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。