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() {@Overridepublic void handleMessage(Message msg) {if (msg.what == 101) {Log.i("MainActivity", "接收到handler消息...");}}};@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);tv = (TextView) findViewById(R.id.tv);tv.setOnClickListener(new View.OnClickListener() {@Overridepublic void onClick(View v) {new Thread() {@Overridepublic 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() {@Overridepublic void onClick(View v) {new Thread() {@Overridepublic void run() {Handler mHandler = new Handler() {@Overridepublic 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() {@Overridepublic void onClick(View v) {new Thread() {@Overridepublic void run() {Looper.prepare();Handler mHandler = new Handler() {@Overridepublic 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(newLogPrinter(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() {@Overridepublic 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() {@Overridepublic 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<? 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;
  }
  

• 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.nextprev.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 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 loggerfinal 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 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;}
  }
  

• 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() {@Overridepublic 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
• 简书：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：yangchong211@163.com
• 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