问题
Scenario:
- there are two stages
- 2nd stage is to be executed only after the 1st one is completed
- 2nd stage is not interested in the 1st stage result but merely in the fact that the first stage is completed
Consider the existing method:
public <U> CompletionStage<U> thenApply(Function<? super T,? extends U> fn);
It doesn't quite satisfy my needs cause the function is aware of the 1st stage result value ? super T
What I would rather like to have is something like:
public <U> CompletionStage<U> thenApply(Supplier<? extends U> fn);
Question: do I understand correctly that there's no out-of-the-box solution for that so I will have to write my own wrapper function in order to achieve the desired behavior?
回答1:
There is no such built-in solution. But you could "abuse" CompletableFuture.supplyAsync and thenCompose:
Supplier<String> sup = ()->"s";
CompletableFuture.supplyAsync(()->4)
.thenCompose(x->CompletableFuture.supplyAsync(sup))
.thenAccept(System.out::println);
BTW, I guess probably there are no such convenience methods because CompletionStage/CompletableFuture already have quite many methods.
回答2:
If the thing you want to run is not expected to return a result, you can use thenRun:
CompletableFuture<Thing> thingsFuture = CompletableFuture.supplyAsync(() -> getThings());
thingsFuture.thenRun(() -> System.out.println("Got ALL THE THINGS!"));
If you need it to return a result, then I'm afraid you're out of luck. Best option you have is just to ignore the argument in your lambda.
来源：https://stackoverflow.com/questions/40410845/completionstage-chaining-when-2nd-stage-is-not-interested-in-the-1st-stage-resul