Message和MessageQueue

消息結構

每個消息用Message表示，Message主要包含以下內容：

這裡的用戶指一般的APP開發者。

一般不用手動設置target,調用Handler.obtainMessage()方法會自動的設置Message的target為當前的Handler。
得到Message之後可以調用sendToTarget(),發送消息給Handler，Handler再把消息放到message queue的尾部。
對Message除了給部分成員變量賦值外的操作都可以交由Handler來處理。

消息池

在通過Handler發送消息時，我們可以通過代碼Message message=new Message();新建一條消息，但是我們並不推薦這樣做，因為這樣每次都會新建一條消息，很容易造成資源浪費。Android中設計了消息池用於避免該現象：

• 獲取消息 obtain()
  從消息池中獲取消息: Message msg=Message.obtain();
  obtain()方法源碼：

  public static Message obtain() {
        synchronized (sPoolSync) {
            if (sPool != null) {
                Message m = sPool;
                sPool = m.next;
                m.next = null; //從sPool中取出一個Message對象，並消息鏈表斷開
                m.flags = 0; // clear in-use flag清除in-use flag
                sPoolSize--;//消息池的可用大小進行-1操作
                return m;
            }
        }
        return new Message();// 當消息池為空時，直接創建Message對象
    }

  從消息池取Message，都是把消息池表頭的Message取走，再把表頭指向下一條消息next;

• 回收消息 recycle()
  把不再使用的消息回收到消息池 mgs.recycle();
  recycle()方法源碼:

  public void recycle() {
        if (isInUse()) {//判斷消息是否正在使用
            if (gCheckRecycle) {
                throw new IllegalStateException("This message cannot be recycled because it "
                        + "is still in use.");
            }
            return;
        }
        recycleUnchecked();
    }
  /**
  
      
  
       
  
  對於不再使用的消息，加入到消息池
  
  
  Recycles a Message that may be in-use.
  
  
  Used internally by the MessageQueue and Looper when disposing of queued Messages.
  */
  void recycleUnchecked() {
  // Mark the message as in use while it remains in the recycled object pool.
  // Clear out all other details.
  //將消息標示位置為IN_USE，並清空消息所有的參數。
  flags = FLAG_IN_USE;
  what = 0;
  arg1 = 0;
  arg2 = 0;
  obj = null;
  replyTo = null;
  sendingUid = -1;
  when = 0;
  target = null;
  callback = null;
  data = null;
  synchronized (sPoolSync) {
  if (sPoolSize < MAX_POOL_SIZE) {//當消息池沒有滿時，將Message對象加入消息池
  next = sPool;
  sPool = this;
  sPoolSize++;//消息池的可用大小進行加1操作
  }
  }
  }
      

消息回收，就是將Message內容重置後，再把Message加到鏈表的表頭，加入到消息池的過程；

MessageQueue

負責管理消息隊列，實際上Message類有一個next字段，會將Message對象串在一起成為一個消息隊列，所以並不需要LinkedList之類的數據結構將Message對象組在一起成為隊列。

• 創建消息隊列

     MessageQueue(boolean quitAllowed) {
        mQuitAllowed = quitAllowed;
        mPtr = nativeInit();//通過native方法初始化消息隊列，其中mPtr是供native代碼使用
    }

  在MessageQueue初始化的時候調用了nativeInit，這是一個Native方法：

  static void android_os_MessageQueue_nativeInit(JNIEnv* env, jobject obj) { 
    NativeMessageQueue* nativeMessageQueue = new NativeMessageQueue(); 
    if (!nativeMessageQueue) { 
        jniThrowRuntimeException(env, "Unable to allocate native queue"); 
        return;
        }
    nativeMessageQueue->incStrong(env); 
    android_os_MessageQueue_setNativeMessageQueue(env, obj, nativeMessageQueue);
  } 
  static void android_os_MessageQueue_setNativeMessageQueue(JNIEnv* env, jobject messageQueueObj,
        NativeMessageQueue* nativeMessageQueue) { 
    env->SetIntField(messageQueueObj, gMessageQueueClassInfo.mPtr, 
    reinterpret_cast<jint>(nativeMessageQueue)); 
  }

  方法名由java層類的包名+類名+方法名組成，這不是標準，是習慣寫法，也可以採用其他名稱組合，具體是什麼名稱由JNINativeMethod方法中Java對象與c++對象的映射決定，此處是JNI方面的內容，不作過多解釋。
  在nativeInit中，new了一個Native層的MessageQueue的對象，並將其地址保存在了Java層MessageQueue的成員mPtr中，Android中有好多這樣的實現，一個類在Java層與Native層都有實現，通過JNI的GetFieldID與SetIntField把Native層的類的實例地址保存到Java層類的實例的mPtr成員中，比如Parcel。

再看NativeMessageQueue的實現：

NativeMessageQueue::NativeMessageQueue() : mInCallback(false), mExceptionObj(NULL) { 
    mLooper = Looper::getForThread();
    if (mLooper == NULL) {
      mLooper = new Looper(false);
      Looper::setForThread(mLooper); 
    }
}

• 消息入隊 enqueueMessage()
  enqueueMessage 用於將Message對象插入消息隊列。MessageQueue永遠是按照Message觸發的時間先後順序排列的，隊頭的消息是將要最早觸發的消息。當有消息需要加入消息隊列時，會從隊列頭開始遍歷，直到找到消息應該插入的合適位置，以保證所有消息的時間順序。
  該方法會被Handler對象調用。
  源碼如下：

   /**
     * 添加一條消息到消息隊列
     * @param msg 要添加的消息
     * @param when 消息處理時間
     * @return 添加成功與否
     */
    boolean enqueueMessage(Message msg, long when) {
        if (msg.target == null) {// 每一個Message必須有一個target
            throw new IllegalArgumentException("Message must have a target.");
        }
        if (msg.isInUse()) {
            throw new IllegalStateException(msg + " This message is already in use.");
        }
    <span class="hljs-keyword">synchronized</span> (<span class="hljs-keyword">this</span>) {
        <span class="hljs-keyword">if</span> (mQuitting) {
            IllegalStateException e = <span class="hljs-keyword">new</span> IllegalStateException(
                    msg.target + <span class="hljs-string">" sending message to a Handler on a dead thread"</span>);
            Log.w(TAG, e.getMessage(), e);
            msg.recycle();
            <span class="hljs-keyword">return</span> <span class="hljs-keyword">false</span>;
        }
  
        msg.markInUse();
        msg.when = when;
        Message p = mMessages;
        <span class="hljs-keyword">boolean</span> needWake;
        <span class="hljs-keyword">if</span> (p == <span class="hljs-keyword">null</span> || when == <span class="hljs-number">0</span> || when &lt; p.when) {
            <span class="hljs-comment">// New head, wake up the event queue if blocked.</span>
            <span class="hljs-comment">//p為null(代表MessageQueue沒有消息） 或者msg的觸發時間是隊列中最早的， 則進入該該分支</span>
            msg.next = p;
            mMessages = msg;
            needWake = mBlocked;
        } <span class="hljs-keyword">else</span> {
            <span class="hljs-comment">//將消息按時間順序插入到MessageQueue。一般地，不需要喚醒事件隊列，除非</span>
            <span class="hljs-comment">//消息隊頭存在barrier，並且同時Message是隊列中最早的異步消息。</span>
  
            <span class="hljs-comment">// Inserted within the middle of the queue.  Usually we don't have to wake</span>
            <span class="hljs-comment">// up the event queue unless there is a barrier at the head of the queue</span>
            <span class="hljs-comment">// and the message is the earliest asynchronous message in the queue.</span>
            needWake = mBlocked &amp;&amp; p.target == <span class="hljs-keyword">null</span> &amp;&amp; msg.isAsynchronous();
            Message prev;
            <span class="hljs-keyword">for</span> (;;) {
                prev = p;
                p = p.next;
                <span class="hljs-keyword">if</span> (p == <span class="hljs-keyword">null</span> || when &lt; p.when) {
                    <span class="hljs-keyword">break</span>;
                }
                <span class="hljs-keyword">if</span> (needWake &amp;&amp; p.isAsynchronous()) {
                    needWake = <span class="hljs-keyword">false</span>;
                }
            }
            msg.next = p; <span class="hljs-comment">// invariant: p == prev.next</span>
            prev.next = msg;
        }
  
        <span class="hljs-comment">// We can assume mPtr != 0 because mQuitting is false.</span>
        <span class="hljs-comment">//消息沒有退出，我們認為此時mPtr != 0</span>
        <span class="hljs-keyword">if</span> (needWake) {
            nativeWake(mPtr);
        }
    }
    <span class="hljs-keyword">return</span> <span class="hljs-keyword">true</span>;
  
  }

• 消息輪詢 next()
  最重要的方法,用於獲取下一個Message對象，如果沒有需要處理的Message對象，該方法將阻塞。MessageQueue用本地方法做同步互斥，因為這樣時間更精準。每個Message對象都有一個什麼時刻處理該Message對象的屬性when，沒到時間都不會處理該Message對象，如果時間不精準的話，會導致系統消息不能及時處理。

   /**
     * 依次從MessageQueue中取出Message
     * @return 消息
     */
    Message next() {
        // Return here if the message loop has already quit and been disposed.
        // This can happen if the application tries to restart a looper after quit
        // which is not supported.
        final long ptr = mPtr;
        if (ptr == 0) {//當消息循環已經退出，則直接返回
            return null;
        }
    <span class="hljs-keyword">int</span> pendingIdleHandlerCount = -<span class="hljs-number">1</span>; <span class="hljs-comment">// -1 only during first iteration  循環迭代的首次為-1</span>
    <span class="hljs-keyword">int</span> nextPollTimeoutMillis = <span class="hljs-number">0</span>;
    <span class="hljs-keyword">for</span> (;;) {
        <span class="hljs-keyword">if</span> (nextPollTimeoutMillis != <span class="hljs-number">0</span>) {
            Binder.flushPendingCommands();
        }
        <span class="hljs-comment">//阻塞操作，當等待nextPollTimeoutMillis時長，或者消息隊列被喚醒，都會返回。</span>
        nativePollOnce(ptr, nextPollTimeoutMillis);
  
        <span class="hljs-keyword">synchronized</span> (<span class="hljs-keyword">this</span>) {
            <span class="hljs-comment">// Try to retrieve the next message.  Return if found.</span>
            <span class="hljs-keyword">final</span> <span class="hljs-keyword">long</span> now = SystemClock.uptimeMillis();
            Message prevMsg = <span class="hljs-keyword">null</span>;
            Message msg = mMessages;
            <span class="hljs-keyword">if</span> (msg != <span class="hljs-keyword">null</span> &amp;&amp; msg.target == <span class="hljs-keyword">null</span>) {
                <span class="hljs-comment">//查詢MessageQueue中的下一條異步消息</span>
                <span class="hljs-comment">// Stalled by a barrier.  Find the next asynchronous message in the queue.</span>
                <span class="hljs-keyword">do</span> {
                    prevMsg = msg;
                    msg = msg.next;
                } <span class="hljs-keyword">while</span> (msg != <span class="hljs-keyword">null</span> &amp;&amp; !msg.isAsynchronous());
            }
            <span class="hljs-keyword">if</span> (msg != <span class="hljs-keyword">null</span>) {
                <span class="hljs-keyword">if</span> (now &lt; msg.when) {
                    <span class="hljs-comment">//設置下一次輪詢消息的超時時間</span>
                    <span class="hljs-comment">// Next message is not ready.  Set a timeout to wake up when it is ready.</span>
                    nextPollTimeoutMillis = (<span class="hljs-keyword">int</span>) Math.min(msg.when - now, Integer.MAX_VALUE);
                } <span class="hljs-keyword">else</span> {
                    <span class="hljs-comment">// 獲取一條消息，並返回</span>
                    <span class="hljs-comment">// Got a message.</span>
                    mBlocked = <span class="hljs-keyword">false</span>;
                    <span class="hljs-keyword">if</span> (prevMsg != <span class="hljs-keyword">null</span>) {
                        prevMsg.next = msg.next;
                    } <span class="hljs-keyword">else</span> {
                        mMessages = msg.next;
                    }
                    msg.next = <span class="hljs-keyword">null</span>;
                    <span class="hljs-keyword">if</span> (DEBUG) Log.v(TAG, <span class="hljs-string">"Returning message: "</span> + msg);
                    msg.markInUse();<span class="hljs-comment">//設置消息flag成使用狀態</span>
                    <span class="hljs-keyword">return</span> msg;<span class="hljs-comment">//成功地獲取MessageQueue中的下一條即將要執行的消息</span>
                }
            } <span class="hljs-keyword">else</span> {
                <span class="hljs-comment">// No more messages.//沒有消息了</span>
                nextPollTimeoutMillis = -<span class="hljs-number">1</span>;
            }
  
            <span class="hljs-comment">// Process the quit message now that all pending messages have been handled.</span>
            <span class="hljs-keyword">if</span> (mQuitting) {<span class="hljs-comment">//消息正在退出，返回null</span>
                dispose();
                <span class="hljs-keyword">return</span> <span class="hljs-keyword">null</span>;
            }
  
            <span class="hljs-comment">// If first time idle, then get the number of idlers to run.</span>
            <span class="hljs-comment">// Idle handles only run if the queue is empty or if the first message</span>
            <span class="hljs-comment">// in the queue (possibly a barrier) is due to be handled in the future.</span>
            <span class="hljs-comment">//當消息隊列為空，或者消息隊列的第一個消息時</span>
            <span class="hljs-keyword">if</span> (pendingIdleHandlerCount &lt; <span class="hljs-number">0</span>
                    &amp;&amp; (mMessages == <span class="hljs-keyword">null</span> || now &lt; mMessages.when)) {
                pendingIdleHandlerCount = mIdleHandlers.size();
            }
            <span class="hljs-keyword">if</span> (pendingIdleHandlerCount &lt;= <span class="hljs-number">0</span>) {
                <span class="hljs-comment">//沒有idle handlers 需要運行，則循環並等待。</span>
                <span class="hljs-comment">// No idle handlers to run.  Loop and wait some more.</span>
                mBlocked = <span class="hljs-keyword">true</span>;
                <span class="hljs-keyword">continue</span>;
            }
  
            <span class="hljs-keyword">if</span> (mPendingIdleHandlers == <span class="hljs-keyword">null</span>) {
                mPendingIdleHandlers = <span class="hljs-keyword">new</span> IdleHandler[Math.max(pendingIdleHandlerCount, <span class="hljs-number">4</span>)];
            }
            mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
        }
  
        <span class="hljs-comment">// Run the idle handlers.</span>
        <span class="hljs-comment">// We only ever reach this code block during the first iteration.</span>
        <span class="hljs-comment">//只有第一次循環時，會運行idle handlers，執行完成後，重置pendingIdleHandlerCount為0.</span>
        <span class="hljs-keyword">for</span> (<span class="hljs-keyword">int</span> i = <span class="hljs-number">0</span>; i &lt; pendingIdleHandlerCount; i++) {
            <span class="hljs-keyword">final</span> IdleHandler idler = mPendingIdleHandlers[i];
            mPendingIdleHandlers[i] = <span class="hljs-keyword">null</span>; <span class="hljs-comment">// release the reference to the handler//去掉handler的引用</span>
  
            <span class="hljs-keyword">boolean</span> keep = <span class="hljs-keyword">false</span>;
            <span class="hljs-keyword">try</span> {
                keep = idler.queueIdle();<span class="hljs-comment">//idle時執行的方法</span>
            } <span class="hljs-keyword">catch</span> (Throwable t) {
                Log.wtf(TAG, <span class="hljs-string">"IdleHandler threw exception"</span>, t);
            }
  
            <span class="hljs-keyword">if</span> (!keep) {
                <span class="hljs-keyword">synchronized</span> (<span class="hljs-keyword">this</span>) {
                    mIdleHandlers.remove(idler);
                }
            }
        }
  
        <span class="hljs-comment">// Reset the idle handler count to 0 so we do not run them again.</span>
        <span class="hljs-comment">//重置idle handler個數為0，以保證不會再次重複運行</span>
        pendingIdleHandlerCount = <span class="hljs-number">0</span>;
  
        <span class="hljs-comment">// While calling an idle handler, a new message could have been delivered</span>
        <span class="hljs-comment">// so go back and look again for a pending message without waiting.</span>
        <span class="hljs-comment">//當調用一個空閒handler時，一個新message能夠被分發，因此無需等待可以直接查詢pending message.</span>
        nextPollTimeoutMillis = <span class="hljs-number">0</span>;
    }
  
  }

  nativePollOnce(ptr, nextPollTimeoutMillis)是一個native方法，是一個阻塞操作。其中nextPollTimeoutMillis代表下一個消息到來前，還需要等待的時長；當nextPollTimeoutMillis = -1時，表示消息隊列中無消息，會一直等待下去。空閒後，往往會執行IdleHandler中的方法。當nativePollOnce()返回後，next()從mMessages中提取一個消息。nativePollOnce()在native做了大量的工作，想深入研究可查看資料： Android消息機制2-Handler(Native層)。

• 移除消息 removeMessages()
  就是將消息從鏈表移除，同時將移除的消息添加到消息池，提供循環複用。
  採用了兩個while循環，第一個循環是從隊頭開始，移除符合條件的消息，第二個循環是從頭部移除完連續的滿足條件的消息之後，再從隊列後面繼續查詢是否有滿足條件的消息需要被移除。

    void removeMessages(Handler h, int what, Object object) {
        if (h == null) {
            return;
        }
    <span class="hljs-keyword">synchronized</span> (<span class="hljs-keyword">this</span>) {
        Message p = mMessages;
        <span class="hljs-comment">//從消息隊列的頭部開始，移除所有符合條件的消息</span>
        <span class="hljs-comment">// Remove all messages at front.</span>
        <span class="hljs-keyword">while</span> (p != <span class="hljs-keyword">null</span> &amp;&amp; p.target == h &amp;&amp; p.what == what
               &amp;&amp; (object == <span class="hljs-keyword">null</span> || p.obj == object)) {
            Message n = p.next;
            mMessages = n;
            p.recycleUnchecked();
            p = n;
        }
  
        <span class="hljs-comment">// Remove all messages after front.</span>
        <span class="hljs-comment">//移除剩餘的符合要求的消息</span>
        <span class="hljs-keyword">while</span> (p != <span class="hljs-keyword">null</span>) {
            Message n = p.next;
            <span class="hljs-keyword">if</span> (n != <span class="hljs-keyword">null</span>) {
                <span class="hljs-keyword">if</span> (n.target == h &amp;&amp; n.what == what
                    &amp;&amp; (object == <span class="hljs-keyword">null</span> || n.obj == object)) {
                    Message nn = n.next;
                    n.recycleUnchecked();
                    p.next = nn;
                    <span class="hljs-keyword">continue</span>;
                }
            }
            p = n;
        }
    }
  
  }

• 退出消息隊列
  消息退出的方式：

  • 當safe =true時，只移除尚未觸發的所有消息，對於正在觸發的消息並不移除；
  • 當safe =flase時，移除所有的消息

  void quit(boolean safe) {
        if (!mQuitAllowed) {// 當mQuitAllowed為false，表示不運行退出，強行調用quit()會拋出異常
            throw new IllegalStateException("Main thread not allowed to quit.");
        }
    <span class="hljs-keyword">synchronized</span> (<span class="hljs-keyword">this</span>) {
        <span class="hljs-keyword">if</span> (mQuitting) { <span class="hljs-comment">//防止多次執行退出操作</span>
            <span class="hljs-keyword">return</span>;
        }
        mQuitting = <span class="hljs-keyword">true</span>;
  
        <span class="hljs-keyword">if</span> (safe) {
            removeAllFutureMessagesLocked();<span class="hljs-comment">//移除尚未觸發的所有消息</span>
        } <span class="hljs-keyword">else</span> {
            removeAllMessagesLocked();<span class="hljs-comment">//移除所有的消息</span>
        }
  
        <span class="hljs-comment">// We can assume mPtr != 0 because mQuitting was previously false.</span>
        <span class="hljs-comment">//mQuitting=false，那麼認定為 mPtr != 0</span>
        nativeWake(mPtr);
    }
  
  }

    </div>