CODEWITHSUNDEEP
javaswingthread-safetyevent-dispatch-thread
Tharwen
Tharwen

Reputation: 3107

How do you use the Event Dispatch Thread?

I learned about how swing isn't thread-safe. Delving deeper, I discovered that every modification to a swing component must be done on the Event Dispatch Thread in order to prevent various issues associated with multithreading. However, the information seemed to completely stop there. There doesn't seem to be a good tutorial that explains how to do this anywhere accessible on the internet.

Patching together information from code posted in relation to other issues, it seemed that I would have to put an untidy block of code around every single swing modification in my program (like this example from my own code):

try {
        SwingUtilities.invokeAndWait(new Runnable() {

            public void run() {
                setTitle("Frame title");
                setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                setVisible(true);

                setSize(800, 480);
                setLocationRelativeTo(null);
                setIconImage(Toolkit.getDefaultToolkit().createImage(ClassLoader.getSystemResource("Frame icon.png")));
            }
        });
    } catch (Exception e) {
        e.printStackTrace();
    }

Basically, is this right? Do I have to put that code (or the equivalent with invokeLater) around every modification to a Swing component in my code?

Also, why doesn't Swing do this automatically?

Upvotes: 10

Views: 12611

Answers (4)

Savvas Dalkitsis
Savvas Dalkitsis

Reputation: 11592

Not all your UI code must be part of a runnable in an invokeLater call. That is simply because a large part of your program will be run on the EDT anyway. You need to dispatch messages to the EDT only when you are on a different thread.

Upvotes: 3

ratchet freak
ratchet freak

Reputation: 48176

note that any code executed from event handlers is already run in the EDT (the Event in the acronym)

this means that for general use (while you don't mess with swingworkers and threadpools and such) you are always inside the EDT

and you can always query if you are in the EDT with SwingUtilities.isEventDispatchThread()

also note that in your code the call to invokeAndWait will fail and throw an error when you are in the EDT already

Upvotes: 4

Bill K
Bill K

Reputation: 62759

The trick is that when swing calls you it will ALWAYS be in the EDT, so you don't have to worry about it.

However if you are in a timer or an action triggered by some other external event, your main thread or any other thread you've created then yes, you have to use invokeLater or invokeAndWait.

In other words, yes swing does do "it" automatically. Needing to use invokeXx is so rare that if swing were to do it internally it would waste too much time.

Many java programmers never figure this out and it can cause some pretty nasty hard-to-find problems with drawing your GUI. I do wish swing threw an exception when you called it not using the EDT--Java would have a better reputation when it came to professional GUIs if it did because there would be less crap out there.

Upvotes: 8

Zak
Zak

Reputation: 7078

Basically, you dont draw or update the GUI from outside of the EDT. You use SwingUtilitis.invokeLater() from another thread to ensure the GUI drawing or updating code is run on the EDT.

Upvotes: 3

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