最近在面試，被面試官問到如果一個Activity有多個handler時候，怎樣知道handler1傳送的訊息不會被handler2接收，同理handler2傳送的訊息不會被handler1接收。

回來以後看原始碼才發現原來是這樣的：
平時我們直接new Handler()，原始碼裡面執行的是

 /**
     * Default constructor associates this handler with the {@link Looper} for the
     * current thread.
     *
     * If this thread does not have a looper, this handler won't be able to receive messages
     * so an exception is thrown.
     */
 

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

    public Handler(Callback callback, boolean async) {
        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 = callback;
        mAsynchronous = async;
    }

我們看到在第二段程式碼中，mLooper = Looper.myLooper();獲取了主執行緒中的那個looper物件，判斷mLooper==null的時候，丟擲了一個異常，說明了不能線上程裡面建立handler，如果必須要用的話，需要呼叫Looper.prepare()方法。mLooper不為null的情況下，mQueue = mLooper.mQueue，獲取的是MessageQueue佇列，mCallback = null,mAsynchronous = fasle;

我們順便看一下Looper.prepare()的原始碼

     /** 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 void prepare() {
        prepare(true);
    }

    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));
    }

可以看到當我們呼叫Looper.prepare()的時候，相當於new Looper(quitAllowed)了一個新Looper設定到新執行緒中。這樣一個thread就有了一個Looper.

繼續看當我們呼叫sendMessage的時候，我們看一下原始碼裡面是怎麼執行的：

 public final boolean sendMessage(Message msg)
    {
        return sendMessageDelayed(msg, 0);
    }

public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
    if (delayMillis < 0) {
        delayMillis = 0;
    }
    return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}

    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);
    }

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
    msg.target = this;
    if (mAsynchronous) {
        msg.setAsynchronous(true);
    }
    return queue.enqueueMessage(msg, uptimeMillis);
}

“
一步一步的往下走我們看到在enqueueMessage（）方法裡面有一個，
msg.target = this;這句話就是重點，就是說我們通過sendMesasge(msg)一層一層的傳遞訊息，最後這個msg.target=this就是把當前的這個handler標記給這個當前我們傳送的msg,然後再把這個打好標記的資訊新增到訊息佇列中。

我們再看看Looper裡面的原始碼

    /**
     * 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.recycleUnchecked();
        }
    }

我們看到 msg.target.dispatchMessage(msg);這一句是，直接將這個訊息交給了傳送這個訊息的handler處理。
繼續看handler中的dispatchMessage原始碼：

    /**
     * Handle system messages here.
     */
    public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }

可以看到Looper把訊息分發給handler, handler最終拿到訊息呼叫handleMessage(msg)，此時我們在前臺呼叫的時候重寫的handleMessage(msg)拿到的這個msg就是我們需要處理的Message.這樣就完成了handler1把訊息傳遞,然後把這個訊息打上這個handler1的標記，最後looper再拿到打上這個handler1標記的訊息，再分發給handler1,此時handler1拿到的就是它上次傳送的那個訊息.

因為這樣就保證了一個Activity中可以有多個handler,同時handler傳送訊息，各自接收各自的訊息，這樣有序的進行，這樣也不會出錯。
看來得經常看看原始碼才能解釋、理解的清楚這些機制。