Handler,Looper,HandlerThread淺析，handlerthreadlooper

Handler,Looper,HandlerThread淺析，handlerthreadlooper

Handler想必在大家寫Android代碼過程中已經運用得爐火純青,特別是在做阻塞操作線程到UI線程的更新上.Handler用得恰當,能防止很多多線程異常.

而Looper大家也肯定有接觸過,只不過寫應用的代碼一般不會直接用到Looper.但實際Handler處理Message的關鍵之處全都在於Looper.

以下是我看了<深入理解Android>的有關章節後,寫的總結.

Handler

先來看看Handler的建構函式.

 

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

 

主要關注Handler的2個成員變數mQueue,mLooper

mLooper可以從建構函式傳入.如果建構函式不傳的話,則直接取當前線程的Looper:mLooper = Looper.myLooper();

mQueue就是mLooper.mQueue.

 

把Message插入訊息佇列

 

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);    }

 

上面兩個正是把Message插入訊息佇列的方法.

從中能看出,Message是被插入到mQueue裡面,實際是mLooper.mQueue.

每個Message.target = this,也就是target被設定成了當前的Handler執行個體.

到此,我們有必要看看Looper是做一些什麼的了.

 

Looper

 這是Looper一個標準的使用例子.

 

class LooperThread extends Thread {        public Handler mHandler;        public void run() {        Looper.prepare();                ......        Looper.loop();       }}

 

 我們再看看Looper.prepare()和Looper.loop()的實現.

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));    }public static Looper myLooper() {        return sThreadLocal.get();    }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();        }    }

prepare()方法給sThreadLocal設定了一個Looper執行個體.

sThreadLocal是Thread Local Variables,執行緒區域變數.

每次調用myLooper()方法就能返回prepare()設定的Looper執行個體.

 

Looper()方法裡面有一個很顯眼的無限For迴圈,它就是用來不斷的處理messageQueue中的Message的.

最終會調用message.target.dispatchMessage(msg)方法.前面介紹過,target是handler的執行個體.下面看看handler.dispatchMessage()方法的實現.

public void dispatchMessage(Message msg) {        if (msg.callback != null) {            handleCallback(msg);        } else {            if (mCallback != null) {                if (mCallback.handleMessage(msg)) {                    return;                }            }            handleMessage(msg);        }    }

實現非常簡單,如果callback不為空白則用handleCallback(msg)來處理message.

而大多數情況下,我們執行個體化Handler的時候都沒有傳callback,所以都會走到handler.handleMessage()方法了.這方法用過Handler的人,都在再熟悉不過了.

這就是Handler和Looper協同工作的原理.訊息佇列的實現都在Looper,Handler更像是一個輔助類.

 

HandlerThread

多數情況下,我們都是用Handler來處理UI介面的更新,這時我們要保證handler的Looper是UI線程的Looper.

只需要這樣子執行個體化Handler就能保證在UI線程處理Message了:Handler handler = new Handler(Looper.getMainLooper());

而當我們不希望Handler在UI線程去處理Message時候,就需要建立一個線程然後把線程的Looper傳給Handler做執行個體化.

也許我們會寫出下面類似的代碼(範例代碼引用<深入理解Android>)

class LooperThread extends Thread {        public Looper myLooper = null;    // 定義一個public 的成員myLooper，初值為空白。        public void run() {         // 假設run 線上程2 中執行                Looper.prepare();                // myLooper 必須在這個線程中賦值                myLooper = Looper.myLooper();                Looper.loop();        }}// 下面這段代碼線上程1 中執行，並且會建立線程2{        LooperThread lpThread= new LooperThread;        lpThread.start();//start 後會建立線程2        Looper looper = lpThread.myLooper;//<====== 注意        // thread2Handler 和線程2 的Looper 掛上鉤        Handler thread2Handler = new Handler(looper);        //sendMessage 發送的訊息將由線程2 處理      threadHandler.sendMessage(...)}

細心的你們可能已經一眼看穿,new Handler(looper);傳進來的looper可能為空白.

原因是Looper looper = lpThread.myLooper時候,lpThread.myLooper可能為空白,因為lpThread還沒有開始執行run()方法.

那要怎麼樣才能保證handler執行個體化時候,looper不為空白呢.

Android給我們提供了完美的解決方案,那就是HandlerThread.

public class HandlerThread extends Thread{        // 線程1 調用getLooper 來獲得新線程的Looper        public Looper getLooper() {                ......                synchronized (this) {                        while (isAlive() && mLooper == null) {                                try {                                        wait();// 如果新線程還未建立Looper，則等待                                } catch (InterruptedException e) {                                }                        }                }                return mLooper;        }        // 線程2 運行它的run 函數，looper 就是在run 線程裡建立的。        public void run() {                mTid = Process.myTid();                Looper.prepare(); // 建立這個線程上的Looper                synchronized (this) {            mLooper = Looper.myLooper();                        notifyAll();// 通知取Looper 的線程1，此時Looper 已經建立好了。                }                Process.setThreadPriority(mPriority);                onLooperPrepared();                Looper.loop();                mTid = -1;        }}

HandlerThread.getLooper()方法會等待mLooper被賦值了才返回.

在handler執行個體化調用handlerThread.getLooper()方法的時候,就能保證得到的Looper一定不為空白了.

HandlerThread handlerThread = new HandlerThread();handlerThread.start();Handler handler = new Handler(handlerThread.getLooper());