Post a Runnable to another thread in plain java

Question

In Android we have Handler#post(Runnable) method to post some code to the main thread from another

Is it possible to make same thing in plain java (for instance while using swing)?

Explaining the question: Assume we have two threads: ThreadA and ThreadB. Both are started already and running side-by-side. And ThreadB wants ThreadA to invoke some method (again, ThreadA is already running).

Addition optional question (you may not answer it): And if it's possible, someone explain me how does exactly Handler do in Android. How it's able to post some code to MainThread? What is a purpose of the Looper?

Answer 1

Addressing your additional question:

You said "In Android we have Handler#post(Runnable) method to post some code to the main thread from another"

It is not exactly correct. You can 'post some code' from any thread A to any thread B provided that thread B is initialized as a Looper and the thread A has a reference to a Handler for the target thread B.

It is very convenient when you need to do something on the UI thread because the UI thread already has a Looper and you can retrieve it from nearly everywhere. The static method Looper.getMainLooper is a way to get a Looper for the main thread. If you initialize a Handler with this Looper you can post a Runnable or send a Message to it (though if you post Runnable it also gets wrapped into a Message) and it will be executed on the UI thread.

Looper, as the name hints, is basically running a non-terminating loop for a thread. The Looper has an associated MessageQueue which it constantly checks for new Messages. Via the Handler initialized with a Looper you can enqueue Messages on this thread. The Messages are processed in a sequential order, depending on the when field of a Message.

Here's a basic implementation of a Looper thread:

  class LooperThread extends Thread {
      public Handler mHandler;

      public void run() {
          Looper.prepare();

          mHandler = new Handler() {
              public void handleMessage(Message msg) {
                  // process incoming messages here
              }
          };

          Looper.loop();
      }
  }

I suggest you read the Chapter 5 of Meike G.B. Android Concurrency. It will give you a comprehensive insight into the Looper/Handler framework. It is also great to browse the source code while you are reading, it is rather simple and self-explanatory.