public Handler(Looper looper, Callback callback, boolean async) {
  mLooper = looper;
  mQueue = looper.mQueue;
  mCallback = callback;
  mAsynchronous = async;
}

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 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();
}

public static Message obtain(Handler h) {
  Message m = obtain();
  m.target = h;

  return m;
}

public Handler(Callback callback, boolean async) {
    ...
    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;
  }

public static @Nullable Looper myLooper() {
    return sThreadLocal.get();
  }

  public T get() {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null) {
      ThreadLocalMap.Entry e = map.getEntry(this);
      if (e != null) {
        @SuppressWarnings("unchecked")
        T result = (T)e.value;
        return result;
      }
    }
    return setInitialValue();
  }

  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 void prepareMainLooper() {
    prepare(false);
    synchronized (Looper.class) {
      if (sMainLooper != null) {
        throw new IllegalStateException("The main Looper has already been prepared.");
      }
      sMainLooper = myLooper();
    }
  }

  public static void main(String[] args) {
    ...
    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");
  }

class LooperThread extends Thread {
   public Handler mHandler;

   public void run() {
     Looper.prepare();

     mHandler = new Handler() {
       public void handleMessage(Message msg) {
         // process incoming messages here
       }
     };

     Looper.loop();
   }
 }

public static void loop() {
    final Looper me = myLooper();//拿到当前线程的looper
    if (me == null) {
      throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
    }
    final MessageQueue queue = me.mQueue;//拿到当前looper的消息队列

    // 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 (;;) {//死循环遍历消息体。如果为null，则休眠。
      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);//此处是真正的分发消息。此处的target即是handler对象
        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();
    }
  }

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