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 e

Answer 1

An old discussion, but if it is a matter of sending request to the main thread (an not the opposite direction) you can also do it with futures. The basic aim is to execute something in background and, when it is finished, to get the result:

public static void main(String[] args) throws InterruptedException, ExecutionException {
    // create the task to execute
    System.out.println("Main: Run thread");
    FutureTask task = new FutureTask(
            new Callable() {

                @Override
                public Integer call() throws Exception {
                    // indicate the beginning of the thread
                    System.out.println("Thread: Start");

                    // decide a timeout between 1 and 5s
                    int timeout = 1000 + new Random().nextInt(4000);

                    // wait the timeout
                    Thread.sleep(timeout);

                    // indicate the end of the thread
                    System.out.println("Thread: Stop after " + timeout + "ms");

                    // return the result of the background execution
                    return timeout;
                }
            });
    new Thread(task).start();
    // here the thread is running in background

    // during this time we do something else
    System.out.println("Main: Start to work on other things...");
    Thread.sleep(2000);
    System.out.println("Main: I have done plenty of stuff, but now I need the result of my function!");

    // wait for the thread to finish if necessary and retrieve the result.
    Integer result = task.get();

    // now we can go ahead and use the result
    System.out.println("Main: Thread has returned " + result);
    // you can also check task.isDone() before to call task.get() to know
    // if it is finished and do somethings else if it is not the case.
}

If your intention is to do several stuff in background and retrieve the results, you can set some queues as said above or you can split the process in several futures (starting all at once or starting a new one when needed, even from another future). If you store each task in a map or a list, initialized in the main thread, you can check the futures that you want at anytime and get their results when they are done.