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Java Community | Help. Code. Learn.•3mo ago

Asyncronous task queue

I have one thread that delegates tasks over to another thread, which runs those tasks in a queue. These tasks are blocking tasks (database), that I want to be run in the correct order, first in first out. Currently using BlockingQueue, but when testing it using junit, it can freeze the whole test. That's because after each test, this thread is waited on until it finishes. I have not found a good way to enable/disable the queue How would one do this properly?

33 Replies

JavaBot•3mo ago

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dan1st•3mo ago

Can you show the relevant code? Just the code that's necessary to reproduce the issue

ThorinwasherOP•3mo ago

Sure, don't laugh at it though 🤣

dan1st•3mo ago

And if possible, please also show where exactly the code blocks the tests - Is it somewhere during the test or after running the tests?

ThorinwasherOP•3mo ago

Oh, too large...

dan1st•3mo ago

I am fairly sure your code is better than the code of many other people - after all you are writing tests you can also try to upload the files

ThorinwasherOP•3mo ago

Good point, I'm not educated enough in concurency though Ah

dan1st•3mo ago

or ideally only include the relevant code

ThorinwasherOP•3mo ago

let me do that then

dan1st•3mo ago

concurrency is hard

ThorinwasherOP•3mo ago

StargateQueuedAsyncT...

dan1st•3mo ago

The most complicated chapter of the JVM specification (which specifies how JVMs have to work) is pretty much about what should happen in multithreaded environments

ThorinwasherOP•3mo ago

I think you need more info than that though

dan1st•3mo ago

Can you show the test and where it blocks?

ThorinwasherOP•3mo ago

Sure, this is one test that blocks everything, but that relates to the test that ran before it It's quite layered though

    @Test
    void portalInvalidBlockPlaceTest() {
        Location bottomLeft = new Location(world, 0, 7, 0);
        Location insidePortal = new Location(world, 0, 9, 0);
        Block signBlock = PortalBlockGenerator.generatePortal(bottomLeft);
        Block irisBlock = new Location(world, 1, 9, 0).getBlock();


        ((Directional) signBlock.getBlockData()).setFacing(BlockFace.SOUTH);
        blockEventListener
                .onSignChange(new SignChangeEvent(signBlock, player, new String[]{"test", "", CUSTOM_NETNAME, ""}));

        BlockPlaceEvent event = new BlockPlaceEvent(irisBlock, irisBlock.getState(), irisBlock,
                new ItemStack(Material.ANDESITE), player, false);
        blockEventListener.onBlockPlace(event);
        blockEventListener.onBlockBreak(new BlockBreakEvent(insidePortal.getBlock(), player));
        Assertions.assertTrue(event.isCancelled());
    }

    @Test
    void portalInvalidBlockPlaceTest() {
        Location bottomLeft = new Location(world, 0, 7, 0);
        Location insidePortal = new Location(world, 0, 9, 0);
        Block signBlock = PortalBlockGenerator.generatePortal(bottomLeft);
        Block irisBlock = new Location(world, 1, 9, 0).getBlock();


        ((Directional) signBlock.getBlockData()).setFacing(BlockFace.SOUTH);
        blockEventListener
                .onSignChange(new SignChangeEvent(signBlock, player, new String[]{"test", "", CUSTOM_NETNAME, ""}));

        BlockPlaceEvent event = new BlockPlaceEvent(irisBlock, irisBlock.getState(), irisBlock,
                new ItemStack(Material.ANDESITE), player, false);
        blockEventListener.onBlockPlace(event);
        blockEventListener.onBlockBreak(new BlockBreakEvent(insidePortal.getBlock(), player));
        Assertions.assertTrue(event.isCancelled());
    }

dan1st•3mo ago

And what's StargateAsyncTask? So it only happens when running multiple tests? Do you know what "suspending" is when it comes to debugging?

ThorinwasherOP•3mo ago

It's an task run in another thread, with some wrapper stuff. (mostly because I want to be sure the tasks get executed whenever the srerver shuts down)= yep

dan1st•3mo ago

When portalInvalidBlockPlaceTest starts, is the queue empty?

ThorinwasherOP•3mo ago

Yeah. Might be a good idea

dan1st•3mo ago

Also is the thread processing the tasks doing something at that point?

ThorinwasherOP•3mo ago

Let's see, good idea to check that

dan1st•3mo ago

My suggesion is the following: - Run both tests so that you run into the blocking issue - Wait until you are sure the blocking issue happens - Suspend the thread running the test and check what it's doing - Check where it has been suspended (Did the test start? Is it somewhere inside the test? Did the test finish?) - Check the content of the BlockingQueue - Check what the working thread is doing (Is it processing a task from the previous thread? Is it waiting for a task (maybe it's waiting even though there are elements in the queue)?)

ThorinwasherOP•3mo ago

Alright, btw do you have a good source I can read up on concurrency? (Your recomendation here will definitely help, thanks btw)

dan1st•3mo ago

If you are into books: There's a book called "Java Concurrency in Practice"

ThorinwasherOP•3mo ago

Alright, thanks (Like my main fallback has just been to let external classes handle this. But in this case, I think I need to get better at concurency)

dan1st•3mo ago

I would also recommend doing that for most cases as it's the best way to avoid issues but it's still good to know what these classes do, what they can and cannot do and how to fix issues btw if you did that and you still need help with it, just say what the result of these things is

ThorinwasherOP•3mo ago

Okay. (need to eat some dinner first) I think I know what's happening now. I made a disable queue item and an enable queue item. These items sets the value of asyncQueueThreadIsEnabled to true or false. It's a way to avoid dealing with concurrency issues. Now to the actual issue this is how I'm cycling through the queue:

do {
    try {
        Runnable runnable = asyncQueue.take();
        runnable.run();
    } catch (InterruptedException e) {
         Thread.currentThread().interrupt();
    } catch (Exception e) {
         Stargate.log(e);
    }
} while (asyncQueueThreadIsEnabled || !asyncQueue.isEmpty());

do {
    try {
        Runnable runnable = asyncQueue.take();
        runnable.run();
    } catch (InterruptedException e) {
         Thread.currentThread().interrupt();
    } catch (Exception e) {
         Stargate.log(e);
    }
} while (asyncQueueThreadIsEnabled || !asyncQueue.isEmpty());

If per say the last item would be a enable queue item in the queue. Then the boolean switch asyncQueueThreadIsEnabled would be true, therefore blocking the thread in the next BlockingQueue#take() call.

dan1st•3mo ago

If the thread is interrupted, you should stop looping interrupts typically mean asking the thread to stop In your case, if a thread is interrupted, it will continue looping but take() will fail every time if the queue is empty because the thread is interrupted

ThorinwasherOP•3mo ago

Oh... that's stupid on my end. I just thought it threw an exception :facepalm: That might partly fix the issue, does still require the thread to be interupted though

dan1st•3mo ago

you can still do that at the end of the test if applicable but note that this may require restarting the thread

ThorinwasherOP•3mo ago

Yes, that's how I want it to be

dan1st•3mo ago

check what the test is waiting for then you need the Thread object of the worker thread and call interrupt() on it

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