How to interrupt CompletableFuture::join?

Question

I've found that CompletableFuture::join seems uninterruptible when not completed:

// CompletableFuture::join implementation from JDK 8 sources
public T join() { 
    Object r;
    return reportJoin((r = result) == null ? waitingGet(false) : r);
}

In above implementation, waitingGet(false) will ignore the interrupt flag of the working Thread and continue waiting. I'm wondering how can I interrupt a Thread in which I call CompletableFuture::join.

Answer 1

Do not use join() if you want to support interruption, use get() instead. Basically they are the same except:

• join() is only defined in CompletableFuture whereas get() comes form interface Future
• join() wraps exceptions in CompletionException whereas get() wraps them in ExecutionException
• get() might be interrupted and would then throw an InterruptedException

Note that what you interrupt is the Thread, not the Future. For example, the following code interrupts the main thread while it is waiting on myFuture.get():

CompletableFuture<Void> myFuture = new CompletableFuture<>();
Thread mainThread = Thread.currentThread();
CompletableFuture.runAsync(() -> {
    try {
        Thread.sleep(1000);
        System.out.println("Interrupting…");
        mainThread.interrupt();
        Thread.sleep(1000);
        System.out.println("Completing");
        myFuture.complete(null);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
});
try {
    myFuture.get();
    System.out.println("Get succeeded");
} catch (Exception e) {
    System.out.println("Get failed");
    e.printStackTrace();
}

Output:

Interrupting…
Get failed
java.lang.InterruptedException
    at java.util.concurrent.CompletableFuture.reportGet(CompletableFuture.java:347)
    at java.util.concurrent.CompletableFuture.get(CompletableFuture.java:1895)
    at CompletableFutureInteruption.main(CompletableFutureInteruption.java:37)
    …

If you replace get() by join(), the interrupt will indeed not work.