Handler,Looper,HandlerThread淺析

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的構造函數.

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;
    }

主要關註Handler的2個成員變數mQueue,mLooper

mLooper可以從構造函數傳入.如果構造函數不傳的話,則直接取當前線程的Looper:mLooper = Looper.myLooper();

mQueue就是mLooper.mQueue.

把Message插入消息隊列

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插入消息隊列的方法.

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被設置成了當前的Handler實例.

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設置了一個Looper實例.

sThreadLocal是Thread Local Variables,線程本地變數.

每次調用myLooper()方法就能返回prepare()設置的Looper實例.

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的實例.下麵看看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不為空則用handleCallback(msg)來處理message.

而大多數情況下,我們實例化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.消息隊列的實現都在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界面的更新,這時我們要保證handler的Looper是UI線程的Looper.

只需要這樣子實例化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,就需要新建一個線程然後把線程的Looper傳給Handler做實例化.

也許我們會寫出下麵類似的代碼(樣例代碼引用<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。    
    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();    
    }
}

// 下麵這段代碼線上程1 中執行，並且會創建線程2
{    
    LooperThread lpThread= new LooperThread;    
    lpThread.start();//start 後會創建線程2    
    Looper looper = lpThread.myLooper;//<====== 註意    
    // thread2Handler 和線程2 的Looper 掛上鉤    
    Handler thread2Handler = new Handler(looper);    
    //sendMessage 發送的消息將由線程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.

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

那要怎麼樣才能保證handler實例化時候,looper不為空呢.

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();// 如果新線程還未創建Looper，則等待                
                } catch (InterruptedException e) {                
                }            
            }        
        }        
        return mLooper;    
    }    

    // 線程2 運行它的run 函數，looper 就是在run 線程里創建的。    
    public void run() {        
        mTid = Process.myTid();        
        Looper.prepare(); // 創建這個線程上的Looper        
        synchronized (this) {
            mLooper = Looper.myLooper();            
            notifyAll();// 通知取Looper 的線程1，此時Looper 已經創建好了。        
        }        
        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實例化調用handlerThread.getLooper()方法的時候,就能保證得到的Looper一定不為空了.

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