Handler,Looper,HandlerThread淺析

Handler想必在你們寫Android代碼過程當中已經運用得爐火純青,特別是在作阻塞操做線程到UI線程的更新上.Handler用得恰當,能防止不少多線程異常.

而Looper你們也確定有接觸過,只不過寫應用的代碼通常不會直接用到Looper.但實際Handler處理Message的關鍵之處全都在於Looper.

如下是我看了<深刻理解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; }

 

主要關注Handler的2個成員變量mQueue,mLooper

mLooper能夠從構造函數傳入.若是構造函數不傳的話,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裏面,實際是mLooper.mQueue.

每一個Message.target = this,也就是target被設置成了當前的Handler實例.

到此,咱們有必要看看Looper是作一些什麼的了.

 

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(); } }

 

 咱們再看看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,線程本地變量.

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

 

Looper()方法裏面有一個很顯眼的無限For循環,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); } }

實現很是簡單,若是callback不爲空則用handleCallback(msg)來處理message.

而大多數狀況下,咱們實例化Handler的時候都沒有傳callback,因此都會走到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協同工做的原理.消息隊列的實現都在Looper,Handler更像是一個輔助類.

 

HandlerThread

多數狀況下,咱們都是用Handler來處理UI界面的更新,這時咱們要保證handler的Looper是UI線程的Looper.

只須要這樣子實例化Handler就能保證在UI線程處理Message了:Handler handler = new Handler(Looper.getMainLooper());

而當咱們不但願Handler在UI線程去處理Message時候,就須要新建一個線程而後把線程的Looper傳給Handler作實例化.

也許咱們會寫出下面相似的代碼(樣例代碼引用<深刻理解Android>)

class LooperThread extends Thread { public Looper myLooper = null; // 定義一個public 的成員myLooper，初值爲空。 
    public void run() { // 假設run 在線程2 中執行 
 Looper.prepare(); // myLooper 必須在這個線程中賦值 
        myLooper = Looper.myLooper(); Looper.loop(); } } // 下面這段代碼在線程1 中執行，而且會建立線程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 發送的消息將由線程2 處理
 threadHandler.sendMessage(...) }

細心的大家可能已經一眼看穿,new Handler(looper);傳進來的looper可能爲空.

緣由是Looper looper = lpThread.myLooper時候,lpThread.myLooper可能爲空,由於lpThread尚未開始執行run()方法.

那要怎麼樣才能保證handler實例化時候,looper不爲空呢.

Android給咱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 來得到新線程的Looper 
    public Looper getLooper() { ...... synchronized (this) { while (isAlive() && mLooper == null) { try { wait();// 若是新線程還未建立Looper，則等待 
                } catch (InterruptedException e) { } } } return mLooper; } // 線程2 運行它的run 函數，looper 就是在run 線程裏建立的。 
    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實例化調用handlerThread.getLooper()方法的時候,就能保證獲得的Looper必定不爲空了.

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