执行完onStart()方法后，将执行onStartCommand()方法。首先我们从消息池中获取一个Message实例，然后给Message对象的arg1、what、obj三个字段赋值。紧接着调用sendMessage(msg)方法，我们跟踪源代码，该方法将会调用sendMessageDelayed(msg, 0)方法，而sendMessageDelayed()方法又会调用sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis)方法，在该方法中我们要注意该句代码msg.target = this，msg的target指向了this，而this就是ServiceHandler对象，因此msg的target字段指向了ServiceHandler对象，同时该方法又调用MessageQueue 的enqueueMessage(msg, uptimeMillis)方法：
final boolean enqueueMessage(Message msg, long when) { if (msg.when != 0) { throw new AndroidRuntimeException(msg + " This message is already in use."); } if (msg.target == null && !mQuitAllowed) { throw new RuntimeException("Main thread not allowed to quit"); } synchronized (this) { if (mQuiting) { RuntimeException e = new RuntimeException( msg.target + " sending message to a Handler on a dead thread"); Log.w("MessageQueue", e.getMessage(), e); return false; } else if (msg.target == null) { mQuiting = true; } msg.when = when; //Log.d("MessageQueue", "Enqueing: " + msg); Message p = mMessages; if (p == null || when == 0 || when < p.when) { msg.next = p; mMessages = msg; this.notify(); } else { Message prev = null; while (p != null && p.when <= when) { prev = p; p = p.next; } msg.next = prev.next; prev.next = msg; this.notify(); } } return true; }该方法主要的任务就是把Message对象的添加到MessageQueue中（数据结构最基础的东西，自己画图理解下）。
handler.sendMessage()-->handler.sendMessageDelayed()-->handler.sendMessageAtTime()-->msg.target = this;queue.enqueueMessage==>把msg添加到消息队列中