前面两篇文章分别讲了Handler的消息发送和Message入列、Message的创建和Message在队列中的存在形式，那么MessageQueue是怎么来的？因为我们在创建Handler和发送Message时并没有创建MessageQueue，那这个消息队列从何而来呢？上源码:
//省略了部分代码
public Handler(Callback callback, boolean async) {
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}以上为Handler的构造方法，Handler的构造方法共有7个，大多数构造方法都指向了这个两个参数的构造方法。从这个构造方法中，我们看到了mQueue = mLooper.mQueue说明消息队列MessageQueue为Looper的一个成员变量，mLooper是有Looper类中的静态方法Looper.myLooper()获取:
//Looper类
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}在Looper.myLooper()方法中调用sThreadLocal.get()，源码继续：
//ThreadLocal类
public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null)
return (T)e.value;
}
return setInitialValue();
}看到这里其实我是懵逼的(相信大家也都有点懵逼吧)。在这个方法中去获取了当前的线程，看到这里我们至少知道Looper对象是和线程相关的。再来看看这段代码:
//Looper类
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}在Looper.prepare()方法中我们看到了sThreadLocal.set(new Looper(quitAllowed));，结合上面的sThreadLocal.get()至少可以看出一个是放入，另一个方法是取出。那为什么我们在没有调用Looper.prepare()方法时还能拿到Looper对象呢？准确的说在主线程为什么我们能拿到Looper对象呢。为此查了下资料。以下资料来自Android中为什么主线程不会因为Looper.loop()里的死循环阻塞？
我们知道APP的入口是在ActivityThread,一个Java类,有着main方法,而且main方法中的代码也不是很多.
public static void main(String[] args) {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
SamplingProfilerIntegration.start();
// CloseGuard defaults to true and can be quite spammy. We
// disable it here, but selectively enable it later (via
// StrictMode) on debug builds, but using DropBox, not logs.
CloseGuard.setEnabled(false);
Environment.initForCurrentUser();
// Set the reporter for event logging in libcore
EventLogger.setReporter(new EventLoggingReporter());
AndroidKeyStoreProvider.install();
// Make sure TrustedCertificateStore looks in the right place for CA certificates
final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
TrustedCertificateStore.setDefaultUserDirectory(configDir);
Process.setArgV0("<pre-initialized>");
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}从上述代码中可以看出，APP在启动时就会在主线程中调用Looper.prepareMainLooper()，所以在主线程中一开始就存在着Looper对象，从Looper.prepare()方法中可以看出，一个线程中只能存在一个Looper对象，所以我们在主线程发送消息时并不需要自己创建新的Looper对象，但是在工作线程中使用Handler就必须手动调用Looper.prepare()方法。这下我们知道了Looper的来源，MessageQueue的由来也非常明确了。
//Looper类
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}在Looper.prepare()方法中调用了Looper的构造方法，而就在Looper的构造方法中创建了MessageQueue。