Handler运行机制

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(){@Overridepublic 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);}};@Overrideprotected 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() {@Overridepublic 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();}

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handlerprivate Handler mHandler = new Handler(){@Overridepublic 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handlerprivate Handler mHandlerThread = null;@Overrideprotected 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() {@Overridepublic 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HandlermHandlerThread = new Handler(){@Overridepublic void handleMessage(Message msg) {super.handleMessage(msg);Log.e("sub thread","---------> msg.what = " + msg.what);}};mHandlerThread.sendEmptyMessage(1);}}).start();}

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Handlerprivate Handler mHandler = new Handler(){@Overridepublic 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Handlerprivate Handler mHandlerThread = null;@Overrideprotected 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() {@Overridepublic 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(){@Overridepublic 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();}

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.nextprev.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 iterationint 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 handlerboolean 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 libcoreEventLogger.setReporter(new EventLoggingReporter());Security.addProvider(new AndroidKeyStoreProvider());// Make sure TrustedCertificateStore looks in the right place for CA certificatesfinal 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(newLogPrinter(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 blockif (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 loggerPrinter 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 laterViewRootImpl.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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