我可以运行两个EDT吗？

Question

This is mostly a theoretical question. The example below is what made me think of it, but it may not be the best example. Please assume that the reason's below are concrete, and can't for the moment be worked around.

The Program I have been running has an optional Debug frame that is created on the program startup, and it made visible by the user pressing buttons/keyboard shortcut.

My issue is that as I have lengthy processes on the EDT, if it is hanging for some reason or fails I'd like to see straight away, and not wait for that thread to end running, to update the Debug Log Frame.

My solution would be to have two separate EDT for two separate GUIs that are updated by a separate thread.

Is this possible, or am I not able to do so? I haven't found any online resourcethat would show me how to. I know that EDTs should be single threaded, but if I keep the threads disentangled, can I have two? Please?

Answer 1

The answer is simple: No you cannot have 2 EDTs , that is not possible. But you are not stuck with a frozen GUI, you have some options available.

First and foremost, two important rules:

1. Never do lengthy calculations in the EDT. Ever.
2. Never manipulate Swing components from outside the EDT. Ever.

Breaking the first rule will result in your GUI being frozen and no events of any sort being processed during that time. This includes updates to the GUI you want to do during the calculations which will not appear until after the calculations are done.

The latter is often ignored which will go by unnoticed most of the time, but it can - and will - bite you in the ass and then most of the time it is a huge pita to go back and fix it. So do it the correct way from the start. What can happen? Components can suddenly display in a broken state, they may appear white, or the whole application can freeze because there is a deadlock between the EDT and your other threads (been there, done that). Adhere to the Oracle Swing Threading Policy !

So, how to avoid doing lengthy calculations on the EDT after for example the user pressed a button? Options:

• use a SwingWorker . Advantage: Has a done() method you can use which is automatically executed in the EDT once the worker is done. Also implements the Future interface and can therefore be used to return a result.
• you can just create your own Runnable and do the calculations in there.
• use any other way Java provides for parallel execution.

Ok, and how to avoid ever manipulating GUI from outside the EDT?

• call SwingUtilities.invokeLater() and execute Swing manipulations in there.
• use the SwingWorkers done() method as described above.

If you follow these two rules, your Swing GUI will be in a much better shape and you can focus more on your application development rather than Swing issues.