Java 线程 - 使函数与线程的多个返回值保持同步

Question

This is just a concept :

• I want to have a int be updated, by a function that is running inside multiple threads.
• My function randomly chooses an int and checks if it's bigger than the existing int.
• If so it updates the variable.

How do I ensure my values never clash or my function is looking at an older value not yet updated?

I'm new to threads so any information is valuable.

Answer 1

This a job for AtomicInteger !

AtomicInteger atomic;
...
int newValue = atomic.accumulateAndGet​(rand, Math::max);

Answer 2

How do I ensure my values never clash?

If you are new to threads, then I would say, use a synchronized block:

final Object lock = new Object();
int global_r = 0;
...
while (true) {
    int r = random.nextInt();
    synchronized(lock) {
        if (r > global_r) {
            global_r = r;
            break;
        }
    }
}

Only one thread at a time can enter the synchronized(lock)... block. A thread that tries when another thread already is in it will be forced to wait.

One should always try to have threads spend as little time as possible inside a synchronized(...) block, which is why I generate the random number outside of the block, and only do the test and the assignment inside it.

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Another answer here suggests that you use AtomicInteger . That works too, but it is not what I would recommend for somebody who is taking their very first steps toward understanding how to use threads.