Android消息循环机制源码深入理解

Android消息循环机制源码

前言：

搞Android的不懂Handler消息循环机制，都不好意思说自己是Android工程师。面试的时候一般也都会问这个知识点，但是我相信大多数码农肯定是没有看过相关源码的，顶多也就是网上搜搜，看看别人的文章介绍。学姐不想把那个万能的关系图拿出来讨论。

近来找了一些关于android线程间通信的资料，整理学习了一下，并制作了一个简单的例子。

　andriod提供了 Handler 和 Looper 来满足线程间的通信。例如一个子线程从网络上下载了一副图片，当它下载完成后会发送消息给主线程，这个消息是通过绑定在主线程的Handler来传递的。

在Android，这里的线程分为有消息循环的线程和没有消息循环的线程，有消息循环的线程一般都会有一个Looper，这个事android的新 概念。我们的主线程（UI线程）就是一个消息循环的线程。针对这种消息循环的机制，我们引入一个新的机制Handle，我们有消息循环，就要往消息循环里 面发送相应的消息，自定义消息一般都会有自己对应的处理，消息的发送和清除，消息的的处理，把这些都封装在Handle里面，注意Handle只是针对那 些有Looper的线程，不管是UI线程还是子线程，只要你有Looper，我就可以往你的消息队列里面添加东西，并做相应的处理。
但是这里还有一点，就是只要是关于UI相关的东西，就不能放在子线程中，因为子线程是不能操作UI的，只能进行数据、系统等其他非UI的操作。

　　在Android，这里的线程分为有消息循环的线程和没有消息循环的线程，有消息循环的线程一般都会有一个Looper，这个是android的新概念。我们的主线程（UI线程）就是一个消息循环的线程。针对这种消息循环的机制，我们引入一个新的机制Handler，我们有消息循环，就要往消息循环里面发送相应的消息，自定义消息一般都会有自己对应的处理，消息的发送和清除，把这些都封装在Handler里面，注意Handler只是针对那 些有Looper的线程，不管是UI线程还是子线程，只要你有Looper，我就可以往你的消息队列里面添加东西，并做相应的处理。

但是这里还有一点，就是只要是关于UI相关的东西，就不能放在子线程中，因为子线程是不能操作UI的，只能进行数据、系统等其他非UI的操作。

先从我们平时的使用方法引出这个机制，再结合源码进行分析。

我们平时使用是这样的：

 //1. 主线程
 Handler handler = new MyHandler();

 //2. 非主线程
 HandlerThread handlerThread = new HandlerThread("handlerThread");
 handlerThread.start();
 Handler handler = new Handler(handlerThread.getLooper());

 //发送消息
 handler.sendMessage(msg);

 //接收消息
 static class MyHandler extends Handler {
  //对于非主线程处理消息需要传Looper，主线程有默认的sMainLooper
  public MyHandler(Looper looper) {
   super(looper);
  }

  @Override
  public void handleMessage(Message msg) {
   super.handleMessage(msg);
  }
 }

那么为什么初始化的时候，我们执行了1或2，后面只需要sendMessage就可处理任务了呢？学姐这里以非主线程为例进行介绍，handlerThread.start()的时候，实际上创建了一个用于消息循环的Looper和消息队列MessageQueue，同时启动了消息循环，并将这个循环传给Handler，这个循环会从MessageQueue中依次取任务出来执行。用户若要执行某项任务，只需要调用handler.sendMessage即可，这里做的事情是将消息添加到MessaeQueue中。对于主线程也类似，只是主线程sMainThread和sMainLooper不需要我们主动去创建，程序启动的时候Application就创建好了，我们只需要创建Handler即可。

我们这里提到了几个概念：

• HandlerThread 支持消息循环的线程
• Handler 消息处理器
• Looper 消息循环对象
• MessageQueue 消息队列
• Message 消息体

对应关系是：一对多，即（一个）HandlerThread、Looper、MessageQueue －> （多个）Handler、Message

源码解析

1. Looper

（1）创建消息循环

prepare()用于创建Looper消息循环对象。Looper对象通过一个成员变量ThreadLocal进行保存。

（2）获取消息循环对象

myLooper()用于获取当前消息循环对象。Looper对象从成员变量ThreadLocal中获取。

（3）开始消息循环

loop()开始消息循环。循环过程如下：

每次从消息队列MessageQueue中取出一个Message

使用Message对应的Handler处理Message

已处理的Message加到本地消息池，循环复用

循环以上步骤，若没有消息表明消息队列停止，退出循环

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();
 }
}

2. Handler

（1）发送消息

Handler支持2种消息类型，即Runnable和Message。因此发送消息提供了post(Runnable r)和sendMessage(Message msg)两个方法。从下面源码可以看出Runnable赋值给了Message的callback，最终也是封装成Message对象对象。学姐个人认为外部调用不统一使用Message，应该是兼容Java的线程任务，学姐认为这种思想也可以借鉴到平常开发过程中。发送的消息都会入队到MessageQueue队列中。

（2）处理消息

Looper循环过程的时候，是通过dispatchMessage(Message msg)对消息进行处理。处理过程：先看是否是Runnable对象，如果是则调用handleCallback(msg)进行处理，最终调到Runnable.run()方法执行线程；如果不是Runnable对象，再看外部是否传入了Callback处理机制，若有则使用外部Callback进行处理；若既不是Runnable对象也没有外部Callback，则调用handleMessage(msg)，这个也是我们开发过程中最常覆写的方法了。

（3）移除消息

removeCallbacksAndMessages()，移除消息其实也是从MessageQueue中将Message对象移除掉。

public void handleMessage(Message msg) {
}

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

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

public final Message obtainMessage()
{
 return Message.obtain(this);
}

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

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

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

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);
}

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

public final void removeCallbacksAndMessages(Object token) {
 mQueue.removeCallbacksAndMessages(this, token);
}

3. MessageQueue

（1）消息入队

消息入队方法enqueueMessage(Message msg, long when)。其处理过程如下：

待入队的Message标记为InUse，when赋值

若消息链表mMessages为空为空，或待入队Message执行时间小于mMessage链表头，则待入队Message添加到链表头

若不符合以上条件，则轮询链表，根据when从低到高的顺序，插入链表合适位置

（2）消息轮询

next()依次从MessageQueue中取出Message

（3）移除消息

removeMessages()可以移除消息，做的事情实际上就是将消息从链表移除，同时将移除的消息添加到消息池，提供循环复用。

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("MessageQueue", 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;
}

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 (false) Log.v("MessageQueue", "Returning message: " + msg);
     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("MessageQueue", "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;
 }
}

void removeMessages(Handler h, int what, Object object) {
 if (h == null) {
  return;
 }

 synchronized (this) {
  Message p = mMessages;

  // Remove all messages at front.
  while (p != null && p.target == h && p.what == what
    && (object == null || p.obj == object)) {
   Message n = p.next;
   mMessages = n;
   p.recycleUnchecked();
   p = n;
  }

  // Remove all messages after front.
  while (p != null) {
   Message n = p.next;
   if (n != null) {
    if (n.target == h && n.what == what
     && (object == null || n.obj == object)) {
     Message nn = n.next;
     n.recycleUnchecked();
     p.next = nn;
     continue;
    }
   }
   p = n;
  }
 }
}

4. Message

（1）消息创建

Message.obtain()创建消息。若消息池链表sPool不为空，则从sPool中获取第一个，flags标记为UnInUse，同时从sPool中移除，sPoolSize减1；若消息池链表sPool为空，则new Message()

（2）消息释放

recycle()将消息释放，从内部实现recycleUnchecked()可知，将flags标记为InUse，其他各种状态清零，同时将Message添加到sPool，且sPoolSize加1

/**
 * Return a new Message instance from the global pool. Allows us to
 * avoid allocating new objects in many cases.
 */
public static Message obtain() {
 synchronized (sPoolSync) {
  if (sPool != null) {
   Message m = sPool;
   sPool = m.next;
   m.next = null;
   m.flags = 0; // clear in-use flag
   sPoolSize--;
   return m;
  }
 }
 return new Message();
}

/**
 * Return a Message instance to the global pool.
 * 

 * You MUST NOT touch the Message after calling this function because it has
 * effectively been freed. It is an error to recycle a message that is currently
 * enqueued or that is in the process of being delivered to a Handler.
 * 
 */
public void recycle() {
 if (isInUse()) {
  if (gCheckRecycle) {
   throw new IllegalStateException("This message cannot be recycled because it "
     + "is still in use.");
  }
  return;
 }
 recycleUnchecked();
}

/**
 * Recycles a Message that may be in-use.
 * Used internally by the MessageQueue and Looper when disposing of queued Messages.
 */
void recycleUnchecked() {
 // Mark the message as in use while it remains in the recycled object pool.
 // Clear out all other details.
 flags = FLAG_IN_USE;
 what = 0;
 arg1 = 0;
 arg2 = 0;
 obj = null;
 replyTo = null;
 sendingUid = -1;
 when = 0;
 target = null;
 callback = null;
 data = null;

 synchronized (sPoolSync) {
  if (sPoolSize < MAX_POOL_SIZE) {
   next = sPool;
   sPool = this;
   sPoolSize++;
  }
 }
}

5. HandlerThread

由于Java中的Thread是没有消息循环机制的，run()方法执行完，线程则结束。HandlerThread通过使用Looper实现了消息循环，只要不主动调用HandlerThread或Looper的quit()方法，循环就是一直走下去。

public class HandlerThread extends Thread {
int mPriority;
int mTid = -1;
Looper mLooper;

public HandlerThread(String name) {
 super(name);
 mPriority = Process.THREAD_PRIORITY_DEFAULT;
}

@Override
public void run() {
 mTid = Process.myTid();
 Looper.prepare();
 synchronized (this) {
  mLooper = Looper.myLooper();
  notifyAll();
 }
 Process.setThreadPriority(mPriority);
 onLooperPrepared();
 Looper.loop();
 mTid = -1;
}

public Looper getLooper() {
 if (!isAlive()) {
  return null;
 }

 // If the thread has been started, wait until the looper has been created.
 synchronized (this) {
  while (isAlive() && mLooper == null) {
   try {
    wait();
   } catch (InterruptedException e) {
   }
  }
 }
 return mLooper;
}

public boolean quit() {
 Looper looper = getLooper();
 if (looper != null) {
  looper.quit();
  return true;
 }
 return false;
}
}

总结

• 关键类：HandlerThread、Handler、Looper、MessageQueue、Messaga
• MessageQueue数据结构，链表。

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