spring事件（一）--内置事件

一、简介

spring框架是java开发中常用的，通常我们需要在spring的某些事件发生时，做某些操作。

在此介绍spring的内置事件，以父抽象类ApplicationContextEvent下的四个子事件类为主。

类图如下：

二、事件类

1、ApplicationContextEvent

是spring内置事件的父抽象类，构造方法传入spring的context容器，同时也有获取spring的context容器的方法。

源码如下：

/**
 * Base class for events raised for an {@code ApplicationContext}.
 *
 * @author Juergen Hoeller
 * @since 2.5
 */
@SuppressWarnings("serial")
public abstract class ApplicationContextEvent extends ApplicationEvent {

   /**
    * Create a new ContextStartedEvent.
    * @param source the {@code ApplicationContext} that the event is raised for
    * (must not be {@code null})
    */
   public ApplicationContextEvent(ApplicationContext source) {
      super(source);
   }

   /**
    * Get the {@code ApplicationContext} that the event was raised for.
    */
   public final ApplicationContext getApplicationContext() {
      return (ApplicationContext) getSource();
   }

}

2、ContextRefreshedEvent

当spring容器初始化或刷新时，会触发此事件。此事件在开发中常用，用于在spring容器启动时，导入自定义的bean实例到spring容器中。

源码如下：

/**
 * Event raised when an {@code ApplicationContext} gets initialized or refreshed.
 *
 * @author Juergen Hoeller
 * @since 04.03.2003
 * @see ContextClosedEvent
 */
@SuppressWarnings("serial")
public class ContextRefreshedEvent extends ApplicationContextEvent {

   /**
    * Create a new ContextRefreshedEvent.
    * @param source the {@code ApplicationContext} that has been initialized
    * or refreshed (must not be {@code null})
    */
   public ContextRefreshedEvent(ApplicationContext source) {
      super(source);
   }

}

3、ContextStartedEvent

当spring启动时，或者说是context调用start()方法时，会触发此事件。

源码如下：

/**
 * Event raised when an {@code ApplicationContext} gets started.
 *
 * @author Mark Fisher
 * @author Juergen Hoeller
 * @since 2.5
 * @see ContextStoppedEvent
 */
@SuppressWarnings("serial")
public class ContextStartedEvent extends ApplicationContextEvent {

   /**
    * Create a new ContextStartedEvent.
    * @param source the {@code ApplicationContext} that has been started
    * (must not be {@code null})
    */
   public ContextStartedEvent(ApplicationContext source) {
      super(source);
   }

}

4、ContextStoppedEvent

当spring停止时，或者说context调用stop()方法时，会触发此事件。

源码如下：

/**
 * Event raised when an {@code ApplicationContext} gets stopped.
 *
 * @author Mark Fisher
 * @author Juergen Hoeller
 * @since 2.5
 * @see ContextStartedEvent
 */
@SuppressWarnings("serial")
public class ContextStoppedEvent extends ApplicationContextEvent {

   /**
    * Create a new ContextStoppedEvent.
    * @param source the {@code ApplicationContext} that has been stopped
    * (must not be {@code null})
    */
   public ContextStoppedEvent(ApplicationContext source) {
      super(source);
   }

}

5、ContextClosedEvent

当spring关闭时，或者说context调用close()方法时，会触发此事件。

源码如下：

/**
 * Event raised when an {@code ApplicationContext} gets closed.
 *
 * @author Juergen Hoeller
 * @since 12.08.2003
 * @see ContextRefreshedEvent
 */
@SuppressWarnings("serial")
public class ContextClosedEvent extends ApplicationContextEvent {

   /**
    * Creates a new ContextClosedEvent.
    * @param source the {@code ApplicationContext} that has been closed
    * (must not be {@code null})
    */
   public ContextClosedEvent(ApplicationContext source) {
      super(source);
   }

}

三、监听类

在spring中，我们通常实现接口ApplicationListener来对事件进行监听，泛型传入事件的类型。

在接口内的方法onApplicationEvent中实现监听到事件时需处理的逻辑。

ApplicationListener接口的源码如下：

/**
 * Interface to be implemented by application event listeners.
 * Based on the standard {@code java.util.EventListener} interface
 * for the Observer design pattern.
 *
 * 
As of Spring 3.0, an ApplicationListener can generically declare the event type
 * that it is interested in. When registered with a Spring ApplicationContext, events
 * will be filtered accordingly, with the listener getting invoked for matching event
 * objects only.
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @param <E> the specific ApplicationEvent subclass to listen to
 * @see org.springframework.context.event.ApplicationEventMulticaster
 */
public interface ApplicationListener<E extends ApplicationEvent> extends EventListener {

   /**
    * Handle an application event.
    * @param event the event to respond to
    */
   void onApplicationEvent(E event);

}

四、代码实例

代码中，我们可以直接实现不带泛型的接口ApplicationListener，在代码中对不同事件做不同处理，如：

@Component
public class SpringContextEventDemo  implements ApplicationListener {
    public void onApplicationEvent(ApplicationEvent event) {
        if(event instanceof ContextRefreshedEvent){
            System.out.println("ContextRefreshedEvent happen");
        }else if(event instanceof ContextStartedEvent){
            System.out.println("ContextStartedEvent happen");
        }else if(event instanceof ContextStoppedEvent){
            System.out.println("ContextStoppedEvent happen");
        }else if(event instanceof ContextClosedEvent){
            System.out.println("ContextClosedEvent happen");
        }
    }
}

也可直接监听特定事件，即实现带泛型接口的ApplicationListener，在此以监听ContextRefreshedEvent事件为例。

1、监听类

@Component
public class SpringContextRefreshedEventDemo implements ApplicationListener {
    public void onApplicationEvent(ContextRefreshedEvent contextRefreshedEvent) {
        ApplicationContext ctx = contextRefreshedEvent.getApplicationContext();
        System.out.println("ContextRefreshedEvent happen");
    }
}

2、测试类

public class SpringSelfEventMain {
    public static void main(String[] args) {
        AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext(SpringConfig.class);
//        ctx.start();  //ContextStartedEvent事件发生
//        ctx.stop();   //ContextStoppedEvent事件发生
//        ctx.close();  //ContextClosedEvent事件发生
    }
}

运行结果（在spring容器启动时，触发ContextRefreshedEvent事件，执行监听逻辑）：

ContextRefreshedEvent happen