Invoking a method which uses CompletableFuture's thenAccept()

Question

I have a rest API function which returns an object of type DeferredResult.

import org.springframework.web.context.request.async.DeferredResult;

public DeferredResult<Object> apiMethod{
CompletableFuture<Object> future = someMethod();
final DeferredResult<Object> response = new DeferredResult<>(); 

future.thenAccept(){
    //logic to populate response
}

return response;
}

I am writing a function which will invoke apiMethod() and use its response. I always end up getting a null response because response is populated in future.thenAccept (). Is there a way to handle this?

Answer 1

The problem is that the method continues the execution while thenAccept runs async. After you call thenAccept , the method just returns response afterwards, independent of if it's already populated.

Imagine the following simple code:

    public static void main(String[] args) {
        AtomicReference<String> result = new AtomicReference<>(null);
        CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> {
            for (int i = 0; i < 100_000_000; i++) {}
            return "Hello World!";
        });
        future.thenAccept(s -> {
            result.compareAndSet(null, s);
        });
        System.out.println(result.get());
    }

You may expect that "Hello World!" is printed, which is not the case; it prints out null . Here's the same problem: the main-thread prints the value, which will be updated async somewhen. You can fix this by joining the future:

    public static void main(String[] args) {
        AtomicReference<String> result = new AtomicReference<>(null);
        CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> {
            for (int i = 0; i < 100_000_000; i++) {}
            return "Hello World!";
        });
        CompletableFuture<Void> end = future.thenAccept(s -> {
            result.compareAndSet(null, s);
        });
        end.join();
        System.out.println(result.get());
    }

Now when we join the async future chain, or rather the one future that sets the value, we will see the main-thread printing out "Hello World!" because it will wait for the future to finish.

Now you just have to apply this fix in your code.