JMX学习笔记(四)：Notifications

Notification

JMX API定义了一种机制，使MBean能生成通知，如当信号状态改变，检测到事件或者一个问题时。

要生成通知，MBean必须实现NotificationEmitter接口或继承NotificationBroadcasterSupport类。要发送一个通知必须构造Notification类的实例或者子类(如NotificationChangedAttribute),通知实例产生后，由NotificationBroadcasterSupport的sendNotification方法发送通知。

例：

Step 1 ：定义HelloMBean接口

package com.jmx.demo5;

public interface HelloMBean {
	public void sayHello();

	public int add(int x, int y);

	public String getName();

	public int getCacheSize();

	public void setCacheSize(int size);

}

Step 2 ：编写HelloMBean的实现类Hello

package com.jmx.demo5;

import javax.management.AttributeChangeNotification;
import javax.management.MBeanNotificationInfo;
import javax.management.Notification;
import javax.management.NotificationBroadcasterSupport;

//MBean must extends NotificationBroadcasterSupport or implements NotificationEmitter
public class Hello extends NotificationBroadcasterSupport implements HelloMBean {
	private final String name = "Reginald";
	private int cacheSize = DEFAULT_CACHE_SIZE;
	private static final int DEFAULT_CACHE_SIZE = 200;
	private long sequenceNumber = 1;

	public void sayHello() {
		System.out.println("hello, world");
	}

	public int add(int x, int y) {
		return x + y;
	}

	public String getName() {
		return this.name;
	}

	public int getCacheSize() {
		return this.cacheSize;
	}

	public synchronized void setCacheSize(int size) {
		int oldSize = this.cacheSize;
		this.cacheSize = size;
		System.out.println("Cache size now " + this.cacheSize);

		/**
		 * To send a notification,you need to construct an instance of class
		 * Notification or a Subclass(such as AttributeChangedNotification),and
		 * pass the instance to NotificationBroadcastSupport.sendNotification.
		 * 
		 */
		Notification n = new AttributeChangeNotification(this,
				sequenceNumber++, System.currentTimeMillis(),
				"CacheSize changed", "CacheSize", "int", oldSize,
				this.cacheSize);

		sendNotification(n);
	}

	@Override
	public MBeanNotificationInfo[] getNotificationInfo() {
		String[] types = new String[] { AttributeChangeNotification.ATTRIBUTE_CHANGE };
		String name = AttributeChangeNotification.class.getName();
		String description = "An attribute of this MBean has changed";
		MBeanNotificationInfo info = new MBeanNotificationInfo(types, name,
				description);
		return new MBeanNotificationInfo[] { info };
	}

}

Step 3：创建MBeanServer,标识MBean,注册MBean,并运行Main方法：

package com.jmx.demo5;

import java.lang.management.ManagementFactory;

import javax.management.MBeanServer;
import javax.management.ObjectName;

public class Main {
	public static void main(String[] args) throws Exception{
		MBeanServer mbs = ManagementFactory.getPlatformMBeanServer();
		ObjectName name = new ObjectName("com.jmx.demo:type=Hello");
		Hello mbean = new Hello();
		mbs.registerMBean(mbean, name);
		System.out.println("Waiting forever...");
		Thread.sleep(Long.MAX_VALUE);
		
	}
}

在Jconsole中查看运行结果：通知选项初始状态没有值，点击订阅后，通知项为0

更改CacheSize的值后，在观察通知选项，变为1

总结：MBean实现通知机制，必须继承NotificationBroadcasterSupport类或实现NotificationEmitter接口，用Notification类构造通知实例，发送通知实例由NotificationBroadcaterSupport.sendNotification方法;