JMX学习笔记(二)-Notification

Notification通知，也可理解为消息，有通知，必然有发送通知的广播，JMX这里采用了一种订阅的方式，类似于观察者模式，注册一个观察者到广播里，当有通知时，广播通过调用观察者，逐一通知.

这里写一个简单的Server配置例子, 首先定义我们的MBean接口:

接着，我们会想第一节那样，去实现这个MBean接口，并且继承NotificationBroadcasterSupport，来提供广播服务：

package com.dxz.mbean;

import java.util.concurrent.atomic.AtomicLong;

import javax.management.AttributeChangeNotification;
import javax.management.NotificationBroadcasterSupport;

public class ServerConfigure extends NotificationBroadcasterSupport implements ServerConfigureMBean {

    private AtomicLong sequenceNumber = new AtomicLong(1);

    private int port;

    private String host;

    public void setPort(int port) {
        int oldPort = this.port;
        this.port = port;
        AttributeChangeNotification notification = new AttributeChangeNotification(this,
                sequenceNumber.getAndIncrement(), System.currentTimeMillis(),
                AttributeChangeNotification.ATTRIBUTE_CHANGE, "Server Port Change", "java.lang.Integer", oldPort + "",
                this.port + "");
        super.sendNotification(notification);
    }

    public int getPort() {
        return port;
    }

    public void setHost(String host) {
        String oldHost = this.host;
        this.host = host;
        AttributeChangeNotification notification = new AttributeChangeNotification(this,
                sequenceNumber.getAndIncrement(), System.currentTimeMillis(),
                AttributeChangeNotification.ATTRIBUTE_CHANGE, "Server Host Change", "java.lang.String", oldHost,
                this.host);
        super.sendNotification(notification);
    }

    public String getHost() {
        return host;
    }

}

在setPort与setHos方法中，首先new了一个AttributeChangeNotification，这个类是javax.management.Notification的子类，而javax.management.Notification这个类又是Java.util.EventObject的子类，由此可以证实上边所说的，JMX通知机制使用了观察者设计模式。

javax.management.Notification是一个JMX的通知核心类，将来需要扩展或者其他JMX自带的消息，均集成自此类.

AttributeChangeNotification根据类名可知，是一个属性改变的通知，造方法参数如下:

Object source,                 // 事件源，一直传递到java.util.EventObject的source

long sequenceNumber,   // 通知序号，标识每次通知的计数器

long timeStamp,              // 通知发出的时间戳

String msg,                     // 通知发送的message

String attributeName,     // 被修改属性名

String attributeType,      // 被修改属性类型

Object oldValue,             // 被修改属性修改以前的值

Object newValue            // 被修改属性修改以后的值

根据观察者模式，由事件与广播组成，所以这里继承了NotificationBroadcasterSupport，来提供广播机制,调用NotificationBroadcasterSupportr的sendNotification(notification) 发送广播，广播会根据注册的观察者来对观察者进行逐一通知.

sendNotification 在JDK1.6是通过Executor来发送通知，默认调用线程同步发送：

1. public NotificationBroadcasterSupport(Executor executor,
                         MBeanNotificationInfo... info) {
       this.executor = (executor != null) ? executor : defaultExecutor;   
   
       notifInfo = info == null ? NO_NOTIFICATION_INFO : info.clone();
       }

private final static Executor defaultExecutor = new Executor() {
        // DirectExecutor using caller thread
        public void execute(Runnable r) {
        r.run();
        }
    }; 

如果想用异步发送通知，大家可以在构造方法中传入异步执行的Executor , 例如 ThreadPoolExecutor.

接下来，还得写一个观察者，来接受我们送出的通知：

package com.dxz.mbean;

import javax.management.Notification;
import javax.management.NotificationListener;

public class ServerConfigureNotificationListener implements NotificationListener {

    public void handleNotification(Notification notification, Object handback) {
        log("SequenceNumber:" + notification.getSequenceNumber());
        log("Type:" + notification.getType());
        log("Message:" + notification.getMessage());
        log("Source:" + notification.getSource());
        log("TimeStamp:" + notification.getTimeStamp());
        log("UserData:" + notification.getUserData());
        log("========="+notification.toString());
    }

    private void log(String message) {
        System.out.println(message);
    }

}

这里只是简单输出了通知内容， 在这个类中我们实现NotificationListener接口，可以看出该接口中只有一个方法，就是处理消息，顺藤摸瓜，在看一下NotificationListener的接口代码：

package javax.management;   

import java.util.EventListener;   

/**
 * Should be implemented by an object that wants to receive notifications.
 *
 * @since 1.5
 */
public interface NotificationListener extends java.util.EventListener   {    

    /**
    * Invoked when a JMX notification occurs.
    * The implementation of this method should return as soon as possible, to avoid
    * blocking its notification broadcaster.
    *
    * @param notification The notification.
    * @param handback An opaque object which helps the listener to associate information
    * regarding the MBean emitter. This object is passed to the MBean during the
    * addListener call and resent, without modification, to the listener. The MBean object
    * should not use or modify the object.
    *
    */
    public void handleNotification(Notification notification, Object handback) ;
}  

可以很清楚的看出继承了java.util.EventListener接口，又一次证实了，JMX通知机制是观察者模式的衍生产品.

好了，所有的功能代码都写完了，下边需要测试一JMX的通知机制：

这里还需要写一个测试用例来支持：

package com.dxz.mbean;

import java.lang.management.ManagementFactory;

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

public class ServerStartup {
    public static void main(String[] args) throws Exception {
        // 创建MBeanServer
        MBeanServer mbs = ManagementFactory.getPlatformMBeanServer();
        // 新建MBean ObjectName, 在MBeanServer里标识注册的MBean
        ObjectName name = new ObjectName("com.dxz.mbean.server:type=ServerConfigure");
        // 创建MBean
        ServerConfigure mbean = new ServerConfigure();
        // 在MBeanServer里注册MBean, 标识为ObjectName(com.dxz.mbean.server:type=ServerConfigure)
        mbs.registerMBean(mbean, name);
        // 自定义观察者
        ServerConfigureNotificationListener listener = new ServerConfigureNotificationListener();
        // 加入MBeanServer
        mbs.addNotificationListener(name, listener, null, null);
        Thread.sleep(Long.MAX_VALUE);
    }
}

最后，我们开始验证成果：

1.打开%JAVA_HOME%/bin/jconsole连接到本地进程：

2. 进入MBean选项框, 点击左边的树，打开通知：

3. 订阅通知

4. 修改属性，产生通知

5. 验证通知

OK, 学习笔记二写完了，回想下一，

1. JMX中要定义接口必须以xxxMBean的规范定义

2. 得有类实现xxxMBean接口

3. 在实现类中可以继承NotificationBroadcasterSupport来支持通知机制

4. 可以通过jconsole来验证