前序：每个APP对应一个进程，该进程内有一个ActivityThread的线程，称为主线程（即UI主线程），此外，还有其他线程，这个再论。
android的消息系统分析。
- 每个Thread只对应一个Looper
- 每个Looper只对应一个MessageQueue
- 每个MessageQueue中有N个Message
- 每个Message中最多指定一个Handler来处理事件
- 一个Thread可以对应多个Handler
Looper负责从消息队列中（MessageQueue）取出消息（Message/Runnable)，交给Handler来处理。
Message：
public final class Message implements Parcelable { public int what;//每个Message可以指定一个Handler，由于可以存在多个Handler，用what来标识具体的Handler public int arg1; public int arg2; public Object obj; public Messenger replyTo; /*package*/ static final int FLAG_IN_USE = 1 << 0; /*package*/ static final int FLAG_ASYNCHRONOUS = 1 << 1; /*package*/ static final int FLAGS_TO_CLEAR_ON_COPY_FROM = FLAG_IN_USE; /*package*/ int flags; /*package*/ long when; /*package*/ Bundle data; /*package*/ Handler target;//指定由哪个Handler来处理本Message /*package*/ Runnable callback; /*package*/ Message next;//消息链 private static final Object sPoolSync = new Object(); private static Message sPool;//消息链表的头，注意是static private static int sPoolSize = 0; private static final int MAX_POOL_SIZE = 50; ...... }
MessageQueue实际上是对Message的一种操作的封装，真正的消息队列是以sPool为首的消息链表：
public class MessageQueue { // True if the message queue can be quit. private final boolean mQuitAllowed; @SuppressWarnings("unused") private int mPtr; // used by native code Message mMessages;//消息... private final ArrayList mIdleHandlers = new ArrayList (); private IdleHandler[] mPendingIdleHandlers; private boolean mQuiting; // Indicates whether next() is blocked waiting in pollOnce() with a non-zero timeout. private boolean mBlocked; // The next barrier token. // Barriers are indicated by messages with a null target whose arg1 field carries the token. private int mNextBarrierToken; //调用Native函数做真正的处理 private native void nativeInit(); private native void nativeDestroy(); private native void nativePollOnce(int ptr, int timeoutMillis); private native void nativeWake(int ptr); ...... }
Looper：
public class Looper { private static final String TAG = "Looper"; static final ThreadLocal sThreadLocal = new ThreadLocal ();//与线程相关的模板类 private static Looper sMainLooper; //static类型，只有一个 final MessageQueue mQueue;//Looper持有的MessageQueue final Thread mThread; volatile boolean mRun; private Printer mLogging; ...... }
Handler：
public class Handler{//真正对处理Message的类：包括处理Message和将消息压到MessageQueue队列里 final MessageQueue mQueue; final Looper mLooper; final Callback mCallback; IMessenger mMessenger;...}
Looper的loop()循环：
public static void loop() { final Looper me = myLooper(); if (me == null) { throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread."); } final MessageQueue queue = me.mQueue; Binder.clearCallingIdentity(); final long ident = Binder.clearCallingIdentity(); for (;;) {//消息循环 Message msg = queue.next(); // might block//取得一个消息 if (msg == null) {//消息体为空则表示退出 // 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 Printer logging = me.mLogging; if (logging != null) { logging.println(">>>>> Dispatching to " + msg.target + " " + msg.callback + ": " + msg.what); } msg.target.dispatchMessage(msg);//调用消息指定的接收者Handler来处理它 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.recycle();//丢弃消息,循环再利用 } } Handler取消息，每次从sPool（Message里的static Message类型）取出一个Message，其调用的是static方法：
public static Message obtain(Handler h, int what, Object obj) { Message m = obtain(); m.target = h; m.what = what; m.obj = obj; return m; } Handler将消息压入消息队列：
public boolean sendMessageAtTime(Message msg, long uptimeMillis) { boolean sent = false; MessageQueue queue = mQueue; if (queue != null) { msg.target = this; sent = queue.enqueueMessage(msg, uptimeMillis);//调用MessageQueue的方法执行压入操作 } else { RuntimeException e = new RuntimeException( this + " sendMessageAtTime() called with no mQueue"); Log.w("Looper", e.getMessage(), e); } return sent; } 小结：Handler取消息是直接取（从sPool里），将消息压入MessageQueue是调用MessageQueue的方法。
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下面分析这些对象都是在什么时候创建的。
Looper是什么时候创建的？
每个线程都只有一个Looper对象。
1.普通线程（非主线程）
一个典型的自己创建的线程使用Looper如：
class LooperThread extends Thread{ public Handler mHandler; public void run(){ Looper.prepare(); mHandler= new Handler(){ public void handleMessage(Message msg){ //处理消息 } }; Looper.loop();//进入消息循环 }} 那么Looper是在什么时候创建的呢？看Looper.prepare()方法：执行完这个方法，就得到了一个本线程独有的Looper。
static final ThreadLocalsThreadLocal = new ThreadLocal ();//这是一个与线程先关的模板类 private static void prepare(boolean quitAllowed) { if (sThreadLocal.get() != null) {//若调用者所在宿主线程已经有Looper，则抛出异常 throw new RuntimeException("Only one Looper may be created per thread"); } sThreadLocal.set(new Looper(quitAllowed));//Java中的线程局部存储，即不同线 }//程都通过sThreadLocal得到自己的不同的Looper对象引用
private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed);//创建Looper要处理的消息队列 mRun = true; mThread = Thread.currentThread();//得到当前线程 }
那么Handler是如何与Looper关联起来的呢？看Handler的构造方法：
public Handler() { if (FIND_POTENTIAL_LEAKS) { final Class klass = getClass(); if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) && (klass.getModifiers() & Modifier.STATIC) == 0) { Log.w(TAG, "The following Handler class should be static or leaks might occur: " + klass.getCanonicalName()); } } mLooper = Looper.myLooper();//通过线程局部存储，获得本线程的Looper if (mLooper == null) { throw new RuntimeException( "Can't create handler inside thread that has not called Looper.prepare()"); } mQueue = mLooper.mQueue; mCallback = null; } public static Looper myLooper() {//获取与本线程相关的Looper return sThreadLocal.get(); }
小结：每个线程都通过sThreadLocal<Looper>来创建一个Looper（进而创建一个MessageQueue），再创建一个Handler（通过sThreadLocal），从而将Handler与Looper“绑定”在一起，形成一个完整的消息循环系统。
2.主线程ActivityThread
主线程是什么时候创建Looper的？在ActivityThread.java的main方法里有：
public static void main(String[] args) { SamplingProfilerIntegration.start(); CloseGuard.setEnabled(false); Process.setArgV0(" "); Looper.prepareMainLooper();//也是sThreadLocal.set(new Looper(quitAllowed))来创建一个线程所属的Looper，并通过sThreadLocal.get()将Looper赋给sMainLooper(private static Looper sMainLooper)，sMainLooper的作用就是，其他线程可以获取到主线程的Looper（因为sMainLooper是static，在主线程中赋值的）。 if (sMainThreadHandler == null) { sMainThreadHandler = new Handler(); } ActivityThread thread = new ActivityThread(); thread.attach(false); AsyncTask.init(); if (false) { Looper.myLooper().setMessageLogging(new LogPrinter(Log.DEBUG, "ActivityThread")); } Looper.loop(); throw new RuntimeException("Main thread loop unexpectedly exited"); }
Looper.loop()是一个static的方法，作为消息循环处理器，不断的取消息，分发消息。