All Downloads are FREE. Search and download functionalities are using the official Maven repository.

org.springframework.context.annotation.Configuration Maven / Gradle / Ivy

The newest version!
/*
 * Copyright 2002-2023 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 } is required in order to * enable {@link ConfigurationClassPostProcessor} and other annotation-related * post processors that facilitate handling {@code @Configuration} classes. * *

Via component scanning

* *

Since {@code @Configuration} is meta-annotated with {@link Component @Component}, * {@code @Configuration} classes are candidates for component scanning — * for example, using {@link ComponentScan @ComponentScan} or Spring XML's * {@code } element — and therefore may also take * advantage of {@link Autowired @Autowired}/{@link jakarta.inject.Inject @Inject} * like any regular {@code @Component}. In particular, if a single constructor is * present, autowiring semantics will be applied transparently for that constructor: * *

 * @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 } or explicitly * in a {@code @Configuration} class via a dedicated {@code static} {@code @Bean} method * (see "a note on BeanFactoryPostProcessor-returning {@code @Bean} methods" of * {@link Bean @Bean}'s javadocs for details). Note, however, that explicit registration * of a {@code PropertySourcesPlaceholderConfigurer} via a {@code static} {@code @Bean} * method is typically only required if you need to customize configuration such as the * placeholder syntax, etc. Specifically, if no bean post-processor (such as a * {@code PropertySourcesPlaceholderConfigurer}) has registered an embedded value * resolver for the {@code ApplicationContext}, Spring will register a default * embedded value resolver which resolves placeholders against property sources * registered in the {@code Environment}. See the section below on composing * {@code @Configuration} classes with Spring XML using {@code @ImportResource}; see * the {@link Value @Value} javadocs; and see the {@link Bean @Bean} javadocs for details * on working with {@code BeanFactoryPostProcessor} types such as * {@code PropertySourcesPlaceholderConfigurer}. * *

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. *

Alias for {@link Component#value}. * @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, for example, 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, for example, 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; }





© 2015 - 2024 Weber Informatics LLC | Privacy Policy