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());