一句话介绍JMX

what?

The Java Management Extensions (JMX) API is a standard API for management and monitoring of resources such as applications, devices, services, and the Java virtual machine.
一句话：对系统资源进行管理的规范API.

where to use?

ypical uses of the JMX technology include:

• Consulting and changing application configuration
• Accumulating statistics about application behavior and making them available
• Notifying of state changes and erroneous conditions.

一句话 典型应用：改变、查看程序配置；收集计算程序行为数据；通知、告警。

import notion

An MBean is a managed Java object, similar to a JavaBeanTM, that follows the design patterns set forth in the instrumentation level of the JMX specification.
（4 kinds MBean）
- standard MBeans
- dynamic MBeans
- open MBeans
- model MBeans.

MBeans expose a management interface: a set of readable and/or writable attributes and a set of invokable operations, along with a self-description. The management interface does not change throughout the life of an MBean instance.

一句话 MBean是JMX的核心，MBean类似与javaBean,我们通过MBean暴漏出的接口去获取MBean所检测程序的各种信息。

standard MBean

A standard MBean is defined by writing a Java interface called SomethingMBean and a Java class called Something that implements that interface.
A standard MBean is composed of the MBean interface which lists the methods for all exposed attributes and operations, and the class which implements this interface and provides the functionality of the instrumented resource.

一句话，standard MBean 就是 一个实现了 接口：***MBean 的实现类(如TimerMBean，自己去看API docs),这里暴漏出获取信息的方法，以便活动你要的各种信息。

例子：

/* HelloMBean.java - MBean interface describing the management
   operations and attributes for the Hello World MBean.  In this case
   there are two operations, "sayHello" and "add", and two attributes,
   "Name" and "CacheSize". */

package com.example.mbeans;

public interface HelloMBean {
    // operations

    public void sayHello();
    public int add(int x, int y);

    // attributes

    // a read-only attribute called Name of type String
    public String getName();

    // a read-write attribute called CacheSize of type int
    public int getCacheSize();
    public void setCacheSize(int size);
}

/* Hello.java - MBean implementation for the Hello World MBean.
   This class must implement all the Java methods declared in the
   HelloMBean interface, with the appropriate behavior for each one.  */

package com.example.mbeans;

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

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

    /* Getter for the Name attribute.  The pattern shown here is
       frequent: the getter returns a private field representing the
       attribute value.  In our case, the attribute value never
       changes, but for other attributes it might change as the
       application runs.  Consider an attribute representing
       statistics such as uptime or memory usage, for example.  Being
       read-only just means that it can‘t be changed through the
       management interface.  */
    public String getName() {
    return this.name;
    }

    /* Getter for the CacheSize attribute.  The pattern shown here is
       frequent: the getter returns a private field representing the
       attribute value, and the setter changes that field.  */
    public int getCacheSize() {
    return this.cacheSize;
    }

    /* Setter for the CacheSize attribute.  To avoid problems with
       stale values in multithreaded situations, it is a good idea
       for setters to be synchronized.  */
    public synchronized void setCacheSize(int size) {
    this.cacheSize = size;

    /* In a real application, changing the attribute would
       typically have effects beyond just modifying the cacheSize
       field.  For example, resizing the cache might mean
       discarding entries or allocating new ones.  The logic for
       these effects would be here.  */
    System.out.println("Cache size now " + this.cacheSize);
    }

    private final String name = "Reginald";
    private int cacheSize = DEFAULT_CACHE_SIZE;
    private static final int DEFAULT_CACHE_SIZE = 200;
}

/* Main.java - main class for Hello World example.  Create the
   HelloWorld MBean, register it, then wait forever (or until the
   program is interrupted).  */

package com.example.mbeans;

import java.lang.management.*;
import javax.management.*;

public class Main {
    /* For simplicity, we declare "throws Exception".  Real programs
       will usually want finer-grained exception handling.  */
    public static void main(String[] args) throws Exception {
    // Get the Platform MBean Server   attention!
    MBeanServer mbs = ManagementFactory.getPlatformMBeanServer();

    // Construct the ObjectName for the MBean we will register
    ObjectName name = new ObjectName("com.example.mbeans:type=Hello");

    // Create the Hello World MBean
    Hello mbean = new Hello();

    // Register the Hello World MBean   attention! 
    mbs.registerMBean(mbean, name);

    // Wait forever
    System.out.println("Waiting forever...");
    Thread.sleep(Long.MAX_VALUE);
    }
}

According to the JMX specification, an MBean interface consists of named and typed attributes that are readable and possibly writable, and named and typed operations that can be invoked by the applications that are managed by the MBean.

**一句话：
接口 以 **MBean 定义,并且这个MBean接口要写明 属性（如 , Name ， CacheSize） 名称，类型，并且暴漏出去获取或设置他们的方法以及一些操作。如例子中add() and sayHello()。
用一个JMX agent去管理 MBean:JMX agent 的核心组件是the MBean server,管理之前要先注册进去。

MBeanServer mbs = ManagementFactory.getPlatformMBeanServer();
通过调用java.lang.management.ManagementFactory.getPlatformMBeanServer()方法 获取运行在platform上的MBean Server.如果platform没有，则会自动获取一个（通过MBeanServerFactory.createMBeanServer()）。

发送消息

MBeans can generate notifications, for example to signal a state change, a detected event, or a problem.

不要忘了，MBean可以监测一个事件，问题，状态，并因此发送消息通知。

待续…….