Question

A Pattern that I’ve seen a few times now and still am unsure if it’s correct: I need to invoke some code on the SWT UI thread via Display.syncExec. The code that runs in the UI thread catches a checked exception and wants to propagate this exception to the waiting thread. The pattern looks like this:

SomeCheckedException[] exc = new SomeCheckedException[] { null };
display.syncExec(new Runnable(){
   public void run(){
      try {
         new Work().something();
      } catch (SomeCheckedException e){
         exc[0] = e;
      }
   }
});

if (exc[0] != null){
 //..
}

Even if there’s no real concurrency here, I still think that this is not really safe in terms of visibility: It could well happen that the UI thread stores an exception, but the invoking thread won’t see this and still read ‘null’ when accessing the array after taking control again. Am I right? Is the best way then just to catch SWTException and check its throwable field via instanceof? EDIT: Hm, that won’t work so easy if it’s a checked exception. I’d need to throw the SWTException from the catch block myself. Better solutions?

Thanks for your help and comments.

Answer 1

This is the sort of case AtomicReference was made for:

  void foo() throws SomeCheckedException {
    final AtomicReference<SomeCheckedException> exRef = new AtomicReference<>();
    display.syncExec(new Runnable() {
      public void run() {
        try {
          new Work().something();
        } catch (SomeCheckedException e) {
          exRef.set(e);
        }
      }
    });
    SomeCheckedException ex = exRef.get();
    if (ex != null) {
      throw ex;
    }
  }