spring-boot/spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

[[boot-features]]
= Spring Boot features

[partintro]
--
This section dives into the details of Spring Boot. Here you can learn about the key
features that you will want to use and customize. If you haven't already, you might want
to read the '<<getting-started.adoc#getting-started>>' and
'<<using-spring-boot.adoc#using-boot>>' sections so that you have a good grounding
of the basics.
--



[[boot-features-spring-application]]
== SpringApplication
The `SpringApplication` class provides a convenient way to bootstrap a Spring application
that will be started from a `main()` method. In many situations you can just delegate to
the static `SpringApplication.run` method:

[source,java,indent=0]
----
	public static void main(String[] args) {
		SpringApplication.run(MySpringConfiguration.class, args);
	}
----

When your application starts you should see something similar to the following:

[indent=0,subs="attributes"]
----
  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::   v{spring-boot-version}

2013-07-31 00:08:16.117  INFO 56603 --- [           main] o.s.b.s.app.SampleApplication            : Starting SampleApplication v0.1.0 on mycomputer with PID 56603 (/apps/myapp.jar started by pwebb)
2013-07-31 00:08:16.166  INFO 56603 --- [           main] ationConfigEmbeddedWebApplicationContext : Refreshing org.springframework.boot.context.embedded.AnnotationConfigEmbeddedWebApplicationContext@6e5a8246: startup date [Wed Jul 31 00:08:16 PDT 2013]; root of context hierarchy
2014-03-04 13:09:54.912  INFO 41370 --- [           main] .t.TomcatEmbeddedServletContainerFactory : Server initialized with port: 8080
2014-03-04 13:09:56.501  INFO 41370 --- [           main] o.s.b.s.app.SampleApplication            : Started SampleApplication in 2.992 seconds (JVM running for 3.658)
----

By default `INFO` logging messages will be shown, including some relevant startup details
such as the user that launched the application.



[[boot-features-customizing-spring-application]]
=== Customizing SpringApplication
If the `SpringApplication` defaults aren't to your taste you can instead create a local
instance and customize it. For example, to turn off the banner you would write:

[source,java,indent=0]
----
	public static void main(String[] args) {
		SpringApplication app = new SpringApplication(MySpringConfiguration.class);
		app.setShowBanner(false);
		app.run(args);
	}
----

NOTE: The constructor arguments passed to `SpringApplication` are configuration sources
for spring beans. In most cases these will be references to `@Configuration` classes, but
they could also be references to XML configuration or to packages that should be scanned.

It is also possible to configure the `SpringApplication` using an `application.properties`
file. See '<<boot-features-external-config>>' for details.

For a complete list of the configuration options, see the
{dc-spring-boot}/SpringApplication.{dc-ext}[`SpringApplication` Javadoc].



[[boot-features-fluent-builder-api]]
=== Fluent builder API
If you need to build an `ApplicationContext` hierarchy (multiple contexts with a
parent/child relationship), or if you just prefer using a ``fluent'' builder API, you
can use the `SpringApplicationBuilder`.

The `SpringApplicationBuilder` allows you to chain together multiple method calls, and
includes `parent` and `child` methods that allow you to create a hierarchy.

For example:
[source,java,indent=0]
----
	new SpringApplicationBuilder()
		.showBanner(false)
		.sources(Parent.class)
		.child(Application.class)
		.run(args);
----

NOTE: There are some restrictions when creating an `ApplicationContext` hierarchy, e.g.
Web components *must* be contained within the child context, and the same `Environment`
will be used for both parent and child contexts. See the
{dc-spring-boot}/builder/SpringApplication.{dc-edit}[`SpringApplicationBuilder` javadoc]
for full details.



[[boot-features-application-events-and-listeners]]
=== Application events and listeners
In addition to the usual Spring Framework events, such as
{spring-javadoc}/context/event/ContextRefreshedEvent.{dc-ext}[`ContextRefreshedEvent`],
a `SpringApplication` sends some additional application events. Some events are actually
triggered before the `ApplicationContext` is created.

You can register event listeners in a number of ways, the most common being
`SpringApplication.addListeners(...)` method.

Application events are sent in the following order, as your application runs:

. An `ApplicationStartedEvent` is sent at the start of a run, but before any
  processing except the registration of listeners and initializers.
. An `ApplicationEnvironmentPreparedEvent` is sent when the `Environment` to be used in
  the context is known, but before the context is created.
. An `ApplicationPreparedEvent` is sent just before the refresh is started, but after bean
  definitions have been loaded.
. An `ApplicationFailedEvent` is sent if there is an exception on startup.

TIP: You often won't need to use application events, but it can be handy to know that they
exist. Internally, Spring Boot uses events to handle a variety of tasks.



[[boot-features-web-environment]]
=== Web environment
A `SpringApplication` will attempt to create the right type of `ApplicationContext` on
your behalf. By default, an `AnnotationConfigApplicationContext` or
`AnnotationConfigEmbeddedWebApplicationContext` will be used, depending on whether you
are developing a web application or not.

The algorithm used to determine a ``web environment'' is fairly simplistic (based on the
presence of a few classes). You can use `setWebEnvironment(boolean webEnvironment)` if
you need to override the default.

It is also possible to take complete control of the `ApplicationContext` type that will
be used by calling `setApplicationContextClass(...)`.

TIP: It is often desirable to call `setWebEnvironment(false)` when using `SpringApplication`
within a JUnit test.



[[boot-features-command-line-runner]]
=== Using the CommandLineRunner
If you want access to the raw command line arguments, or you need to run some specific code
once the `SpringApplication` has started you can implement the `CommandLineRunner`
interface. The `run(String... args)` method will be called on all spring beans
implementing this interface.

[source,java,indent=0]
----
	import org.springframework.boot.*
	import org.springframework.stereotype.*

	@Component
	public class MyBean implements CommandLineRunner {

	    public void run(String... args) {
	        // Do something...
	    }

	}
----

You can additionally implement the `org.springframework.core.Ordered` interface or use the
`org.springframework.core.annotation.Order` annotation if several `CommandLineRunner`
beans are defined that must be called in a specific order.



[[boot-features-application-exit]]
=== Application exit
Each `SpringApplication` will register a shutdown hook with the JVM to ensure that the
`ApplicationContext` is closed gracefully on exit. All the standard Spring lifecycle
callbacks (such as the `DisposableBean` interface, or the `@PreDestroy` annotation) can
be used.

In addition, beans may implement the `org.springframework.boot.ExitCodeGenerator`
interface if they wish to return a specific exit code when the application ends.



[[boot-features-external-config]]
== Externalized Configuration
Spring Boot likes you to externalize your configuration so you can work with the same
application code in different environments. You can use properties files, YAML files,
environment variables and command-line arguments to externalize configuration. Property
values can be injected directly into your beans using the `@Value` annotation, accessed
via Spring's `Environment` abstraction or bound to structured objects.

Spring Boot uses a very particular `PropertySource` order that is designed to allow
sensible overriding of values, properties are considered in the the following order:

. Command line arguments.
. Java System properties (`System.getProperties()`).
. OS environment variables.
. `@PropertySource` annotations on your `@Configuration` classes.
. Application properties outside of your packaged jar (`application.properties`
  including YAML and profile variants).
. Application properties packaged inside your jar (`application.properties`
  including YAML and profile variants).
. Default properties (specified using `SpringApplication.setDefaultProperties`).

To provide a concrete example, suppose you develop a `@Component` that uses a
`name` property:

[source,java,indent=0]
----
	import org.springframework.stereotype.*
	import org.springframework.beans.factory.annotation.*

	@Component
	public class MyBean {

	    @Value("${name}")
	    private String name;

	    // ...

	}
----

You can bundle an `application.properties` inside your jar that provides a sensible
default `name`. When running in production, an `application.properties` can be provided
outside of your jar that overrides `name`; and for one off testing, you can launch with
a specific command line switch (e.g. `java -jar app.jar --name="Spring"`).



[[boot-features-external-config-command-line-args]]
=== Accessing command line properties
By default SpringApplication will convert any command line option arguments (starting
with ``--'', e.g. `--server.port=9000`) to a `property` and add it to the Spring
`Environment`. As mentioned above, command line properties always take precedence over
other property sources.

If you don't want command line properties to be added to the `Environment` you can disable
them using `SpringApplication.setAddCommandLineProperties(false)`.



[[boot-features-external-config-application-property-files]]
=== Application property files
`SpringApplication` will load properties from `application.properties` files in the
following locations and add them to the Spring `Environment`:

. A `/config` subdir of the current directory.
. The current directory
. A classpath `/config` package
. The classpath root

The list is ordered by precedence (locations higher in the list override lower items).

NOTE: You can also <<boot-features-external-config-yaml, use YAML ('.yml') files>> as
an alternative to '.properties'.

If you don't like `application.properties` as the configuration file name you can switch
to another by specifying a `spring.config.name` environment property. You can also refer
to an explicit location using the `spring.config.location` environment property (comma-
separated list of directory locations, or file paths).

[indent=0]
----
	$ java -jar myproject.jar --spring.config.name=myproject
----

or

[indent=0]
----
	$ java -jar myproject.jar --spring.config.location=classpath:/default.properties,classpath:/override.properties
----

If `spring.config.location` contains directories (as opposed to files)
they should end in "/" (and will be appended with the names generated
from `spring.config.name` before being loaded). The default search
path `classpath:,classpath:/config,file:,file:config/` is always used,
irrespective of the value of `spring.config.location`. In that way you
can set up default values for your app in `application.properties` (or
whatever other basename you choose with `spring.config.name`) and
override it at runtime with a different file, keeping the defaults.


[[boot-features-external-config-profile-specific-properties]]
=== Profile specific properties
In addition to `application.properties` files, profile specific properties can also be
defined using the naming convention `application-{profile}.properties`.

Profile specific properties are loaded from the same locations as standard
`application.properties`, with profiles specific files overriding the default ones.



[[boot-features-external-config-placeholders-in-properties]]
=== Placeholders in properties
The values in `application.properties` are filtered through the existing `Environment`
when they are used so you can refer back to previously defined values (e.g. from System
properties).

[source,properties,indent=0]
----
	app.name=MyApp
	app.description=${app.name} is a Spring Boot application
----

TIP: You can also use this technique to create ``short'' variants of existing Spring Boot
properties. See the '<<howto.adoc#howto-use-short-command-line-arguments>>' how-to
for details.



[[boot-features-external-config-yaml]]
=== Using YAML instead of Properties
http://yaml.org[YAML] is a superset of JSON, and as such is a very convenient format
for specifying hierarchical configuration data. The `SpringApplication` class will
automatically support YAML as an alternative to properties whenever you have the
http://code.google.com/p/snakeyaml/[SnakeYAML] library on your classpath.

NOTE: If you use ``starter POMs'' SnakeYAML will be automatically provided via
`spring-boot-starter`.



[[boot-features-external-config-loading-yaml]]
==== Loading YAML
Spring Boot provides two convenient classes that can be used to load YAML documents. The
`YamlPropertiesFactoryBean` will load YAML as `Properties` and the `YamlMapFactoryBean`
will load YAML as a `Map`.

For example, the following YAML document:

[source,yaml,indent=0]
----
	dev:
		url: http://dev.bar.com
		name: Developer Setup
	prod:
		url: http://foo.bar.com
		name: My Cool App
----

Would be transformed into these properties:

[source,properties,indent=0]
----
	environments.dev.url=http://dev.bar.com
	environments.dev.name=Developer Setup
	environments.prod.url=http://foo.bar.com
	environments.prod.name=My Cool App
----

YAML lists are represented as comma-separated values (useful for simple String values)
and also as property keys with `[index]` dereferencers, for example this YAML:

[source,yaml,indent=0]
----
	 servers:
 		- dev.bar.com
 		- foo.bar.com
----

Would be transformed into these properties:

[source,properties,indent=0]
----
	servers=dev.bar.com,foo.bar.com
	servers[0]=dev.bar.com
	servers[1]=foo.bar.com
----



[[boot-features-external-config-exposing-yaml-to-spring]]
==== Exposing YAML as properties in the Spring Environment
The `YamlPropertySourceLoader` class can be used to expose YAML as a `PropertySource`
in the Spring `Environment`. This allows you to use the familiar `@Value` annotation with
placeholders syntax to access YAML properties.



[[boot-features-external-config-multi-profile-yaml]]
==== Multi-profile YAML documents
You can specify multiple profile-specific YAML document in a single file by
by using a `spring.profiles` key to indicate when the document applies. For example:

[source,yaml,indent=0]
----
	server:
		address: 192.168.1.100
	---
	spring:
		profiles: development
	server:
		address: 127.0.0.1
	---
	spring:
		profiles: production
	server:
		address: 192.168.1.120
----



[[boot-features-external-config-yaml-shortcomings]]
==== YAML shortcomings
YAML files can't be loaded via the `@PropertySource` annotation. So in the
case that you need to load values that way, you need to use a properties file.



[[boot-features-external-config-typesafe-configuration-properties]]
=== Typesafe Configuration Properties
Using the `@Value("${property}")` annotation to inject configuration properties can
sometimes be cumbersome, especially if you are working with multiple properties or
your data is hierarchical in nature. Spring Boot provides an alternative method
of working with properties that allows strongly typed beans to govern and validate
the configuration of your application. For example:

[source,java,indent=0]
----
	@Component
	@ConfigurationProperties(name="connection")
	public class ConnectionSettings {

		private String username;

		private InetAddress remoteAddress;

		// ... getters and setters

	}
----

When the `@EnableConfigurationProperties` annotation is applied to your `@Configuration`,
any beans annotated with `@ConfigurationProperties` will be automatically configured
from the `Environment` properties. This style of configuration works particularly well
with the `SpringApplication` external YAML configuration:

[source,yaml,indent=0]
----
	# application.yml

	connection:
		username: admin
		remoteAddress: 192.168.1.1

	# additional configuration as required
----

To work with `@ConfigurationProperties` beans you can just inject them in the same way
as any other bean.

[source,java,indent=0]
----
	@Service
	public class MyService {

		@Autowired
		private ConnectionSettings connection;

	 	//...

		@PostConstruct
		public void openConnection() {
			Server server = new Server();
			this.connection.configure(server);
		}

	}
----

It is also possible to shortcut the registration of `@ConfigurationProperties` bean
definitions by simply listing the properties classes directly in the
`@EnableConfigurationProperties` annotation:

[source,java,indent=0]
----
	@Configuration
	@EnableConfigurationProperties(ConnectionSettings.class)
	public class MyConfiguration {
	}
----



[[boot-features-external-config-relaxed-binding]]
==== Relaxed binding
Spring Boot uses some relaxed rules for binding `Environment` properties to
`@ConfigurationProperties` beans, so there doesn't need to be an exact match between
the `Environment` property name and the bean property name.  Common examples where this
is useful include underscore separated (e.g. `context_path` binds to `contextPath`), and
capitalized (e.g. `PORT` binds to `port`) environment properties.

Spring will attempt to coerce the external application properties to the right type when
it binds to the `@ConfigurationProperties` beans. If you need custom type conversion you
can provide a `ConversionService` bean (with bean id `conversionService`) or custom
property editors (via a `CustomEditorConfigurer` bean).



[[boot-features-external-config-validation]]
==== @ConfigurationProperties Validation
Spring Boot will attempt to validate external configuration, by default using JSR-303
(if it is on the classpath). You can simply add JSR-303 `javax.valididation` constraint
annotations to your `@ConfigurationProperties` class:

[source,java,indent=0]
----
	@Component
	@ConfigurationProperties(name="connection")
	public class ConnectionSettings {

		@NotNull
		private InetAddress remoteAddress;

		// ... getters and setters

	}
----

You can also add a custom Spring `Validator` by creating a bean definition called
`configurationPropertiesValidator`.

TIP: The `spring-boot-actuator` module includes an endpoint that exposes all
`@ConfigurationProperties` beans. Simply point your web browser to `/configprops`
or use the equivalent JMX endpoint. See the
'<<production-ready-features.adoc#production-ready-endpoints, Production ready features>>'.
section for details.


[[boot-features-profiles]]
== Profiles
Spring Profiles provide a way to segregate parts of your application configuration and
make it only available in certain environments.  Any `@Component` or `@Configuration` can
be marked with `@Profile` to limit when it is loaded:

[source,java,indent=0]
----
	@Configuration
	@Profile("production")
	public class ProductionConfiguraiton {

		// ...

	}
----

In the normal Spring way, you can use a `spring.profiles.active`
`Environment` property to specify which profiles are active. You can
specify the property in any of the usual ways, for example you could
include it in your `application.properties`:

[source,properties,indent=0]
----
	spring.profiles.active=dev,hsqldb
----

or specify on the command line using the switch `--spring.profiles.active=dev,hsqldb`.



[[boot-features-adding-active-profiles]]
=== Adding active profiles
The `spring.profiles.active` property follows the same ordering rules as other
properties, the highest `PropertySource` will win. This means that you can specify
active profiles in `application.properties` then *replace* them using the command line
switch.

Sometimes it is useful to have profile specific properties that *add* to the active
profiles rather than replace them. The `spring.profiles.include` property can be used
to unconditionally add active profiles. The `SpringApplication` entry point also has
a Java API for setting additional profiles (i.e. on top of those activated by the
`spring.profiles.active` property): see the `setAdditionalProfiles()` method.

For example, when an application with following properties is run using the switch
`--spring.profiles.active=prod` the `proddb` and `prodmq` profiles will also be activated:

[source,yaml,indent=0]
----
	---
	my.property: fromyamlfile
	---
	spring.profiles: prod
	spring.profiles.include: proddb,prodmq
----



[[boot-features-programmatically-setting-profiles]]
=== Programmatically setting profiles
You can programmatically set active profiles by calling
`SpringApplication.setAdditionalProfiles(...)` before your application runs. It is also
possible to activate profiles using Spring's `ConfigurableEnvironment` interface.



[[boot-features-profile-specific-configuration]]
=== Profile specific configuration files
Profile specific variants of both `application.properties` (or `application.yml`) and
files referenced via `@ConfigurationProperties` are considered as files are loaded.
See '<<boot-features-external-config-profile-specific-properties>>' for details.



[[boot-features-logging]]
== Logging
Spring Boot uses http://commons.apache.org/logging[Commons Logging] for all internal
logging, but leaves the underlying log implementation open. Default configurations are
provided for
http://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[Java Util Logging],
http://logging.apache.org/log4j/[Log4J] and
http://logback.qos.ch/[Logback].
In each case there is console output and file output (rotating, 10 Mb file size).

By default, If you use the ``Starter POMs'', Logback will be used for logging. Appropriate
Logback routing is also included to ensure that dependent libraries that use
Java Util Logging, Commons Logging, Log4J or SLF4J will all work correctly.

TIP: There are a lot of logging frameworks available for Java. Don't worry if the above
list seems confusing, generally you won't need to change your logging dependencies and
the Spring Boot defaults will work just fine.



[[boot-features-logging-format]]
=== Log format
The default log output from Spring Boot looks like this:

[indent=0]
----
2014-03-05 10:57:51.112  INFO 45469 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet Engine: Apache Tomcat/7.0.52
2014-03-05 10:57:51.253  INFO 45469 --- [ost-startStop-1] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext
2014-03-05 10:57:51.253  INFO 45469 --- [ost-startStop-1] o.s.web.context.ContextLoader            : Root WebApplicationContext: initialization completed in 1358 ms
2014-03-05 10:57:51.698  INFO 45469 --- [ost-startStop-1] o.s.b.c.e.ServletRegistrationBean        : Mapping servlet: 'dispatcherServlet' to [/]
2014-03-05 10:57:51.702  INFO 45469 --- [ost-startStop-1] o.s.b.c.embedded.FilterRegistrationBean  : Mapping filter: 'hiddenHttpMethodFilter' to: [/*]
----

The following items are output:

* Date and Time -- Millesecond precision and easily sortable.
* Log Level -- `ERROR`, `WARN`, `INFO`, `DEBUG` or `TRACE`.
* Process ID.
* A `---` separator to distinguish the start of actual log messages.
* Logger name -- This is usually the source class name (often abbreviated).
* The log message.



[[boot-features-logging-console-output]]
=== Console output
The default log configuration will echo messages to the console as they written. By
default `ERROR`, `WARN` and `INFO` level messages are logged. To also log `DEBUG` level
messages to the console you can start your application with a `--debug` flag.

[indent=0]
----
	$ java -jar myapp.jar --debug
----

If your terminal supports ANSI, color output will be used to aid readability.



[[boot-features-logging-file-output]]
=== File output
By default, log files are written to `spring.log` in your `temp` directory and rotate at
10 Mb. You can easily customize the output folder by setting the `logging.path` property
(for example in your `application.properties`). It is also possible to change the filename
using a `logging.file` property.

As with console output, `ERROR`, `WARN` and `INFO` level messages are logged by default.



[[boot-features-custom-log-configuration]]
=== Custom log configuration

The various logging systems can be activated by including the appropriate libraries on
the classpath, and further customized by providing a suitable configuration file in the
root of the classpath, or in a location specified by the Spring `Environment` property
`logging.config`.

Depending on your logging system, the following files will be loaded:

|===
|Logging System |Customization

|Logback
|`logback.xml`

|Log4j
|`log4j.properties` or `log4j.xml`

|JDK (Java Util Logging)
|`logging.properties`
|===

To help with the customization some other properties are transferred from the Spring
`Environment` to System properties:

|===
|Spring Environment |System Property |Comments

|`logging.file`
|`LOG_FILE`
|Used in default log configuration if defined.

|`logging.path`
|`LOG_PATH`
|Used in default log configuration if defined.

|`PID`
|`PID`
|The current process ID (discovered if possible and when not already defined as an OS
 environment variable).
|===

All the logging systems supported can consult System properties when parsing their
configuration files.  See the default configurations in `spring-boot.jar` for examples.

WARNING: There are know classloading issues with Java Util Logging that cause problems
when running from an ``executable jar''. We recommend that you avoid it if at all
possible.



[[boot-features-developing-web-applications]]
== Developing web applications
Spring Boot is well suited for web application development. You can easily create a
self-contained HTTP server using embedded Tomcat or Jetty. Most web applications will
use the `spring-boot-starter-web` module to get up and running quickly.

If you haven't yet developed a Spring Boot web application you can follow the
"Hello World!" example in the
'<<getting-started.adoc#getting-started-first-application, Getting started>>' section.



[[boot-features-spring-mvc]]
=== The ``Spring Web MVC framework''
The Spring Web MVC framework (often referred to as simply ``Spring MVC'') is a rich
``model view controller'' web framework. Spring MVC lets you create special `@Controller`
or `@RestController` beans to handle incoming HTTP requests. Methods in your controller
are mapped to HTTP using `@RequestMapping` annotations.

Here is a typical example `@RestController` to serve JSON data:

[source,java,indent=0]
----
	@RestController
	@RequestMapping(value="/users")
	public class MyRestController {

		@RequestMapping(value="/{user}", method=RequestMethod.GET)
		public User getUser(@PathVariable Long user) {
			// ...
		}

		@RequestMapping(value="/{user}/customers", method=RequestMethod.GET)
		List<Customer> getUserCustomers(@PathVariable Long user) {
			// ...
		}

		@RequestMapping(value="/{user}", method=RequestMethod.DELETE)
		public User deleteUser(@PathVariable Long user) {
			// ...
		}

	}
----

Spring MVC is part of the core Spring Framework and detailed information is available in
the  {spring-reference}#mvc[reference documentation]. There are also several guides
available at http://spring.io/guides that cover Spring MVC.



[[boot-features-spring-mvc-auto-configuration]]
==== Spring MVC auto-configuration
Spring Boot provides auto-configuration for Spring MVC that works well with most
applications. If you want to take complete control of Spring MVC you can add your
own `@Configuration` annotated with `@EnableWebMvc`.

The auto-configuration adds the following features on top of Spring's defaults:

* Inclusion of `ContentNegotiatingViewResolver` and `BeanNameViewResolver` beans.
* Support for serving static resources, including support for WebJars (see below).
* Automatic registration of `Converter`, `GenericConverter`, `Formatter` beans.
* Support for `HttpMessageConverters` (see below).
* Static `index.html` support.
* Custom `Favicon` support.



[[boot-features-spring-mvc-message-converters]]
==== HttpMessageConverters
Spring MVC uses the `HttpMessageConverter` interface to convert HTTP requests and
responses. Sensible defaults are included out of the box, for example Objects can be
automatically converted to JSON (using the Jackson library) or XML (using JAXB).

If you need to add or customize converters you can use Spring Boot's
`HttpMessageConverters` class:
[source,java,indent=0]
----
	import org.springframework.boot.autoconfigure.web.HttpMessageConverters;
	import org.springframework.context.annotation.*;
	import org.springframework.http.converter.*;

	@Configuration
	public class MyConfiguration {

		@Bean
		public HttpMessageConverters customConverters() {
			HttpMessageConverter<?> additional = ...
			HttpMessageConverter<?> another = ...
			return new HttpMessageConverters(additional, another);
		}

	}
----



[[boot-features-spring-mvc-static-content]]
==== Static Content
By default Spring Boot will serve static content from a folder called `/static` (or
`/public` or `/resources` or `/META-INF/resources`) in the classpath or from the root
of the `ServeltContext`.  It uses the `ResourceHttpRequestHandler` from Spring MVC so you
can modify that behavior by adding your own `WebMvcConfigurerAdapter` and overriding the
`addResourceHandlers` method.

In a stand-alone web application the default servlet from the container is also
enabled, and acts as a fallback, serving content from the root of the `ServletContext` if
Spring decides not to handle it. Most of the time this will not happen (unless you modify
the default MVC configuration) because Spring will always be able to handle requests
through the `DispatcherServlet`.

In addition to the ``standard'' static resource locations above, a special case is made for
http://www.webjars.org/[Webjars content]. Any resources with a path in `/webjars/**` will
be served from jar files if they are packaged in the Webjars format.

TIP: Do not use the `src/main/webapp` folder if your application will be packaged as a
jar. Although this folder is a common standard, it will *only* work with war packaging
and it will be silently ignored by most build tools if you generate a jar.



[[boot-features-spring-mvc-template-engines]]
==== Template engines
As well as REST web services, you can also use Spring MVC to serve dynamic HTML content.
Spring MVC supports a variety of templating technologies including: velocity, freemarker,
and JSPs. Many other templating engines also ship their own Spring MVC integrations.

Spring Boot includes auto-configuration support for the Thymeleaf templating engine.
Thymeleaf is an XML/XHTML/HTML5 template engine that can work both in web and non-web
environments. If allows you to create natural templates, that can be correctly displayed
by browsers and therefore work also as static prototypes. Thymeleaf templates will be
picked up automatically from `src/main/resources/templates`.

TIP: JSPs should be avoided if possible, there are several
<<boot-features-jsp-limitations, known limitations>> when using them with embedded
servlet containers.



[[boot-features-embedded-container]]
=== Embedded servlet container support
Spring Boot includes support for embedded Tomcat and Jetty servers. Most developers will
simply use the appropriate ``Starter POM'' to obtain a fully configured instance. By
default both Tomcat and Jetty will listen for HTTP requests on port `8080`.



[[boot-features-embedded-container-servlets-and-filters]]
==== Servlets and Filters
When using an embedded servlet container you can register Servlets and Filters directly as
Spring beans. This can be particularly convenient if you want to refer to a value from
your `application.properties` during configuration.

By default, if the context contains only a single Servlet it will be mapped to `/`. In
the case of multiple Servlets beans the bean name will be used as a path prefix. Filters
will map to `/*`.

If convention based mapping is not flexible enough you can use the
`ServletRegistrationBean` and `FilterRegistrationBean` classes for complete control. You
can also register items directly if your bean implements the `ServletContextInitializer`
interface.



[[boot-features-embedded-container-application-context]]
==== The EmbeddedWebApplicationContext
Under the hood Spring Boot uses a new type of `ApplicationContext` for embedded
servlet container support.  The `EmbeddedWebApplicationContext` is a special
type of `WebApplicationContext` that bootstraps itself by searching for a single
`EmbeddedServletContainerFactory` bean. Usually a `TomcatEmbeddedServletContainerFactory`
or `JettyEmbeddedServletContainerFactory` will have been auto-configured.

NOTE: You usually won't need to be aware of these implementation classes. Most
applications will be auto-configured and the appropriate `ApplicationContext` and
`EmbeddedServletContainerFactory` will be created on your behalf.



[[boot-features-customizing-embedded-containers]]
==== Customizing embedded servlet containers
Common servlet container settings can be configured using Spring `Environment`
properties. Usually you would define the properties in your `application.properties`
file.

Common server settings include:

* `server.port` -- The listen port for incoming HTTP requests.
* `server.address` -- The interface address to bind to.
* `server.sessionTimeout` -- A session timeout.

See the {sc-spring-boot-autoconfigure}/web/ServerProperties.{sc-ext}[`ServerProperties`]
class for a complete list.



[[boot-features-programmatic-embedded-container-customization]]
===== Programmatic customization
If you need to configure your embdedded servlet container programmatically you can register
a Spring bean that implements the `EmbeddedServletContainerCustomizer` interface.
`EmbeddedServletContainerCustomizer` provides access to the
`ConfigurableEmbeddedServletContainerFactory` which includes numerous customization
setter methods.

[source,java,indent=0]
----
	import org.springframework.boot.context.embedded.*;
	import org.springframework.stereotype.Component;

	@Component
	public class CustomizationBean implements EmbeddedServletContainerCustomizer {

		@Override
		public void customize(ConfigurableEmbeddedServletContainer container) {
			container.setPort(9000);
		}

	}
----



[[boot-features-customizing-configurableembeddedservletcontainerfactory-directly]]
===== Customizing ConfigurableEmbeddedServletContainerFactory directly
If the above customization techniques are too limited, you can register the
`TomcatEmbeddedServletContainerFactory` or `JettyEmbeddedServletContainerFactory` bean
yourself.

[source,java,indent=0]
----
	@Bean
	public EmbeddedServletContainerFactory servletContainer() {
		TomcatEmbeddedServletContainerFactory factory = new TomcatEmbeddedServletContainerFactory();
		factory.setPort(9000);
		factory.setSessionTimeout(10, TimeUnit.MINUTES);
		factory.addErrorPages(new ErrorPage(HttpStatus.404, "/notfound.html");
		return factory;
	}
----

Setters are provided for many configuration options. Several protected method
``hooks'' are also provided should you need to do something more exotic. See the
source code documentation for details.



[[boot-features-jsp-limitations]]
==== JSP limitations
When running a Spring Boot application that uses an embedded servlet container (and is
packaged as an executable archive), there are some limitations in the JSP support.

* With Tomcat it should work if you use war packaging, i.e. an executable war will work,
  and will also be deployable to a standard container (not limited to, but including
  Tomcat). An executable jar will not work because of a hard coded file pattern in Tomcat.

* Jetty does not currently work as an embedded container with JSPs.

There is a {github-code}/spring-boot-samples/spring-boot-sample-web-jsp[JSP sample] so
you can see how to set things up.



[[boot-features-sql]]
== Working with SQL databases
The Spring Framework provides extensive support for working with SQL databases. From
direct JDBC access using `JdbcTemplate` to complete ``object relational mapping''
technologies such as Hibernate. Spring Data provides an additional level of functionality,
creating `Repository` implementations directly from interfaces and using conventions to
generate queries from your method names.



[[boot-features-configure-datasource]]
=== Configure a DataSource
Java's `javax.sql.DataSource` interface provides a standard method of working with
database connections. Traditionally a DataSource uses a `URL` along with some
credentials to establish a database connection.



[[boot-features-embedded-database-support]]
==== Embedded Database Support
It's often convenient to develop applications using an in-memory embedded database.
Obviously, in-memory databases do not provide persistent storage; you will need to
populate your database when your application starts and be prepared to throw away
data when your application ends.

TIP: The ``How-to'' section includes a '<<howto.adoc#howto-database-initialization, section
on how to initialize a database>>'

Spring Boot can auto-configure embedded http://www.h2database.com[H2],
http://hsqldb.org/[HSQL] and http://db.apache.org/derby/[Derby] databases. You don't
need to provide any connection URLs, simply include a build dependency to the
embedded database that you want to use.

For example, typical POM dependencies would be:

[source,xml,indent=0]
----
	<dependency>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-data-jpa</artifactId>
	</dependency>
	<dependency>
		<groupId>org.hsqldb</groupId>
		<artifactId>hsqldb</artifactId>
		<scope>runtime</scope>
	</dependency>
----

NOTE: You need a dependency on `spring-jdbc` for an embedded database to be
auto-configured. In this example it's pulled in transitively via
`spring-boot-starter-data-jpa`.



[[boot-features-connect-to-production-database]]
==== Connection to a production database
Production database connections can also be auto-configured using a pooling
`DataSource`.  Here's the algorithm for choosing a specific implementation.

* We prefer the Tomcat pooling `DataSource` for its performance and concurrency, so if
  that is available we always choose it.
* If commons-dbcp is available we will use that, but we don't recommend it in production.

If you use the `spring-boot-starter-jdbc` or `spring-boot-starter-data-jpa`
``starter POMs'' you will automcatically get a dependency to `tomcat-jdbc`.

NOTE: Additional connection pools can always be configured manually. If you define your
own `DataSource` bean, auto-configuration will not occur.

DataSource configuration is controlled by external configuration properties in
`spring.datasource.*`. For example, you might declare the following section
in `application.properties`:

[source,properties,indent=0]
----
	spring.datasource.url=jdbc:mysql://localhost/test
	spring.datasource.username=dbuser
	spring.datasource.password=dbpass
	spring.datasource.driverClassName=com.mysql.jdbc.Driver
----

See {sc-spring-boot-autoconfigure}/jdbc/AbstractDataSourceConfiguration.{sc-ext}[`AbstractDataSourceConfiguration`]
for more of the supported options.

NOTE: For a pooling `DataSource` to be created we need to be able to verify that a valid
`Driver` class is available, so we check for that before doing anything. I.e. if you set
`spring.datasource.driverClassName=com.mysql.jdbc.Driver` then that class has to be
loadable.

[[boot-features-using-jdbc-template]]
=== Using JdbcTemplate
Spring's `JdbcTemplate` and `NamedParameterJdbcTemplate` classes are auto-configured and
you can `@Autowire` them directly into your own beans:

[source,java,indent=0]
----
	import org.springframework.beans.factory.annotation.Autowired;
	import org.springframework.jdbc.core.JdbcTemplate;
	import org.springframework.stereotype.Component;

	@Component
	public class MyBean {

		private final JdbcTemplate jdbcTemplate;

		@Autowired
		public MyBean(JdbcTemplate jdbcTemplate) {
			this.jdbcTemplate = jdbcTemplate;
		}

		// ...

	}
----



[[boot-features-jpa-and-spring-data]]
=== JPA and ``Spring Data''
The Java Persistence API is a standard technology that allows you to ``map'' objects to
relational databases. The `spring-boot-starter-data-jpa` POM provides a quick way to get
started. It provides the following key dependencies:

* Hibernate -- One of the most popular JPA implementations.
* Spring Data JPA -- Makes it easy to easily implement JPA based repositories.
* Spring ORMs -- Core ORM support from the Spring Framework.

TIP: We won't go into too many details of JPA or Spring Data here. You can follow the
http://spring.io/guides/gs/accessing-data-jpa/[``Accessing Data with JPA''] guide from
http://spring.io and read the http://projects.spring.io/spring-data-jpa/[Spring Data JPA]
and  http://hibernate.org/orm/documentation/[Hibernate] reference documentation.



[[boot-features-entity-classes]]
==== Entity Classes
Traditionally, JPA ``Entity'' classes are specified in a `persistence.xml` file. With
Spring Boot this file is not necessary and instead ``Entity Scanning'' is used. By
default all packages below your main configuration class (the one annotated with
`@EnableAutoConfiguration`) will be searched.

Any classes annotated with `@Entity`, `@Embeddable` or `@MappedSuperclass` will be
considered. A typical entity class would look something like this:

[source,java,indent=0]
----
	package com.example.myapp.domain;

	import java.io.Serializable;
	import javax.persistence.*;

	@Entity
	public class City implements Serializable {

		@Id
		@GeneratedValue
		private Long id;

		@Column(nullable = false)
		private String name;

		@Column(nullable = false)
		private String state;

		// ... additional members, often include @OneToMany mappings

		protected City() {
			// no-args constructor required by JPA spec
			// this one is protected since it shouldn't be used directly
		}

		public City(String name, String state) {
			this.name = name;
			this.country = country;
		}

		public String getName() {
			return this.name;
		}

		public String getState() {
			return this.state;
		}

		// ... etc

	}
----

TIP: You can customize entity scanning locations using the `@EntityScan` annotation.
See the '<<howto.adoc#howto-separate-entity-definitions-from-spring-configuration>>'
how-to.



[[boot-features-spring-data-jpa-repositories]]
==== Spring Data JPA Repositories
Spring Data JPA repositories are interfaces that you can define to access data. JPA
queries are created automatically from your method names. For example, a `CityRepository`
interface might declare a `findAllByState(String state)` method to find all cities
in a given state.

For more complex queries you can annotate your method using Spring Data's
{spring-data-javadoc}/repository/Query.html[`Query`] annotation.

Spring Data repositories usually extend from the
{spring-data-commons-javadoc}/repository/Repository.html[`Repository`] or
{spring-data-commons-javadoc}/repository/CrudRepository.html[`CrudRepository`] interfaces. If you are using
auto-configuration, repositories will be searched from the package containing your
main configuration class (the one annotated with `@EnableAutoConfiguration`) down.

Here is a typical Spring Data repository:

[source,java,indent=0]
----
	package com.example.myapp.domain;

	import org.springframework.data.domain.*;
	import org.springframework.data.repository.*;

	public interface CityRepository extends Repository<City, Long> {

		Page<City> findAll(Pageable pageable);

		City findByNameAndCountryAllIgnoringCase(String name, String country);

	}
----

TIP: We have barely scratched the surface of Spring Data JPA. For complete details check
their http://projects.spring.io/spring-data-jpa/[reference documentation].



[[boot-features-creating-and-dropping-jpa-databases]]
==== Creating and dropping JPA databases
By default JPA database will be automatically created *only* if you use an embedded
database (H2, HSQL or Derby). You can explicitly configure JPA settings using
`spring.jpa.*` properties. For example, to create and drop tables you can add the
following to your `application.properties`.

[indent=0]
----
	spring.jpa.hibernate.ddl-auto="create-drop"
----



[[boot-features-nosql]]
== Working with NoSQL technologies
Spring Data provides additional projects that help you access a variety of NoSQL
technologies including
http://projects.spring.io/spring-data-mongodb/[MongoDB],
http://projects.spring.io/spring-data-neo4j/[Neo4J],
http://projects.spring.io/spring-data-redis/[Redis],
http://projects.spring.io/spring-data-gemfire/[Gemfire],
http://projects.spring.io/spring-data-couchbase/[Couchbase] and
http://projects.spring.io/spring-data-cassandra/[Cassandra].
Spring Boot provides auto-configuration for MongoDB; you can make use of the other
projects, but you will need to configure them yourself. Refer to the appropriate
reference documentation at http://projects.spring.io/spring-data.



[[boot-features-mongodb]]
=== MongoDB
http://www.mongodb.com/[MongoDB] is an open-source NoSQL document database that uses a
JSON-like schema instead of traditional table-based relational data. Spring Boot offers
several conveniences for working with MongoDB, including the The
`spring-boot-starter-data-mongodb` ``Starter POM''.



[[boot-features-connecting-to-mongodb]]
==== Connecting to a MongoDB database
You can inject an auto-configured `com.mongodb.Mongo` instance as you would any other
Spring Bean. By default the instance will attempt to connect to a MongoDB server using
the URL `mongodb://localhost/test`:

[source,java,indent=0]
----
	import org.springframework.beans.factory.annotation.Autowired;
	import org.springframework.stereotype.Component;

	import com.mongodb.Mongo;

	@Component
	public class MyBean {

		private final Mongo mongo;

		@Autowired
		public MyBean(Mongo mongo) {
			this.mongo = mongo;
		}

		// ...

	}
----

You can set `spring.data.mongodb.url` property to change the `url`, or alternatively
specify a `host`/`port`. For example, you might declare the following in your
`application.properties`:

[source,properties,indent=0]
----
	spring.data.mongodb.host=mongoserver
	spring.data.mongodb.port=27017
----

TIP: If `spring.data.mongodb.port` is not specified the default of `27017` is used. You
could simply delete this line from the sample above.

You can also declare your own `Mongo` `@Bean` if you want to take complete control of
establishing the MongoDB connection.



[[boot-features-mongo-template]]
==== MongoTemplate
Spring Data Mongo provides a {spring-data-mongo-javadoc}/core/MongoTemplate.html[`MongoTemplate`]
class that is very similar in its design to Spring's `JdbcTemplate`. As with
`JdbcTemplate` Spring Boot auto-configures a bean for you to simply inject:

[source,java,indent=0]
----
	import org.springframework.beans.factory.annotation.Autowired;
	import org.springframework.data.mongodb.core.MongoTemplate;
	import org.springframework.stereotype.Component;

	@Component
	public class MyBean {

		private final MongoTemplate mongoTemplate;

		@Autowired
		public MyBean(MongoTemplate mongoTemplate) {
			this.mongoTemplate = mongoTemplate;
		}

		// ...

	}
----

See the `MongoOperations` Javadoc for complete details.



[[boot-features-spring-data-mongo-repositories]]
==== Spring Data MongoDB repositories
Spring Data includes repository support for MongoDB. As with the JPA repositories
discussed earlier, the basic principle is that queries are constructed for you
automatically based on method names.

In fact, both Spring Data JPA and Spring Data MongoDB share the same common
infrastructure; so you could take the JPA example from earlier and, assuming that
`City` is now a Mongo data class rather than a JPA `@Entity`, it will work in the
same way.

[source,java,indent=0]
----
	package com.example.myapp.domain;

	import org.springframework.data.domain.*;
	import org.springframework.data.repository.*;

	public interface CityRepository extends Repository<City, Long> {

		Page<City> findAll(Pageable pageable);

		City findByNameAndCountryAllIgnoringCase(String name, String country);

	}
----

TIP: For complete details of Spring Data MongoDB, including its rich object mapping
technologies, refer to their http://projects.spring.io/spring-data-mongodb/[reference
documentation].



[[boot-features-testing]]
== Testing
Spring Boot provides a number of useful tools for testing your application. The
`spring-boot-starter-parent` POM provides JUnit, Hamcrest and Mockito ``test'' `scope`
dependencies. There are also useful test utilities in the core `spring-boot` module
under the `org.springframework.boot.test` package. There is also a
`spring-boot-starter-test` ``Starter POM''.



[[boot-features-test-scope-dependencies]]
=== Test scope dependencies
If you extend your Maven project from the `spring-boot-starter-parent` POM, or use the
`spring-boot-starter-test` ``Starter POM'' (in the `test` `scope`), you will find
the following provided libraries:

* Junit -- The de-facto standard for unit testing Java applications.
* Hamcrest -- A library of matcher objects (also known as constraints or predicates)
  allowing `assertThat` style JUnit assertions.
* Mockito -- A Java mocking framework.

These are common libraries that we generally find useful when writing Tests. You are free
to add additional test dependencies of your own if these don't suit your needs.



[[boot-features-testing-spring-applications]]
=== Testing Spring applications
One of the major advantages of dependency injection is that it should make your code
easier to unit test. You can simply instantiate objects using the `new` operator without
even involving Spring. You can also use _mock objects_ instead of real dependencies.

Often you need to move beyond ``unit testing'' and start ``integration testing'' (with
a Spring `ApplicationContext` actually involved in the process). It's useful to be able
to perform integration testing without requiring deployment of your application or
needing to connect to other infrastructure.

The Spring Framework includes a dedicated test module for just such integration testing.
You can declare a dependency directly to `org.springframework:spring-test` or use the
`spring-boot-starter-test` ``Starter POM'' to pull it in transitively.

If you have not use the `spring-test` module before you should start by reading the
{spring-reference}/#testing[relevant section] of the Spring Framework reference
documentation.



[[boot-features-testing-spring-boot-applications]]
=== Testing Spring Boot applications
A Spring Boot application is just a Spring `ApplicationContext` so nothing very special
has to be done to test it beyond what you would normally do with a vanilla Spring context.
One thing to watch out for though is that the external properties, logging and other
features of Spring Boot are only installed in the context by default if you use
`SpringApplication` to create it.

Spring Boot provides a `@SpringApplicationConfiguration` annotation as an alternative
to the standard `spring-test` `@ContextConfiguration` annotation. If you use
`@SpringApplicationConfiguration` to configure the `ApplicationContext` used in your
tests, it will be created via `SpringApplication` and you will get the additional Spring
Boot features.

For example:
[source,java,indent=0,subs="verbatim,quotes,attributes"]
----
	@RunWith(SpringJUnit4ClassRunner.class)
	@SpringApplicationConfiguration(classes = SampleDataJpaApplication.class)
	public class CityRepositoryIntegrationTests {

		@Autowired
		CityRepository repository;

		// ...

	}
----

TIP: The context loader guesses whether you want to test a web application or not (e.g.
with `MockMVC`) by looking for the `@WebAppConfiguration` annotation.  (`MockMVC` and
`@WebAppConfiguration` are part of `spring-test`).

If you want a web application to start up and listen on its normal port, so you can test
it with HTTP (e.g. using `RestTemplate`), annotate your test class (or one of its
superclasses) with `@IntegrationTest`. This can be very useful because it means you can
test the full stack of your application, but also inject its components into the test
class and use them to assert the internal state of the application after an HTTP
interaction. For Example:

[source,java,indent=0,subs="verbatim,quotes,attributes"]
----
	@RunWith(SpringJUnit4ClassRunner.class)
	@SpringApplicationConfiguration(classes = SampleDataJpaApplication.class)
	@WebApplication
	@IntegrationTest
	public class CityRepositoryIntegrationTests {

		@Autowired
		CityRepository repository;

		RestTemplate restTemplate = RestTemplates.get();

		// ... interact with the running server

	}
----

[[boot-features-test-utilities]]
=== Test utilities
A few test utility classes are packaged as part of `spring-boot` that are generally
useful when testing your application.



[[boot-features-configfileapplicationcontextinitializer-test-utility]]
==== ConfigFileApplicationContextInitializer
`ConfigFileApplicationContextInitializer` is an `ApplicationContextInitializer` that
can apply to your tests to load Spring Boot `application.properties` files. You can use
this when you don't need the full features provided by `@SpringApplicationConfiguration`.

[source,java,indent=0]
----
	@ContextConfiguration(classes = Config.class,
		initializers = ConfigFileApplicationContextInitializer.class)
----



[[boot-features-environment-test-utilities]]
==== EnvironmentTestUtils
`EnvironmentTestUtils` allows you to quickly add properties to a
`ConfigurableEnvironment` or `ConfigurableApplicationContext`. Simply call it with
`key=value` strings:

[source,java,indent=0]
----
EnvironmentTestUtils.addEnvironment(env, "org=Spring", "name=Boot");
----



[[boot-features-output-capture-test-utility]]
==== OutputCapture
`OutputCapture` is a JUnit `Rule` that you can use to capture `System.out` and
`System.err` output. Simply declare the capture as a `@Rule` then use `toString()`
for assertions:

[source,java,indent=0]
----
import org.junit.Rule;
import org.junit.Test;
import org.springframework.boot.test.OutputCapture;

import static org.hamcrest.Matchers.*;
import static org.junit.Assert.*;

public class MyTest {

	@Rule
	public OutputCapture capture = new OutputCapture();

	@Test
	public void testName() throws Exception {
		System.out.println("Hello World!");
		assertThat(capture.toString(), containsString("World"));
	}

}
----

[[boot-features-rest-templates-test-utility]]
==== RestTemplates

`RestTemplates` is a static convenience factory for instances of `RestTemplate` that are
useful in integration tests.  You can get a vanilla template or one that sends Basic HTTP
authentication (with a username and password). And in either case the template will behave
in a friendly way for testing, not following redirects (so you can assert the response
location), ignoring cookies (so the template is stateless), and not throwing exceptions
on server-side errors. It is recommended, but not mandatory, to use Apache HTTP Client
(version 4.3.2 or better), and if you have that on your classpath the `RestTemplates` will
respond by configuring the client appropriately.

[source,java,indent=0]
----
public class MyTest {

	RestTemplate template = RestTemplates.get();

	@Test
	public void testRequest() throws Exception {
		HttpHeaders headers = template.getForEntity("http://myhost.com", String.class).getHeaders();
		assertThat(headers.getLocation().toString(), containsString("myotherhost"));
	}

}
----



[[boot-features-developing-auto-configuration]]
== Developing auto-configuration and using conditions
If you work in a company that develops shared libraries, or if you work on an open-source
or commercial library, you might want to develop your own auto-configuration.
Auto-configuration classes can be bundled in external jars and still be picked-up by
Spring Boot.



[[boot-features-understanding-auto-configured-beans]]
=== Understanding auto-configured beans
Under the hood, auto-configuration is implemented with standard `@Configuration` classes.
Additional `@Conditional` annotations are used to constrain when the auto-configuration
should apply. Usually auto-configuration classes use `@ConditionalOnClass` and
`@ConditionalOnMissingBean` annotations. This ensures that auto-configuration only
applies when relevant classes are found and when you have not declared your own
`@Configuration`.

You can browse the source code of `spring-boot-autoconfigure` to see the `@Configuration`
classes that we provide (see the `META-INF/spring.factories` file).



[[boot-features-locating-auto-configuration-candidates]]
=== Locating auto-configuration candidates
Spring Boot checks for the presence of a `META-INF/spring.factories` file within your
published jar. The file should list your configuration classes under the
`EnableAutoConfiguration` key.

[indent=0]
----
	org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
	com.mycorp.libx.autoconfigure.LibXAutoConfiguration,\
	com.mycorp.libx.autoconfigure.LibXWebAutoConfiguration
----

You can use the
{sc-spring-boot-autoconfigure}/AutoConfigureAfter.{sc-ext}[`@AutoConfigureAfter`] or
{sc-spring-boot-autoconfigure}/AutoConfigureBefore.{sc-ext}[`@AutoConfigureBefore`]
annotations if your configuration needs to be applied in a specific order. For example,
if you provide web specific configuration, your class may need to be applied after
`WebMvcAutoConfiguration`.



[[boot-features-condition-annotations]]
=== Condition annotations
You almost always want to include one or more `@Condition` annotations on your
auto-configuration class. The `@ConditionalOnMissingBean` is one common example that is
used to allow developers to ``override'' auto-configuration if they are not happy with
your defaults.

Spring Boot includes a number of `@Conditional` annotations that you can reuse in your own
code by annotating `@Configuration` classes or individual `@Bean` methods.



[[boot-features-class-conditions]]
==== Class conditions
The `@ConditionalOnClass` and `@ConditionalOnMissingClass` annotations allows configuration
to be skipped based on the presence or absence of specific classes. Due to the fact that
annotation meta-data is parsed using http://asm.ow2.org/[ASM] you can actually use the
`value` attribute to refer to the real class, even though that class might not actually
appear on the running application classpath. You can also use the `name` attribute if you
prefer to specify the class name using a `String` value.



[[boot-features-bean-conditions]]
==== Bean conditions
The `@ConditionalOnBean` and `@ConditionalOnMissingBean` annotations allow configurations
to be skipped based on the presence or absence of specific beans. You can use the `value`
attribute to specify beans by type, or `name` to specify beans by name. The `search`
attribute allows you to limit the `ApplicationContext` hierarchy that should be considered
when searching for beans.

NOTE: `@Conditional` annotations are processed when `@Configuration` classes are
parsed. Auto-configure `@Configuration` is always parsed last (after any user defined
beans), however, if you are using these annotations on regular `@Configuration` classes,
care must be taken not to refer to bean definitions that have not yet been created.



[[boot-features-resource-conditions]]
==== Resource conditions
The `@ConditionalOnResource` annotation allows configuration to be included only when a
specific resource is present. Resources can be specified using the usual Spring
conventions, for example, `file:/home/user/test.dat`.



[[boot-features-web-application-conditions]]
==== Web Application Conditions
The `@ConditionalOnWebApplication` and `@ConditionalOnNotWebApplication` annotations
allow configuration to be skipped depending on whether the application is a
'web application'. A web application is any application that is using a Spring
`WebApplicationContext`, defines a `session` scope or has a `StandardServletEnvironment`.



[[boot-features-spel-conditions]]
==== SpEL expression conditions
The `@ConditionalOnExpression` annotation allows configuration to be skipped based on the
result of a {spring-reference}/#expressions[SpEL expression].



[[boot-features-whats-next]]
== What to read next
If you want to learn more about any of the classes discussed in this section you can
check out the {dc-root}[Spring Boot API documentation] or you can browse the
{github-code}[source code directly]. If you have specific questions, take a look at the
<<howto.aoc#howto, how-to>> section.

If you are comfortable with Spring Boot's core features, you can carry on and read
about <<production-ready-features.adoc#production-ready, production-ready features>>.