Java question

[Munky]

Right now I have a piece of code that gets a list of file names in a directory like shown in the code below:

However, doing it this way causes the GUI thread to block until all the files have been listed in a certain directory. My question is - how do I get a list of files in a directory so that the list is updated one element at a time, and the GUI can display the elements one at a time instead of waiting for the whole list to be completed?

 

[mundane]

I'd suggest spawning it off on its own thread, and having it callback a local method to update your UI. It will result in a UI update all at once, but at least your GUI thread won't block.

Pseudo-code follows; it probably won't compile as is, but you should get the main idea.

 

[Thyme]

Spawn a thread to get the list using a thread-safe data structure. Then in your main thread, you can check to see when there are new items in the structure and process based on that.

 

[kamper]

diego, doesn't you solution have two different threads accessing the gui? I don't believe that's allowed in swing (I assume that's what we're talking about).

Thyme, are you proposing to have the gui thread block on the thread-safe data structure (probably a queue)? If root.list() is the slow call, I don't think that will make any difference, as the call won't return anything until it's all done and then it'll be fast. I don't think it's possible to get a one-by-one update without using something different than root.list().

 

[Thyme]

Originally posted by: kamper
Thyme, are you proposing to have the gui thread block on the thread-safe data structure (probably a queue)? If root.list() is the slow call, I don't think that will make any difference, as the call won't return anything until it's all done and then it'll be fast. I don't think it's possible to get a one-by-one update without using something different than root.list().

Yes, you're right. He'll need to use a different method to populate the data structure, but I don't know what there is available.

 

[mundane]

Originally posted by: kamper
diego, doesn't you solution have two different threads accessing the gui? I don't believe that's allowed in swing (I assume that's what we're talking about).

I think it's highly discouraged, as the GUI work should be done within the event dispatch thread. I believe it still does work, although (or at least has in some of the quick GUIs I've made).

This (Java Tutorial, Swing/Threads) should provide the OP with a starting point on doing it the correct way.

 

[Munky]

Ok, thanks for the tips, guys. I had a hunch that I'd have to use something instead of root.list().

 

[kamper]

Originally posted by: diegoalcatraz
I believe it still does work, although (or at least has in some of the quick GUIs I've made).

That's a really bad strategy when it comes to threaded code 😉 The core .net gui components actually verify that you're using the right thread and throw an exception if not. I think that's a good idea.

 

[mundane]

Originally posted by: kamper

Originally posted by: diegoalcatraz
I believe it still does work, although (or at least has in some of the quick GUIs I've made).

That's a really bad strategy when it comes to threaded code 😉 The core .net gui components actually verify that you're using the right thread and throw an exception if not. I think that's a good idea.

Yeah, I ran into that with C#. Unfortunately, Java doesn't come right out and fail like that, so I had assumed (incorrectly) that such an approach was allowed. It wasn't until this discussion and finding subsequent documentation that I've fully realized my mistake.

 

[kamper]

I actually don't see why gui libraries don't make an effort to be thread safe. Yes, they say it's for performance reasons, but I have trouble believing that the overhead is that great, given that not a lot of time gets spent actually executing gui code. At the very least they should try to provide a mechanism to allow a programmer to access the components safely from any thread, even if it requires explicit synchronization.