Android Handler实例线程安全

Question

Is Android Handler class instance itself thread safe?

For example, is it safe to post from multiple threads to the main thread with the example code below, or one has to instantiate two handlers (one in each thread)?

Handler singleHandler = new Handler(Looper.getMainLooper());

new Thread(new Runnable() {
    public void run() {
        Runnable runnable1 = new MyRunnable();
        singleHandler.postDelayed(runnable1, 100);
    }
}).start();

new Thread(new Runnable() {
    public void run() {
        Runnable runnable2 = new MyRunnable();
        singleHandler.postDelayed(runnable2, 150);
    }
}).start();

There's a similar question on SO, but it deals with thread safety of message processing (not posting), which is slightly different Are android handlers thread safe?

Answer 1

Yes, it completely fine to post from any numbers of threads to single instance of Main Handler. As @Onik mentioned Handler job is to deliver the message to mentioned Looper , and there is at any given time only one MainLooper for your process. So a single instance of handler will post to that. To support answer, if you look at View.Java , it initialises one mHandler inside AttachInfo and pretty much uses that to post all the messages to MainLooper .