Handler,Looper,HandlerThread浅析

Handler想必在大家写Android代码过程中已经运用得炉火纯青,特别是在做阻塞操作线程到UI线程的更新上.Handler用得恰当,能防止很多多线程异常.

而Looper大家也肯定有接触过,只不过写应用的代码一般不会直接用到Looper.但实际Handler处理Message的关键之处全都在于Looper.

以下是我看了<深入理解Android>的有关章节后,写的总结.

Handler

先来看看Handler的构造函数.

 

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; }

 

主要关注Handler的2个成员变量mQueue,mLooper

mLooper可以从构造函数传入.如果构造函数不传的话,则直接取当前线程的Looper:mLooper = Looper.myLooper();

mQueue就是mLooper.mQueue.

 

把Message插入消息队列

 

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); }

 

上面两个正是把Message插入消息队列的方法.

从中能看出,Message是被插入到mQueue里面,实际是mLooper.mQueue.

每个Message.target = this,也就是target被设置成了当前的Handler实例.

到此,我们有必要看看Looper是做一些什么的了.

 

Looper

 这是Looper一个标准的使用例子.

 

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

 

 我们再看看Looper.prepare()和Looper.loop()的实现.

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()方法给sThreadLocal设置了一个Looper实例.

sThreadLocal是Thread Local Variables,线程本地变量.

每次调用myLooper()方法就能返回prepare()设置的Looper实例.

 

Looper()方法里面有一个很显眼的无限For循环,它就是用来不断的处理messageQueue中的Message的.

最终会调用message.target.dispatchMessage(msg)方法.前面介绍过,target是handler的实例.下面看看handler.dispatchMessage()方法的实现.

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

实现非常简单,如果callback不为空则用handleCallback(msg)来处理message.

而大多数情况下,我们实例化Handler的时候都没有传callback,所以都会走到handler.handleMessage()方法了.这方法用过Handler的人,都在再熟悉不过了.

这就是Handler和Looper协同工作的原理.消息队列的实现都在Looper,Handler更像是一个辅助类.

 

HandlerThread

多数情况下,我们都是用Handler来处理UI界面的更新,这时我们要保证handler的Looper是UI线程的Looper.

只需要这样子实例化Handler就能保证在UI线程处理Message了:Handler handler = new Handler(Looper.getMainLooper());

而当我们不希望Handler在UI线程去处理Message时候,就需要新建一个线程然后把线程的Looper传给Handler做实例化.

也许我们会写出下面类似的代码(样例代码引用<深入理解Android>)

class LooperThread extends Thread { public Looper myLooper = null; // 定义一个public 的成员myLooper，初值为空。 
    public void run() { // 假设run 在线程2 中执行 
 Looper.prepare(); // myLooper 必须在这个线程中赋值 
        myLooper = Looper.myLooper(); Looper.loop(); } } // 下面这段代码在线程1 中执行，并且会创建线程2
{ LooperThread lpThread= new LooperThread; lpThread.start();//start 后会创建线程2 
    Looper looper = lpThread.myLooper;//<====== 注意 // thread2Handler 和线程2 的Looper 挂上钩 
    Handler thread2Handler = new Handler(looper); //sendMessage 发送的消息将由线程2 处理
 threadHandler.sendMessage(...) }

细心的你们可能已经一眼看穿,new Handler(looper);传进来的looper可能为空.

原因是Looper looper = lpThread.myLooper时候,lpThread.myLooper可能为空,因为lpThread还没有开始执行run()方法.

那要怎么样才能保证handler实例化时候,looper不为空呢.

Android给我们提供了完美的解决方案,那就是HandlerThread.

public class HandlerThread extends Thread{ // 线程1 调用getLooper 来获得新线程的Looper 
    public Looper getLooper() { ...... synchronized (this) { while (isAlive() && mLooper == null) { try { wait();// 如果新线程还未创建Looper，则等待 
                } catch (InterruptedException e) { } } } return mLooper; } // 线程2 运行它的run 函数，looper 就是在run 线程里创建的。 
    public void run() { mTid = Process.myTid(); Looper.prepare(); // 创建这个线程上的Looper 
        synchronized (this) { mLooper = Looper.myLooper(); notifyAll();// 通知取Looper 的线程1，此时Looper 已经创建好了。 
 } Process.setThreadPriority(mPriority); onLooperPrepared(); Looper.loop(); mTid = -1; } }

HandlerThread.getLooper()方法会等待mLooper被赋值了才返回.

在handler实例化调用handlerThread.getLooper()方法的时候,就能保证得到的Looper一定不为空了.

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