要理解原理， read the fucking source

1、從HandlerThread入手。

HandlerThread是android系統提供的類，繼承Thread，是一個線程。請看run方法。

     public void run() {
        mTid = Process.myTid();
        Looper.prepare();// #1
        synchronized (this) {
            mLooper = Looper.myLooper();// #2
            notifyAll();
        }
        Process.setThreadPriority(mPriority);
        onLooperPrepared();
        Looper.loop();// #3
        mTid = -1;
    }

2、Thread和Looper的關系

在代碼段的第1個標示中，是調用了Looper的prepare方法。

     private static void prepare(boolean quitAllowed) {
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        sThreadLocal.set(new Looper(quitAllowed));// #4
    }

在代碼段的第4個標示中，將Looper實例存放在線程局部變量ThreadLocal中，將Looper和當前線程綁定。

在代碼段的第2個標示中，獲取了與當前線程綁定的Looper實例，當前線程就擁有了Looper實例。

在代碼段的第3個標示中，調用了Looper的loop()方法，loop()方法代碼：

    /**
     * Run the message queue in this thread. Be sure to call
     * {@link #quit()} to end the loop.
     */
    public static void loop() {
        final Looper me = myLooper();
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
        final MessageQueue queue = me.mQueue;

        // Make sure the identity of this thread is that of the local process,
        // and keep track of what that identity token actually is.
        Binder.clearCallingIdentity();
        final long ident = Binder.clearCallingIdentity();

        for (;;) {
            Message msg = queue.next(); // might block
            if (msg == null) {
                // No message indicates that the message queue is quitting.
                return;
            }

            // This must be in a local variable, in case a UI event sets the logger
            Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }

            msg.target.dispatchMessage(msg);

            if (logging != null) {
                logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
            }

            // Make sure that during the course of dispatching the
            // identity of the thread wasn't corrupted.
            final long newIdent = Binder.clearCallingIdentity();
            if (ident != newIdent) {
                Log.wtf(TAG, "Thread identity changed from 0x"
                        + Long.toHexString(ident) + " to 0x"
                        + Long.toHexString(newIdent) + " while dispatching to "
                        + msg.target.getClass().getName() + " "
                        + msg.callback + " what=" + msg.what);
            }

            msg.recycle();
        }
    }

loop()方法的作用是：從Looper中獲取MessageQueue隊列，再從MessageQueue中取出Message,通過Handler發送出去，直到取完。或許有人會問，Message取完之後，在哪裡喚醒該線程，然後繼續循環獲取Message呢？沒錯，是在調用Handler的sendMessage後，向MessageQueue中插入消息的時候喚醒，調用本地方法nativeWake(mPtr)。

3、Handler和Looper的關系

最常見的是，我們都會在Activity中創建一個Handler對象，用於更新UI界面。

   private Handler handler = new Handler(){
        @Override
        public void handleMessage(Message msg)
        {
            // TODO Auto-generated method stub
            super.handleMessage(msg);
        }
    };

看看Handler的構造方法。

   public Handler() {
        this(null, false);
    }

   public Handler(Callback callback, boolean async) {
        if (FIND_POTENTIAL_LEAKS) {
            final Class 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 = callback;
        mAsynchronous = async;
    }

大家可以看到，在構造方法中，Handler已經擁有了當前線程的Looper實例、Looper對象的MessageQueue隊列。

說到這裡，大家應該也差不多明白了他們究竟是怎麼樣的關系，線程間是如何交換消息的了。Handler發送消息的方法，其實就是往MessageQueue隊列中插入消息。

 public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
        MessageQueue queue = mQueue;
        if (queue == null) {
            RuntimeException e = new RuntimeException(
                    this + " sendMessageAtTime() called with no mQueue");
            Log.w("Looper", e.getMessage(), e);
            return false;
        }
        return enqueueMessage(queue, msg, uptimeMillis);
    }

而在調用了Handler的enqueueMessage方法時候會調用MessageQueue的enqueueMessage

final boolean enqueueMessage(Message msg, long when) {
        if (msg.isInUse()) {
            throw new AndroidRuntimeException(msg + " This message is already in use.");
        }
        if (msg.target == null) {
            throw new AndroidRuntimeException("Message must have a target.");
        }

        boolean needWake;
        synchronized (this) {
            if (mQuiting) {
                RuntimeException e = new RuntimeException(
                        msg.target + " sending message to a Handler on a dead thread");
                Log.w("MessageQueue", e.getMessage(), e);
                return false;
            }

            msg.when = when;
            Message p = mMessages;
            if (p == null || when == 0 || when < p.when) {
                // New head, wake up the event queue if blocked.
                msg.next = p;
                mMessages = msg;
                needWake = mBlocked;
            } else {
                // Inserted within the middle of the queue.  Usually we don't have to wake
                // up the event queue unless there is a barrier at the head of the queue
                // and the message is the earliest asynchronous message in the queue.
                needWake = mBlocked && p.target == null && msg.isAsynchronous();
                Message prev;
                for (;;) {
                    prev = p;
                    p = p.next;
                    if (p == null || when < p.when) {
                        break;
                    }
                    if (needWake && p.isAsynchronous()) {
                        needWake = false;
                    }
                }
                msg.next = p; // invariant: p == prev.next
                prev.next = msg;
            }
        }
        if (needWake) {
            nativeWake(mPtr);// #5
        }
        return true;
    }

在代碼片段標示5中，調用了nativeWake(mPtr)方法，該方法是本地方法，用於喚醒Thread線程。

參考資料：

android的消息隊列機制