CompletableFuture not working as expected

Question

I am trying to understand the working of CompletableFuture from Java 8. Below code works as expected

CompletableFuture.supplyAsync(() -> {
    System.out.println("supplyAsync Thread name " + Thread.currentThread().getName());
    return "str";
}).thenApply(str -> {
    System.out.println("thenApply Thread name " + Thread.currentThread().getName());
    return str;
}).thenApply(str1 -> {
    System.out.println("thenApply Thread name " + Thread.currentThread().getName());
    return str1;
}).thenAccept(str3 -> {
    System.out.println("thenAccept Thread name " + Thread.currentThread().getName());
});
System.out.println("Thread name " + Thread.currentThread().getName());

Output:

supplyAsync Thread name ForkJoinPool.commonPool-worker-1
thenApply Thread name main
thenApply Thread name main
thenAccept Thread name main
Thread name main

But when I put in some computation, it doesn't work as expected please correct me if I am missing something.

CompletableFuture.supplyAsync(() -> {
    System.out.println("supplyAsync Thread name " + Thread.currentThread().getName());
    long val = 0;
    for (long i = 0; i < 1000000; i++) {
        val++;
    }
    return "str";
}).thenApply(str -> {
    System.out.println("thenApply Thread name " + Thread.currentThread().getName());
    long val = 0;
    for (long i = 0; i < 1000000; i++) {
        val++;
    }
    return str;
}).thenApply(str1 -> {
    System.out.println("thenApply Thread name " + Thread.currentThread().getName());
    long val = 0;
    for (long i = 0; i < 1000000; i++) {
        val++;
    }
    return str1;
}).thenAccept(str3 -> {
    System.out.println("thenAccept Thread name " + Thread.currentThread().getName());
    long val = 0;
    for (long i = 0; i < 1000000; i++) {
        val++;
    }
});

System.out.println("Thread name " + Thread.currentThread().getName());

Output is:

supplyAsync Thread name ForkJoinPool.commonPool-worker-1
Thread name main

I agree that I am not joining the child thread to main thread. My understanding is child thread should print the statements independently of main thread. The question is why is it not printing at all.

Answer 1

Explanation

You're not joining the child thread ForkJoinPool.commonPool-worker-1 to the main thread. So it gets killed once thread main finishes.

Solution

Try calling .join() on your completable future at some point in your code. Note that this method is blocking the main thread. So the execution after the join point will be suspended until the child thread finishes its execution.

CompletableFuture.supplyAsync(() -> {
    System.out.println("=> supplyAsync Thread name " + Thread.currentThread().getName());
    // ...
}).thenApply(str -> {
    System.out.println("thenApply Thread name " + Thread.currentThread().getName());
    // ...
}).thenApply(str1 -> {
    System.out.println("thenApply Thread name " + Thread.currentThread().getName());
    // ...
}).thenAccept(str3 -> {
    System.out.println("thenAccept Thread name " + Thread.currentThread().getName());
    // ...
}).join()

Will print:

supplyAsync Thread name ForkJoinPool.commonPool-worker-1
thenApply Thread name ForkJoinPool.commonPool-worker-1
thenApply Thread name ForkJoinPool.commonPool-worker-1
thenAccept Thread name ForkJoinPool.commonPool-worker-1
Thread name main

If you want the last System.out.println(...) not depend from execution of the child thread then assign your CompletableFuture to a variable and join it in the very end of main:

CompletableFuture<Void> future = CompletableFuture.supplyAsync(...) ... // 

System.out.println("Thread name " + Thread.currentThread().getName());

future.join();