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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
"http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd">
<chapter id="remoting">
<title>Remoting and web services using Spring</title>
<section id="remoting-introduction">
<title>Introduction</title>
<para>Spring features integration classes for remoting support using various
technologies. The remoting support eases the development of remote-enabled
services, implemented by your usual (Spring) POJOs. Currently, Spring supports
four remoting technologies:
<itemizedlist>
<listitem>
<para><emphasis>Remote Method Invocation (RMI)</emphasis>. Through the use
of the <classname>RmiProxyFactoryBean</classname> and the
<classname>RmiServiceExporter</classname> Spring supports both traditional
RMI (with <interfacename>java.rmi.Remote</interfacename> interfaces and
<exceptionname>java.rmi.RemoteException</exceptionname>) and
transparent remoting via RMI invokers (with any Java interface).</para>
</listitem>
<listitem>
<para><emphasis>Spring's HTTP invoker</emphasis>. Spring provides a special
remoting strategy which allows for Java serialization via HTTP,
supporting any Java interface (just like the RMI invoker). The corresponding
support classes are <classname>HttpInvokerProxyFactoryBean</classname> and
<classname>HttpInvokerServiceExporter</classname>.</para>
</listitem>
<listitem>
<para><emphasis>Hessian</emphasis>. By using Spring's
<classname>HessianProxyFactoryBean</classname> and the
<classname>HessianServiceExporter</classname> you can transparently
expose your services using the lightweight binary HTTP-based protocol
provided by Caucho.</para>
</listitem>
<listitem>
<para><emphasis>Burlap</emphasis>. Burlap is Caucho's XML-based
alternative to Hessian. Spring provides support classes such
as <classname>BurlapProxyFactoryBean</classname> and
<classname>BurlapServiceExporter</classname>.</para>
</listitem>
<listitem>
<para><emphasis>JAX-RPC</emphasis>. Spring provides remoting support
for web services via JAX-RPC (J2EE 1.4's web service API).</para>
</listitem>
<listitem>
<para><emphasis>JAX-WS</emphasis>. Spring provides remoting support
for web services via JAX-WS (the successor of JAX-RPC, as introduced
in Java EE 5 and Java 6).</para>
</listitem>
<listitem>
<para><emphasis>JMS</emphasis>. Remoting using JMS as the underlying protocol
is supported via the <classname>JmsInvokerServiceExporter</classname> and
<classname>JmsInvokerProxyFactoryBean</classname> classes.</para>
</listitem>
</itemizedlist></para>
<para>While discussing the remoting capabilities of Spring, we'll use the following domain
model and corresponding services:</para>
<programlisting language="java"><![CDATA[public class Account implements Serializable{
private String name;
public String getName();
public void setName(String name) {
this.name = name;
}
}]]></programlisting>
<programlisting language="java"><![CDATA[public interface AccountService {
public void insertAccount(Account account);
public List getAccounts(String name);
}]]></programlisting>
<programlisting language="java"><![CDATA[public interface RemoteAccountService extends Remote {
public void insertAccount(Account account) throws RemoteException;
public List getAccounts(String name) throws RemoteException;
}]]></programlisting>
<programlisting language="java"><lineannotation>// the implementation doing nothing at the moment</lineannotation><![CDATA[
public class AccountServiceImpl implements AccountService {
public void insertAccount(Account acc) {
]]><lineannotation>// do something...</lineannotation><![CDATA[
}
public List getAccounts(String name) {
]]><lineannotation>// do something...</lineannotation><![CDATA[
}
}]]></programlisting>
<para>We will start exposing the service to a remote client by using RMI and
talk a bit about the drawbacks of using RMI. We'll then continue to show
an example using Hessian as the protocol.</para>
</section>
<section id="remoting-rmi">
<title>Exposing services using RMI</title>
<para>Using Spring's support for RMI, you can transparently expose your services through
the RMI infrastructure. After having this set up, you basically have a configuration
similar to remote EJBs, except for the fact that there is no standard support for
security context propagation or remote transaction propagation. Spring does provide
hooks for such additional invocation context when using the RMI invoker, so you can
for example plug in security frameworks or custom security credentials here.</para>
<section id="remoting-rmi-server">
<title>Exporting the service using the <classname>RmiServiceExporter</classname></title>
<para>Using the <classname>RmiServiceExporter</classname>, we can expose the interface
of our AccountService object as RMI object. The interface can be accessed by using
<classname>RmiProxyFactoryBean</classname>, or via plain RMI in case of a traditional
RMI service. The <classname>RmiServiceExporter</classname> explicitly supports the
exposing of any non-RMI services via RMI invokers.
</para>
<para>Of course, we first have to set up our service in the Spring container:</para>
<programlisting language="xml"><![CDATA[<bean id="accountService" class="example.AccountServiceImpl">
]]><lineannotation>&lt;!-- any additional properties, maybe a DAO? --&gt;</lineannotation><![CDATA[
</bean>]]></programlisting>
<para>Next we'll have to expose our service using the <classname>RmiServiceExporter</classname>:</para>
<programlisting language="xml"><![CDATA[<bean class="org.springframework.remoting.rmi.RmiServiceExporter">
]]><lineannotation>&lt;!-- does not necessarily have to be the same name as the bean to be exported --&gt;</lineannotation><![CDATA[
<property name="serviceName" value="AccountService"/>
<property name="service" ref="accountService"/>
<property name="serviceInterface" value="example.AccountService"/>
]]><lineannotation>&lt;!-- defaults to <literal>1099</literal> --&gt;</lineannotation><![CDATA[
<property name="registryPort" value="1199"/>
</bean>]]></programlisting>
<para>As you can see, we're overriding the port for the RMI registry. Often,
your application server also maintains an RMI registry and it is wise
to not interfere with that one. Furthermore, the service name is used to bind the
service under. So right now, the service will be bound at
<literal>'rmi://HOST:1199/AccountService'</literal>. We'll use the URL later on to
link in the service at the client side.</para>
<note>
<para>The <literal>servicePort</literal> property has been omitted (it defaults to 0).
This means that an anonymous port will be used to communicate with the service.</para>
</note>
</section>
<section id="remoting-rmi-client">
<title>Linking in the service at the client</title>
<para>Our client is a simple object using the <interfacename>AccountService</interfacename>
to manage accounts:</para>
<programlisting language="java"><![CDATA[public class SimpleObject {
private AccountService accountService;
public void setAccountService(AccountService accountService) {
this.accountService = accountService;
}
}]]></programlisting>
<para>To link in the service on the client, we'll create a separate Spring container,
containing the simple object and the service linking configuration bits:</para>
<programlisting language="xml"><![CDATA[<bean class="example.SimpleObject">
<property name="accountService" ref="accountService"/>
</bean>
<bean id="accountService" class="org.springframework.remoting.rmi.RmiProxyFactoryBean">
<property name="serviceUrl" value="rmi://HOST:1199/AccountService"/>
<property name="serviceInterface" value="example.AccountService"/>
</bean>]]></programlisting>
<para>That's all we need to do to support the remote account service on the client.
Spring will transparently create an invoker and remotely enable the account
service through the <classname>RmiServiceExporter</classname>. At the client
we're linking it in using the <classname>RmiProxyFactoryBean</classname>.</para>
</section>
</section>
<section id="remoting-caucho-protocols">
<title>Using Hessian or Burlap to remotely call services via HTTP</title>
<para>Hessian offers a binary HTTP-based remoting protocol. It is developed by
Caucho and more information about Hessian itself can be found at
<ulink url="http://www.caucho.com"/>.</para>
<section id="remoting-caucho-protocols-hessian">
<title>Wiring up the <classname>DispatcherServlet</classname> for Hessian and co.</title>
<para>Hessian communicates via HTTP and does so using a custom servlet.
Using Spring's <classname>DispatcherServlet</classname> principles, as known
from Spring Web MVC usage, you can easily wire up such a servlet exposing
your services. First we'll have to create a new servlet in your application
(this an excerpt from <filename>'web.xml'</filename>):</para>
<programlisting language="xml"><![CDATA[<servlet>
<servlet-name>remoting</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>remoting</servlet-name>
<url-pattern>/remoting/*</url-pattern>
</servlet-mapping>]]></programlisting>
<para>You're probably familiar with Spring's <classname>DispatcherServlet</classname>
principles and if so, you know that now you'll have to create a Spring container
configuration resource named <filename>'remoting-servlet.xml'</filename> (after
the name of your servlet) in the <filename class="directory">'WEB-INF'</filename>
directory. The application context will be used in the next section.</para>
<para>Alternatively, consider the use of Spring's simpler
<classname>HttpRequestHandlerServlet</classname>.
This allows you to embed the remote exporter definitions in your root application
context (by default in <filename>'WEB-INF/applicationContext.xml'</filename>),
with individual servlet definitions pointing to specific exporter beans.
Each servlet name needs to match the bean name of its target exporter in this case.</para>
</section>
<section id="remoting-caucho-protocols-hessian-server">
<title>Exposing your beans by using the <classname>HessianServiceExporter</classname></title>
<para>In the newly created application context called <literal>remoting-servlet.xml</literal>,
we'll create a <classname>HessianServiceExporter</classname> exporting your services:</para>
<programlisting language="xml"><![CDATA[<bean id="accountService" class="example.AccountServiceImpl">
]]><lineannotation>&lt;!-- any additional properties, maybe a DAO? --&gt;</lineannotation><![CDATA[
</bean>
<bean name="/AccountService" class="org.springframework.remoting.caucho.HessianServiceExporter">
<property name="service" ref="accountService"/>
<property name="serviceInterface" value="example.AccountService"/>
</bean>]]></programlisting>
<para>Now we're ready to link in the service at the client. No explicit handler mapping
is specified, mapping request URLs onto services, so <classname>BeanNameUrlHandlerMapping</classname>
will be used: Hence, the service will be exported at the URL indicated through
its bean name within the containing <classname>DispatcherServlet</classname>'s
mapping (as defined above): <literal>'http://HOST:8080/remoting/AccountService'</literal>.
</para>
<para>Alternatively, create a <classname>HessianServiceExporter</classname> in your
root application context (e.g. in <filename>'WEB-INF/applicationContext.xml'</filename>):</para>
<programlisting language="xml"><![CDATA[<bean name="accountExporter" class="org.springframework.remoting.caucho.HessianServiceExporter">
<property name="service" ref="accountService"/>
<property name="serviceInterface" value="example.AccountService"/>
</bean>]]></programlisting>
<para>In the latter case, define a corresponding servlet for this exporter
in <filename>'web.xml'</filename>, with the same end result: The exporter
getting mapped to the request path <literal>/remoting/AccountService</literal>.
Note that the servlet name needs to match the bean name of the target exporter.</para>
<programlisting language="xml"><![CDATA[<servlet>
<servlet-name>accountExporter</servlet-name>
<servlet-class>org.springframework.web.context.support.HttpRequestHandlerServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>accountExporter</servlet-name>
<url-pattern>/remoting/AccountService</url-pattern>
</servlet-mapping>]]></programlisting>
</section>
<section id="remoting-caucho-protocols-hessian-client">
<title>Linking in the service on the client</title>
<para>Using the <classname></classname> we can link in the service
at the client. The same principles apply as with the RMI example. We'll create
a separate bean factory or application context and mention the following beans
where the <classname>SimpleObject</classname> is using the
<interfacename>AccountService</interfacename> to manage accounts:</para>
<programlisting language="xml"><![CDATA[<bean class="example.SimpleObject">
<property name="accountService" ref="accountService"/>
</bean>
<bean id="accountService" class="org.springframework.remoting.caucho.HessianProxyFactoryBean">
<property name="serviceUrl" value="http://remotehost:8080/remoting/AccountService"/>
<property name="serviceInterface" value="example.AccountService"/>
</bean>]]></programlisting>
</section>
<section id="remoting-caucho-protocols-burlap">
<title>Using Burlap</title>
<para>We won't discuss Burlap, the XML-based equivalent of Hessian, in detail here,
since it is configured and set up in exactly the same way as the Hessian
variant explained above. Just replace the word <literal>Hessian</literal>
with <literal>Burlap</literal> and you're all set to go.</para>
</section>
<section id="remoting-caucho-protocols-security">
<title>Applying HTTP basic authentication to a service exposed through Hessian or Burlap</title>
<para>One of the advantages of Hessian and Burlap is that we can easily apply HTTP basic
authentication, because both protocols are HTTP-based. Your normal HTTP server security
mechanism can easily be applied through using the <literal>web.xml</literal> security
features, for example. Usually, you don't use per-user security credentials here, but
rather shared credentials defined at the <literal>Hessian/BurlapProxyFactoryBean</literal> level
(similar to a JDBC <interfacename>DataSource</interfacename>).</para>
<programlisting language="xml"><![CDATA[<bean class="org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping">
<property name="interceptors" ref="authorizationInterceptor"/>
</bean>
<bean id="authorizationInterceptor"
class="org.springframework.web.servlet.handler.UserRoleAuthorizationInterceptor">
<property name="authorizedRoles" value="administrator,operator"/>
</bean>]]></programlisting>
<para>This an example where we explicitly mention the <classname>BeanNameUrlHandlerMapping</classname>
and set an interceptor allowing only administrators and operators to call
the beans mentioned in this application context.</para>
<note>
<para>Of course, this example doesn't show a flexible kind of security
infrastructure. For more options as far as security is concerned,
have a look at the Acegi Security System for Spring, to be found at
<ulink url="http://acegisecurity.sourceforge.net"/>.</para>
</note>
</section>
</section>
<section id="remoting-httpinvoker">
<title>Exposing services using HTTP invokers</title>
<para>As opposed to Burlap and Hessian, which are both lightweight protocols using their
own slim serialization mechanisms, Spring Http invokers use the standard
Java serialization mechanism to expose services through HTTP. This has a huge
advantage if your arguments and return types are complex types that cannot be
serialized using the serialization mechanisms Hessian and Burlap use (refer to the
next section for more considerations when choosing a remoting technology).</para>
<para>Under the hood, Spring uses either the standard facilities provided by J2SE to
perform HTTP calls or Commons <classname>HttpClient</classname>. Use the latter if you need more advanced
and easy-to-use functionality. Refer to
<ulink url="http://jakarta.apache.org/commons/httpclient">jakarta.apache.org/commons/httpclient</ulink>
for more info.</para>
<section id="remoting-httpinvoker-server">
<title>Exposing the service object</title>
<para>Setting up the HTTP invoker infrastructure for a service objects much resembles
the way you would do using Hessian or Burlap. Just as Hessian support provides
the <classname>HessianServiceExporter</classname>, Spring's HttpInvoker support provides
the <classname>org.springframework.remoting.httpinvoker.HttpInvokerServiceExporter</classname>.
</para>
<para>To expose the <literal>AccountService</literal> (mentioned above) within a
Spring Web MVC <classname>DispatcherServlet</classname>, the following configuration
needs to be in place in the dispatcher's application context:</para>
<programlisting language="xml"><![CDATA[<bean name="/AccountService" class="org.springframework.remoting.httpinvoker.HttpInvokerServiceExporter">
<property name="service" ref="accountService"/>
<property name="serviceInterface" value="example.AccountService"/>
</bean>
]]></programlisting>
<para>Such an exporter definition will be exposed through the
<classname>DispatcherServlet</classname>'s standard mapping facilities,
as explained in the section on Hessian.</para>
<para>Alternatively, create an <classname>HttpInvokerServiceExporter</classname> in your
root application context (e.g. in <filename>'WEB-INF/applicationContext.xml'</filename>):</para>
<programlisting language="xml"><![CDATA[<bean name="accountExporter" class="org.springframework.remoting.httpinvoker.HttpInvokerServiceExporter">
<property name="service" ref="accountService"/>
<property name="serviceInterface" value="example.AccountService"/>
</bean>]]></programlisting>
<para>In addition, define a corresponding servlet for this exporter in
<filename>'web.xml'</filename>, with the servlet name matching the bean
name of the target exporter:</para>
<programlisting language="xml"><![CDATA[<servlet>
<servlet-name>accountExporter</servlet-name>
<servlet-class>org.springframework.web.context.support.HttpRequestHandlerServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>accountExporter</servlet-name>
<url-pattern>/remoting/AccountService</url-pattern>
</servlet-mapping>]]></programlisting>
</section>
<section id="remoting-httpinvoker-client">
<title>Linking in the service at the client</title>
<para>Again, linking in the service from the client much resembles the way you would
do it when using Hessian or Burlap. Using a proxy, Spring will be able to
translate your calls to HTTP POST requests to the URL pointing to the exported
service.</para>
<programlisting language="xml"><![CDATA[<bean id="httpInvokerProxy" class="org.springframework.remoting.httpinvoker.HttpInvokerProxyFactoryBean">
<property name="serviceUrl" value="http://remotehost:8080/remoting/AccountService"/>
<property name="serviceInterface" value="example.AccountService"/>
</bean>
]]></programlisting>
<para>As mentioned before, you can choose what HTTP client you want to use.
By default, the <classname>HttpInvokerProxy</classname> uses the J2SE HTTP functionality, but
you can also use the Commons <classname>HttpClient</classname> by setting the
<literal>httpInvokerRequestExecutor</literal> property:</para>
<programlisting language="xml"><![CDATA[<property name="httpInvokerRequestExecutor">
<bean class="org.springframework.remoting.httpinvoker.CommonsHttpInvokerRequestExecutor"/>
</property>
]]></programlisting>
</section>
</section>
<section id="remoting-web-services">
<title>Web services</title>
<para>Spring provides full support for standard Java web services APIs:</para>
<itemizedlist>
<listitem><para>Exposing web services using JAX-RPC</para></listitem>
<listitem><para>Accessing web services using JAX-RPC</para></listitem>
<listitem><para>Exposing web services using JAX-WS</para></listitem>
<listitem><para>Accessing web services using JAX-WS</para></listitem>
</itemizedlist>
<note>
<para>Why two standard Java web services APIs?</para>
<para>JAX-RPC 1.1 is the standard web service API in J2EE 1.4.
As its name indicates, it focuses on on RPC bindings, which became
less and less popular in the past couple of years. As a consequence,
it has been superseded by JAX-WS 2.0 in Java EE 5, being more flexible
in terms of bindings but also being heavily annotation-based. JAX-WS 2.1
is also included in Java 6 (or more specifically, in Sun's JDK 1.6.0_04
and above; previous Sun JDK 1.6.0 releases included JAX-WS 2.0),
integrated with the JDK's built-in HTTP server.</para>
<para>Spring can work with both standard Java web services APIs.
The choice is effectively dependent on the runtime platform:
On JDK 1.4 / J2EE 1.4, the only option is JAX-RPC. On Java EE 5 / Java 6,
the obvious choice is JAX-WS. On J2EE 1.4 environments that run on Java 5,
you might have the option to plug in a JAX-WS provider; check your J2EE
server's documentation.</para>
</note>
<para>In addition to stock support for JAX-RPC and JAX-WS in Spring Core, the Spring portfolio also
features <ulink url="http://www.springframework.org/spring-ws">Spring Web Services</ulink>, a solution
for contract-first, document-driven web services - highly recommended for building modern, future-proof
web services. Last but not least, <ulink url="http://xfire.codehaus.org">XFire</ulink> also allows
you to export Spring-managed beans as a web service, through built-in Spring support.</para>
<section id="remoting-web-services-jaxrpc-export">
<title>Exposing servlet-based web services using JAX-RPC</title>
<para>Spring provides a convenience base class for JAX-RPC servlet endpoint implementations -
<classname>ServletEndpointSupport</classname>. To expose our
<interfacename>AccountService</interfacename> we extend Spring's
<classname>ServletEndpointSupport</classname> class and implement our business
logic here, usually delegating the call to the business layer.</para>
<programlisting language="java"><lineannotation>/**
* JAX-RPC compliant RemoteAccountService implementation that simply delegates
* to the AccountService implementation in the root web application context.
*
* This wrapper class is necessary because JAX-RPC requires working with dedicated
* endpoint classes. If an existing service needs to be exported, a wrapper that
* extends ServletEndpointSupport for simple application context access is
* the simplest JAX-RPC compliant way.
*
* This is the class registered with the server-side JAX-RPC implementation.
* In the case of Axis, this happens in "server-config.wsdd" respectively via
* deployment calls. The web service engine manages the lifecycle of instances
* of this class: A Spring application context can just be accessed here.
*/</lineannotation><![CDATA[import org.springframework.remoting.jaxrpc.ServletEndpointSupport;
public class AccountServiceEndpoint extends ServletEndpointSupport implements RemoteAccountService {
private AccountService biz;
protected void onInit() {
this.biz = (AccountService) getWebApplicationContext().getBean("accountService");
}
public void insertAccount(Account acc) throws RemoteException {
biz.insertAccount(acc);
}
public Account[] getAccounts(String name) throws RemoteException {
return biz.getAccounts(name);
}
}]]></programlisting>
<para>Our <classname>AccountServletEndpoint</classname> needs to run in the same web
application as the Spring context to allow for access to Spring's facilities. In case of
Axis, copy the <classname>AxisServlet</classname> definition into your
<filename>'web.xml'</filename>, and set up the endpoint in
<filename>'server-config.wsdd'</filename> (or use the deploy tool). See the sample
application JPetStore where the <interfacename>OrderService</interfacename> is exposed as
a web service using Axis.</para>
</section>
<section id="remoting-web-services-jaxrpc-access">
<title>Accessing web services using JAX-RPC</title>
<para>Spring provides two factory beans to create JAX-RPC web service proxies,
namely <classname>LocalJaxRpcServiceFactoryBean</classname> and
<classname>JaxRpcPortProxyFactoryBean</classname>. The former can only return a JAX-RPC
service class for us to work with. The latter is the full-fledged version that can return
a proxy that implements our business service interface. In this example we use the latter
to create a proxy for the <interfacename>AccountService</interfacename> endpoint we exposed
in the previous section. You will see that Spring has great support for web services
requiring little coding efforts - most of the setup is done in the Spring configuration
file as usual:</para>
<programlisting language="xml"><![CDATA[<bean id="accountWebService" class="org.springframework.remoting.jaxrpc.JaxRpcPortProxyFactoryBean">
<property name="serviceInterface" value="example.RemoteAccountService"/>
<property name="wsdlDocumentUrl" value="http://localhost:8080/account/services/accountService?WSDL"/>
<property name="namespaceUri" value="http://localhost:8080/account/services/accountService"/>
<property name="serviceName" value="AccountService"/>
<property name="portName" value="AccountPort"/>
</bean>]]></programlisting>
<para>Where <literal>serviceInterface</literal> is our remote business interface the clients will use.
<literal>wsdlDocumentUrl</literal> is the URL for the WSDL file. Spring needs this a startup time to create the JAX-RPC Service.
<literal>namespaceUri</literal> corresponds to the targetNamespace in the .wsdl file.
<literal>serviceName</literal> corresponds to the service name in the .wsdl file.
<literal>portName</literal> corresponds to the port name in the .wsdl file.
</para>
<para>Accessing the web service is now very easy as we have a bean factory for it that will expose it
as <literal>RemoteAccountService</literal> interface. We can wire this up in Spring:</para>
<programlisting language="xml"><![CDATA[<bean id="client" class="example.AccountClientImpl">
...
<property name="service" ref="accountWebService"/>
</bean>]]></programlisting>
<para>From the client code we can access the web service just as if it
was a normal class, except that it throws <exceptionname>RemoteException</exceptionname>.</para>
<programlisting language="java"><![CDATA[public class AccountClientImpl {
private RemoteAccountService service;
public void setService(RemoteAccountService service) {
this.service = service;
}
public void foo() {
try {
service.insertAccount(...);
}
catch (RemoteException ex) {
]]><lineannotation>// ouch</lineannotation><![CDATA[
}
}
}
]]></programlisting>
<para>We can get rid of the checked <exceptionname>RemoteException</exceptionname> since
Spring supports automatic conversion to its corresponding unchecked
<exceptionname>RemoteException</exceptionname>. This requires that we provide a non-RMI
interface also. Our configuration is now:</para>
<programlisting language="xml"><![CDATA[<bean id="accountWebService" class="org.springframework.remoting.jaxrpc.JaxRpcPortProxyFactoryBean">
<property name="serviceInterface" value="example.AccountService"/>
<property name="portInterface" value="example.RemoteAccountService"/>
</bean>]]></programlisting>
<para>Where <literal>serviceInterface</literal> is changed to our non RMI interface. Our RMI
interface is now defined using the property <literal>portInterface</literal>. Our client
code can now avoid handling <exceptionname>java.rmi.RemoteException</exceptionname>:</para>
<programlisting language="java"><![CDATA[public class AccountClientImpl {
private AccountService service;
public void setService(AccountService service) {
this.service = service;
}
public void foo() {
service.insertAccount(...);
}
}]]></programlisting>
<para>Note that you can also drop the "portInterface" part and specify a plain
business interface as "serviceInterface". In this case,
<classname>JaxRpcPortProxyFactoryBean</classname> will automatically switch
to the JAX-RPC "Dynamic Invocation Interface", performing dynamic invocations
without a fixed port stub. The advantage is that you don't even need to have
an RMI-compliant Java port interface around (e.g. in case of a non-Java target
web service); all you need is a matching business interface. Check out
<classname>JaxRpcPortProxyFactoryBean</classname>'s javadoc for details
on the runtime implications.</para>
</section>
<section id="remoting-web-services-jaxrpc-mapping-registration">
<title>Registering JAX-RPC Bean Mappings</title>
<para>To transfer complex objects over the wire such as <classname>Account</classname> we must
register bean mappings on the client side.</para>
<note>
<para>On the server side using Axis registering bean mappings is usually done in
the <filename>'server-config.wsdd'</filename> file.</para>
</note>
<para>We will use Axis to register bean mappings on the client side. To do this we need to
register the bean mappings programmatically:</para>
<programlisting language="java"><![CDATA[public class AxisPortProxyFactoryBean extends JaxRpcPortProxyFactoryBean {
protected void postProcessJaxRpcService(Service service) {
TypeMappingRegistry registry = service.getTypeMappingRegistry();
TypeMapping mapping = registry.createTypeMapping();
registerBeanMapping(mapping, Account.class, "Account");
registry.register("http://schemas.xmlsoap.org/soap/encoding/", mapping);
}
protected void registerBeanMapping(TypeMapping mapping, Class type, String name) {
QName qName = new QName("http://localhost:8080/account/services/accountService", name);
mapping.register(type, qName,
new BeanSerializerFactory(type, qName),
new BeanDeserializerFactory(type, qName));
}
}]]></programlisting>
</section>
<section id="remoting-web-services-jaxrpc-handler-registration">
<title>Registering your own JAX-RPC Handler</title>
<para>In this section we will register our own
<interfacename>javax.rpc.xml.handler.Handler</interfacename> to the web service proxy
where we can do custom code before the SOAP message is sent over the wire.
The <interfacename>Handler</interfacename> is a callback interface. There is a convenience
base class provided in <filename class="libraryfile">jaxrpc.jar</filename>, namely
<classname>javax.rpc.xml.handler.GenericHandler</classname> that we will extend:</para>
<programlisting language="java"><![CDATA[public class AccountHandler extends GenericHandler {
public QName[] getHeaders() {
return null;
}
public boolean handleRequest(MessageContext context) {
SOAPMessageContext smc = (SOAPMessageContext) context;
SOAPMessage msg = smc.getMessage();
try {
SOAPEnvelope envelope = msg.getSOAPPart().getEnvelope();
SOAPHeader header = envelope.getHeader();
...
}
catch (SOAPException ex) {
throw new JAXRPCException(ex);
}
return true;
}
}]]></programlisting>
<para>What we need to do now is to register our AccountHandler to JAX-RPC Service so it would
invoke <methodname>handleRequest(..)</methodname> before the message is sent over the wire.
Spring has at this time of writing no declarative support for registering handlers, so we must
use the programmatic approach. However Spring has made it very easy for us to do this as we can
override the <methodname>postProcessJaxRpcService(..)</methodname> method that is designed for
this:</para>
<programlisting language="java"><![CDATA[public class AccountHandlerJaxRpcPortProxyFactoryBean extends JaxRpcPortProxyFactoryBean {
protected void postProcessJaxRpcService(Service service) {
QName port = new QName(this.getNamespaceUri(), this.getPortName());
List list = service.getHandlerRegistry().getHandlerChain(port);
list.add(new HandlerInfo(AccountHandler.class, null, null));
logger.info("Registered JAX-RPC AccountHandler on port " + port);
}
}]]></programlisting>
<para>The last thing we must remember to do is to change the Spring configuration to use our
factory bean:</para>
<programlisting language="xml"><![CDATA[<bean id="accountWebService" class="example.AccountHandlerJaxRpcPortProxyFactoryBean">
...
</bean>]]></programlisting>
</section>
<section id="remoting-web-services-jaxws-export-servlet">
<title>Exposing servlet-based web services using JAX-WS</title>
<para>Spring provides a convenient base class for JAX-WS servlet endpoint implementations -
<classname>SpringBeanAutowiringSupport</classname>. To expose our
<interfacename>AccountService</interfacename> we extend Spring's
<classname>SpringBeanAutowiringSupport</classname> class and implement our business
logic here, usually delegating the call to the business layer.
We'll simply use Spring 2.5's <literal>@Autowired</literal>
annotation for expressing such dependencies on Spring-managed beans.</para>
<programlisting language="java"><lineannotation>/**
* JAX-WS compliant AccountService implementation that simply delegates
* to the AccountService implementation in the root web application context.
*
* This wrapper class is necessary because JAX-WS requires working with dedicated
* endpoint classes. If an existing service needs to be exported, a wrapper that
* extends SpringBeanAutowiringSupport for simple Spring bean autowiring (through
* the @Autowired annotation) is the simplest JAX-WS compliant way.
*
* This is the class registered with the server-side JAX-WS implementation.
* In the case of a Java EE 5 server, this would simply be defined as a servlet
* in web.xml, with the server detecting that this is a JAX-WS endpoint and reacting
* accordingly. The servlet name usually needs to match the specified WS service name.
*
* The web service engine manages the lifecycle of instances of this class.
* Spring bean references will just be wired in here.
*/</lineannotation><![CDATA[import org.springframework.web.context.support.SpringBeanAutowiringSupport;
@WebService(serviceName="AccountService")
public class AccountServiceEndpoint extends SpringBeanAutowiringSupport {
@Autowired
private AccountService biz;
@WebMethod
public void insertAccount(Account acc) {
biz.insertAccount(acc);
}
@WebMethod
public Account[] getAccounts(String name) {
return biz.getAccounts(name);
}
}]]></programlisting>
<para>Our <classname>AccountServletEndpoint</classname> needs to run in the same web
application as the Spring context to allow for access to Spring's facilities. This is
the case by default in Java EE 5 environments, using the standard contract for JAX-WS
servlet endpoint deployment. See Java EE 5 web service tutorials for details.</para>
</section>
<section id="remoting-web-services-jaxws-export-standalone">
<title>Exporting standalone web services using JAX-WS</title>
<para>The built-in JAX-WS provider that comes with Sun's JDK 1.6 supports exposure
of web services using the built-in HTTP server that's included in JDK 1.6 as well.
Spring's <classname>SimpleJaxWsServiceExporter</classname> detects all
<literal>@WebService</literal> annotated beans in the Spring application context,
exporting them through the default JAX-WS server (the JDK 1.6 HTTP server).</para>
<para>In this scenario, the endpoint instances are defined and managed as Spring beans
themselves; they will be registered with the JAX-WS engine but their lifecycle
will be up to the Spring application context. This means that Spring functionality
like explicit dependency injection may be applied to the endpoint instances.
Of course, annotation-driven injection through <literal>@Autowired</literal>
will work as well.
</para>
<programlisting language="xml"><![CDATA[<bean class="org.springframework.remoting.jaxws.SimpleJaxWsServiceExporter">
<property name="baseAddress" value="http://localhost:9999/"/>
</bean>
<bean id="accountServiceEndpoint" class="example.AccountServiceEndpoint">
...
</bean>
...]]></programlisting>
<para>The <classname>AccountServiceEndpoint</classname> may derive from
Spring's <classname>SpringBeanAutowiringSupport</classname> but doesn't
have to since the endpoint is a fully Spring-managed bean here.
This means that the endpoint implementation may look like as follows,
without any superclass declared - and Spring's <literal>@Autowired</literal>
configuration annotation still being honored:</para>
<programlisting language="java"><![CDATA[@WebService(serviceName="AccountService")
public class AccountServiceEndpoint {
@Autowired
private AccountService biz;
@WebMethod
public void insertAccount(Account acc) {
biz.insertAccount(acc);
}
@WebMethod
public Account[] getAccounts(String name) {
return biz.getAccounts(name);
}
}]]></programlisting>
</section>
<section id="remoting-web-services-jaxws-export-ri">
<title>Exporting web services using the JAX-WS RI's Spring support</title>
<para>Sun's JAX-WS RI, developed as part of the GlassFish project, ships Spring support
as part of its JAX-WS Commons project. This allows for defining JAX-WS endpoints as
Spring-managed beans, similar to the standalone mode discussed in the previous section
- but this time in a Servlet environment. <emphasis>Note that this is not portable
in a Java EE 5 environment; it is mainly intended for non-EE environments such as Tomcat,
embedding the JAX-WS RI as part of the web application.</emphasis></para>
<para>The difference to the standard style of exporting servlet-based endpoints is
that the lifecycle of the endpoint instances themselves will be managed by Spring here,
and that there will be only one JAX-WS servlet defined in <literal>web.xml</literal>.
With the standard Java EE 5 style (as illustrated above), you'll have one servlet
definition per service endpoint, with each endpoint typically delegating to Spring
beans (through the use of <literal>@Autowired</literal>, as shown above).</para>
<para>Check out
<ulink url="https://jax-ws-commons.dev.java.net/spring/">https://jax-ws-commons.dev.java.net/spring/</ulink>
for the details on setup and usage style.</para>
</section>
<section id="remoting-web-services-jaxws-access">
<title>Accessing web services using JAX-WS</title>
<para>Analogous to the JAX-RPC support, Spring provides two factory beans
to create JAX-WS web service proxies, namely <classname>LocalJaxWsServiceFactoryBean</classname> and
<classname>JaxWsPortProxyFactoryBean</classname>. The former can only return a JAX-WS
service class for us to work with. The latter is the full-fledged version that can return
a proxy that implements our business service interface. In this example we use the latter
to create a proxy for the <interfacename>AccountService</interfacename> endpoint (again):
</para>
<programlisting language="xml"><![CDATA[<bean id="accountWebService" class="org.springframework.remoting.jaxws.JaxWsPortProxyFactoryBean">
<property name="serviceInterface" value="example.AccountService"/>
<property name="wsdlDocumentUrl" value="http://localhost:8080/account/services/accountService?WSDL"/>
<property name="namespaceUri" value="http://localhost:8080/account/services/accountService"/>
<property name="serviceName" value="AccountService"/>
<property name="portName" value="AccountPort"/>
</bean>]]></programlisting>
<para>Where <literal>serviceInterface</literal> is our business interface the clients will use.
<literal>wsdlDocumentUrl</literal> is the URL for the WSDL file. Spring needs this a startup time to create the JAX-WS Service.
<literal>namespaceUri</literal> corresponds to the targetNamespace in the .wsdl file.
<literal>serviceName</literal> corresponds to the service name in the .wsdl file.
<literal>portName</literal> corresponds to the port name in the .wsdl file.
</para>
<para>Accessing the web service is now very easy as we have a bean factory for it that will expose it
as <literal>AccountService</literal> interface. We can wire this up in Spring:</para>
<programlisting language="xml"><![CDATA[<bean id="client" class="example.AccountClientImpl">
...
<property name="service" ref="accountWebService"/>
</bean>]]></programlisting>
<para>From the client code we can access the web service just as if it
was a normal class:</para>
<programlisting language="java"><![CDATA[public class AccountClientImpl {
private AccountService service;
public void setService(AccountService service) {
this.service = service;
}
public void foo() {
service.insertAccount(...);
}
}]]></programlisting>
<para><emphasis>NOTE:</emphasis> The above is slightly simplified in that JAX-WS
requires endpoint interfaces and implementation classes to be annotated with
<literal>@WebService</literal>, <literal>@SOAPBinding</literal> etc annotations.
This means that you cannot (easily) use plain Java interfaces and implementation
classes as JAX-WS endpoint artifacts; you need to annotate them accordingly first.
Check the JAX-WS documentation for details on those requirements.</para>
</section>
<section id="remoting-web-services-xfire">
<title>Exposing web services using XFire</title>
<para>XFire is a lightweight SOAP library, hosted by Codehaus. Exposing XFire is done using a
XFire context that shipping with XFire itself in combination with a RemoteExporter-style bean
you have to add to your <interfacename>WebApplicationContext</interfacename>. As with all
methods that allow you to expose service, you have to create a
<classname>DispatcherServlet</classname> with a corresponding
<interfacename>WebApplicationContext</interfacename> containing the services you will be
exposing:</para>
<programlisting language="xml"><![CDATA[<servlet>
<servlet-name>xfire</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
</servlet>]]></programlisting>
<para>You also have to link in the XFire configuration. This is done by adding a context
file to the <literal>contextConfigLocations</literal> context parameter picked up by the
<classname>ContextLoaderListener</classname> (or <classname>ContextLoaderServlet</classname>
for that matter).</para>
<programlisting language="xml"><![CDATA[<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:org/codehaus/xfire/spring/xfire.xml</param-value>
</context-param>
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>]]></programlisting>
<para>After you added a servlet mapping (mapping <literal>/*</literal> to the XFire servlet
declared above) you only have to add one extra bean to expose the service using XFire.
Add for example the following configuration in your <filename>'xfire-servlet.xml'</filename>
file:</para>
<programlisting language="xml"><![CDATA[<beans>
<bean name="/Echo" class="org.codehaus.xfire.spring.remoting.XFireExporter">
<property name="serviceInterface" value="org.codehaus.xfire.spring.Echo"/>
<property name="serviceBean">
<bean class="org.codehaus.xfire.spring.EchoImpl"/>
</property>
]]><lineannotation>&lt;!-- the XFire bean is defined in the <filename>xfire.xml</filename> file --&gt;</lineannotation><![CDATA[
<property name="xfire" ref="xfire"/>
</bean>
</beans>]]></programlisting>
<para>XFire handles the rest. It introspects your service interface and generates a WSDL from it.
Parts of this documentation have been taken from the XFire site; for more detailed information
on XFire Spring integration, navigate to
<ulink url="http://docs.codehaus.org/display/XFIRE/Spring">http://docs.codehaus.org/display/XFIRE/Spring</ulink>.</para>
</section>
</section>
<section id="remoting-jms">
<title>JMS</title>
<para>It is also possible to expose services transparently using JMS as the underlying
communication protocol. The JMS remoting support in the Spring Framework is pretty basic -
it sends and receives on the <interfacename>same thread</interfacename> and in the
<emphasis>same non-transactional</emphasis> <interfacename>Session</interfacename>, and as
such throughput will be very implementation dependent.</para>
<para>The following interface is used on both the server and the client side.</para>
<programlisting language="java"><![CDATA[package com.foo;
public interface CheckingAccountService {
public void cancelAccount(Long accountId);
}]]></programlisting>
<para>The following simple implementation of the above interface is used on the server-side.</para>
<programlisting language="java"><![CDATA[package com.foo;
public class SimpleCheckingAccountService implements CheckingAccountService {
public void cancelAccount(Long accountId) {
System.out.println("Cancelling account [" + accountId + "]");
}
}]]></programlisting>
<para>This configuration file contains the JMS-infrastructure beans that are shared on both
the client and server.</para>
<programlisting language="xml"><![CDATA[<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-2.5.xsd">
<bean id="connectionFactory" class="org.apache.activemq.ActiveMQConnectionFactory">
<property name="brokerURL" value="tcp://ep-t43:61616"/>
</bean>
<bean id="queue" class="org.apache.activemq.command.ActiveMQQueue">
<constructor-arg value="mmm"/>
</bean>
</beans>]]></programlisting>
<section id="remoting-jms-server">
<title>Server-side configuration</title>
<para>On the server, you just need to expose the service object using the
<classname>JmsInvokerServiceExporter</classname>.</para>
<programlisting language="xml"><![CDATA[<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-2.5.xsd">
<bean id="checkingAccountService"
class="org.springframework.jms.remoting.JmsInvokerServiceExporter">
<property name="serviceInterface" value="com.foo.CheckingAccountService"/>
<property name="service">
<bean class="com.foo.SimpleCheckingAccountService"/>
</property>
</bean>
<bean class="org.springframework.jms.listener.SimpleMessageListenerContainer">
<property name="connectionFactory" ref="connectionFactory"/>
<property name="destination" ref="queue"/>
<property name="concurrentConsumers" value="3"/>
<property name="messageListener" ref="checkingAccountService"/>
</bean>
</beans>]]></programlisting>
<programlisting language="java"><![CDATA[package com.foo;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class Server {
public static void main(String[] args) throws Exception {
new ClassPathXmlApplicationContext(new String[]{"com/foo/server.xml", "com/foo/jms.xml"});
}
}]]></programlisting>
</section>
<section id="remoting-jms-client">
<title>Client-side configuration</title>
<para>The client merely needs to create a client-side proxy that will implement the agreed
upon interface (<interfacename>CheckingAccountService</interfacename>). The resulting
object created off the back of the following bean definition can be injected into other
client side objects, and the proxy will take care of forwarding the call to the
server-side object via JMS.</para>
<programlisting language="xml"><![CDATA[<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-2.5.xsd">
<bean id="checkingAccountService"
class="org.springframework.jms.remoting.JmsInvokerProxyFactoryBean">
<property name="serviceInterface" value="com.foo.CheckingAccountService"/>
<property name="connectionFactory" ref="connectionFactory"/>
<property name="queue" ref="queue"/>
</bean>
</beans>]]></programlisting>
<programlisting language="java"><![CDATA[package com.foo;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class Client {
public static void main(String[] args) throws Exception {
ApplicationContext ctx = new ClassPathXmlApplicationContext(
new String[] {"com/foo/client.xml", "com/foo/jms.xml"});
CheckingAccountService service = (CheckingAccountService) ctx.getBean("checkingAccountService");
service.cancelAccount(new Long(10));
}
}]]></programlisting>
</section>
<para>You may also wish to investigate the support provided by the
<ulink url="http://lingo.codehaus.org/">Lingo</ulink> project, which (to quote the
homepage blurb) <quote><emphasis>... is a lightweight POJO based remoting and messaging library
based on the Spring Framework's remoting libraries which extends it to support JMS.</emphasis></quote></para>
</section>
<section id="remoting-autodection-remote-interfaces">
<title>Auto-detection is not implemented for remote interfaces</title>
<para>The main reason why auto-detection of implemented interfaces does not occur for
remote interfaces is to avoid opening too many doors to remote callers. The target
object might implement internal callback interfaces like <interfacename>InitializingBean</interfacename> or
<interfacename>DisposableBean</interfacename> which one would not want to expose to callers.
</para>
<para>Offering a proxy with all interfaces implemented by the target usually does not
matter in the local case. But when exporting a remote service, you should expose
a specific service interface, with specific operations intended for remote usage.
Besides internal callback interfaces, the target might implement multiple business
interfaces, with just one of them intended for remote exposure. For these reasons,
we <emphasis>require</emphasis> such a service interface to be specified.
</para>
<para>This is a trade-off between configuration convenience and the risk of accidental
exposure of internal methods. Always specifying a service interface is not too much
effort, and puts you on the safe side regarding controlled exposure of specific methods.
</para>
</section>
<section id="remoting-considerations">
<title>Considerations when choosing a technology</title>
<para>Each and every technology presented here has its drawbacks. You should carefully
consider you needs, the services your exposing and the objects you'll be sending
over the wire when choosing a technology.
</para>
<para>When using RMI, it's not possible to access the objects through the HTTP protocol,
unless you're tunneling the RMI traffic. RMI is a fairly heavy-weight protocol
in that it support full-object serialization which is important when using a
complex data model that needs serialization over the wire. However, RMI-JRMP
is tied to Java clients: It is a Java-to-Java remoting solution.
</para>
<para>Spring's HTTP invoker is a good choice if you need HTTP-based remoting but also
rely on Java serialization. It shares the basic infrastructure with RMI invokers,
just using HTTP as transport. Note that HTTP invokers are not only limited to
Java-to-Java remoting but also to Spring on both the client and server side.
(The latter also applies to Spring's RMI invoker for non-RMI interfaces.)
</para>
<para>Hessian and/or Burlap might provide significant value when operating in a
heterogeneous environment, because they explicitly allow for non-Java clients.
However, non-Java support is still limited. Known issues include the serialization
of Hibernate objects in combination with lazily-initialized collections. If you
have such a data model, consider using RMI or HTTP invokers instead of Hessian.
</para>
<para>JMS can be useful for providing clusters of services and allowing the JMS broker
to take care of load balancing, discovery and auto-failover.
By default: Java serialization is used when using JMS remoting but
the JMS provider could use a different mechanism for the wire formatting,
such as XStream to allow servers to be implemented in other technologies.
</para>
<para>Last but not least, EJB has an advantage over RMI in that it supports standard
role-based authentication and authorization and remote transaction propagation.
It is possible to get RMI invokers or HTTP invokers to support security context
propagation as well, although this is not provided by core Spring: There are
just appropriate hooks for plugging in third-party or custom solutions here.
</para>
</section>
</chapter>
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