Throwing exception from CompletableFuture

Question

I have the following code:

// How to throw the ServerException?
public void myFunc() throws ServerException{
    // Some code
    CompletableFuture<A> a = CompletableFuture.supplyAsync(() -> {
        try {
            return someObj.someFunc();
        } catch(ServerException ex) {
            // throw ex; gives an error here.
        }
    }));
    // Some code
}

someFunc() throws a ServerException. I don't want to handle this here but throw the exception from someFunc() to caller of myFunc().

Answer 1

Your code suggests that you are using the result of the asynchronous operation later in the same method, so you’ll have to deal with CompletionException anyway, so one way to deal with it, is

public void myFunc() throws ServerException {
    // Some code
    CompletableFuture<A> a = CompletableFuture.supplyAsync(() -> {
        try { return someObj.someFunc(); }
        catch(ServerException ex) { throw new CompletionException(ex); }
    });
    // Some code running in parallel to someFunc()

    A resultOfA;
    try {
        resultOfA = a.join();
    }
    catch(CompletionException ex) {
        try {
            throw ex.getCause();
        }
        catch(Error|RuntimeException|ServerException possible) {
            throw possible;
        }
        catch(Throwable impossible) {
            throw new AssertionError(impossible);
        }
    }
    // some code using resultOfA
}

All exceptions thrown inside the asynchronous processing of the Supplier will get wrapped into a CompletionException when calling join, except the ServerException we have already wrapped in a CompletionException.

When we re-throw the cause of the CompletionException, we may face unchecked exceptions, i.e. subclasses of Error or RuntimeException, or our custom checked exception ServerException. The code above handles all of them with a multi-catch which will re-throw them. Since the declared return type of getCause() is Throwable, the compiler requires us to handle that type despite we already handled all possible types. The straight-forward solution is to throw this actually impossible throwable wrapped in an AssertionError.

Alternatively, we could use an alternative result future for our custom exception:

public void myFunc() throws ServerException {
    // Some code
    CompletableFuture<ServerException> exception = new CompletableFuture<>();
    CompletableFuture<A> a = CompletableFuture.supplyAsync(() -> {
        try { return someObj.someFunc(); }
        catch(ServerException ex) {
            exception.complete(ex);
            throw new CompletionException(ex);
        }
    });
    // Some code running in parallel to someFunc()

    A resultOfA;
    try {
        resultOfA = a.join();
    }
    catch(CompletionException ex) {
        if(exception.isDone()) throw exception.join();
        throw ex;
    }

    // some code using resultOfA
}

This solution will re-throw all “unexpected” throwables in their wrapped form, but only throw the custom ServerException in its original form passed via the exception future. Note that we have to ensure that a has been completed (like calling join() first), before we query the exception future, to avoid race conditions.