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Looper、MessageQueue、Handler机制简要分析

2017-04-17

Looper、MessageQueue、Handler机制简要分析,在Android开发中Handler、Message是非常常见的,常用于定时执行任务、主线程更新UI等操作。之前自己一直都是在停留在使用上,这次有空翻翻源码自己了解下相关原理,对自己也会有一定的提高。

前言:

Android开发中Handler、Message是非常常见的,常用于定时执行任务、主线程更新UI等操作。之前自己一直都是在停留在使用上,这次有空翻翻源码自己了解下相关原理,对自己也会有一定的提高。
首先整个应用的入口函数是ActivityThread的main函数,UI线程的消息循环也是在这个方法中创建的:

public static void main(String[] args) {
        SamplingProfilerIntegration.start();

        // CloseGuard defaults to true and can be quite spammy.  We
        // disable it here, but selectively enable it later (via
        // StrictMode) on debug builds, but using DropBox, not logs.
        CloseGuard.setEnabled(false);

        Environment.initForCurrentUser();

        // Set the reporter for event logging in libcore
        EventLogger.setReporter(new EventLoggingReporter());

        Security.addProvider(new AndroidKeyStoreProvider());

        // Make sure TrustedCertificateStore looks in the right place for CA certificates
        final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
        TrustedCertificateStore.setDefaultUserDirectory(configDir);

        Process.setArgV0("");

        Looper.prepareMainLooper();//创建消息循环器Looper

        ActivityThread thread = new ActivityThread();
        thread.attach(false);

        if (sMainThreadHandler == null) {
            sMainThreadHandler = thread.getHandler();//拿到系统级别的Handler
        }

        if (false) {
            Looper.myLooper().setMessageLogging(new
                    LogPrinter(Log.DEBUG, "ActivityThread"));
        }

        Looper.loop();//执行消息循环器

        throw new RuntimeException("Main thread loop unexpectedly exited");
    }

Looper.prepareMainLooper()

    public static void prepareMainLooper() {
        prepare(false);
        synchronized (Looper.class) {
            if (sMainLooper != null) {
                throw new IllegalStateException("The main Looper has already been prepared.");
            }
            sMainLooper = myLooper();
        }
    }

   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));
    }

    /**
     * Return the Looper object associated with the current thread.  Returns
     * null if the calling thread is not associated with a Looper.
     */
    public static @Nullable Looper myLooper() {
        return sThreadLocal.get();  //获取到关联目前这个线程的looper
    }

prepareMainLooper方法中,调用了prepare()方法,在这个方法中创建了一个Looper对象并将该对象设置给了ThreadLocal(这样队列就与线程关联上了!不同的线程是不能访问对方的消息队列的。)
thread.getHandler():

  final Handler getHandler() {
        return mH;
    }

mH是系统级别的Handler:

  final H mH = new H();

H是Handler的一个子类:

public void handleMessage(Message msg) {
            if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
            switch (msg.what) {
                case LAUNCH_ACTIVITY: {
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
                    final ActivityClientRecord r = (ActivityClientRecord) msg.obj;

                    r.packageInfo = getPackageInfoNoCheck(
                            r.activityInfo.applicationInfo, r.compatInfo);
                    handleLaunchActivity(r, null);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                } break;
                case RELAUNCH_ACTIVITY: {
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityRestart");
                    ActivityClientRecord r = (ActivityClientRecord)msg.obj;
                    handleRelaunchActivity(r);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                } break;
                case PAUSE_ACTIVITY:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityPause");
                    handlePauseActivity((IBinder)msg.obj, false, (msg.arg1&1) != 0, msg.arg2,
                            (msg.arg1&2) != 0);
                    maybeSnapshot();
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case PAUSE_ACTIVITY_FINISHING:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityPause");
                    handlePauseActivity((IBinder)msg.obj, true, (msg.arg1&1) != 0, msg.arg2,
                            (msg.arg1&1) != 0);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case STOP_ACTIVITY_SHOW:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStop");
                    handleStopActivity((IBinder)msg.obj, true, msg.arg2);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case STOP_ACTIVITY_HIDE:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStop");
                    handleStopActivity((IBinder)msg.obj, false, msg.arg2);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
......

上面这个handlerMessage是H中的处理消息的方法,当我看到这里面的switch时,貌似明白了什么,switch当中写了相当多的筛选条件,仔细一看基本全都是activity、service、broadcast的生命周期方法,由此明白了原来这些生命周期方法的执行是通过系统级别的Handler(H)进行消息的处理的。
Looper.loop():

    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 (;;) {  //死循环不断取获取消息
            Message msg = queue.next(); // might block   //获取消息
            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
            final Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }

            final long traceTag = me.mTraceTag;
            if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
            }
            try {
                msg.target.dispatchMessage(msg);//消息处理事件分发
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }

            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&#39;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();
        }
    }

Looper.loop方法中实质上就是建立一个死循环,然后通过从消息队列中挨个取出消息,最后处理消息的过程。重点来看queue.next()方法,MessageQueue的初始化是在prepareMainLooper中,通过new Looper(quitAllowed)方法进行初始化:

    Message next() {
        // Return here if the message loop has already quit and been disposed.
        // This can happen if the application tries to restart a looper after quit
        // which is not supported.
        final long ptr = mPtr;
        if (ptr == 0) {
            return null;
        }

        int pendingIdleHandlerCount = -1; // -1 only during first iteration
        int nextPollTimeoutMillis = 0;
        for (;;) {
            if (nextPollTimeoutMillis != 0) {
                Binder.flushPendingCommands();
            }

            nativePollOnce(ptr, nextPollTimeoutMillis);

            synchronized (this) {
                // Try to retrieve the next message.  Return if found.
                final long now = SystemClock.uptimeMillis();
                Message prevMsg = null;
                Message msg = mMessages;
                if (msg != null && msg.target == null) {
                    // Stalled by a barrier.  Find the next asynchronous message in the queue.
                    do {
                        prevMsg = msg;
                        msg = msg.next;
                    } while (msg != null && !msg.isAsynchronous());
                }
                if (msg != null) {
                    if (now < msg.when) {
                        // Next message is not ready.  Set a timeout to wake up when it is ready.
                        nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
                    } else {
                        // Got a message.
                        mBlocked = false;
                        if (prevMsg != null) {
                            prevMsg.next = msg.next;
                        } else {
                            mMessages = msg.next;
                        }
                        msg.next = null;
                        if (DEBUG) Log.v(TAG, "Returning message: " + msg);
                        msg.markInUse();
                        return msg;
                    }
                } else {
                    // No more messages.
                    nextPollTimeoutMillis = -1;
                }

                // Process the quit message now that all pending messages have been handled.
                if (mQuitting) {
                    dispose();
                    return null;
                }

                // If first time idle, then get the number of idlers to run.
                // Idle handles only run if the queue is empty or if the first message
                // in the queue (possibly a barrier) is due to be handled in the future.
                if (pendingIdleHandlerCount < 0
                        && (mMessages == null || now < mMessages.when)) {
                    pendingIdleHandlerCount = mIdleHandlers.size();
                }
                if (pendingIdleHandlerCount <= 0) {
                    // No idle handlers to run.  Loop and wait some more.
                    mBlocked = true;
                    continue;
                }

                if (mPendingIdleHandlers == null) {
                    mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
                }
                mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
            }

            // Run the idle handlers.
            // We only ever reach this code block during the first iteration.
            for (int i = 0; i < pendingIdleHandlerCount; i++) {
                final IdleHandler idler = mPendingIdleHandlers[i];
                mPendingIdleHandlers[i] = null; // release the reference to the handler

                boolean keep = false;
                try {
                    keep = idler.queueIdle();
                } catch (Throwable t) {
                    Log.wtf(TAG, "IdleHandler threw exception", t);
                }

                if (!keep) {
                    synchronized (this) {
                        mIdleHandlers.remove(idler);
                    }
                }
            }

            // Reset the idle handler count to 0 so we do not run them again.
            pendingIdleHandlerCount = 0;

            // While calling an idle handler, a new message could have been delivered
            // so go back and look again for a pending message without waiting.
            nextPollTimeoutMillis = 0;
        }
    }

next()方法里面主要做的是对消息队列进行消息的获取,这里大概介绍下,消息队列中的消息是按照时间的先后顺序去排列的(当然它会去判断now和msg.when的时间是不是相等,如果相等就获取返回这个消息,如果now < msg.when,就说明这个消息并没有准备好必须等待处理),消息队列是可以插队的,队头的消息是将要最早触发的消息。当有消息需要加入消息队列时,会从队列头开始遍历,直到找到消息应该插入的合适位置,以保证所有消息的时间顺序。

上面整理完了looper和MessageQueue之间的关系,总结一下:主线程通过Looper.prepare()来创建Looper对象(消息队列封装在Looper对象中),并且保存在sThreadLoal中,然后通过Looper.loop()来执行消息循环。

接下来轮到消息处理类Handler出场了:

Handler:
我们在子线程中执行完耗时操作后很多情况下我们需要更新UI,但我们都知道,不能在子线程中更新UI。此时最常用的手段就是通过Handler将一个消息post到UI线程中,然后再在Handler的handleMessage方法中进行处理。但是有一个点要注意,那就是该Handler必须在主线程中创建!!
使用实例:

class MyHandler extends Handler {  
    @Override  
    public void handleMessage(Message msg) {  
      // 更新UI  

    }  
}   

MyHandler mHandler = new MyHandler() ;  
// 开启新的线程  
new Thread(){  
        public void run() {  
            // 耗时操作  
            mHandler.sendEmptyMessage("message") ;  
        };  
    }.start();  

为什么必须要这么做呢?其实每个Handler都会关联一个消息队列,消息队列被封装在Lopper中,而每个Looper又会关联一个线程(ThreadLocal),也就是每个消息队列会关联一个线程,一般应用中只会存在一个消息队列。Handler就是一个消息处理器,将消息投递给消息队列,然后再由对应的线程从消息队列中挨个取出消息,并且执行。默认情况下,消息队列只有一个,即主线程的消息队列,这个消息队列是在ActivityThread.main方法中创建的,通过Lopper.prepareMainLooper()来创建,然后最后执行Looper.loop()来启动消息循环。那么Handler是如何关联消息队列以及线程的呢?我们看看如下源码 :

    public Handler(Callback callback, boolean async) {
        if (FIND_POTENTIAL_LEAKS) {
            final Class 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();// 获取Looper  
        if (mLooper == null) {
            throw new RuntimeException(
                "Can&#39;t create handler inside thread that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;// 获取消息队列  
        mCallback = callback;
        mAsynchronous = async;
    }

从Handler默认的构造函数中我们可以看到,Handler会在内部通过Looper.getLooper()来获取Looper对象,并且与之关联,最重要的就是消息队列。那么Looper.getLooper()又是如何工作的呢?我们继续往下看:

    /**
     * Return the Looper object associated with the current thread.  Returns
     * null if the calling thread is not associated with a Looper.
     */
    public static @Nullable Looper myLooper() {
        return sThreadLocal.get();
    }

    /**
     * Initialize the current thread as a looper, marking it as an
     * application&#39;s main looper. The main looper for your application
     * is created by the Android environment, so you should never need
     * to call this function yourself.  See also: {@link #prepare()}
     */
    public static void prepareMainLooper() {
        prepare(false);
        synchronized (Looper.class) {
            if (sMainLooper != null) {
                throw new IllegalStateException("The main Looper has already been prepared.");
            }
            sMainLooper = myLooper();
        }
    }

 /** Initialize the current thread as a looper. 
  * This gives you a chance to create handlers that then reference 
  * this looper, before actually starting the loop. Be sure to call 
  * {@link #loop()} after calling this method, and end it by calling 
  * {@link #quit()}. 
  */  
public static void prepare() {  
    if (sThreadLocal.get() != null) {  
        throw new RuntimeException("Only one Looper may be created per thread");  
    }  
    sThreadLocal.set(new Looper());  
}  

我们看到myLooper()方法是通过sThreadLocal.get()来获取的,那么Looper对象又是什么时候存储在sThreadLocal中的呢?上面其实也讲相关代码:

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));
    }

prepare方法中就是,这个方法中new了一个looper对象,并且将对象设置给sThreadLocal,这时队列就与线程关联上了!不同的线程是不能访问对方的消息队列的,消息队列通过Looper与线程关联上,而Handler又与Looper关联,因此Handler最终就和线程、线程的消息队列关联上了。
那如何回答“为什么要更新UI的Handler必须要在主线程中创建?”
就是因为Handler要与主线程的消息队列关联上,这样handleMessage才会执行在UI线程,此时更新UI才是线程安全的!!!

问题:子线程中创建Handler为何会抛出异常 ?

new Thread(){  
    Handler handler = null;  
    public void run() {  
        handler = new Handler();  
    };  
}.start();  

看下面的源码:

    public Handler(Callback callback, boolean async) {
        if (FIND_POTENTIAL_LEAKS) {
            final Class 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&#39;t create handler inside thread that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }

当mLooper为空的时候,就会抛出异常,这是因为该线程中的Looper对象还没有创建,因此sThreadLocal.get()会返回null。
如何解决:

new Thread(){  
    Handler handler = null;  
    public void run() {  
        Looper.prepare();    // 1、创建Looper,并且会绑定到ThreadLocal中  
        handler = new Handler();  
        Looper.loop();       // 2、启动消息循环  
    };  
}.start();  

上面的代码中,一是通过Looper.prepare()来创建Looper,二是通过Looper.loop()来启动消息循环。这样该线程就有了自己的Looper,也就是有了自己的消息队列。如果之前创建Looper,而不启动消息循环,虽然不会抛出异常,但是你通过handler来post或者sendMessage也不会有效,因为虽然消息被追加到消息队列了,但是并没有启动消息循环,也就不会从消息队列中获取消息并且执行了!

总结:
在应用启动时,会开启一个主线程(UI线程),创建主线程Looper和消息队列,并且启动消息循环,应用不停地从该消息队列中取出、处理消息达到程序运行的效果。Looper对象封装了消息队列,Looper对象是ThreadLocal的,不同线程之间的Looper对象不能共享与访问。而Handler通过与Looper对象绑定来实现与执行线程的绑定,handler会把Runnable(包装成Message)或者Message对象追加到与线程关联的消息队列中,然后在消息循环中挨个取出消息,并且处理消息。当Handler绑定的Looper是主线程的Looper,则该Handler可以在handleMessage中更新UI,否则更新UI则会抛出异常,所以每个线程必须要自己的looper和自己的Handler同时有才不会发生问题。

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