Re: Help! Swing & Threads

Thomas Hawtin <>
Thu, 01 Jun 2006 09:20:49 +0100
Mark Space wrote:

Basically, Swing makes a special thread to invoke various Swing events
such as ActionEvents, yes?

Well, AWT does. The Event Dispatch Thread (EDT). The significant methods
are java.awt.EventQueue isDispatchThread (useful for asserts),
invokeLater (useful) and invokeAndWait (watch out for deadlocks).

So what happens when you of your listeners invokes a long action, or
blocks? Doesn't the rest of the Swing components have to stop until
that listener unblocks? It wouldn't be safe for Java to just make a new
thread and call some more events...

The GUI blocks and doesn't so much as repaint, so don't do that.
Dispatching events in multiple threads would be exceptionally difficult
to code for.

Ok, but I see a lot of examples where a menu item, File->Open for
 example, invokes JFileChooser which does indeed block. (For example,
O'Reilly's Learning Java does this in it's example for the
JFileChooser.) What the heck is going on here?

JFileChooser creates a thread to do the directory listing in. That
thread then sends events to the EDT to update the GUI model.

I guess my question really is serveral questions:

1. Is invoking from a listener JFileChooser safe?

Yes. In fact JFileChooser should only be used from the EDT.

2. Should I be spawning my on thread instead?

You may wish to for your own file access. However, you need a decision
as to whether you really need to. Top quality of implementation
applications wont block the EDT, but you probably wont notice a little
local file I/O or database access. You can still write top quality code
for programs that block. An application that blocks will be cheaper to
write, and probably perform better and more reliably.

3. Does Swing do anything fancy or special with JFileChooser to prevent
deadlocks when the listener thread blocks?

If the EDT blocks, then Swing components wont be doing anything.

4. What the best general case solution for "long tasks" inside listeners?

Take the task out of the listener. Run it in a different thread
(possibly from a thread pool). Send events with EventQueue.invokeLater
to notify the GUI side of changes.

5. Is there a good source for questions like this one? Most of the
tutorials seem very simplified.

I think the Swing threads tutorial tells you what you need to know about
threads in Swing. Threads in general is a difficult subject. The
traditional recommended book is Concurrent Programming in Java: Design
Principles and Patterns by Doug Lea. There is now Java Concurrency in
Practice by Brian Goetz, Joshua Bloch, Joseph Bowbeer, Doug Lea, David
Holmes and Tim Peierls, but I'm still waiting for Amazon to dispatch myu

Tom Hawtin
Unemployed English Java programmer

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