Android异步消息机制

目录介绍

• 1.Handler的常见的使用方式
• 2.如何在子线程中定义Handler
• 3.主线程如何自动调用Looper.prepare()
• 4.Looper.prepare()方法源码分析
• 5.Looper中用什么存储消息
• 6.Handler发送消息如何运作
• 7.Looper.loop()方法源码分析
• 8.runOnUiThread如何实现子线程更新UI
• 9.Handler的post方法和view的post方法
• 10.得出部分结论

好消息

• 博客笔记大汇总【16年3月到至今】，包括Java基础及深入知识点，Android技术博客，Python学习笔记等等，还包括平时开发中遇到的bug汇总，当然也在工作之余收集了大量的面试题，长期更新维护并且修正，持续完善……开源的文件是markdown格式的！同时也开源了生活博客，从12年起，积累共计47篇[近20万字]，转载请注明出处，谢谢！
• 链接地址：https://github.com/yangchong211/YCBlogs
• 如果觉得好，可以star一下，谢谢！当然也欢迎提出建议，万事起于忽微，量变引起质变！
• 00.Android异步消息机制
  • 如何在子线程中定义Handler，主线程如何自动调用Looper.prepare()，Looper.prepare()方法源码分析，Looper中用什么存储消息，Looper.loop()方法源码分析，runOnUiThread如何实现子线程更新UI等等
• 01.Handler消息机制
  • 为什么不允许在子线程中访问UI，Handler消息机制作用，避免子线程手动创建looper，ActivityThread源码分析，ActivityThread源码分析，Looper死循环为什么不会导致应用卡死，会消耗大量资源吗？

1.Handler的常见的使用方式

• handler机制大家都比较熟悉呢。在子线程中发送消息，主线程接受到消息并且处理逻辑。如下所示
  • 一般handler的使用方式都是在主线程中定义Handler，然后在子线程中调用mHandler.sendXx()方法，这里有一个疑问可以在子线程中定义Handler吗？

  public class MainActivity extends AppCompatActivity {
  
      private TextView tv ;
  
      /**
       * 在主线程中定义Handler，并实现对应的handleMessage方法
       */
      public static Handler mHandler = new Handler() {
          @Override
          public void handleMessage(Message msg) {
              if (msg.what == 101) {
                  Log.i("MainActivity", "接收到handler消息...");
              }
          }
      };
  
      @Override
      protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_main);
          tv = (TextView) findViewById(R.id.tv);
          tv.setOnClickListener(new View.OnClickListener() {
              @Override
              public void onClick(View v) {
                  new Thread() {
                      @Override
                      public void run() {
                          // 在子线程中发送异步消息
                          mHandler.sendEmptyMessage(1);
                      }
                  }.start();
              }
          });
      }
  }
  

2.如何在子线程中定义Handler

• 直接在子线程中创建handler，看看会出现什么情况？
  • 运行后可以得出在子线程中定义Handler对象出错，难道Handler对象的定义或者是初始化只能在主线程中？其实不是这样的，错误信息中提示的已经很明显了，在初始化Handler对象之前需要调用Looper.prepare()方法

  tv.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
          new Thread() {
              @Override
              public void run() {
                  Handler mHandler = new Handler() {
                      @Override
                      public void handleMessage(Message msg) {
                          if (msg.what == 1) {
                              Log.i(TAG, "在子线程中定义Handler，接收并处理消息");
                          }
                      }
                  };
              }
          }.start();
      }
  });
  

  • 如何正确运行。在这里问一个问题，在子线程中可以吐司吗？答案是可以的，只不过又条件，详细可以看这篇文章02.Toast源码深度分析
    • 这样程序已经不会报错，那么这说明初始化Handler对象的时候我们是需要调用Looper.prepare()的，那么主线程中为什么可以直接初始化Handler呢？难道是主线程创建handler对象的时候，会自动调用Looper.prepare()方法的吗？

  tv.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
          new Thread() {
              @Override
              public void run() {
                  Looper.prepare();
                  Handler mHandler = new Handler() {
                      @Override
                      public void handleMessage(Message msg) {
                          if (msg.what == 1) {
                              Log.i(TAG, "在子线程中定义Handler，接收并处理消息");
                          }
                      }
                  };
                  Looper.loop();
              }
          }.start();
      }
  });
  

3.主线程如何自动调用Looper.prepare()

• 首先直接可以看在App初始化的时候会执行ActivityThread的main方法中的代码，如下所示
  • 可以看到Looper.prepare()方法在这里调用，所以在主线程中可以直接初始化Handler了。

  public static void main(String[] args) {
      //省略部分代码
      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"));
      }
      Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
      Looper.loop();
      throw new RuntimeException("Main thread loop unexpectedly exited");
  }
  

• 并且可以看到还调用了：Looper.loop()方法，可以知道一个Handler的标准写法其实是这样的

  Looper.prepare();
  Handler mHandler = new Handler() {
     @Override
     public void handleMessage(Message msg) {
        if (msg.what == 101) {
           Log.i(TAG, "在子线程中定义Handler，并接收到消息");
         }
     }
  };
  Looper.loop();
  

4.Looper.prepare()方法源码分析

• 源码如下所示
  • 可以看到Looper中有一个ThreadLocal成员变量，熟悉JDK的同学应该知道，当使用ThreadLocal维护变量时，ThreadLocal为每个使用该变量的线程提供独立的变量副本，所以每一个线程都可以独立地改变自己的副本，而不会影响其它线程所对应的副本。

  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));
  }
  

• 思考：Looper.prepare()能否调用两次或者多次
  • 如果运行，则会报错，并提示prepare中的Excetion信息。由此可以得出在每个线程中Looper.prepare()能且只能调用一次

  //这里Looper.prepare()方法调用了两次
  Looper.prepare();
  Looper.prepare();
  Handler mHandler = new Handler() {
     @Override
     public void handleMessage(Message msg) {
         if (msg.what == 1) {
            Log.i(TAG, "在子线程中定义Handler，并接收到消息。。。");
         }
     }
  };
  Looper.loop();
  

5.Looper中用什么存储消息

• 先看一下下面得源代码
  • 看Looper对象的构造方法，可以看到在其构造方法中初始化了一个MessageQueue对象。MessageQueue也称之为消息队列，特点是先进先出，底层实现是单链表数据结构

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

• 得出结论
  • Looper.prepare()方法初始话了一个Looper对象并关联在一个MessageQueue对象，并且一个线程中只有一个Looper对象，只有一个MessageQueue对象。

6.Handler发送消息如何运作

• 首先看看构造方法
  • 可以看出在Handler的构造方法中，主要初始化了一下变量，并判断Handler对象的初始化不应再内部类，静态类，匿名类中，并且保存了当前线程中的Looper对象。

  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't create handler inside thread that has not called Looper.prepare()");
      }
      mQueue = mLooper.mQueue;
      mCallback = callback;
      mAsynchronous = async;
  }
  

• 看handler.sendMessage(msg)方法
  • 关于下面得源码，是步步追踪，看enqueueMessage这个方法，原来msg.target就是Handler对象本身；而这里的queue对象就是我们的Handler内部维护的Looper对象关联的MessageQueue对象。

  handler.sendMessage(message);
  
  //追踪到这一步
  public final boolean sendMessage(Message msg){
      return sendMessageDelayed(msg, 0);
  }
  
  
  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);
  }
  

• 看MessageQueue对象的enqueueMessage方法
  • 看到这里MessageQueue并没有使用列表将所有的Message保存起来，而是使用Message.next保存下一个Message，从而按照时间将所有的Message排序

  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;
  }
  

7.Looper.loop()方法源码分析

• 看看里面得源码，如下所示
  • 看到Looper.loop()方法里起了一个死循环，不断的判断MessageQueue中的消息是否为空，如果为空则直接return掉，然后执行queue.next()方法

  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;
      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
          final Printer logging = me.mLogging;
          if (logging != null) {
              logging.println(">>>>> Dispatching to " + msg.target + " " +
                      msg.callback + ": " + msg.what);
          }
          final long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
          final long traceTag = me.mTraceTag;
          if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
              Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
          }
          final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
          final long end;
          try {
              msg.target.dispatchMessage(msg);
              end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
          } finally {
              if (traceTag != 0) {
                  Trace.traceEnd(traceTag);
              }
          }
          if (slowDispatchThresholdMs > 0) {
              final long time = end - start;
              if (time > slowDispatchThresholdMs) {
                  Slog.w(TAG, "Dispatch took " + time + "ms on "
                          + Thread.currentThread().getName() + ", h=" +
                          msg.target + " cb=" + msg.callback + " msg=" + msg.what);
              }
          }
          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();
      }
  }
  

• 看queue.next()方法源码
  • 大概的实现逻辑就是Message的出栈操作，里面可能对线程，并发控制做了一些限制等。获取到栈顶的Message对象之后开始执行：msg.target.dispatchMessage(msg)

  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;
      }
  }
  

• 那么msg.target是什么呢？通过追踪可以知道就是定义的Handler对象，然后查看一下Handler类的dispatchMessage方法：
  • 可以看到，如果我们设置了callback（Runnable对象）的话，则会直接调用handleCallback方法
  • 在初始化Handler的时候设置了callback（Runnable）对象，则直接调用run方法。

  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();
  }
  

8.runOnUiThread如何实现子线程更新UI

• 看看源码，如下所示
  • 如果msg.callback为空的话，会直接调用我们的mCallback.handleMessage(msg)，即handler的handlerMessage方法。由于Handler对象是在主线程中创建的，所以handler的handlerMessage方法的执行也会在主线程中。
  • 在runOnUiThread程序首先会判断当前线程是否是UI线程，如果是就直接运行，如果不是则post，这时其实质还是使用的Handler机制来处理线程与UI通讯。

  public void dispatchMessage(Message msg) {
      if (msg.callback != null) {
          handleCallback(msg);
      } else {
          if (mCallback != null) {
              if (mCallback.handleMessage(msg)) {
                  return;
              }
          }
          handleMessage(msg);
      }
  }
  
  @Override
  public final void runOnUiThread(Runnable action) {
      if (Thread.currentThread() != mUiThread) {
          mHandler.post(action);
      } else {
          action.run();
      }
  }
  

9.Handler的post方法和view的post方法

• Handler的post方法实现很简单，如下所示

  mHandler.post(new Runnable() {
      @Override
      public void run() {
  
      }
  });
  
  public final boolean post(Runnable r){
     return  sendMessageDelayed(getPostMessage(r), 0);
  }
  

• view的post方法也很简单，如下所示
  • 可以发现其调用的就是activity中默认保存的handler对象的post方法

  public boolean post(Runnable action) {
      final AttachInfo attachInfo = mAttachInfo;
      if (attachInfo != null) {
          return attachInfo.mHandler.post(action);
      }
      ViewRootImpl.getRunQueue().post(action);
      return true;
  }
  
  public void post(Runnable action) {
      postDelayed(action, 0);
  }
  
  public void postDelayed(Runnable action, long delayMillis) {
      final HandlerAction handlerAction = new HandlerAction(action, delayMillis);
  
      synchronized (this) {
          if (mActions == null) {
              mActions = new HandlerAction[4];
          }
          mActions = GrowingArrayUtils.append(mActions, mCount, handlerAction);
          mCount++;
      }
  }
  

10.得出部分结论

• 得出得结论如下所示
  • 1.主线程中定义Handler对象，ActivityThread的main方法中会自动创建一个looper，并且与其绑定。如果是子线程中直接创建handler对象，则需要手动创建looper。不过手动创建不太友好，需要手动调用quit方法结束looper。这个后面再说
  • 2.一个线程中只存在一个Looper对象，只存在一个MessageQueue对象，可以存在N个Handler对象，Handler对象内部关联了本线程中唯一的Looper对象，Looper对象内部关联着唯一的一个MessageQueue对象。
  • 3.MessageQueue消息队列不是通过列表保存消息（Message）列表的，而是通过Message对象的next属性关联下一个Message从而实现列表的功能，同时所有的消息都是按时间排序的。

关于其他内容介绍

01.关于博客汇总链接

• 1.技术博客汇总
• 2.开源项目汇总
• 3.生活博客汇总
• 4.喜马拉雅音频汇总
• 5.其他汇总

02.关于我的博客

• 我的个人站点：www.yczbj.org，www.ycbjie.cn
• github：https://github.com/yangchong211
• 知乎：https://www.zhihu.com/people/yang-chong-69-24/pins/posts
• ：http://www.jianshu.com/u/b7b2c6ed9284
• csdn：http://my.csdn.net/m0_37700275
• 喜马拉雅听书：http://www.ximalaya.com/zhubo/71989305/
• 开源中国：https://my.oschina.net/zbj1618/blog
• 泡在网上的日子：http://www.jcodecraeer.com/member/content_list.php?channelid=1
• 邮箱：[email protected]
• 阿里云博客：https://yq.aliyun.com/users/article?spm=5176.100- 239.headeruserinfo.3.dT4bcV
• segmentfault头条：https://segmentfault.com/u/xiangjianyu/articles