Threads101

[granite fiber]

How come this code doesn't throw InterruptedException?? it should throw the exception but it

[vagrant saddle]

<@&987246399047479336> please have a look, thanks.

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[sweet mural]

Why should it throw ?

[minor sigil]

the doc of join doesn't say it would throw if the time runs out. if u expected that

[tough shuttle]

Click here to open the online javadoc in your browser

public final void join(long millis) throws InterruptedException

Waits at most millis milliseconds for this thread to terminate. A timeout of 0 means to wait forever. This method returns immediately, without waiting, if the thread has not been #start().

throws InterruptedException

if any thread has interrupted the current thread. The interrupted status of the current thread is cleared when this exception is thrown.

implNote

For platform threads, the implementation uses a loop of this.wait calls conditioned on this.isAlive. As a thread terminates the this.notifyAll method is invoked. It is recommended that applications not use wait, notify, or notifyAll on Thread instances.

param millis

the time to wait in milliseconds

throws IllegalArgumentException

if the value of millis is negative

[minor sigil]

see, nothing here says it would throw

as u can see, it will only throw if someone interrupts the thread

u never do that

[granite fiber]

Smh, idk the slides says:

[granite fiber]

How does that occur? "someone interrupts the thread"

[graceful cedar]

You can call something like Thread.getGroup() and interrupt

Or something like that

Definitely butchered the name

[granite fiber]

Also, is this wrong? or did I interpret it wrong?

[graceful cedar]

Ah it’s

[granite fiber]

@graceful cedar But usually with interruption we don't use that in code, it's based on the OS when it wants to interrupt running threads?

[graceful cedar]

Thread.currentThread().getThreadGroup().interrupt()

If you run 2 threads and have one interrupt the group, it will throw this error

[tough shuttle]

Click here to open the online javadoc in your browser

public void interrupt()

Interrupts this thread.

Unless the current thread is interrupting itself, which is always permitted, the #checkAccess() method of this thread is invoked, which may cause a SecurityException to be thrown.

If this thread is blocked in an invocation of the #wait(), #wait(long), or wait(long, int)

[minor sigil]

u just call the interrupt method on the thread u want to interrupt

couldn't be any simpler...

[minor sigil]

it is wrong. u should get used to consulting the official doc first

[granite fiber]

I tried calling .interrupt() but for some reason "hi" got printed?

[graceful cedar]

interrupt() does not actually kill the thread

[minor sigil]

it just sets the interrupted flag

[granite fiber]

Oh

[minor sigil]

which the thread can now check with interrupted()

[graceful cedar]

public class Main implements Runnable{
    public static void main(String[] args) throws InterruptedException {
        for(int i = 0; i < 2; i++){
            Thread thread = new Thread(new Main());
            thread.start();
            Thread.sleep(1);
        }
    }

    @Override
    public void run() {
        for(int i = 0; i < 100; i++){
            System.out.println("hey");
            Thread.currentThread().getThreadGroup().interrupt();
        }
    }
}

[minor sigil]

and then maybe use that to stop activity

[graceful cedar]

oh it's so bad

sorry let me edit

[minor sigil]

some methods like sleep will throw the exception if interrupted in the meantime

or join

but other than those, interrupting itself just sets a flag

[minor sigil]

why do u interrupt via the group? u can literally call it on the thread itself

[sweet mural]

Note that thread group is legacy and shouldn't be used, also most of its methods are deprecated for removal

[graceful cedar]

If you just call Thread.currentThread().interrupt() it won't throw the exception

[sweet mural]

No

That's unrelated

It will only throw an exception the next time you do a sleep of any kind

I use it the Jshell backend project if you want to see an use case

[minor sigil]

the idea of interrupt is that a flag is set, which the thread can now periodically check and then react to it. for example stop its while(true) loop and do a clean shutdown

[granite fiber]

If the interrupted statute is set to true, what does that mean? that this thread doesn't mind getting interrupted by other threads?

[minor sigil]

but some methods adopted this concept to stop a blocking waiting action exceptionally

[minor sigil]

the thread could now check that flag and react to it

how u use that concept is totally up to u

methods like sleep or join use it to stop that action

there is no magic here. those methods literally have if (interrupted) throw coded into some loop

[graceful cedar]

ah is this what you meant

    @Override
    public void run() {
        try {
            Thread.currentThread().interrupt();
            Thread.sleep(10);
        } catch (InterruptedException e) {
            System.out.println(":(");
        }
    }

[granite fiber]

Do I place join method before the start method or vice versa?

What do most ppl do?

I figured I should always have the join before the start.. but idk if that's how am supposed to do it or not..

^ on the main method

If I placed .join() inside the run method, it's literally giving me an infinite loop..

[graceful cedar]

I'm pretty sure the reason for this

is that you are joining your thread inside the run

which will just wait until that thread is finished

which it never finishes because it's waiting

[granite fiber]

So it's always best to join the thread where the thread is started? if yes, should I do it before or after the .start() call?

[minor sigil]

if u join after starting, ur rendering threads pointless

the purpose of a thread is to compute sth while u can do sth else

if u literally wait on it, u didn't gain anything

[graceful cedar]

Yeah, do you understand the purpose of join()?

List<Thread> threads = new ArrayList<>();
for(int i = 0; i < 3; i++){
    Thread thread = new Thread(new Main());
    threads.add(thread);
    thread.start();
    thread.join();
}

Let's say we have this dumby code

[granite fiber]

Waiting for a specific thread to complete

Or terminates

[graceful cedar]

Looking at the code I just posted

what do you think the problem is?

[granite fiber]

Umm when a new thread is created it will do all its work and then when it terminates, another thread will be created which will do all its work until termination and then 3rd thread will be created...

[graceful cedar]

Yeah exactly

[granite fiber]

Is that a problem?

[graceful cedar]

So what Zab said, it pretty much negates the purpose of creating threads

I created three threads, but really only getting the performance of one thread

[granite fiber]

Right

So what's the point of using join?

[graceful cedar]

Zab might chew me out on this

but

Let's say you need the completion of the thread parts done before moving on

public static void main(String[] args){
    List<Thread> threads = new ArrayList<>();
    for(int i = 0; i < 3; i++){
        Thread thread = new Thread(new Main());
        threads.add(thread);
        thread.start();
    }
    threads.forEach(x -> {
        try {
            x.join();
        } catch (InterruptedException e) {
            System.out.println(":(");
        }
    });
}

this allows each thread to be created and ran

but it won't continue main, as it will wait for each thread to finish

So let's say you have a 1000x1000 matrix, (pretty big)

You create 4 threads to handle traversing the matrix, giving each thread a quarter of the matrix for calculation purposes

This solution allows each thread to run at the same time

If you call join() like the previous example, each thread is not doing calculations at the same time

[granite fiber]

So can I say the idea of using join(), is to prevent the interference of more than one thread on the same data that they share, so that I don't get inaccurate results?

[graceful cedar]

There are some cases like the above code that I posted in which join() is beneficial, but I wouldn't say it is to protect race cases or whatever

I'm not an expert on threads so maybe someone else can chime in, but if you are using join() such that each thread takes a turn, and not running it together, the results will not be faster

In that case, you might as well only have one thread

[sweet mural]

join will make the current thread wait until the other thread finishes

[graceful cedar]

There are better ways to prevent accessing the same data such as locks or semaphores or whatever, but I am pretty sure that stuff is outdated

[granite fiber]

No for real, isn't that the point of using join? to protect from the race condition? besides using locks/synchronization..

[graceful cedar]

For multithreading, if you are sharing data, you want the critical part of the code to be the least. If you are using join, the entire code is the critical zone

[sweet mural]

no

join is useful because you usually have a master thread

and you want to process stuff in parallel

and then once all of them are over, collect the result

[graceful cedar]

^ Like the code I posted above

[sweet mural]

how do you make so you wait that all the tasks are over ? by using join

[granite fiber]

👍🏻 👍🏻 👍🏻 thanks guys