java - Running code on the main thread from a secondary thread?

Question

This is a general Java question and not an Android one first off!

I'd like to know how to run code on the main thread, from the context of a secondary thread. For example:

new Thread(new Runnable() {
        public void run() {
            //work out pi to 1,000 DP (takes a while!)

            //print the result on the main thread
        }
    }).start();

That sort of thing - I realise my example is a little poor since in Java you don't need to be in the main thread to print something out, and that Swing has an event queue also - but the generic situation where you might need to run say a Runnable on the main thread while in the context of a background thread.

EDIT: For comparison - here's how I'd do it in Objective-C:

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0UL), ^{
    //do background thread stuff

    dispatch_async(dispatch_get_main_queue(), ^{
        //update UI
    });
});

Thanks in advance!

Answer 1

There is no universal way to just send some code to another running thread and say "Hey, you, do this." You would need to put the main thread into a state where it has a mechanism for receiving work and is waiting for work to do.

Here's a simple example of setting up the main thread to wait to receive work from other threads and run it as it arrives. Obviously you would want to add a way to actually end the program and so forth...!

public static final BlockingQueue<Runnable> queue = new LinkedBlockingQueue<Runnable>();

public static void main(String[] args) throws Exception {
    new Thread(new Runnable(){
        @Override
        public void run() {
            final int result;
            result = 2+3;
            queue.add(new Runnable(){
                @Override
                public void run() {
                    System.out.println(result);
                }
            });
        }
    }).start();

    while(true) {
        queue.take().run();
    }
}