java CompletableFuture.thenCombine returns CompletableFuture of CompletableFuture

Question

I have 3 different methods in my application. All are returning CompletableFuture<SomeType>. I want to execute method 1 and 2 in parallel. On completion of method 1 and 2, I want to trigger method 3 with parameters from method 1 and 2 return values.

Code example :

CompletableFuture<Request> future1 = RequestConverter.Convert(requestDto);
CompletableFuture<String> future2 = tokenProvider.getAuthToken();

CompletableFuture<CompletableFuture<String>> future3 = 
future1.thenCombine(future2,(request,token) -> 
requestProcessor.Process(request,token));

But the problem with the above code is that I am getting a CompletableFuture of CompletableFuture. I want to avoid this and get a simple CompletableFuture<String> without blocking. Is this possible?

My understanding is "thenCompose" can be used only if we need result from only one CompletableFuture. But in this case i need result of both the CompletableFuture to trigger my third method. — VinothNair, Jan 09 '18 at 12:42
I am getting a compile time exception if i use thenCompose. I am new to Java, so maybe i am missing some syntax. Can you please post the future3 syntax with your suggestion. — VinothNair, Jan 09 '18 at 13:22
I will try as per your suggestion. Please post your code as well, so that i can cross check whether my code is inline. Thanks — VinothNair, Jan 09 '18 at 13:29
Note that java standards recommend that method and variable names start with a lowercase letter while class names start with an uppercase one. The syntax highlighting gets a bit messed up here since this convention is not followed. — Didier L, Jan 09 '18 at 23:40

score 6 · Answer 1 · answered Jan 09 '18 at 23:34

6

As there is no method that combines thenCombine() and thenCompose(), you can use thenCompose() to unwrap nested CompletableFutures coming from your thenCombine():

CompletableFuture<CompletableFuture<String>> future3 = 
    future1.thenCombine(future2, (request,token) -> 
        requestProcessor.Process(request,token));

CompletableFuture<String> future4 = future3.thenCompose(c -> c);

answered Jan 09 '18 at 23:34

Didier L

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This avoids the extra class. Thanks. – VinothNair Jan 10 '18 at 06:06

zhh · Accepted Answer · 2018-01-09T13:35:22.843

Suppose you have the Pair class.

class Pair<L, R> {
    public final L left;
    public final R right;

    public Pair(L left, R right) {
        this.left = left;
        this.right = right;
    }
}

Then you can first combine to a pair then compose.

CompletableFuture<Request> future1 = RequestConverter.Convert(requestDto);
CompletableFuture<String> future2 = tokenProvider.getAuthToken();

CompletableFuture<String> future3 = future1
    .thenCombine(future2, Pair::new)
    .thenCompose(pair -> requestProcessor.Process(pair.left, pair.right));

java CompletableFuture.thenCombine returns CompletableFuture of CompletableFuture

2 Answers2