Do different handlers use the same MessageQueue in the main thread?

Analysis of this problem requires a certain understanding of MessagQueue creation, check the source code in Looper, MessageQueue creation code is as follows

  private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
    }
Copy the code

This is the only Looper method that creates messageQueue. There is only one call to this method: perpare(Boolean quitAllowed)

  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));
    }
Copy the code

From the RuntimeException error, it can be concluded that each thread can have only one Looper and only one MessageQueue. So Handler and Looer Message are many-to-one. Perpare (Boolean quitAllowed) calls perpare(Boolean quitAllowed) in two places. One is the perpare() that creates the Handler in the child thread.

 public static void prepare(a) {
        prepare(true);
    }
Copy the code

The other place to call is prepareMainLooper(), which is called in the main method of activityThread.class and SystemServer.class.

 public static void prepareMainLooper(a) {
        prepare(false);
        synchronized (Looper.class) {
            if(sMainLooper ! =null) {
                throw new IllegalStateException("The main Looper has already been prepared."); } sMainLooper = myLooper(); }}Copy the code