Java Community | Help. Code. Learn.•2mo ago

How would I have a thread wait on a lambda?

This is my first time messing with threads. The project takes user input with a lambda, so I need to wait until that lambda gets valid data to resume the thread. From my research I havent found a good way to do that other than while (var == null) {} which doesn't seem ideal.

29 Replies

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nikcho-kouhai•2mo ago

what input are you taking?

blockgoblin31OP•2mo ago

Right now just a string from terminal, but thats for testing.

nikcho-kouhai•2mo ago

well what do you mean by valid data?

blockgoblin31OP•2mo ago

oh it needs to be one of three strings(selecting an option) specifically im looking for one of "draft", "set", or "random"

nikcho-kouhai•2mo ago

why don't you just check if the input is equal to one of those things ?

blockgoblin31OP•2mo ago

because I can't stop the main thread until the user gives input(This is eventally going to be server side code on something with multiple users)

nikcho-kouhai•2mo ago

so let me see if I got this right you have a main thread which shouldn't stop running so you have delegated the user input to another thread and you are now wondering how to notify the main thread when valid input has been provided?

blockgoblin31OP•2mo ago

no, user input is handled on the main thread. I'm passing the thing that handles user input a lambda to run the message through and I want to continue the side thread when that lambda has gotten valid input

nikcho-kouhai•2mo ago

you just said you can't stop the main thread until the user gives input

blockgoblin31OP•2mo ago

I'm not

nikcho-kouhai•2mo ago

can the main thread block or can it not block?

blockgoblin31OP•2mo ago

the main thread is getting a lambda that extends a specific interface for it to run messages through as a simplified example:

SelectionType type;
//runner handles the user input
runner.captureMessagesFrom(user, (message) -> {
    switch (message.toLowerCase()) {
        case "draft" -> type = SelectionType.DRAFT;
        case "set" -> type = SelectionType.SET;
        case "random" -> type = SelectionType.RANDOM;
        default -> return false;
    }
    return true;
}
//use type for something

SelectionType type;
//runner handles the user input
runner.captureMessagesFrom(user, (message) -> {
    switch (message.toLowerCase()) {
        case "draft" -> type = SelectionType.DRAFT;
        case "set" -> type = SelectionType.SET;
        case "random" -> type = SelectionType.RANDOM;
        default -> return false;
    }
    return true;
}
//use type for something

nikcho-kouhai•2mo ago

who runs this function? is it the main thread or some other thread

blockgoblin31OP•2mo ago

its run on the main thread when it gets a message and it gets the message from side threads that are actually doing the listening for user input

nikcho-kouhai•2mo ago

right so what is the problem with validating the input before sending the message off to the main thread?

blockgoblin31OP•2mo ago

the problem is I need to know that type is not null before the code continues, and that requires the specific user to send a valid message. the main thread can't just wait forever because then we wouldnt handle messages from other users so I need the code to wait until the value is set, and I'm not sure how to do that

nikcho-kouhai•2mo ago

this is confusing

blockgoblin31OP•2mo ago

(well other than while (type == null) {})

nikcho-kouhai•2mo ago

in the first place you can't set type from inside the lambda

blockgoblin31OP•2mo ago

yeah I know, that was just quicker for my example

nikcho-kouhai•2mo ago

how does the main thread handle this message? Is it running an event loop? like run me through what happens when some thread receives some data from somewhere why can't it validate that data before sending it off to the main thread

blockgoblin31OP•2mo ago

So let me phrase this differently(I don’t know exactly how the main code works, I’m writing an addon with an api), all messages go through the main thread and this is triggered by a message. So for other messages to be processed the main thread has to be freed, and this needs another message before it can finish its task.

nikcho-kouhai•2mo ago

so im assuming the main thread is ran by this api then? again, what is the problem with validating the input in the current thread before captureMessageFrom wait a second do you want the side thread to wait for type to be set in this example? in other words:

SelectionType type;
//runner handles the user input
runner.captureMessagesFrom(user, (message) -> {
    switch (message.toLowerCase()) {
        case "draft" -> type = SelectionType.DRAFT;
        case "set" -> type = SelectionType.SET;
        case "random" -> type = SelectionType.RANDOM;
        default -> return false;
    }
    return true;
}
//wait until type !=null
//use type for something

SelectionType type;
//runner handles the user input
runner.captureMessagesFrom(user, (message) -> {
    switch (message.toLowerCase()) {
        case "draft" -> type = SelectionType.DRAFT;
        case "set" -> type = SelectionType.SET;
        case "random" -> type = SelectionType.RANDOM;
        default -> return false;
    }
    return true;
}
//wait until type !=null
//use type for something

is this what you are trying to do?

blockgoblin31OP•2mo ago

Yes That lambda is where we get the input (it’s the message parameter), and it’s run on the main thread later

nikcho-kouhai•2mo ago

okay you can do this multiple ways the simplest is with a CountDownLatch

        SelectionType type;
        final CountDownLatch latch = new CountDownLatch(1);

        runner.captureMessagesFrom(user, (message) -> {
            try{
                switch (message.toLowerCase()) {
                    case "draft" -> type = SelectionType.DRAFT;
                    case "set" -> type = SelectionType.SET;
                    case "random" -> type = SelectionType.RANDOM;
                    default -> return false;
                }
                return true;                
            }finally {
                latch.countDown();
            }

        });
        try {
            latch.await();
        } catch (InterruptedException e) {
            //handle interrupt as you see fit
        }

//use type for something

        SelectionType type;
        final CountDownLatch latch = new CountDownLatch(1);

        runner.captureMessagesFrom(user, (message) -> {
            try{
                switch (message.toLowerCase()) {
                    case "draft" -> type = SelectionType.DRAFT;
                    case "set" -> type = SelectionType.SET;
                    case "random" -> type = SelectionType.RANDOM;
                    default -> return false;
                }
                return true;                
            }finally {
                latch.countDown();
            }

        });
        try {
            latch.await();
        } catch (InterruptedException e) {
            //handle interrupt as you see fit
        }

//use type for something

the thread will block on await() until another calls countDown() you can find out more about the CountDownLatch here: https://docs.oracle.com/javase/6/docs/api/java/util/concurrent/CountDownLatch.html I suggest before jumping into multi-threaded code to have a look at synchronization blocks, locks and atomic variables it's also important to keep in mind that depending on your code the thread on await() might block indefinitely if the main thread shuts down before processing the message just learn a bit more about this before doing multi-threaded stuff

blockgoblin31OP•2mo ago

Thank you, this worked!

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How would I have a thread wait on a lambda?

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