Spring事件處理

你已經看到了所有的章節了Spring的核心是ApplicationContext，它管理bean的整個生命周期。ApplicationContext中加載bean時發布某些類型的事件。例如，一個ContextStartedEvent是當所述上下文被啟動並發布，上下文被停止ContextStoppedEvent。

事件處理中ApplicationContext 通過了ApplicationEvent類和ApplicationListener接口提供。所以，如果一個bean實現了ApplicationListener，然後每anApplicationEvent被發布到ApplicationContext的時候通知bean。

Spring提供了以下標準的事件：

S.N.	Spring 內置活動及說明
1	ContextRefreshedEvent This event is published when the ApplicationContext is either initialized or refreshed. This can also be raised using the refresh() method on the ConfigurableApplicationContext interface.
2	ContextStartedEvent This event is published when the ApplicationContext is started using the start() method on theConfigurableApplicationContext interface. You can poll your database or you can re/start any stopped application after receiving this event.
3	ContextStoppedEvent This event is published when the ApplicationContext is stopped using the stop() method on the ConfigurableApplicationContext interface. You can do required housekeep work after receiving this event.
4	ContextClosedEvent This event is published when the ApplicationContext is closed using the close() method on theConfigurableApplicationContext interface. A closed context reaches its end of life; it cannot be refreshed or restarted.
5	RequestHandledEvent This is a web-specific event telling all beans that an HTTP request has been serviced.

Spring事件處理是單線程的，所以如果一個事件被發布，直至及除非所有的接收器得到的消息，該進程被阻塞並且流程將不會繼續。因此，應注意在設計應用程序時，如果事件處理被使用。

監聽上下文事件：

要監聽上下文事件，一個bean應該實現ApplicationListener接口，隻有一個方法onApplicationEvent()。因此，讓我們寫一個例子，看看如何傳播的事件，以及如何可以把代碼來執行基於某些事件所需的任務。

讓我們使用Eclipse IDE，然後按照下麵的步驟來創建一個Spring應用程序：

步驟	描述
1	Create a project with a name SpringExample and create a package com.yiibai under the src folder in the created project.
2	Add required Spring libraries using Add External JARs option as explained in the Spring Hello World Example chapter.
3	Create Java classes HelloWorld, CStartEventHandler, CStopEventHandler and MainAppunder the com.yiibai package.
4	Create Beans configuration file Beans.xml under the src folder.
5	The final step is to create the content of all the Java files and Bean Configuration file and run the application as explained below.

這裡是HelloWorld.java 文件的內容：

package com.yiibai;

public class HelloWorld {
   private String message;

   public void setMessage(String message){
      this.message  = message;
   }

   public void getMessage(){
      System.out.println("Your Message : " + message);
   }
}

以下是CStartEventHandler.java文件的內容：

package com.yiibai;

import org.springframework.context.ApplicationListener;
import org.springframework.context.event.ContextStartedEvent;

public class CStartEventHandler 
   implements ApplicationListener<ContextStartedEvent>{

   public void onApplicationEvent(ContextStartedEvent event) {
      System.out.println("ContextStartedEvent Received");
   }
}

以下是CStopEventHandler.java文件的內容：

package com.yiibai;

import org.springframework.context.ApplicationListener;
import org.springframework.context.event.ContextStoppedEvent;

public class CStopEventHandler 
   implements ApplicationListener<ContextStoppedEvent>{

   public void onApplicationEvent(ContextStoppedEvent event) {
      System.out.println("ContextStoppedEvent Received");
   }
}

以下是MainApp.java文件的內容：

package com.yiibai;

import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class MainApp {
   public static void main(String[] args) {
      ConfigurableApplicationContext context = 
      new ClassPathXmlApplicationContext("Beans.xml");

      // Let us raise a start event.
      context.start();
	  
      HelloWorld obj = (HelloWorld) context.getBean("helloWorld");

      obj.getMessage();

      // Let us raise a stop event.
      context.stop();
   }
}

以下是配置文件beans.xml文件：

<?xml version="1.0" encoding="UTF-8"?>

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">

   <bean id="helloWorld" class="com.yiibai.HelloWorld">
       <property name="message" value="Hello World!"/>
   </bean>

   <bean id="cStartEventHandler" 
         class="com.yiibai.CStartEventHandler"/>

   <bean id="cStopEventHandler" 
         class="com.yiibai.CStopEventHandler"/>

</beans>

創建源程序和bean配置文件完成，讓我們運行應用程序。如果一切順利，這將打印以下信息：

ContextStartedEvent Received
Your Message : Hello World!
ContextStoppedEvent Received

如果喜歡，你可以發布自己的自定義事件，以後你可以捕捉相同處理那些自定義事件動作。如果你有興趣在編寫自己的自定義事件，可以查看 Spring自定義事件