CompletableFuture dependent chain to run asynchronously - java-8

Question

How can I execute this full CompletableFuture chain to run asynchronously using a separate executor
.thenApply(r -> {
return validateStudents();
})
.thenCompose(r -> {
return fetchAll(r);
})
.thenCompose(r -> {
return processAll(r);
})
.whenComplete((r, t) -> {
});
});

Answer 1

You can use the Async methods from CompletableFuture with the default ForkJoinPool
All async methods without an explicit Executor argument are performed using the ForkJoinPool.commonPool() (unless it does not support a parallelism level of at least two, in which case, a new Thread is created to run each task)
System.out.println(Thread.currentThread().getName());
CompletableFuture.supplyAsync(()->{
System.out.println(Thread.currentThread().getName());
return "supplyAsync";
}).thenApplyAsync(supply->{
System.out.println(Thread.currentThread().getName()+"----"+supply);
return "applyAsync";
}).thenComposeAsync(compose->{
System.out.println(Thread.currentThread().getName()+"----"+compose);
return CompletableFuture.completedStage("composeAsync");
});
Output :
main
ForkJoinPool.commonPool-worker-3
ForkJoinPool.commonPool-worker-3----supplyAsync
ForkJoinPool.commonPool-worker-3----applyAsync
You can also define custom thread pool and you can use that thread pool
ExecutorService pool = Executors.newFixedThreadPool(1);
System.out.println(Thread.currentThread().getName());
CompletableFuture.supplyAsync(()->{
System.out.println(Thread.currentThread().getName());
return "supplyAsync";
},pool).thenApplyAsync(supply->{
System.out.println(Thread.currentThread().getName()+"----"+supply);
return "applyAsync";
},pool).thenComposeAsync(compose->{
System.out.println(Thread.currentThread().getName()+"----"+compose);
return CompletableFuture.completedStage("composeAsync");
},pool);
Output :
main
pool-1-thread-1
pool-1-thread-1----supplyAsync
pool-1-thread-1----applyAsync