第一次接触Android应用程序（这里指的是JAVA层的UI程序，也难怪了，Google放出的API就只支持JAVA应用程序了），很难搞明白内部是如何实现的。但是，从原理上分析，应该是有一个消息循环，一个消息队列，然后主线程不断得从消息队列中取得消息并处理之。
然而，google封装得太厉害了，所以一时半会还是搞不清楚到底是怎么做的。本文将分析android内的looper，这个是用来封装消息循环和消息队列的一个类，handler其实可以看做是一个工具类，用来向消息队列中插入消息的。好比是Windows API的SendMessage中的HANDLE，这个handle是窗口句柄。
//Looper类分析 //没找到合适的分析代码的办法，只能这么来了。每个重要行的上面都会加上注释 //功能方面的代码会在代码前加上一段分析 public class Looper { //static变量，判断是否打印调试信息。 private static final boolean DEBUG = false; private static final boolean localLOGV = DEBUG ? Config.LOGD : Config.LOGV; // sThreadLocal.get() will return null unless you've called prepare(). //线程本地存储功能的封装，TLS，thread local storage,什么意思呢？因为存储要么在栈上，例如函数内定义的内部变量。要么在堆上，例如new或者malloc出来的东西 //但是现在的系统比如Linux和windows都提供了线程本地存储空间，也就是这个存储空间是和线程相关的，一个线程内有一个内部存储空间，这样的话我把线程相关的东西就存储到 //这个线程的TLS中，就不用放在堆上而进行同步操作了。 private static final ThreadLocal sThreadLocal = new ThreadLocal(); //消息队列，MessageQueue，看名字就知道是个queue.. final MessageQueue mQueue; volatile boolean mRun; //和本looper相关的那个线程，初始化为null Thread mThread; private Printer mLogging = null; //static变量，代表一个UI Process（也可能是service吧，这里默认就是UI）的主线程 private static Looper mMainLooper = null; /** 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()}. */ //往TLS中设上这个Looper对象的，如果这个线程已经设过了ｌｏｏｐｅｒ的话就会报错 //这说明，一个线程只能设一个looper public static final void prepare() { if (sThreadLocal.get() != null) { throw new RuntimeException("Only one Looper may be created per thread"); } sThreadLocal.set(new Looper()); } /** 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. * {@link #prepare()} */ //由framework设置的UI程序的主消息循环，注意，这个主消息循环是不会主动退出的 // public static final void prepareMainLooper() { prepare(); setMainLooper(myLooper()); //判断主消息循环是否能退出.... //通过quit函数向looper发出退出申请 if (Process.supportsProcesses()) { myLooper().mQueue.mQuitAllowed = false; } } private synchronized static void setMainLooper(Looper looper) { mMainLooper = looper; } /** Returns the application's main looper, which lives in the main thread of the application. */ public synchronized static final Looper getMainLooper() { return mMainLooper; } /** * Run the message queue in this thread. Be sure to call * {@link #quit()} to end the loop. */ //消息循环，整个程序就在这里while了。 //这个是static函数喔！ public static final void loop() { Looper me = myLooper();//从该线程中取出对应的looper对象 MessageQueue queue = me.mQueue;//取消息队列对象... while (true) { Message msg = queue.next(); // might block取消息队列中的一个待处理消息.. //if (!me.mRun) {//是否需要退出？mRun是个volatile变量，跨线程同步的，应该是有地方设置它。 // break; //} if (msg != null) { if (msg.target == null) { // No target is a magic identifier for the quit message. return; } if (me.mLogging!= null) me.mLogging.println( ">>>>> Dispatching to " + msg.target + " " + msg.callback + ": " + msg.what ); msg.target.dispatchMessage(msg); if (me.mLogging!= null) me.mLogging.println( "<<<<< Finished to " + msg.target + " " + msg.callback); msg.recycle(); } } } /** * Return the Looper object associated with the current thread. Returns * null if the calling thread is not associated with a Looper. */ //返回和线程相关的looper public static final Looper myLooper() { return (Looper)sThreadLocal.get(); } /** * Control logging of messages as they are processed by this Looper. If * enabled, a log message will be written to <var>printer</var> * at the beginning and ending of each message dispatch, identifying the * target Handler and message contents. * * @param printer A Printer object that will receive log messages, or * null to disable message logging. */ //设置调试输出对象，looper循环的时候会打印相关信息，用来调试用最好了。 public void setMessageLogging(Printer printer) { mLogging = printer; } /** * Return the {@link MessageQueue} object associated with the current * thread. This must be called from a thread running a Looper, or a * NullPointerException will be thrown. */ public static final MessageQueue myQueue() { return myLooper().mQueue; } //创建一个新的looper对象， //内部分配一个消息队列，设置mRun为true private Looper() { mQueue = new MessageQueue(); mRun = true; mThread = Thread.currentThread(); } public void quit() { Message msg = Message.obtain(); // NOTE: By enqueueing directly into the message queue, the // message is left with a null target. This is how we know it is // a quit message. mQueue.enqueueMessage(msg, 0); } /** * Return the Thread associated with this Looper. */ public Thread getThread() { return mThread; } //后面就简单了，打印，异常定义等。 public void dump(Printer pw, String prefix) { pw.println(prefix + this); pw.println(prefix + "mRun=" + mRun); pw.println(prefix + "mThread=" + mThread); pw.println(prefix + "mQueue=" + ((mQueue != null) ? mQueue : "(null")); if (mQueue != null) { synchronized (mQueue) { Message msg = mQueue.mMessages; int n = 0; while (msg != null) { pw.println(prefix + " Message " + n + ": " + msg); n++; msg = msg.next; } pw.println(prefix + "(Total messages: " + n + ")"); } } } public String toString() { return "Looper{" + Integer.toHexString(System.identityHashCode(this)) + "}"; } static class HandlerException extends Exception { HandlerException(Message message, Throwable cause) { super(createMessage(cause), cause); } static String createMessage(Throwable cause) { String causeMsg = cause.getMessage(); if (causeMsg == null) { causeMsg = cause.toString(); } return causeMsg; } } }