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。
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<IdleHandler> mIdleHandlers = new ArrayList<IdleHandler>();
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<Looper> sThreadLocal = new ThreadLocal<Looper>();//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 ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();//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<? extends Handler> 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("<pre-initialized>");
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。
原文：http://www.cnblogs.com/littlefishxu/p/3985130.html