[[beans-java-composing-configuration-classes]]
= Composing Java-based Configurations

Spring's Java-based configuration feature lets you compose annotations, which can reduce
the complexity of your configuration.


[[beans-java-using-import]]
== Using the `@Import` Annotation

Much as the `<import/>` element is used within Spring XML files to aid in modularizing
configurations, the `@Import` annotation allows for loading `@Bean` definitions from
another configuration class, as the following example shows:

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
----
	@Configuration
	public class ConfigA {

		@Bean
		public A a() {
			return new A();
		}
	}

	@Configuration
	@Import(ConfigA.class)
	public class ConfigB {

		@Bean
		public B b() {
			return new B();
		}
	}
----

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
----
	@Configuration
	class ConfigA {

		@Bean
		fun a() = A()
	}

	@Configuration
	@Import(ConfigA::class)
	class ConfigB {

		@Bean
		fun b() = B()
	}
----
======

Now, rather than needing to specify both `ConfigA.class` and `ConfigB.class` when
instantiating the context, only `ConfigB` needs to be supplied explicitly, as the
following example shows:

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
----
	public static void main(String[] args) {
		ApplicationContext ctx = new AnnotationConfigApplicationContext(ConfigB.class);

		// now both beans A and B will be available...
		A a = ctx.getBean(A.class);
		B b = ctx.getBean(B.class);
	}
----

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
----
	import org.springframework.beans.factory.getBean

	fun main() {
		val ctx = AnnotationConfigApplicationContext(ConfigB::class.java)

		// now both beans A and B will be available...
		val a = ctx.getBean<A>()
		val b = ctx.getBean<B>()
	}
----
======

This approach simplifies container instantiation, as only one class needs to be dealt
with, rather than requiring you to remember a potentially large number of
`@Configuration` classes during construction.

TIP: As of Spring Framework 4.2, `@Import` also supports references to regular component
classes, analogous to the `AnnotationConfigApplicationContext.register` method.
This is particularly useful if you want to avoid component scanning, by using a few
configuration classes as entry points to explicitly define all your components.

[[beans-java-injecting-imported-beans]]
=== Injecting Dependencies on Imported `@Bean` Definitions

The preceding example works but is simplistic. In most practical scenarios, beans have
dependencies on one another across configuration classes. When using XML, this is not an
issue, because no compiler is involved, and you can declare
`ref="someBean"` and trust Spring to work it out during container initialization.
When using `@Configuration` classes, the Java compiler places constraints on
the configuration model, in that references to other beans must be valid Java syntax.

Fortunately, solving this problem is simple. As
xref:core/beans/java/bean-annotation.adoc#beans-java-dependencies[we already discussed],
a `@Bean` method can have an arbitrary number of parameters that describe the bean
dependencies. Consider the following more real-world scenario with several `@Configuration`
classes, each depending on beans declared in the others:

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
----
	@Configuration
	public class ServiceConfig {

		@Bean
		public TransferService transferService(AccountRepository accountRepository) {
			return new TransferServiceImpl(accountRepository);
		}
	}

	@Configuration
	public class RepositoryConfig {

		@Bean
		public AccountRepository accountRepository(DataSource dataSource) {
			return new JdbcAccountRepository(dataSource);
		}
	}

	@Configuration
	@Import({ServiceConfig.class, RepositoryConfig.class})
	public class SystemTestConfig {

		@Bean
		public DataSource dataSource() {
			// return new DataSource
		}
	}

	public static void main(String[] args) {
		ApplicationContext ctx = new AnnotationConfigApplicationContext(SystemTestConfig.class);
		// everything wires up across configuration classes...
		TransferService transferService = ctx.getBean(TransferService.class);
		transferService.transfer(100.00, "A123", "C456");
	}
----

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
----
	import org.springframework.beans.factory.getBean

	@Configuration
	class ServiceConfig {

		@Bean
		fun transferService(accountRepository: AccountRepository): TransferService {
			return TransferServiceImpl(accountRepository)
		}
	}

	@Configuration
	class RepositoryConfig {

		@Bean
		fun accountRepository(dataSource: DataSource): AccountRepository {
			return JdbcAccountRepository(dataSource)
		}
	}

	@Configuration
	@Import(ServiceConfig::class, RepositoryConfig::class)
	class SystemTestConfig {

		@Bean
		fun dataSource(): DataSource {
			// return new DataSource
		}
	}


	fun main() {
		val ctx = AnnotationConfigApplicationContext(SystemTestConfig::class.java)
		// everything wires up across configuration classes...
		val transferService = ctx.getBean<TransferService>()
		transferService.transfer(100.00, "A123", "C456")
	}
----
======

There is another way to achieve the same result. Remember that `@Configuration` classes are
ultimately only another bean in the container: This means that they can take advantage of
`@Autowired` and `@Value` injection and other features the same as any other bean.

[WARNING]
====
Make sure that the dependencies you inject that way are of the simplest kind only. `@Configuration`
classes are processed quite early during the initialization of the context, and forcing a dependency
to be injected this way may lead to unexpected early initialization. Whenever possible, resort to
parameter-based injection, as in the preceding example.

Avoid access to locally defined beans within a `@PostConstruct` method on the same configuration
class. This effectively leads to a circular reference since non-static `@Bean` methods semantically
require a fully initialized configuration class instance to be called on. With circular references
disallowed (e.g. in Spring Boot 2.6+), this may trigger a `BeanCurrentlyInCreationException`.

Also, be particularly careful with `BeanPostProcessor` and `BeanFactoryPostProcessor` definitions
through `@Bean`. Those should usually be declared as `static @Bean` methods, not triggering the
instantiation of their containing configuration class. Otherwise, `@Autowired` and `@Value` may not
work on the configuration class itself, since it is possible to create it as a bean instance earlier than
{spring-framework-api}/beans/factory/annotation/AutowiredAnnotationBeanPostProcessor.html[`AutowiredAnnotationBeanPostProcessor`].
====

The following example shows how one bean can be autowired to another bean:

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
----
	@Configuration
	public class ServiceConfig {

		@Autowired
		private AccountRepository accountRepository;

		@Bean
		public TransferService transferService() {
			return new TransferServiceImpl(accountRepository);
		}
	}

	@Configuration
	public class RepositoryConfig {

		private final DataSource dataSource;

		public RepositoryConfig(DataSource dataSource) {
			this.dataSource = dataSource;
		}

		@Bean
		public AccountRepository accountRepository() {
			return new JdbcAccountRepository(dataSource);
		}
	}

	@Configuration
	@Import({ServiceConfig.class, RepositoryConfig.class})
	public class SystemTestConfig {

		@Bean
		public DataSource dataSource() {
			// return new DataSource
		}
	}

	public static void main(String[] args) {
		ApplicationContext ctx = new AnnotationConfigApplicationContext(SystemTestConfig.class);
		// everything wires up across configuration classes...
		TransferService transferService = ctx.getBean(TransferService.class);
		transferService.transfer(100.00, "A123", "C456");
	}
----

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
----
	import org.springframework.beans.factory.getBean

	@Configuration
	class ServiceConfig {

		@Autowired
		lateinit var accountRepository: AccountRepository

		@Bean
		fun transferService(): TransferService {
			return TransferServiceImpl(accountRepository)
		}
	}

	@Configuration
	class RepositoryConfig(private val dataSource: DataSource) {

		@Bean
		fun accountRepository(): AccountRepository {
			return JdbcAccountRepository(dataSource)
		}
	}

	@Configuration
	@Import(ServiceConfig::class, RepositoryConfig::class)
	class SystemTestConfig {

		@Bean
		fun dataSource(): DataSource {
			// return new DataSource
		}
	}

	fun main() {
		val ctx = AnnotationConfigApplicationContext(SystemTestConfig::class.java)
		// everything wires up across configuration classes...
		val transferService = ctx.getBean<TransferService>()
		transferService.transfer(100.00, "A123", "C456")
	}
----
======

TIP: Constructor injection in `@Configuration` classes is only supported as of Spring
Framework 4.3. Note also that there is no need to specify `@Autowired` if the target
bean defines only one constructor.

.[[beans-java-injecting-imported-beans-fq]]Fully-qualifying imported beans for ease of navigation
--
In the preceding scenario, using `@Autowired` works well and provides the desired
modularity, but determining exactly where the autowired bean definitions are declared is
still somewhat ambiguous. For example, as a developer looking at `ServiceConfig`, how do
you know exactly where the `@Autowired AccountRepository` bean is declared? It is not
explicit in the code, and this may be just fine. Remember that the
{spring-site-tools}[Spring Tools for Eclipse] provides tooling that
can render graphs showing how everything is wired, which may be all you need. Also,
your Java IDE can easily find all declarations and uses of the `AccountRepository` type
and quickly show you the location of `@Bean` methods that return that type.

In cases where this ambiguity is not acceptable and you wish to have direct navigation
from within your IDE from one `@Configuration` class to another, consider autowiring the
configuration classes themselves. The following example shows how to do so:

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
----
	@Configuration
	public class ServiceConfig {

		@Autowired
		private RepositoryConfig repositoryConfig;

		@Bean
		public TransferService transferService() {
			// navigate 'through' the config class to the @Bean method!
			return new TransferServiceImpl(repositoryConfig.accountRepository());
		}
	}
----

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
----
@Configuration
class ServiceConfig {

	@Autowired
	private lateinit var repositoryConfig: RepositoryConfig

	@Bean
	fun transferService(): TransferService {
		// navigate 'through' the config class to the @Bean method!
		return TransferServiceImpl(repositoryConfig.accountRepository())
	}
}
----
======

In the preceding situation, where `AccountRepository` is defined is completely explicit.
However, `ServiceConfig` is now tightly coupled to `RepositoryConfig`. That is the
tradeoff. This tight coupling can be somewhat mitigated by using interface-based or
abstract class-based `@Configuration` classes. Consider the following example:

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
----
	@Configuration
	public class ServiceConfig {

		@Autowired
		private RepositoryConfig repositoryConfig;

		@Bean
		public TransferService transferService() {
			return new TransferServiceImpl(repositoryConfig.accountRepository());
		}
	}

	@Configuration
	public interface RepositoryConfig {

		@Bean
		AccountRepository accountRepository();
	}

	@Configuration
	public class DefaultRepositoryConfig implements RepositoryConfig {

		@Bean
		public AccountRepository accountRepository() {
			return new JdbcAccountRepository(...);
		}
	}

	@Configuration
	@Import({ServiceConfig.class, DefaultRepositoryConfig.class})  // import the concrete config!
	public class SystemTestConfig {

		@Bean
		public DataSource dataSource() {
			// return DataSource
		}

	}

	public static void main(String[] args) {
		ApplicationContext ctx = new AnnotationConfigApplicationContext(SystemTestConfig.class);
		TransferService transferService = ctx.getBean(TransferService.class);
		transferService.transfer(100.00, "A123", "C456");
	}
----

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
----
	import org.springframework.beans.factory.getBean

	@Configuration
	class ServiceConfig {

		@Autowired
		private lateinit var repositoryConfig: RepositoryConfig

		@Bean
		fun transferService(): TransferService {
			return TransferServiceImpl(repositoryConfig.accountRepository())
		}
	}

	@Configuration
	interface RepositoryConfig {

		@Bean
		fun accountRepository(): AccountRepository
	}

	@Configuration
	class DefaultRepositoryConfig : RepositoryConfig {

		@Bean
		fun accountRepository(): AccountRepository {
			return JdbcAccountRepository(...)
		}
	}

	@Configuration
	@Import(ServiceConfig::class, DefaultRepositoryConfig::class)  // import the concrete config!
	class SystemTestConfig {

		@Bean
		fun dataSource(): DataSource {
			// return DataSource
		}

	}

	fun main() {
		val ctx = AnnotationConfigApplicationContext(SystemTestConfig::class.java)
		val transferService = ctx.getBean<TransferService>()
		transferService.transfer(100.00, "A123", "C456")
	}
----
======

Now `ServiceConfig` is loosely coupled with respect to the concrete
`DefaultRepositoryConfig`, and built-in IDE tooling is still useful: You can easily
get a type hierarchy of `RepositoryConfig` implementations. In this
way, navigating `@Configuration` classes and their dependencies becomes no different
than the usual process of navigating interface-based code.
--


[[beans-java-startup]]
== Influencing the Startup of `@Bean`-defined Singletons

If you want to influence the startup creation order of certain singleton beans, consider
declaring some of them as `@Lazy` for creation on first access instead of on startup.

`@DependsOn` forces certain other beans to be initialized first, making sure that
the specified beans are created before the current bean, beyond what the latter's
direct dependencies imply.

[[beans-java-startup-background]]
=== Background Initialization

As of 6.2, there is a background initialization option: `@Bean(bootstrap=BACKGROUND)`
allows for singling out specific beans for background initialization, covering the
entire bean creation step for each such bean on context startup.

Dependent beans with non-lazy injection points automatically wait for the bean instance
to be completed. All regular background initializations are forced to complete at the
end of context startup. Only for beans additionally marked as `@Lazy`, the completion
is allowed to happen later (up until first actual access).

Background initialization typically goes together with `@Lazy` (or `ObjectProvider`)
injection points in dependent beans. Otherwise, the main bootstrap thread is going to
block when an actual background-initialized bean instance needs to be injected early.

This form of concurrent startup applies to individual beans: If such a bean depends on
other beans, they need to have been initialized already, either simply through being
declared earlier or through `@DependsOn` which is enforcing initialization in the main
bootstrap thread before background initialization for the affected bean is triggered.

Note that a `bootstrapExecutor` bean of type `Executor` needs to be declared for
background bootstrapping to be actually active. Otherwise, the background markers are
going to be ignored at runtime. The bootstrap executor may be a bounded executor just
for startup purposes or a shared thread pool which serves for other purposes as well.


[[beans-java-conditional]]
== Conditionally Include `@Configuration` Classes or `@Bean` Methods

It is often useful to conditionally enable or disable a complete `@Configuration` class
or even individual `@Bean` methods, based on some arbitrary system state. One common
example of this is to use the `@Profile` annotation to activate beans only when a specific
profile has been enabled in the Spring `Environment` (see xref:core/beans/environment.adoc#beans-definition-profiles[Bean Definition Profiles]
for details).

The `@Profile` annotation is actually implemented by using a much more flexible annotation
called {spring-framework-api}/context/annotation/Conditional.html[`@Conditional`].
The `@Conditional` annotation indicates specific
`org.springframework.context.annotation.Condition` implementations that should be
consulted before a `@Bean` is registered.

Implementations of the `Condition` interface provide a `matches(...)`
method that returns `true` or `false`. For example, the following listing shows the actual
`Condition` implementation used for `@Profile`:

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
----
	@Override
	public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
		// Read the @Profile annotation attributes
		MultiValueMap<String, Object> attrs = metadata.getAllAnnotationAttributes(Profile.class.getName());
		if (attrs != null) {
			for (Object value : attrs.get("value")) {
				if (context.getEnvironment().acceptsProfiles(((String[]) value))) {
					return true;
				}
			}
			return false;
		}
		return true;
	}
----

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
----
	override fun matches(context: ConditionContext, metadata: AnnotatedTypeMetadata): Boolean {
		// Read the @Profile annotation attributes
		val attrs = metadata.getAllAnnotationAttributes(Profile::class.java.name)
		if (attrs != null) {
			for (value in attrs["value"]!!) {
				if (context.environment.acceptsProfiles(Profiles.of(*value as Array<String>))) {
					return true
				}
			}
			return false
		}
		return true
	}
----
======

See the {spring-framework-api}/context/annotation/Conditional.html[`@Conditional`]
javadoc for more detail.


[[beans-java-combining]]
== Combining Java and XML Configuration

Spring's `@Configuration` class support does not aim to be a 100% complete replacement
for Spring XML. Some facilities, such as Spring XML namespaces, remain an ideal way to
configure the container. In cases where XML is convenient or necessary, you have a
choice: either instantiate the container in an "`XML-centric`" way by using, for example,
`ClassPathXmlApplicationContext`, or instantiate it in a "`Java-centric`" way by using
`AnnotationConfigApplicationContext` and the `@ImportResource` annotation to import XML
as needed.

[[beans-java-combining-xml-centric]]
=== XML-centric Use of `@Configuration` Classes

It may be preferable to bootstrap the Spring container from XML and include
`@Configuration` classes in an ad-hoc fashion. For example, in a large existing codebase
that uses Spring XML, it is easier to create `@Configuration` classes on an
as-needed basis and include them from the existing XML files. Later in this section, we cover the
options for using `@Configuration` classes in this kind of "`XML-centric`" situation.

.[[beans-java-combining-xml-centric-declare-as-bean]]Declaring `@Configuration` classes as plain Spring `<bean/>` elements
--
Remember that `@Configuration` classes are ultimately bean definitions in the
container. In this series examples, we create a `@Configuration` class named `AppConfig` and
include it within `system-test-config.xml` as a `<bean/>` definition. Because
`<context:annotation-config/>` is switched on, the container recognizes the
`@Configuration` annotation and processes the `@Bean` methods declared in `AppConfig`
properly.

The following example shows an ordinary configuration class in Java:

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
----
	@Configuration
	public class AppConfig {

		@Autowired
		private DataSource dataSource;

		@Bean
		public AccountRepository accountRepository() {
			return new JdbcAccountRepository(dataSource);
		}

		@Bean
		public TransferService transferService() {
			return new TransferService(accountRepository());
		}
	}
----

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
----
	@Configuration
	class AppConfig {

		@Autowired
		private lateinit var dataSource: DataSource

		@Bean
		fun accountRepository(): AccountRepository {
			return JdbcAccountRepository(dataSource)
		}

		@Bean
		fun transferService() = TransferService(accountRepository())
	}
----
======

The following example shows part of a sample `system-test-config.xml` file:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<beans>
		<!-- enable processing of annotations such as @Autowired and @Configuration -->
		<context:annotation-config/>
		<context:property-placeholder location="classpath:/com/acme/jdbc.properties"/>

		<bean class="com.acme.AppConfig"/>

		<bean class="org.springframework.jdbc.datasource.DriverManagerDataSource">
			<property name="url" value="${jdbc.url}"/>
			<property name="username" value="${jdbc.username}"/>
			<property name="password" value="${jdbc.password}"/>
		</bean>
	</beans>
----

The following example shows a possible `jdbc.properties` file:

[literal,subs="verbatim,quotes"]
----
jdbc.url=jdbc:hsqldb:hsql://localhost/xdb
jdbc.username=sa
jdbc.password=
----

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
----
	public static void main(String[] args) {
		ApplicationContext ctx = new ClassPathXmlApplicationContext("classpath:/com/acme/system-test-config.xml");
		TransferService transferService = ctx.getBean(TransferService.class);
		// ...
	}
----

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
----
	fun main() {
		val ctx = ClassPathXmlApplicationContext("classpath:/com/acme/system-test-config.xml")
		val transferService = ctx.getBean<TransferService>()
		// ...
	}
----
======


NOTE: In `system-test-config.xml` file, the `AppConfig` `<bean/>` does not declare an `id`
element. While it would be acceptable to do so, it is unnecessary, given that no other bean
ever refers to it, and it is unlikely to be explicitly fetched from the container by name.
Similarly, the `DataSource` bean is only ever autowired by type, so an explicit bean `id`
is not strictly required.
--

.[[beans-java-combining-xml-centric-component-scan]] Using <context:component-scan/> to pick up `@Configuration` classes
--
Because `@Configuration` is meta-annotated with `@Component`, `@Configuration`-annotated
classes are automatically candidates for component scanning. Using the same scenario as
described in the previous example, we can redefine `system-test-config.xml` to take advantage of component-scanning.
Note that, in this case, we need not explicitly declare
`<context:annotation-config/>`, because `<context:component-scan/>` enables the same
functionality.

The following example shows the modified `system-test-config.xml` file:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<beans>
		<!-- picks up and registers AppConfig as a bean definition -->
		<context:component-scan base-package="com.acme"/>
		<context:property-placeholder location="classpath:/com/acme/jdbc.properties"/>

		<bean class="org.springframework.jdbc.datasource.DriverManagerDataSource">
			<property name="url" value="${jdbc.url}"/>
			<property name="username" value="${jdbc.username}"/>
			<property name="password" value="${jdbc.password}"/>
		</bean>
	</beans>
----
--

[[beans-java-combining-java-centric]]
=== `@Configuration` Class-centric Use of XML with `@ImportResource`

In applications where `@Configuration` classes are the primary mechanism for configuring
the container, it is still likely necessary to use at least some XML. In these
scenarios, you can use `@ImportResource` and define only as much XML as you need. Doing
so achieves a "`Java-centric`" approach to configuring the container and keeps XML to a
bare minimum. The following example (which includes a configuration class, an XML file
that defines a bean, a properties file, and the `main` class) shows how to use
the `@ImportResource` annotation to achieve "`Java-centric`" configuration that uses XML
as needed:

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
----
	@Configuration
	@ImportResource("classpath:/com/acme/properties-config.xml")
	public class AppConfig {

		@Value("${jdbc.url}")
		private String url;

		@Value("${jdbc.username}")
		private String username;

		@Value("${jdbc.password}")
		private String password;

		@Bean
		public DataSource dataSource() {
			return new DriverManagerDataSource(url, username, password);
		}
	}
----

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
----
	@Configuration
	@ImportResource("classpath:/com/acme/properties-config.xml")
	class AppConfig {

		@Value("\${jdbc.url}")
		private lateinit var url: String

		@Value("\${jdbc.username}")
		private lateinit var username: String

		@Value("\${jdbc.password}")
		private lateinit var password: String

		@Bean
		fun dataSource(): DataSource {
			return DriverManagerDataSource(url, username, password)
		}
	}
----
======

[source,xml,indent=0,subs="verbatim,quotes"]
----
	properties-config.xml
	<beans>
		<context:property-placeholder location="classpath:/com/acme/jdbc.properties"/>
	</beans>
----

[literal,subs="verbatim,quotes"]
----
jdbc.properties
jdbc.url=jdbc:hsqldb:hsql://localhost/xdb
jdbc.username=sa
jdbc.password=
----

[tabs]
======
Java::
+
[source,java,indent=0,subs="verbatim,quotes",role="primary"]
----
	public static void main(String[] args) {
		ApplicationContext ctx = new AnnotationConfigApplicationContext(AppConfig.class);
		TransferService transferService = ctx.getBean(TransferService.class);
		// ...
	}
----

Kotlin::
+
[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
----
	import org.springframework.beans.factory.getBean

	fun main() {
		val ctx = AnnotationConfigApplicationContext(AppConfig::class.java)
		val transferService = ctx.getBean<TransferService>()
		// ...
	}
----
======



