org.springframework.context.annotation.Configuration Maven / Gradle / Ivy
/*
* Copyright 2002-2022 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* https://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.springframework.context.annotation;
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.core.annotation.AliasFor;
import org.springframework.stereotype.Component;
/**
* Indicates that a class declares one or more {@link Bean @Bean} methods and
* may be processed by the Spring container to generate bean definitions and
* service requests for those beans at runtime, for example:
*
*
* @Configuration
* public class AppConfig {
*
* @Bean
* public MyBean myBean() {
* // instantiate, configure and return bean ...
* }
* }
*
* Bootstrapping {@code @Configuration} classes
*
* Via {@code AnnotationConfigApplicationContext}
*
* {@code @Configuration} classes are typically bootstrapped using either
* {@link AnnotationConfigApplicationContext} or its web-capable variant,
* {@link org.springframework.web.context.support.AnnotationConfigWebApplicationContext
* AnnotationConfigWebApplicationContext}. A simple example with the former follows:
*
*
* AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
* ctx.register(AppConfig.class);
* ctx.refresh();
* MyBean myBean = ctx.getBean(MyBean.class);
* // use myBean ...
*
*
* See the {@link AnnotationConfigApplicationContext} javadocs for further details, and see
* {@link org.springframework.web.context.support.AnnotationConfigWebApplicationContext
* AnnotationConfigWebApplicationContext} for web configuration instructions in a
* {@code Servlet} container.
*
*
Via Spring {@code } XML
*
* As an alternative to registering {@code @Configuration} classes directly against an
* {@code AnnotationConfigApplicationContext}, {@code @Configuration} classes may be
* declared as normal {@code } definitions within Spring XML files:
*
*
* <beans>
* <context:annotation-config/>
* <bean class="com.acme.AppConfig"/>
* </beans>
*
*
* In the example above, {@code
Via component scanning
*
* {@code @Configuration} is meta-annotated with {@link Component @Component}, therefore
* {@code @Configuration} classes are candidates for component scanning (typically using
* Spring XML's {@code
* @Configuration
* public class AppConfig {
*
* private final SomeBean someBean;
*
* public AppConfig(SomeBean someBean) {
* this.someBean = someBean;
* }
*
* // @Bean definition using "SomeBean"
*
* }
*
* {@code @Configuration} classes may not only be bootstrapped using
* component scanning, but may also themselves configure component scanning using
* the {@link ComponentScan @ComponentScan} annotation:
*
*
* @Configuration
* @ComponentScan("com.acme.app.services")
* public class AppConfig {
* // various @Bean definitions ...
* }
*
* See the {@link ComponentScan @ComponentScan} javadocs for details.
*
*
Working with externalized values
*
* Using the {@code Environment} API
*
* Externalized values may be looked up by injecting the Spring
* {@link org.springframework.core.env.Environment} into a {@code @Configuration}
* class — for example, using the {@code @Autowired} annotation:
*
*
* @Configuration
* public class AppConfig {
*
* @Autowired Environment env;
*
* @Bean
* public MyBean myBean() {
* MyBean myBean = new MyBean();
* myBean.setName(env.getProperty("bean.name"));
* return myBean;
* }
* }
*
* Properties resolved through the {@code Environment} reside in one or more "property
* source" objects, and {@code @Configuration} classes may contribute property sources to
* the {@code Environment} object using the {@link PropertySource @PropertySource}
* annotation:
*
*
* @Configuration
* @PropertySource("classpath:/com/acme/app.properties")
* public class AppConfig {
*
* @Inject Environment env;
*
* @Bean
* public MyBean myBean() {
* return new MyBean(env.getProperty("bean.name"));
* }
* }
*
* See the {@link org.springframework.core.env.Environment Environment}
* and {@link PropertySource @PropertySource} javadocs for further details.
*
*
Using the {@code @Value} annotation
*
* Externalized values may be injected into {@code @Configuration} classes using
* the {@link Value @Value} annotation:
*
*
* @Configuration
* @PropertySource("classpath:/com/acme/app.properties")
* public class AppConfig {
*
* @Value("${bean.name}") String beanName;
*
* @Bean
* public MyBean myBean() {
* return new MyBean(beanName);
* }
* }
*
* This approach is often used in conjunction with Spring's
* {@link org.springframework.context.support.PropertySourcesPlaceholderConfigurer
* PropertySourcesPlaceholderConfigurer} that can be enabled automatically
* in XML configuration via {@code
Composing {@code @Configuration} classes
*
* With the {@code @Import} annotation
*
* {@code @Configuration} classes may be composed using the {@link Import @Import} annotation,
* similar to the way that {@code } works in Spring XML. Because
* {@code @Configuration} objects are managed as Spring beans within the container,
* imported configurations may be injected — for example, via constructor injection:
*
*
* @Configuration
* public class DatabaseConfig {
*
* @Bean
* public DataSource dataSource() {
* // instantiate, configure and return DataSource
* }
* }
*
* @Configuration
* @Import(DatabaseConfig.class)
* public class AppConfig {
*
* private final DatabaseConfig dataConfig;
*
* public AppConfig(DatabaseConfig dataConfig) {
* this.dataConfig = dataConfig;
* }
*
* @Bean
* public MyBean myBean() {
* // reference the dataSource() bean method
* return new MyBean(dataConfig.dataSource());
* }
* }
*
* Now both {@code AppConfig} and the imported {@code DatabaseConfig} can be bootstrapped
* by registering only {@code AppConfig} against the Spring context:
*
*
* new AnnotationConfigApplicationContext(AppConfig.class);
*
* With the {@code @Profile} annotation
*
* {@code @Configuration} classes may be marked with the {@link Profile @Profile} annotation to
* indicate they should be processed only if a given profile or profiles are active:
*
*
* @Profile("development")
* @Configuration
* public class EmbeddedDatabaseConfig {
*
* @Bean
* public DataSource dataSource() {
* // instantiate, configure and return embedded DataSource
* }
* }
*
* @Profile("production")
* @Configuration
* public class ProductionDatabaseConfig {
*
* @Bean
* public DataSource dataSource() {
* // instantiate, configure and return production DataSource
* }
* }
*
* Alternatively, you may also declare profile conditions at the {@code @Bean} method level
* — for example, for alternative bean variants within the same configuration class:
*
*
* @Configuration
* public class ProfileDatabaseConfig {
*
* @Bean("dataSource")
* @Profile("development")
* public DataSource embeddedDatabase() { ... }
*
* @Bean("dataSource")
* @Profile("production")
* public DataSource productionDatabase() { ... }
* }
*
* See the {@link Profile @Profile} and {@link org.springframework.core.env.Environment}
* javadocs for further details.
*
*
With Spring XML using the {@code @ImportResource} annotation
*
* As mentioned above, {@code @Configuration} classes may be declared as regular Spring
* {@code } definitions within Spring XML files. It is also possible to
* import Spring XML configuration files into {@code @Configuration} classes using
* the {@link ImportResource @ImportResource} annotation. Bean definitions imported from
* XML can be injected — for example, using the {@code @Inject} annotation:
*
*
* @Configuration
* @ImportResource("classpath:/com/acme/database-config.xml")
* public class AppConfig {
*
* @Inject DataSource dataSource; // from XML
*
* @Bean
* public MyBean myBean() {
* // inject the XML-defined dataSource bean
* return new MyBean(this.dataSource);
* }
* }
*
* With nested {@code @Configuration} classes
*
* {@code @Configuration} classes may be nested within one another as follows:
*
*
* @Configuration
* public class AppConfig {
*
* @Inject DataSource dataSource;
*
* @Bean
* public MyBean myBean() {
* return new MyBean(dataSource);
* }
*
* @Configuration
* static class DatabaseConfig {
* @Bean
* DataSource dataSource() {
* return new EmbeddedDatabaseBuilder().build();
* }
* }
* }
*
* When bootstrapping such an arrangement, only {@code AppConfig} need be registered
* against the application context. By virtue of being a nested {@code @Configuration}
* class, {@code DatabaseConfig} will be registered automatically. This avoids
* the need to use an {@code @Import} annotation when the relationship between
* {@code AppConfig} and {@code DatabaseConfig} is already implicitly clear.
*
*
Note also that nested {@code @Configuration} classes can be used to good effect
* with the {@code @Profile} annotation to provide two options of the same bean to the
* enclosing {@code @Configuration} class.
*
*
Configuring lazy initialization
*
* By default, {@code @Bean} methods will be eagerly instantiated at container
* bootstrap time. To avoid this, {@code @Configuration} may be used in conjunction with
* the {@link Lazy @Lazy} annotation to indicate that all {@code @Bean} methods declared
* within the class are by default lazily initialized. Note that {@code @Lazy} may be used
* on individual {@code @Bean} methods as well.
*
*
Testing support for {@code @Configuration} classes
*
* The Spring TestContext framework available in the {@code spring-test} module
* provides the {@code @ContextConfiguration} annotation which can accept an array of
* component class references — typically {@code @Configuration} or
* {@code @Component} classes.
*
*
* @ExtendWith(SpringExtension.class)
* @ContextConfiguration(classes = {AppConfig.class, DatabaseConfig.class})
* class MyTests {
*
* @Autowired MyBean myBean;
*
* @Autowired DataSource dataSource;
*
* @Test
* void test() {
* // assertions against myBean ...
* }
* }
*
* See the
* TestContext framework
* reference documentation for details.
*
*
Enabling built-in Spring features using {@code @Enable} annotations
*
* Spring features such as asynchronous method execution, scheduled task execution,
* annotation driven transaction management, and even Spring MVC can be enabled and
* configured from {@code @Configuration} classes using their respective "{@code @Enable}"
* annotations. See
* {@link org.springframework.scheduling.annotation.EnableAsync @EnableAsync},
* {@link org.springframework.scheduling.annotation.EnableScheduling @EnableScheduling},
* {@link org.springframework.transaction.annotation.EnableTransactionManagement @EnableTransactionManagement},
* {@link org.springframework.context.annotation.EnableAspectJAutoProxy @EnableAspectJAutoProxy},
* and {@link org.springframework.web.servlet.config.annotation.EnableWebMvc @EnableWebMvc}
* for details.
*
*
Constraints when authoring {@code @Configuration} classes
*
*
* - Configuration classes must be provided as classes (i.e. not as instances returned
* from factory methods), allowing for runtime enhancements through a generated subclass.
*
- Configuration classes must be non-final (allowing for subclasses at runtime),
* unless the {@link #proxyBeanMethods() proxyBeanMethods} flag is set to {@code false}
* in which case no runtime-generated subclass is necessary.
*
- Configuration classes must be non-local (i.e. may not be declared within a method).
*
- Any nested configuration classes must be declared as {@code static}.
*
- {@code @Bean} methods may not in turn create further configuration classes
* (any such instances will be treated as regular beans, with their configuration
* annotations remaining undetected).
*
*
* @author Rod Johnson
* @author Chris Beams
* @author Juergen Hoeller
* @since 3.0
* @see Bean
* @see Profile
* @see Import
* @see ImportResource
* @see ComponentScan
* @see Lazy
* @see PropertySource
* @see AnnotationConfigApplicationContext
* @see ConfigurationClassPostProcessor
* @see org.springframework.core.env.Environment
* @see org.springframework.test.context.ContextConfiguration
*/
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Configuration {
/**
* Explicitly specify the name of the Spring bean definition associated with the
* {@code @Configuration} class. If left unspecified (the common case), a bean
* name will be automatically generated.
* The custom name applies only if the {@code @Configuration} class is picked
* up via component scanning or supplied directly to an
* {@link AnnotationConfigApplicationContext}. If the {@code @Configuration} class
* is registered as a traditional XML bean definition, the name/id of the bean
* element will take precedence.
* @return the explicit component name, if any (or empty String otherwise)
* @see AnnotationBeanNameGenerator
*/
@AliasFor(annotation = Component.class)
String value() default "";
/**
* Specify whether {@code @Bean} methods should get proxied in order to enforce
* bean lifecycle behavior, e.g. to return shared singleton bean instances even
* in case of direct {@code @Bean} method calls in user code. This feature
* requires method interception, implemented through a runtime-generated CGLIB
* subclass which comes with limitations such as the configuration class and
* its methods not being allowed to declare {@code final}.
*
The default is {@code true}, allowing for 'inter-bean references' via direct
* method calls within the configuration class as well as for external calls to
* this configuration's {@code @Bean} methods, e.g. from another configuration class.
* If this is not needed since each of this particular configuration's {@code @Bean}
* methods is self-contained and designed as a plain factory method for container use,
* switch this flag to {@code false} in order to avoid CGLIB subclass processing.
*
Turning off bean method interception effectively processes {@code @Bean}
* methods individually like when declared on non-{@code @Configuration} classes,
* a.k.a. "@Bean Lite Mode" (see {@link Bean @Bean's javadoc}). It is therefore
* behaviorally equivalent to removing the {@code @Configuration} stereotype.
* @since 5.2
*/
boolean proxyBeanMethods() default true;
/**
* Specify whether {@code @Bean} methods need to have unique method names,
* raising an exception otherwise in order to prevent accidental overloading.
*
The default is {@code true}, preventing accidental method overloads which
* get interpreted as overloaded factory methods for the same bean definition
* (as opposed to separate bean definitions with individual conditions etc).
* Switch this flag to {@code false} in order to allow for method overloading
* according to those semantics, accepting the risk for accidental overlaps.
* @since 6.0
*/
boolean enforceUniqueMethods() default true;
}