如何为所有CompletableFuture全局设置线程池

Question

I am trying to mimic what single threaded async programming in Javascript in Java with the use of async / await library by EA ( ea-async ). This is mainly because I do not have long-lasting CPU bound computations in my program and I want to code single thread lock free code in Java.

ea-async library heavily relies on the CompletableFuture in Java and underneath Java seems to use ForkJoinPool to run the async callbacks. This puts me into multi threaded environment as my CPU is multi-core. It seems for every CompletableFuture task, I can supply async with my custom thread pool executor. I can supply Executors.newSingleThreadExecutor() for this but I need a way to set this globally so that all CompletableFuture will be using this executor within the single JVM process. How do I do this?

Answer 1

ea-async library heavily relies on the CompletableFuture in Java and underneath Java seems to use ForkJoinPool to run the async callbacks.

That is the default behavior of CompleteableFuture :

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). This may be overridden for non-static methods in subclasses by defining method defaultExecutor() .

That's a defined characteristic of the class, so if you're using class CompleteableFuture , not a subclass, and generating instances without specifying an Executor explicitly, then a ForkJoinPool is what you're going to get.

Of course, if you are in control of the CompletableFuture s provided to ea-async then you have the option to provide instances of a subclass that defines defaultExecutor() however you like. Alternatively, you can create your CompleteableFuture objects via the static factory methods that allow you to explicitly specify the Executor to use, such as runAsync​(Runnable, Executor) .

But that's probably not what you really want to do .

If you use an executor with only one thread, then your tasks can be executed asynchronously with respect to the thread that submits them, yes, but they will be serialized with respect to each other . You do get only one thread working on them, but it will at any time be working on a specific one, sticking with that one only until it finishes, regardless of the order in which the responses actually arrive. If that's satisfactory, then it's unclear why you want async operations at all.

This puts me into multi threaded environment as my CPU is multi-core.

It puts you in multiple threads regardless of how many cores your CPU has. That's what Executor s do , even Executors.newSingleThreadExecutor() . That's the sense of "asynchronous" they provide.

If I understand correctly, you are instead looking to use one thread to multiplex I/O to multiple remote web applications. That is what java.nio.channels.Selector is for, but using that generally requires either managing the I/O operations yourself or using interfaces designed to interoperate with selectors. If you are locked in to third-party interfaces that do not afford use of a Selector , then multithreading and multiprocessing are your only viable alternatives.

In comments you wrote:

I'm starting to think maybe BlockingQueue might do the job in consolidating all API responses into one queue as tasks where a single thread will work on them.

Again, I don't think that you want everything that comes with that, and if in fact you do, then I don't see why it wouldn't be even better and easier to work synchronously instead of asynchronously.