如何处理Java 8 CompletableFuture中的错误？

Question

I am trying to implement a simple RPC-like (or request-response) system over WebSockets in Java (there will be JS on the front-end but I am working on the back-end for now).

I am trying to apply the Java CompletableFuture pattern to handle sending messages asynchronously. But I am currnently stuck on error handling.

I have a class (let's call it the rpc class) that is responsible to send the message over a WebSocket session (using Spring WebSocket support classes here), then to wait for "reply" type messages, and matches them with the pending request and returning the content to the caller.

The flow is :

• Client code calls the method on the rpc class, specifying the name of the procedure to call on the remote process, the session to which to send the message, and a map of arguments to send along.
• The rpc class uses another lower level class to sends the message asynchronously using an Executor (thread pool), and receives a CompletableFuture<Void> for the "send the message" operation
• It stores the pending request in a map, builds a CompletableFuture<Map<String, Object>> and associates it with the pending request, and stores them in a map. It returns the completable future.
• When a "reply" type message is received, a method is called on the same class, this method tries to match the response with one of the pending requests (they have an ID for this) and then completes the CompletableFuture with the content received in the response.

So there are 3 threads involved : the caller thread, the thread that sends the message, and the thread that receives the message and completes the future.

Now, how should I handle an error in the sending of the message (eg IO error) in order to make the returned completableFuture also fail (or maybe implement a retry strategy, and a time out...) ?

Here is the code of the rpc class method that sends the message :

/**
 * Invoke a remote procedure over WS on the specified session, with the given arguments.
 * @param session The target session on which to send the RPC message
 * @param target The name of the procedure to call
 * @param arguments The arguments to be sent in the message
 * @return
 */
public CompletableFuture<Map<String,Object>> invoke(WebSocketSession session, String target, Map<String, Object> arguments){
    Invocation invocationMessage = new Invocation(target, arguments);
    invocationMessage.setId(getNextId());

    // completeable future for the result. It does nothing, will be completed when reply is received which happen in a different thread, see completeInvocation
    CompletableFuture<Map<String, Object>> invocationFuture = new CompletableFuture<>();

    CompletableFuture<Void> senderFuture = sender.sendMessage(session, invocationMessage);

    // handle problem in the sending of the message
    senderFuture.exceptionally(e -> {
        // is this correct ??
        invocationFuture.completeExceptionally(e);
        return null;
    });

    // store the pending invocation in the registry
    registry.addPendingInvocation(new PendingInvocation(invocationMessage, session, invocationFuture));

    // return the future so the caller can have access to the result once it is ready
    return invocationFuture;
}

Answer 1

The simplest way to do it would be to simply chain the futures using thencompose() :

// completeable future for the result. It does nothing, will be completed when reply is received which happen in a different thread, see completeInvocation
CompletableFuture<Map<String, Object>> invocationFuture = new CompletableFuture<>();

CompletableFuture<Void> senderFuture = sender.sendMessage(session, invocationMessage);

// store the pending invocation in the registry
registry.addPendingInvocation(new PendingInvocation(invocationMessage, session, invocationFuture));

// return the future so the caller can have access to the result once it is ready
return senderFuture.thenCompose(__ -> invocationFuture);

In case of exceptional completion of senderFuture , the returned future will be completed exceptionally as well, with a CompletionException holding the exception as its cause (cf CompletionStage api ).

Note that there are other concerns you might want to tackle as well:

• Shouldn't you also cancel the pending invocation in case of exception?
• What happens if the response arrives before addPendingInvocation is called? Shouldn't you call it before calling sendMessage to avoid issues?
• Since you don't do anything with invocationFuture , wouldn't it be better to create it inside addPenidngInvocation ?