虽然MessageQueue提供了直接读/写的函数接口。但对于程序猿来说，一般不直接读/写消息队列。之前了解到，在Looper.loop()函数中。当取出消息后，会回调msg.target对象的handleMessage()函数。而msg.target的类型正是Handler。

    /**
     *  Run the message queue in this thread. Be sure to call
     * {@link #quit()} to end the loop.
     */
    public static final void loop() {
        Looper me = myLooper();
        MessageQueue queue = me.mQueue;
        while (true) {
            Message msg = queue.next(); // might block
            //if (!me.mRun) {
            //    break;
            //}
            if (msg != null) {
                if (msg.target == null) {
                    // No target is a magic identifier for the quit message.
                    return;
                }
                if (me.mLogging!= null) me.mLogging.println(
                        ">>>>> Dispatching to " + msg.target + " "
                        + msg.callback + ": " + msg.what
                        );
                msg.target.dispatchMessage(msg);
                if (me.mLogging!= null) me.mLogging.println(
                        "<<<<< Finished to    " + msg.target + " "
                        + msg.callback);
                msg.recycle();
            }
        }
    }

一般使用Handler类向消息队列中发送消息，并重载Handler类的handleMessage()函数加入消息处理代码。

Handler对象仅仅能加入到有消息队列的线程中，否则会发生异常。以下代码是Handler类的构造函数：

    /**
     * Default constructor associates this handler with the queue for the
     * current thread.
     *
     * If there isn‘t one, this handler won‘t be able to receive messages.
     */
    public Handler() {
        if (FIND_POTENTIAL_LEAKS) {
            final Class<?
 extends Handler> klass = getClass();
            if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
                    (klass.getModifiers() & Modifier.STATIC) == 0) {
                Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
                    klass.getCanonicalName());
            }
        }

        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can‘t create handler inside thread that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;
        mCallback = null;
    }

注意这句代码：

        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can‘t create handler inside thread that has not called Looper.prepare()");
        }

由以上代码能够得出结论，在构造Handler对象前。必须已经运行过Looper.prepare()，但prepare()不能被运行两次。

以下是Looper.prepare()的代码：

     /** Initialize the current thread as a looper.
      * This gives you a chance to create handlers that then reference
      * this looper, before actually starting the loop. Be sure to call
      * {@link #loop()} after calling this method, and end it by calling
      * {@link #quit()}.
      */
    public static final void prepare() {
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        sThreadLocal.set(new Looper());
    }

创建Handler对象能够在运行Looper.loop()函数之前。也能够在运行之后。

在以往的应用程序开发中，一般在Activity的初始化代码中加入Handler对象。其实，在Activity对象被构造前，Activity所在的线程已经运行了Looper.prepare()函数。详细可查看以下的链接。

http://blog.csdn.net/manoel/article/details/39499747

一个线程中能够包括多个Handler对象。在Looper.loop()函数中。不同的Message相应不同的Handler对象，从而回调不同的handleMessage()函数。