/**
* Enqueue a message into the message queue after all pending messages
* before the absolute time (in milliseconds) <var>uptimeMillis</var>.
* <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
* You will receive it in {@link #handleMessage}, in the thread attached
* to this handler.
*
* @param uptimeMillis The absolute time at which the message should be
* delivered, using the
* {@link android.os.SystemClock#uptimeMillis} time-base.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting. Note that a
* result of true does not mean the message will be processed -- if
* the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
public boolean sendMessageAtTime(Message msg, long uptimeMillis)
{
boolean sent = false;
MessageQueue queue = mQueue;
if (queue != null) {
msg.target = this;//注意这行代码后面会用，把Handler赋值给Msg的target对象
sent = queue.enqueueMessage(msg, uptimeMillis);//把msg放到MsgQueue中
}
else {
RuntimeException e = new RuntimeException(
this + " sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
}
return sent;
}
上面的方法第二个是延时毫秒数，queue.enqueueMessage把消息发送到MessageQueue后剩下的就是等待消息被处理，前面不是说了Looper.loop()方法开始轮询消息队列吗，你发送的消息就是在loop方法中读取到的，读取到后谁去处理呢？在loop()方法中有一句代码：
msg.target.dispatchMessage(msg);
msg就是你发送到MessageQueue的消息，被读取后调用target.dispatchMessage()，这个target就是上面Handler发送消息是赋值的，就是发送消息的Handler本身，然后Handler调用自己的下面方法就行消息处理：
/**
* 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);//在这会调用到上面重写的handleMessage方法。
}
}
因为在new Message的时候callback为空，并且Handler的mCallback = null，所以会调用到你上面new Handler时重写的handleMessage方法。
总结：
每一个线程中都对应一个Looper，每一个Looper都对应一个MessageQueue，这个Looper是用来管理消息队列的，主要是读取消息队列和把消息发送给Message的target去处理。到这你应该清除Thread、Handler、Message、MessageQueue和Looper他们之间的关系了吧。