Java 8 parallel stream + anyMatch - do threads get interrupted once a match is found?

Question

If I have a parallel stream in java 8, and I terminate with an anyMatch, and my collection has an element that matches the predicate, I'm trying to figure out what happens when one thread processes this element. I know that anyMatch is short circuiting, so that I wouldn't expect further elements to be processed once the matching element is processed. My confusion is about what happens to the other threads, that are presumably in the middle of processing elements. I can think of 3 plausible scenarios: a) Do they get interrupted? b) Do they keep processing the element that they are working on, and then, once all the threads are doing nothing, I get my result? c) Do I get my result, but the threads that were processing other elements continue processing those elements (but don't take on other elements once they are done)? I have a long running predicate, where it is very useful to terminate quickly as soon as I know that one element matches. I worry a bit since I can't find this informat…

Accepted Answer

After some digging through the Java source code I think I found the answer.

The other threads periodically check to see if another thread has found the answer and if so, then they stop working and cancel any not yet running nodes.

`java.util.Stream.FindOps$FindTask` has this method:

```
private void foundResult(O answer) {
        if (isLeftmostNode())
            shortCircuit(answer);
        else
            cancelLaterNodes();
    }
```

Its parent class, `AbstractShortcircuitTask` implements `shortCircuit` like this:

```
 /**
 * Declares that a globally valid result has been found.  If another task has
 * not already found the answer, the result is installed in
 * {@code sharedResult}.  The {@code compute()} method will check
 * {@code sharedResult} before proceeding with computation, so this causes
 * the computation to terminate early.
 *
 * @param result the result found
 */
protected void shortCircuit(R result) {
    if (result != null)
        sharedResult.compareAndSet(null, result);
}
```

And the actual `compute()` method that does the work has this important line:

```
 AtomicReference<R> sr = sharedResult;
    R result;
    while ((result = sr.get()) == null) {
        ...//does the actual fork stuff here
    }
```

where `sharedResult` is updated by the `shortCircuit()` method so the compute will see it the next time it checks the while loop condition.

EDIT So in summary:

- Threads are not interrupted

- Instead, they will periodically check to see if someone has found the answer and will stop further processing if the answer has been found.

- No new threads will be started once the answer has been found.

Java 8 parallel stream + anyMatch - do threads get interrupted once a match is found?

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