通过注释也能看到上面的函数会找到ActivityThread的main函数并且执行。main函数中创建了Looper，Looper的作用就是利用线程创建一个消息处理队列，并且维护这个消息队列：
public static void main(String[] args) {
Looper.prepareMainLooper();//创建Looper
if (sMainThreadHandler == null) {
sMainThreadHandler = new Handler();
}
ActivityThread thread = new ActivityThread();
thread.attach(false);//应用所有的逻辑都在这个方法中
Looper.loop();//开启一个消息循环，不断的读取MessageQueue中的Message。
}
Looper:
Looper.prepareMainLooper()的代码如下：
/**
* Initialize the current thread as a looper, marking it as an
* application's main looper. The main looper for your application
* is created by the Android environment, so you should never need
* to call this function yourself. See also: {@link #prepare()}
*/
public static void prepareMainLooper() {
prepare();
setMainLooper(myLooper());
myLooper().mQueue.mQuitAllowed = false;
}
上面的方法注释已经说的很明白，创建了主线程的Looper，这段代码是系统调用的。先看prepare方法做了什么操作。
/** Initialize the current thread as a looper.
* This gives you a chance to create handlers that then reference
* this looper, before actually starting the loop. Be sure to call
* {@link #loop()} after calling this method, and end it by calling
* {@link #quit()}.
*/
public static void prepare() {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper());
}
private Looper() {
mQueue = new MessageQueue();
mRun = true;
mThread = Thread.currentThread();//获取当前线程
}
创建主线程的Looper，每一个Looper对应一个Thread、一个MessageQueue，创建Looper的时候会创建一个MessageQueue。到目前位置创建了应用的主线程（Thread）、Looper、MessageQueue，调用Looper.loop()，开始不断的从MessageQueue中读取Message并处理，如果没有消息则等待。现在有了消息循环，有了管理消息循环的Looper就差发送消息和处理消息的Handler了。
Handler:
这个时候你在你的应用中创建一个Handler，一般都是下面的代码：
private static final Handler mHandler = new Handler() {
public void handleMessage(Message msg) {
..........
}
}
};
这个Handler是在主线程中创建的，Handler的构造函数如下：
/**
* Default constructor associates this handler with the queue for the
* current thread.
*
* If there isn't one, this handler won't be able to receive messages.
*/
public Handler() {
mLooper = Looper.myLooper();//获取上面在主线程创建的Looper
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;//获取Looper的MessageQueue
mCallback = null;//默认为null在后面处理msg时会就行检查
}
创建完Handler你就可以用了，比如你发一个消息：
Java
mHandler.sendEmptyMessage(MSG_WHAT);
在系统中会走最终走到Handler.java下面的方法：