Android消息机制底层原理

1.概述

Android的消息机制主要是指Handler的运行机制，Handler的运行需要底层的MessageQueue和Looper的支撑。MessageQueue是消息队列。他的内存存储了一组消息，以队列的形式对外提供插入和删除的工作。他的内部存储结构并不是真正的队列，而是采用单链表的数据结构来存储消息列表。Looper为消息循环，由于MessageQueue只是一个消息的存储单元，它不能去处理消息，而Looper就填补了这个功能，Looper会以无限循环的形式去查找是否有新的消息，如果有的话就处理消息，否则就一直等待，Looper还有一个特殊的概念，那就是ThreadLocal，ThreadLocal并不是线程，它的作用可以在每个线程中存储数据。我们知道，Handler创建的时候会采用当前线程的Looper来构造消息循环系统，那么Handler内部如何获取到当前线程的Looper呢？这就要使用ThreadLocal了，ThreadLocal可以在不同的线程中互不干扰地存储并提供数据，通过ThreadLocal可以轻松获取每个线程的Looper。当然需要注意的是，线程是默认没有Looper的，如果需要使用Handler就必须为线程创建Looper。我们经常提到的主线程，也就是UI线程，它就是ActivityThread，ActivityThread被创建时会初始化Looper，这也是在主线程中默认可以使用Handler的原因。

2.ThreadLocal-线程局部变量

ThreadLocal是一个现场内部的数据存储类，通过它可以在指定的线程中存储数据，数据存储以后，只有在指定线程中可以获取到存储的数据。对于Handler来说，它需要获取当前线程的Looper，很显然Looper的作用域就是线程并且不同线程具有不同的Looper，这个时候通过ThreadLocal就可以轻松实现Looper在线程中的存储。ThreadLocal是一个泛型类。

1）存储机制

在localValues内部有一个数组；private Object[]table，ThreadLocal的值就存在这个table数组中，ThreadLocal的值在table数组中的存储位置总是为ThreadLocal的reference字段所标识的对象的下一个位置，比如ThreadLocal的reference对象在table数组中的索引为index，那么ThreadLocal的值在table数组中的索引就是index+1.最终ThreadLocal的值将会被存储在table数组中：table[index+1]=value

//value加入数据
 void put(ThreadLocal key, Object value) {
  cleanUp();

  // Keep track of first tombstone. That's where we want to go back
  // and add an entry if necessary.
  int firstTombstone = -1;

  for (int index = key.hash & mask;; index = next(index)) {
  Object k = table[index];

  if (k == key.reference) {
  // Replace existing entry.
  table[index + 1] = value;
  return;
  }

  if (k == null) {
  if (firstTombstone == -1) {
  // Fill in null slot.
  table[index] = key.reference;
  table[index + 1] = value;
  size++;
  return;
  }

  // Go back and replace first tombstone.
  table[firstTombstone] = key.reference;
  table[firstTombstone + 1] = value;
  tombstones--;
  size++;
  return;
  }

  // Remember first tombstone.
  if (firstTombstone == -1 && k == TOMBSTONE) {
  firstTombstone = index;
  }
  }
  }

//获取当前线程的数据
  Values values(Thread current) {
  return current.localValues;//当前线程存储的数组
  }

//初始化当前线程的数据
Values initializeValues(Thread current) {
  return current.localValues = new Values();
  }

2）set

  public void set(T value) {
  Thread currentThread = Thread.currentThread();//获取当前的线程
  Values values = values(currentThread);//
  if (values == null) {
  values = initializeValues(currentThread);
  }
  values.put(this, value);
  }

3）get

 public T get() {
  // Optimized for the fast path.
  Thread currentThread = Thread.currentThread();
  Values values = values(currentThread);
  if (values != null) {
  Object[] table = values.table;
  int index = hash & values.mask;
  if (this.reference == table[index]) {
  return (T) table[index + 1];
  }
  } else {
  values = initializeValues(currentThread);
  }

  return (T) values.getAfterMiss(this);
  }

从ThreadLocal的set和get方法可以看出，他们所操作的对象都是当前线程localValues对象的table数组，因此在不同线程中访问同一个ThreadLocal的set和get方法，他们对ThreadLocal所做的读写操作仅限于各自线程的内部。

3.MessageQueue-消息队列

消息队列在Android中指的是MessageQueue，MessageQueue主要包含两个操作：插入和读取。读取操作本身会伴随着删除操作，插入和读取对应的方法分别为enqueueMessage和next，其中enqueueMessage的作用是往消息队列中 插入一条消息，而next的作用是从消息队列中取出一条消息并将其从消息队列中移除。MessageQueue内部是通过一个单链表的数据结构来维护消息列表，当链表在插入和删除上比较有优势。

1）enqueueMessage插入消息

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;
  }

2）next获取消息

  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;
  }
  }

next方法是一个无限循环的方法，如果消息队列中没有消息，那么next方法会一直堵塞在这里。当有新消息到来时，next方法会返回这条消息并将其从链表中移除

4.Message- 消息实体

需要注意Message的一些成员变量

 Handler target;  //对应的Handler

 Runnable callback; //对应的回调

Message next;//单链表引用

5.Looper-消息循环

Looper在Android的消息机制中扮演着消息循环的角色，具体来说就是他会不停地从MessageQueue中查看是否有新消息，如果有新消息就会立刻处理，否则就一直阻塞在哪里。

Looper处理prepare方法外，还提供了prepareMainLooper方法，这个方法主要是给主线程也就是ActivityThread创建Looper使用的，其本质也是通过prepare方法来实现。由于主线程的Looper比较特殊，所以Looper提供一个getMainLooper方法，通过它可以在任何地方获取主线程的Looper。Looper也是可以退出的，Looper提供勒quit和quitSafely来退出一个Looper。quit会直接退出Looper，而quitSafely只是设定一个退出标记，然后把消息队列的已有消息处理完毕后才安全退出。Looper退出后，通过Handler发送的消息会失败，这个时候Handler的send方法会返回false。在子线程，如果手动为其创建了Looper，那么所有的事情完成以后应该调用quit方法来终止消息循环，否则这个子线程就会一直处理等待的状态。

Looper最重要的一个方法是Loop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;

  // 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();
  }
  }

loop方法是一个死循环，唯一跳出循环的方式是MessageQueue的next方法返回了null。当Looper的quit方法被调用时，Looper就会调用MessageQueue的quit或者quitSafely方法来通知消息队列退出，当消息队列被标记为退出状态时，他的next方法会返回null。loop方法会调用MessageQueue的next方法来获取新消息，而next是一个阻塞操作，当没有消息时，next方法会一直阻塞在哪里，这也导致loop方法一直阻塞在哪里。若有新消息，Looper会调用msg。target。dispatchMessage（msg），这里的msg.target是发送这条消息的Handler对象，这样Handler发送的消息最终又交给它的dispatchMessage方法来处理了。但是这里不同的是，Handler的dispatchMessage方法是在创建Handler时所使用的Looper中执行，这样就成功将代码逻辑切换到指定的线程中去执行了。

6.Handle-消息处理

Handler的工作主要包含消息的发送和接收过程。消息的发送可以通过post的一系列方法以及send的一系列方法来实现，post的一系列方法最终是通过send的一系列方法来实现的。

1）创建

使用Handler必须要有Looper，不然会报异常

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

  /**
  * Use the provided {@link Looper} instead of the default one.
  *
  * @param looper The looper, must not be null.
  */
  public Handler(Looper looper) {
  this(looper, null, false);
  }

  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;
  }

2   发送

Handler发送消息的过程仅仅是向消息队列中插入了一条消息

 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);
  }

3）接收

 当消息队列插入消息后，MessageQueue的next方法就会返回这条消息给Looper，Looper收到消息后就开始处理了，最终消息由Looper交由Handler处理

 public interface Callback {
  public boolean handleMessage(Message msg);
  }

  /**
  * Subclasses must implement this to receive messages.
  */
  public void handleMessage(Message msg) {
  }

  /**
  * 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);
  }
  }

7.主线程的消息循环

Android的主线程就是ActivityThread，主线程的入口方法为main，在main方法中系统会通过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

 public static void main(String[] args) {
  Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
  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());

  AndroidKeyStoreProvider.install();

  // Make sure TrustedCertificateStore looks in the right place for CA certificates
  final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
  TrustedCertificateStore.setDefaultUserDirectory(configDir);

  Process.setArgV0("");

  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"));
  }

  // End of event ActivityThreadMain.
  Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
  Looper.loop();

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

主线程的消息循环开始以后，ActivityThread还需要一个Handler来和消息队列进行交互，这个Handler就是ActivityThread.H，他的内部定义了一组消息类型，主要管理Activity的生命周期及四大组件的启动和停止过程等

 private class H extends Handler {
  public static final int LAUNCH_ACTIVITY = 100;
  public static final int PAUSE_ACTIVITY = 101;
  public static final int PAUSE_ACTIVITY_FINISHING= 102;
  public static final int STOP_ACTIVITY_SHOW = 103;
  public static final int STOP_ACTIVITY_HIDE = 104;
  public static final int SHOW_WINDOW = 105;
  public static final int HIDE_WINDOW = 106;
  public static final int RESUME_ACTIVITY = 107;
  public static final int SEND_RESULT = 108;
  public static final int DESTROY_ACTIVITY = 109;
  public static final int BIND_APPLICATION = 110;
  public static final int EXIT_APPLICATION = 111;
  public static final int NEW_INTENT = 112;
  public static final int RECEIVER = 113;
  public static final int CREATE_SERVICE = 114;
  public static final int SERVICE_ARGS = 115;
  public static final int STOP_SERVICE = 116;

  public static final int CONFIGURATION_CHANGED = 118;
  public static final int CLEAN_UP_CONTEXT = 119;
  public static final int GC_WHEN_IDLE = 120;
  public static final int BIND_SERVICE = 121;
  public static final int UNBIND_SERVICE = 122;
  public static final int DUMP_SERVICE = 123;
  public static final int LOW_MEMORY = 124;
  public static final int ACTIVITY_CONFIGURATION_CHANGED = 125;
  public static final int RELAUNCH_ACTIVITY = 126;
  public static final int PROFILER_CONTROL = 127;
  public static final int CREATE_BACKUP_AGENT = 128;
  public static final int DESTROY_BACKUP_AGENT = 129;
  public static final int SUICIDE = 130;
  public static final int REMOVE_PROVIDER = 131;
  public static final int ENABLE_JIT = 132;
  public static final int DISPATCH_PACKAGE_BROADCAST = 133;
  public static final int SCHEDULE_CRASH = 134;
  public static final int DUMP_HEAP = 135;
  public static final int DUMP_ACTIVITY = 136;
  public static final int SLEEPING = 137;
  public static final int SET_CORE_SETTINGS = 138;
  public static final int UPDATE_PACKAGE_COMPATIBILITY_INFO = 139;
  public static final int TRIM_MEMORY = 140;
  public static final int DUMP_PROVIDER = 141;
  public static final int UNSTABLE_PROVIDER_DIED = 142;
  public static final int REQUEST_ASSIST_CONTEXT_EXTRAS = 143;
  public static final int TRANSLUCENT_CONVERSION_COMPLETE = 144;
  public static final int INSTALL_PROVIDER = 145;
  public static final int ON_NEW_ACTIVITY_OPTIONS = 146;
  public static final int CANCEL_VISIBLE_BEHIND = 147;
  public static final int BACKGROUND_VISIBLE_BEHIND_CHANGED = 148;
  public static final int ENTER_ANIMATION_COMPLETE = 149;
}

另外经常使用的runOnUIThread(Runable action),通过源码分析也是使用了mHandler，而mHandler的Looper也是使用的UI线程的mainLooper。

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