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Handler运行机制

时间:2016-04-29 19:00:16      阅读:233      评论:0      收藏:0      [点我收藏+]

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Handler的运行机制

Handler的作用:

当我们需要在子线程处理耗时的操作(例如访问网络,数据库的操作),而当耗时的操作完成后,需要更新UI,这就需要使用Handler来处理,因为子线程不能做更新UI的操作。Handler能帮我们很容易的把任务(在子线程处理)切换回它所在的线程。简单理解,Handler就是解决线程和线程之间的通信的。

Handler的使用

使用的handler的两种形式:
1.在主线程使用handler;
2.在子线程使用handler。

在主线程使用handler的示例:

    public class TestHandlerActivity extends AppCompatActivity {


        private static final String TAG = "TestHandlerActivity";

        private Handler mHandler = new Handler(){
            @Override
            public void handleMessage(Message msg) {
                super.handleMessage(msg);
                //获得刚才发送的Message对象,然后在这里进行UI操作
                Log.e(TAG,"------------> msg.what = " + msg.what);
            }
        };


        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_handler_test);
            initData();
        }

        private void initData() {

            //开启一个线程模拟处理耗时的操作
            new Thread(new Runnable() {
                @Override
                public void run() {

                    SystemClock.sleep(2000);
                    //通过Handler发送一个消息切换回主线程(mHandler所在的线程)
                    mHandler.sendEmptyMessage(0);
                }
            }).start();

        }   

技术分享

在主线程使用handler很简单,只需在主线程创建一个handler对象,在子线程通过在主线程创建的handler对象发送Message,在handleMessage()方法中接受这个Message对象进行处理。通过handler很容易的从子线程切换回主线程了。

那么来看看在子线程中使用是否也是如此。

 public class TestHandlerActivity extends AppCompatActivity {


        private static final String TAG = "TestHandlerActivity";
        //主线程中的handler
        private Handler mHandler = new Handler(){
            @Override
            public void handleMessage(Message msg) {
                super.handleMessage(msg);
                //获得刚才发送的Message对象,然后在这里进行UI操作
                Log.e(TAG,"------------> msg.what = " + msg.what);
            }
        };
        //子线程中的handler
        private Handler mHandlerThread = null;

        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_handler_test);
            initData();
        }

        private void initData() {

            //开启一个线程模拟处理耗时的操作
            new Thread(new Runnable() {
                @Override
                public void run() {

                    SystemClock.sleep(2000);
                    //通过Handler发送一个消息切换回主线程(mHandler所在的线程)
                    mHandler.sendEmptyMessage(0);
                    //在子线程中创建Handler
                    mHandlerThread = new Handler(){
                        @Override
                        public void handleMessage(Message msg) {
                            super.handleMessage(msg);
                            Log.e("sub thread","---------> msg.what = " + msg.what);
                        }
                    };

                    mHandlerThread.sendEmptyMessage(1);
                }
            }).start();

        }

技术分享

程序崩溃了。报的错误是没有在子线程调用Looper.prepare()的方法。而为什么在主线程中使用不会报错?通过源码的分析可以解析这个问题。

在子线程中正确的使用Handler应该是这样的。

 public class TestHandlerActivity extends AppCompatActivity {


        private static final String TAG = "TestHandlerActivity";

        //主线程的Handler
        private Handler mHandler = new Handler(){
            @Override
            public void handleMessage(Message msg) {
                super.handleMessage(msg);
                //获得刚才发送的Message对象,然后在这里进行UI操作
                Log.e(TAG,"------------> msg.what = " + msg.what);
            }
        };
        //子线程中的Handler
        private Handler mHandlerThread = null;

        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_handler_test);
            initData();
        }

        private void initData() {

            //开启一个线程模拟处理耗时的操作
            new Thread(new Runnable() {
                @Override
                public void run() {

                    SystemClock.sleep(2000);
                    //通过Handler发送一个消息切换回主线程(mHandler所在的线程)
                    mHandler.sendEmptyMessage(0);

                    //调用Looper.prepare()方法
                    Looper.prepare();

                    mHandlerThread = new Handler(){
                        @Override
                        public void handleMessage(Message msg) {
                            super.handleMessage(msg);
                            Log.e("sub thread","---------> msg.what = " + msg.what);
                        }
                    };

                    mHandlerThread.sendEmptyMessage(1);

                    //调用Looper.loop()方法
                    Looper.loop();
                }
            }).start();

        }

技术分享

可以看到,通过调用Looper.prepare()运行正常,handleMessage方法中就可以接收到发送的Message。

至于为什么要调用这个方法呢?去看看源码。

Handler的源码分析

Handler的消息处理主要有五个部分组成,Message,Handler,Message Queue,Looper和ThreadLocal。首先简要的了解这些对象的概念

Message:Message是在线程之间传递的消息,它可以在内部携带少量的数据,用于线程之间交换数据。Message有四个常用的字段,what字段,arg1字段,arg2字段,obj字段。what,arg1,arg2可以携带整型数据,obj可以携带object对象。

Handler:它主要用于发送和处理消息的发送消息一般使用sendMessage()方法,还有其他的一系列sendXXX的方法,但最终都是调用了sendMessageAtTime方法,除了sendMessageAtFrontOfQueue()这个方法

而发出的消息经过一系列的辗转处理后,最终会传递到Handler的handleMessage方法中。

Message Queue:MessageQueue是消息队列的意思,它主要用于存放所有通过Handler发送的消息,这部分的消息会一直存在于消息队列中,等待被处理。每个线程中只会有一个MessageQueue对象。

Looper:每个线程通过Handler发送的消息都保存在,MessageQueue中,Looper通过调用loop()的方法,就会进入到一个无限循环当中,然后每当发现Message Queue中存在一条消息,就会将它取出,并传递到Handler的handleMessage()方法中。每个线程中只会有一个Looper对象。

ThreadLocal:MessageQueue对象,和Looper对象在每个线程中都只会有一个对象,怎么能保证它只有一个对象,就通过ThreadLocal来保存。Thread Local是一个线程内部的数据存储类,通过它可以在指定线程中存储数据,数据存储以后,只有在指定线程中可以获取到存储到数据,对于其他线程来说则无法获取到数据。

了解了这些基本概念后,我们深入源码来了解Handler的工作机制。

MessageQueue的工作原理

MessageQueue消息队列是通过一个单链表的数据结构来维护消息列表的。下面主要看enqueueMessage方法和next()方法。如下:

    boolean enqueueMessage(Message msg, long when) {
            if (msg.target == null) {
                throw new IllegalArgumentException("Message must have a target.");
            }
            if (msg.isInUse()) {
                throw new IllegalStateException(msg + " This message is already in use.");
            }

            synchronized (this) {
                if (mQuitting) {
                    IllegalStateException e = new IllegalStateException(
                            msg.target + " sending message to a Handler on a dead thread");
                    Log.w(TAG, e.getMessage(), e);
                    msg.recycle();
                    return false;
                }

                msg.markInUse();
                msg.when = when;
                Message p = mMessages;
                boolean needWake;
                if (p == null || when == 0 || when < p.when) {
                    // New head, wake up the event queue if blocked.
                    msg.next = p;
                    mMessages = msg;
                    needWake = mBlocked;
                } else {
                    // Inserted within the middle of the queue.  Usually we don‘t have to wake
                    // up the event queue unless there is a barrier at the head of the queue
                    // and the message is the earliest asynchronous message in the queue.
                    needWake = mBlocked && p.target == null && msg.isAsynchronous();
                    Message prev;
                    for (;;) {
                        prev = p;
                        p = p.next;
                        if (p == null || when < p.when) {
                            break;
                        }
                        if (needWake && p.isAsynchronous()) {
                            needWake = false;
                        }
                    }
                    msg.next = p; // invariant: p == prev.next
                    prev.next = msg;
                }

                // We can assume mPtr != 0 because mQuitting is false.
                if (needWake) {
                    nativeWake(mPtr);
                }
            }
            return true;
        }

可以看出,在这个方法里主要是根据时间的顺序向单链表中插入一条消息。

next()方法。如下

    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;
            }
        }

在next方法是一个无限循环的方法,如果有消息返回这条消息并从链表中移除,而没有消息则一直阻塞在这里。

Looper的工作原理

每个程序都有一个入口,而Android程序是基于java的,java的程序入口是静态的main函数,因此Android程序的入口也应该为静态的main函数,在android程序中这个静态的main在ActivityThread类中。我们来看一下这个main方法,如下:

     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("<pre-initialized>");
            //######
            Looper.prepareMainLooper();

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

            if (sMainThreadHandler == null) {
                sMainThreadHandler = thread.getHandler();
            }

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

            Looper.loop();

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

在main方法中系统调用了 Looper.prepareMainLooper();来创建主线程的Looper以及MessageQueue,并通过Looper.loop()来开启主线程的消息循环。来看看Looper.prepareMainLooper()是怎么创建出这两个对象的。如下:

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

可以看到,在这个方法中调用了 prepare(false);方法和 myLooper();方法,我在进入这个两个方法中,如下:

     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));
        }

在这里可以看出,sThreadLocal对象保存了一个Looper对象,首先判断是否已经存在Looper对象了,以防止被调用两次。sThreadLocal对象是ThreadLocal类型,因此保证了每个线程中只有一个Looper对象。Looper对象是什么创建的,我们进入看看,如下:

  private Looper(boolean quitAllowed) {
            mQueue = new MessageQueue(quitAllowed);
            mThread = Thread.currentThread();
        }

可以看出,这里在Looper构造函数中创建出了一个MessageQueue对象和保存了当前线程。从上面可以看出一个线程中只有一个Looper对象,而Message Queue对象是在Looper构造函数创建出来的,因此每一个线程也只会有一个MessageQueue对象。

对prepare方法还有一个重载的方法:如下

  public static void prepare() {
            prepare(true);
        }

prepare()仅仅是对prepare(boolean quitAllowed) 的封装而已,在这里就很好解释了在主线程为什么不用调用Looper.prepare()方法了。因为在主线程启动的时候系统已经帮我们自动调用了Looper.prepare()方法。

在Looper.prepareMainLooper()方法中还调用了一个方法myLooper(),我们进去看看,如下:

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

在调用prepare()方法中在当前线程保存一个Looper对象sThreadLocal.set(new Looper(quitAllowed));my Looper()方法就是取出当前线程的Looper对象,保存在sMainLooper引用中。

在main()方法中还调用了Looper.loop()方法,如下:

    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.recycle();
            }
    }

在这个方法里,进入一个无限循环,不断的从MessageQueue的next方法获取消息,而next方法是一个阻塞操作,当没有消息的时候一直在阻塞,当有消息通过 msg.target.dispatchMessage(msg);这里的msg.target其实就是发送给这条消息的Handler对象。

Handler的运行机制

看看Handler的构造方法。如下:

    public Handler(Callback callback) {
            this(callback, false);
        }

        public Handler(Looper looper) {
            this(looper, null, false);
        }

        public Handler(Looper looper, Callback callback) {
            this(looper, callback, false);
        }

我们去看看没有Looper 对象的构造方法:

     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;
        }

可以看到,到looper对象为null,抛出 “Can’t create handler inside thread that has not called Looper.prepare()”异常由这里可以知道,当我们在子线程使用Handler的时候要手动调用Looper.prepare()创建一个Looper对象,之所以主线程不用,是系统启动的时候帮我们自动调用了Looper.prepare()方法。

handler的工作主要包含发送和接收过程。消息的发送主要通过post和send的一系列方法,而post的一系列方法是最终是通过send的一系列方法来实现的。而send的一系列方法最终是通过sendMessageAtTime方法来实现的,除了sendMessageAtFrontOfQueue()这个方法。去看看这些一系列send的方法,如下:

    public final boolean sendMessage(Message msg)
        {
            return sendMessageDelayed(msg, 0);
        }

        public final boolean sendEmptyMessage(int what)
        {
            return sendEmptyMessageDelayed(what, 0);
        }  

        public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {
            Message msg = Message.obtain();
            msg.what = what;
            return sendMessageAtTime(msg, uptimeMillis);
        }

        public final boolean sendMessageDelayed(Message msg, long delayMillis)
        {
            if (delayMillis < 0) {
                delayMillis = 0;
            }
            return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
        }

        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);
        }

        public final boolean sendMessageAtFrontOfQueue(Message msg) {
            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, 0);
        }

        private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
            msg.target = this;
            if (mAsynchronous) {
                msg.setAsynchronous(true);
            }
            return queue.enqueueMessage(msg, uptimeMillis);
        }

可以看出,handler发送一条消息其实就是在消息队列插入一条消息。在Looper的loop方法中,从Message Queue中取出消息调msg.target.dispatchMessage(msg);这里其实就是调用了Handler的dispatchMessage(msg)方法,进去看看,如下:

      /**
         * Handle system messages here.
         */
        public void dispatchMessage(Message msg) {
            if (msg.callback != null) {
                handleCallback(msg);
            } else {
                if (mCallback != null) {
                    if (mCallback.handleMessage(msg)) {
                        return;
                    }
                }
                handleMessage(msg);
            }
        }

判断msg.callback是否为空,不为空调用 handleCallback(msg);来处理消息。其实callback是一个Runnable对象,就是Handler发送post消息传过来的对象。

     public final boolean post(Runnable r)
        {
           return  sendMessageDelayed(getPostMessage(r), 0);
        }

         public final boolean postAtTime(Runnable r, long uptimeMillis)
        {
            return sendMessageAtTime(getPostMessage(r), uptimeMillis);
        }

        public final boolean postAtTime(Runnable r, Object token, long uptimeMillis)
        {
            return sendMessageAtTime(getPostMessage(r, token), uptimeMillis);
        }


        public final boolean postDelayed(Runnable r, long delayMillis)
        {
            return sendMessageDelayed(getPostMessage(r), delayMillis);
        }


        public final boolean postAtFrontOfQueue(Runnable r)
        {
            return sendMessageAtFrontOfQueue(getPostMessage(r));
        }

        private static Message getPostMessage(Runnable r) {
            Message m = Message.obtain();
            m.callback = r;
            return m;
        }

进去handleCallback方法看看怎么处理消息的,如下:

      private static void handleCallback(Message message) {
            message.callback.run();
        }

可以看出,其实就是回调Runnable对象的run方法。Activity的runOnUiThread,View的postDelayed方法也是同样的原理,我们先看看runOnUiThread方法,如下:

    public final void runOnUiThread(Runnable action) {
            if (Thread.currentThread() != mUiThread) {
                mHandler.post(action);
            } else {
                action.run();
            }
        }

View的postDelayed方法。如下:

 public boolean postDelayed(Runnable action, long delayMillis) {
            final AttachInfo attachInfo = mAttachInfo;
            if (attachInfo != null) {
                return attachInfo.mHandler.postDelayed(action, delayMillis);
            }
            // Assume that post will succeed later
            ViewRootImpl.getRunQueue().postDelayed(action, delayMillis);
            return true;
        }

实质上都是在UI线程中执行了Runnable的run方法。

如果msg.callback是否为null,判断mCallback是否为null?mCallback是一个接口,如下:

       /**
         * Callback interface you can use when instantiating a Handler to avoid
         * having to implement your own subclass of Handler.
         *
         * @param msg A {@link android.os.Message Message} object
         * @return True if no further handling is desired
         */
        public interface Callback {
            public boolean handleMessage(Message msg);
        }

CallBack其实提供了另一种使用Handler的方式,可以派生子类重写handleMessage()方法,也可以通过设置CallBack来实现。

我们梳理一下我们在主线程使用Handler的过程。

首先在主线程创建一个Handler对象 ,并重写handleMessage()方法。然后当在子线程中需要进行更新UI的操作,我们就创建一个Message对象,并通过handler发送这条消息出去。之后这条消息被加入到MessageQueue队列中等待被处理,通过Looper对象会一直尝试从Message Queue中取出待处理的消息,最后分发会Handler的handler Message()方法中。

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END…

Handler运行机制

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原文地址:http://blog.csdn.net/u012827296/article/details/51236614

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