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