may remain unresolved because
the user is connected to a different server. In such cases you can configure a
destination to broadcast unresolved messages to so that other servers have a chance to try.
This can be done through the `userDestinationBroadcast` property of the
`MessageBrokerRegistry` in Java config and the `user-destination-broadcast` attribute
of the `message-broker` element in XML.



[[websocket-stomp-ordered-messages]]
=== Order of Messages

Messages from the broker are published to the "clientOutboundChannel" from where they are
written to WebSocket sessions. As the channel is backed by a `ThreadPoolExecutor` messages
are processed in different threads, and the resulting sequence received by the client may
not match the exact order of publication.

If this is an issue, enable the following flag:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebSocketMessageBroker
	public class MyConfig implements WebSocketMessageBrokerConfigurer {

		@Override
		protected void configureMessageBroker(MessageBrokerRegistry registry) {
			// ...
			registry.setPreservePublishOrder(true);
		}

	}
----

The same in XML:

[source,xml,indent=0]
[subs="verbatim,quotes,attributes"]
----
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns:websocket="http://www.springframework.org/schema/websocket"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/websocket
			http://www.springframework.org/schema/websocket/spring-websocket.xsd">

		<websocket:message-broker preserve-publish-order="true">
			<!-- ... -->
		</websocket:message-broker>

	</beans>
----

When the flag is set, messages within the same client session are published to the
"clientOutboundChannel" one at a time, so that the order of publication is guaranteed.
Note that this incurs a small performance overhead, so enable it only if required.



[[websocket-stomp-appplication-context-events]]
=== Events

Several `ApplicationContext` events (listed below) are published and can be
received by implementing Spring's `ApplicationListener` interface.

* `BrokerAvailabilityEvent` -- indicates when the broker becomes available/unavailable.
While the "simple" broker becomes available immediately on startup and remains so while
the application is running, the STOMP "broker relay" may lose its connection
to the full featured broker, for example if the broker is restarted. The broker relay
has reconnect logic and will re-establish the "system" connection to the broker
when it comes back, hence this event is published whenever the state changes from connected
to disconnected and vice versa. Components using the `SimpMessagingTemplate` should
subscribe to this event and avoid sending messages at times when the broker is not
available. In any case they should be prepared to handle `MessageDeliveryException`
when sending a message.
* `SessionConnectEvent` -- published when a new STOMP CONNECT is received
indicating the start of a new client session. The event contains the message representing the
connect including the session id, user information (if any), and any custom headers the client
may have sent. This is useful for tracking client sessions. Components subscribed
to this event can wrap the contained message using `SimpMessageHeaderAccessor` or
`StompMessageHeaderAccessor`.
* `SessionConnectedEvent` -- published shortly after a `SessionConnectEvent` when the
broker has sent a STOMP CONNECTED frame in response to the CONNECT. At this point the
STOMP session can be considered fully established.
* `SessionSubscribeEvent` -- published when a new STOMP SUBSCRIBE is received.
* `SessionUnsubscribeEvent` -- published when a new STOMP UNSUBSCRIBE is received.
* `SessionDisconnectEvent` -- published when a STOMP session ends. The DISCONNECT may
have been sent from the client, or it may also be automatically generated when the
WebSocket session is closed. In some cases this event may be published more than once
per session. Components should be idempotent with regard to multiple disconnect events.

[NOTE]
====
When using a full-featured broker, the STOMP "broker relay" automatically reconnects the
"system" connection in case the broker becomes temporarily unavailable. Client connections
however are not automatically reconnected. Assuming heartbeats are enabled, the client
will typically notice the broker is not responding within 10 seconds. Clients need to
implement their own reconnect logic.
====



[[websocket-stomp-interceptors]]
=== Interception

<<websocket-stomp-appplication-context-events>> provide notifications for the lifecycle
of a STOMP connection and not for every client message. Applications can also register a
`ChannelInterceptor` to intercept any message, and in any part of the processing chain.
For example to intercept inbound messages from clients:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebSocketMessageBroker
	public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {

		@Override
		public void configureClientInboundChannel(ChannelRegistration registration) {
			registration.setInterceptors(new MyChannelInterceptor());
		}
	}
----

A custom `ChannelInterceptor` can use `StompHeaderAccessor` or `SimpMessageHeaderAccessor`
to access information about the message.

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	public class MyChannelInterceptor implements ChannelInterceptor {

		@Override
		public Message<?> preSend(Message<?> message, MessageChannel channel) {
			StompHeaderAccessor accessor = StompHeaderAccessor.wrap(message);
			StompCommand command = accessor.getStompCommand();
			// ...
			return message;
		}
	}
----

Applications may also implement `ExecutorChannelInterceptor` which is a sub-interface
of `ChannelInterceptor` with callbacks in the thread in which the messages are handled.
While a `ChannelInterceptor` is invoked once for per message sent to a channel, the
`ExecutorChannelInterceptor` provides hooks in the thread of each `MessageHandler`
subscribed to messages from the channel.

Note that just like with the `SesionDisconnectEvent` above, a DISCONNECT message
may have been sent from the client, or it may also be automatically generated when
the WebSocket session is closed. In some cases an interceptor may intercept this
message more than once per session. Components should be idempotent with regard to
multiple disconnect events.



[[websocket-stomp-client]]
=== STOMP Client

Spring provides a STOMP over WebSocket client and a STOMP over TCP client.

To begin create and configure `WebSocketStompClient`:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	WebSocketClient webSocketClient = new StandardWebSocketClient();
	WebSocketStompClient stompClient = new WebSocketStompClient(webSocketClient);
	stompClient.setMessageConverter(new StringMessageConverter());
	stompClient.setTaskScheduler(taskScheduler); // for heartbeats
----

In the above example `StandardWebSocketClient` could be replaced with `SockJsClient`
since that is also an implementation of `WebSocketClient`. The `SockJsClient` can
use WebSocket or HTTP-based transport as a fallback. For more details see
<<websocket-fallback-sockjs-client>>.

Next establish a connection and provide a handler for the STOMP session:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	String url = "ws://127.0.0.1:8080/endpoint";
	StompSessionHandler sessionHandler = new MyStompSessionHandler();
	stompClient.connect(url, sessionHandler);
----

When the session is ready for use the handler is notified:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
public class MyStompSessionHandler extends StompSessionHandlerAdapter {

	@Override
	public void afterConnected(StompSession session, StompHeaders connectedHeaders) {
		// ...
	}
}
----

Once the session is established any payload can be sent and that will be
serialized with the configured `MessageConverter`:
 
[source,java,indent=0]
[subs="verbatim,quotes"]
----
session.send("/topic/foo", "payload");
----

You can also subscribe to destinations. The `subscribe` methods require a handler
for messages on the subscription and return a `Subscription` handle that can be
used to unsubscribe. For each received message the handler can specify the target
Object type the payload should be deserialized to:
 
[source,java,indent=0]
[subs="verbatim,quotes"]
----
session.subscribe("/topic/foo", new StompFrameHandler() {

	@Override
	public Type getPayloadType(StompHeaders headers) {
		return String.class;
	}

	@Override
	public void handleFrame(StompHeaders headers, Object payload) {
		// ...
	}

});
----

To enable STOMP heartbeat configure `WebSocketStompClient` with a `TaskScheduler`
and optionally customize the heartbeat intervals, 10 seconds for write inactivity
which causes a heartbeat to be sent and 10 seconds for read inactivity which
closes the connection.

[NOTE]
====
When using `WebSocketStompClient` for performance tests to simulate thousands
of clients from the same machine consider turning off heartbeats since each
connection schedules its own heartbeat tasks and that's not optimized for a
a large number of clients running on the same machine.
====


The STOMP protocol also supports receipts where the client must add a "receipt"
header to which the server responds with a RECEIPT frame after the send or
subscribe are processed. To support this the `StompSession` offers
`setAutoReceipt(boolean)` that causes a "receipt" header to be
added on every subsequent send or subscribe.
Alternatively you can also manually add a "receipt" header to the `StompHeaders`.
Both send and subscribe return an instance of `Receiptable`
that can be used to register for receipt success and failure callbacks.
For this feature the client must be configured with a `TaskScheduler`
and the amount of time before a receipt expires (15 seconds by default).

Note that `StompSessionHandler` itself is a `StompFrameHandler` which allows
it to handle ERROR frames in addition to the `handleException` callback for
exceptions from the handling of messages, and `handleTransportError` for
transport-level errors including `ConnectionLostException`.



[[websocket-stomp-websocket-scope]]
=== WebSocket Scope

Each WebSocket session has a map of attributes. The map is attached as a header to
inbound client messages and may be accessed from a controller method, for example:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
@Controller
public class MyController {

	@MessageMapping("/action")
	public void handle(SimpMessageHeaderAccessor headerAccessor) {
		Map<String, Object> attrs = headerAccessor.getSessionAttributes();
		// ...
	}
}
----

It is also possible to declare a Spring-managed bean in the `websocket` scope.
WebSocket-scoped beans can be injected into controllers and any channel interceptors
registered on the "clientInboundChannel". Those are typically singletons and live
longer than any individual WebSocket session. Therefore you will need to use a
scope proxy mode for WebSocket-scoped beans:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Component
	@Scope(scopeName = "websocket", proxyMode = ScopedProxyMode.TARGET_CLASS)
	public class MyBean {

		@PostConstruct
		public void init() {
			// Invoked after dependencies injected
		}

		// ...

		@PreDestroy
		public void destroy() {
			// Invoked when the WebSocket session ends
		}
	}

	@Controller
	public class MyController {

		private final MyBean myBean;

		@Autowired
		public MyController(MyBean myBean) {
			this.myBean = myBean;
		}

		@MessageMapping("/action")
		public void handle() {
			// this.myBean from the current WebSocket session
		}
	}
----

As with any custom scope, Spring initializes a new `MyBean` instance the first
time it is accessed from the controller and stores the instance in the WebSocket
session attributes. The same instance is returned subsequently until the session
ends. WebSocket-scoped beans will have all Spring lifecycle methods invoked as
shown in the examples above.



[[websocket-stomp-configuration-performance]]
=== Performance

There is no silver bullet when it comes to performance. Many factors may
affect it including the size of messages, the volume, whether application
methods perform work that requires blocking, as well as external factors
such as network speed and others. The goal of this section is to provide
an overview of the available configuration options along with some thoughts
on how to reason about scaling.

In a messaging application messages are passed through channels for asynchronous
executions backed by thread pools. Configuring such an application requires
good knowledge of the channels and the flow of messages. Therefore it is
recommended to review <<websocket-stomp-message-flow>>.

The obvious place to start is to configure the thread pools backing the
`"clientInboundChannel"` and the `"clientOutboundChannel"`. By default both
are configured at twice the number of available processors.

If the handling of messages in annotated methods is mainly CPU bound then the
number of threads for the `"clientInboundChannel"` should remain close to the
number of processors. If the work they do is more IO bound and requires blocking
or waiting on a database or other external system then the thread pool size
will need to be increased.

[NOTE]
====
`ThreadPoolExecutor` has 3 important properties. Those are the core and
the max thread pool size as well as the capacity for the queue to store
tasks for which there are no available threads.

A common point of confusion is that configuring the core pool size (e.g. 10)
and max pool size (e.g. 20) results in a thread pool with 10 to 20 threads.
In fact if the capacity is left at its default value of Integer.MAX_VALUE
then the thread pool will never increase beyond the core pool size since
all additional tasks will be queued.

Please review the Javadoc of `ThreadPoolExecutor` to learn how these
properties work and understand the various queuing strategies.
====

On the `"clientOutboundChannel"` side it is all about sending messages to WebSocket
clients. If clients are on a fast network then the number of threads should
remain close to the number of available processors. If they are slow or on
low bandwidth they will take longer to consume messages and put a burden on the
thread pool. Therefore increasing the thread pool size will be necessary.

While the workload for the "clientInboundChannel" is possible to predict --
after all it is based on what the application does -- how to configure the
"clientOutboundChannel" is harder as it is based on factors beyond
the control of the application. For this reason there are two additional
properties related to the sending of messages. Those are the `"sendTimeLimit"`
and the `"sendBufferSizeLimit"`. Those are used to configure how long a
send is allowed to take and how much data can be buffered when sending
messages to a client.

The general idea is that at any given time only a single thread may be used
to send to a client. All additional messages meanwhile get buffered and you
can use these properties to decide how long sending a message is allowed to
take and how much data can be buffered in the mean time. Please review the
Javadoc and documentation of the XML schema for this configuration for
important additional details.

Here is example configuration:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebSocketMessageBroker
	public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {

		@Override
		public void configureWebSocketTransport(WebSocketTransportRegistration registration) {
			registration.setSendTimeLimit(15 * 1000).setSendBufferSizeLimit(512 * 1024);
		}

		// ...

	}
----

[source,xml,indent=0]
[subs="verbatim,quotes,attributes"]
----
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns:websocket="http://www.springframework.org/schema/websocket"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/websocket
			http://www.springframework.org/schema/websocket/spring-websocket.xsd">

		<websocket:message-broker>
			<websocket:transport send-timeout="15000" send-buffer-size="524288" />
			<!-- ... -->
		</websocket:message-broker>

	</beans>
----

The WebSocket transport configuration shown above can also be used to configure the
maximum allowed size for incoming STOMP messages. Although in theory a WebSocket
message can be almost unlimited in size, in practice WebSocket servers impose
limits -- for example, 8K on Tomcat and 64K on Jetty. For this reason STOMP clients
such as the JavaScript https://github.com/JSteunou/webstomp-client[webstomp-client]
and others split larger STOMP messages at 16K boundaries and send them as multiple
WebSocket  messages thus requiring the server to buffer and re-assemble.

Spring's STOMP over WebSocket support does this so applications can configure the
maximum size for STOMP messages irrespective of WebSocket server specific message
sizes. Do keep in mind that the WebSocket message size will be automatically
adjusted if necessary to ensure they can carry 16K WebSocket messages at a
minimum.

Here is example configuration:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebSocketMessageBroker
	public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {

		@Override
		public void configureWebSocketTransport(WebSocketTransportRegistration registration) {
			registration.setMessageSizeLimit(128 * 1024);
		}

		// ...

	}
----

[source,xml,indent=0]
[subs="verbatim,quotes,attributes"]
----
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns:websocket="http://www.springframework.org/schema/websocket"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/websocket
			http://www.springframework.org/schema/websocket/spring-websocket.xsd">

		<websocket:message-broker>
			<websocket:transport message-size="131072" />
			<!-- ... -->
		</websocket:message-broker>

	</beans>
----

An important point about scaling is using multiple application instances.
Currently it is not possible to do that with the simple broker.
However when using a full-featured broker such as RabbitMQ, each application
instance connects to the broker and messages broadcast from one application
instance can be broadcast through the broker to WebSocket clients connected
through any other application instances.



[[websocket-stomp-stats]]
=== Monitoring

When using `@EnableWebSocketMessageBroker` or `<websocket:message-broker>` key
infrastructure components automatically gather stats and counters that provide
important insight into the internal state of the application. The configuration
also declares a bean of type `WebSocketMessageBrokerStats` that gathers all
available information in one place and by default logs it at `INFO` level once
every 30 minutes. This bean can be exported to JMX through Spring's
`MBeanExporter` for viewing at runtime, for example through JDK's `jconsole`.
Below is a summary of the available information.

Client WebSocket Sessions::
	Current::: indicates how many client sessions there are
	currently with the count further broken down by WebSocket vs HTTP
	streaming and polling SockJS sessions.
	Total::: indicates how many total sessions have been established.
	Abnormally Closed:::
		Connect Failures:::: these are sessions that got established but were
		closed after not having received any messages within 60 seconds. This is
		usually an indication of proxy or network issues.
		Send Limit Exceeded:::: sessions closed after exceeding the configured send
		timeout or the send buffer limits which can occur with slow clients
		(see previous section).
		Transport Errors:::: sessions closed after a transport error such as
		failure to read or write to a WebSocket connection or
		HTTP request/response.
	STOMP Frames::: the total number of CONNECT, CONNECTED, and DISCONNECT frames
	processed indicating how many clients connected on the STOMP level. Note that
	the DISCONNECT count may be lower when sessions get closed abnormally or when
	clients close without sending a DISCONNECT frame.
STOMP Broker Relay::
	TCP Connections::: indicates how many TCP connections on behalf of client
	WebSocket sessions are established to the broker. This should be equal to the
	number of client WebSocket sessions + 1 additional shared "system" connection
	for sending messages from within the application.
	STOMP Frames::: the total number of CONNECT, CONNECTED, and DISCONNECT frames
	forwarded to or received from the broker on behalf of clients. Note that a
	DISCONNECT frame is sent to the broker regardless of how the client WebSocket
	session was closed. Therefore a lower DISCONNECT frame count is an indication
	that the broker is pro-actively closing connections, may be because of a
	heartbeat that didn't arrive in time, an invalid input frame, or other.
Client Inbound Channel:: stats from thread pool backing the "clientInboundChannel"
	providing insight into the health of incoming message processing. Tasks queueing
	up here is an indication the application may be too slow to handle messages.
	If there I/O bound tasks (e.g. slow database query, HTTP request to 3rd party
	REST API, etc) consider increasing the thread pool size.
Client Outbound Channel:: stats from the thread pool backing the "clientOutboundChannel"
	providing insight into the health of broadcasting messages to clients. Tasks
	queueing up here is an indication clients are too slow to consume messages.
	One way to address this is to increase the thread pool size to accommodate the
	number of concurrent slow clients expected. Another option is to reduce the
	send timeout and send buffer size limits (see the previous section).
SockJS Task Scheduler:: stats from thread pool of the SockJS task scheduler which
	is used to send heartbeats. Note that when heartbeats are negotiated on the
	STOMP level the SockJS heartbeats are disabled.



[[websocket-stomp-testing]]
=== Testing

There are two main approaches to testing applications using Spring's STOMP over
WebSocket support. The first is to write server-side tests verifying the functionality
of controllers and their annotated message handling methods. The second is to write
full end-to-end tests that involve running a client and a server.

The two approaches are not mutually exclusive. On the contrary each has a place
in an overall test strategy. Server-side tests are more focused and easier to write
and maintain. End-to-end integration tests on the other hand are more complete and
test much more, but they're also more involved to write and maintain.

The simplest form of server-side tests is to write controller unit tests. However
this is not useful enough since much of what a controller does depends on its
annotations. Pure unit tests simply can't test that.

Ideally controllers under test should be invoked as they are at runtime, much like
the approach to testing controllers handling HTTP requests using the Spring MVC Test
framework. i.e. without running a Servlet container but relying on the Spring Framework
to invoke the annotated controllers. Just like with Spring MVC Test here there are two
two possible alternatives, either using a "context-based" or "standalone" setup:

1. Load the actual Spring configuration with the help of the
Spring TestContext framework, inject "clientInboundChannel" as a test field, and
use it to send messages to be handled by controller methods.

2. Manually set up the minimum Spring framework infrastructure required to invoke
controllers (namely the `SimpAnnotationMethodMessageHandler`) and pass messages for
controllers directly to it.

Both of these setup scenarios are demonstrated in the
https://github.com/rstoyanchev/spring-websocket-portfolio/tree/master/src/test/java/org/springframework/samples/portfolio/web[tests for the stock portfolio]
sample application.

The second approach is to create end-to-end integration tests. For that you will need
to run a WebSocket server in embedded mode and connect to it as a WebSocket client
sending WebSocket messages containing STOMP frames.
The https://github.com/rstoyanchev/spring-websocket-portfolio/tree/master/src/test/java/org/springframework/samples/portfolio/web[tests for the stock portfolio]
sample application also demonstrates this approach using Tomcat as the embedded
WebSocket server and a simple STOMP client for test purposes.