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();     } } 

Answer 2

In case you are on Android, using a Handler should do the job?

new Handler(Looper.getMainLooper()).post(new Runnable () {     @Override     public void run () {         ...     } });