配置-代码

1. 前言

Spring 的代码配置 Java-configuration，核心是通过@Configuration修饰的配置类搭配@Bean修饰的方法来支持的：

• The @Bean annotation is used to indicate that a method instantiates, configures, and initializes a new object to be managed by the Spring IoC container.

• Annotating a class with @Configuration indicates that its primary purpose is as a source of bean definitions. Furthermore, @Configuration classes let inter-bean dependencies be defined by calling other @Bean methods in the same class.

• 以下两者等价：

  • 代码配置

    @Configuration
    public class AppConfig {
    
        @Bean
        public MyService myService() {
            return new MyServiceImpl();
        }
    }

  • XML 配置

    <beans>
        <bean id="myService" class="com.acme.services.MyServiceImpl"/>
    </beans>

When @Configuration classes are provided as input, the @Configuration class itself is registered as a bean definition and all declared @Bean methods within the class are also registered as bean definitions.

When @Component and JSR-330 classes are provided, they are registered as bean definitions, and it is assumed that DI metadata such as @Autowired or @Inject are used within those classes where necessary.

2. Full @Configuration vs lite @Bean mode?

详见 配置-注解：@Configuration vs @Component

3. 构建 IoC 容器

3.1 实例化 IoC 容器——AnnotationConfigApplicationContext

两种方式。

In much the same way that Spring XML files are used as input when instantiating a ClassPathXmlApplicationContext, you can use @Configuration classes as input when instantiating an AnnotationConfigApplicationContext.

public static void main(String[] args) {
    ApplicationContext ctx = new AnnotationConfigApplicationContext(AppConfig.class);
    MyService myService = ctx.getBean(MyService.class);
    myService.doStuff();
}

You can instantiate an AnnotationConfigApplicationContext by using a no-arg constructor and then configure it by using the register() method.

public static void main(String[] args) {
    AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
    ctx.register(AppConfig.class, OtherConfig.class);
    ctx.register(AdditionalConfig.class);
    ctx.refresh();
    MyService myService = ctx.getBean(MyService.class);
    myService.doStuff();
}

3.2 设置包扫描路径

除了在 @Configuration 配置类上使用 @ComponentScan 注解外，也可以使用 AnnotationConfigApplicationContext 暴露的scan(String...)方法。以下两种方式等价：

• 注解：

  @Configuration
  @ComponentScan(basePackages = "com.acme") 
  public class AppConfig  {
      // ...
  }

• scan 方法：

  public static void main(String[] args) {
      AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
      ctx.scan("com.acme");
      ctx.refresh();
      MyService myService = ctx.getBean(MyService.class);
  }

  Remember that @Configuration classes are meta-annotated with @Component, so they are candidates for component-scanning.

  In the preceding example, assuming that AppConfig is declared within the com.acme package (or any package underneath), it is picked up during the call to scan(). Upon refresh(), all its @Bean methods are processed and registered as bean definitions within the container.

3.3 AnnotationConfigWebApplicationContext

A WebApplicationContext variant of AnnotationConfigApplicationContext is available with AnnotationConfigWebApplicationContext. You can use this implementation when configuring the Spring ContextLoaderListener servlet listener, Spring MVC DispatcherServlet, and so forth.

4. @Profile, @Conditional

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 Bean Definition Profiles for details).

The @Profile annotation is actually implemented by using a much more flexible annotation called @Conditional. The @Conditional annotation indicates specific org.springframework.context.annotation.Condition implementations that should be consulted before a @Bean is registered.

package org.springframework.context.annotation;

@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional({ProfileCondition.class})
public @interface Profile {
    String[] value();
}

package org.springframework.context.annotation;

@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Conditional {
    Class<? extends Condition>[] value();
}