Java CompletableFuture assigning executor

I am having confusion with defining executor in CompletableFuture. I am not sure how to tell CompletableFuture to run it in that particular executor. Thanks in advance.

//Suppose I have an executor
ExecutorService myExecutor=Executors.newFixedThreadPool(2);

//If I create a future like this
CompletableFuture.runAsync(() ->
//Do something
, myExecutor); // I can put the executor here and say the future to this executor

//But I do not know where to put executor if I create my future in method style like this

private final CompletableFuture<Void> myMethod(String something)
//Do something
return null;


//and use it like this
.thenCompose(this::myMethod); //How can I specify the executor in this case?


myMethod

thenComposeAsync​

thenCompose

myMethod

2 Answers
2

you can do something like this:

ExecutorService es = Executors.newFixedThreadPool(4);
List<Runnable> tasks = getTasks();
CompletableFuture<?> futures = tasks.stream()
.map(task -> CompletableFuture.runAsync(task, es))
.toArray(CompletableFuture::new);
CompletableFuture.allOf(futures).join();
es.shutdown();


CompletableFuture

CompletableFutures

In your example, you have 3 CompletableFutures that are at play:

CompletableFuture

runAsync()

myMethod()

thenCompose()

You also have 4 tasks that need to be run:

runAsync()

myMethod()

thenCompose()

thenComposeAsync()

myMethod()

myMethod()

thenCompose()

myMethod()

As you can see, several tasks and executors are involved, but you can always control the executors used in dependent stages using *Async() variants. The only case where you don't really control it is the 4th case, but it is a cheap operation as long as dependent stages use the *Async() variants as well.

*Async()

*Async()

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