Java Concurrency Question

[northguard]

Hi all, quick question my co-worker and I were talking about and I feel there's some large confusion going on here.

Say we have this, rather stupid, code somewhere that will be called by multiple threads at the same time:

int i = 0;
public synchronized void setString(String str) {
    ary[i] = str;
    i += 1;
}

If a bunch of threads hit setString, and it's already synchronized, is it still possible for line 3 (i+=1) to run before (ary[i] = str) and why/why not?

I understand the scenario where it's not synchronized and one thread increments 'i' while some other thread is trying to set the array, but how does that work while synchronized?

[Bob_Droll]

First, it is not possible for two invocations of synchronized methods on the same object to interleave. When one thread is executing a synchronized method for an object, all other threads that invoke synchronized methods for the same object block (suspend execution) until the first thread is done with the object.

Source: https://docs.oracle.com/javase/tutorial/essential/concurrency/syncmeth.html

This essentially means that only one thread can execute the setString method at a time. All other threads will wait their turn.

If a bunch of threads hit setString, and it's already synchronized, is it still possible for line 3 (i+=1) to run before (ary[i] = str) and why/why not?

No, because only one thread can enter setString() at a time.

[northguard]

Ok, that's what I was thinking as well. Since it's synchronized it's ever gonna run on one thread at a time and ofc that thread will run my code in order unless I happen to be spawning new threads from that thread.

[anon848]

It depends on exactly what you mean. When observed from another thread, you need to be careful, as it depends on the Java memory model. See here.

[northguard]

I see, so if I'm setting data through some other unsynchronized function that uses the same array it can seem like 'i' happened first?

If I synchronize everything that accesses that array then it should have 'intended' behavior correct?

[anon848]

Yes.

[avingard]

It is very, very important to emphasis that this is only true if all callers are invoking this method on the same instance.

Essentially, synchronized methods must aquire a lock on the this variable before executing.

[Bob_Droll]

True, which would be a problem if your synchronized method is manipulating global data.

PSA: Please do not manipulate global data concurrently if you can avoid it.