当请求 url 相同并使用 CompletableFuture 处理时，为什么 Tomcat 使用不同的线程？

Question

I have used this sample in github using CompletableFuture.

I tried running /asyncCompletable endpoint from several tabs in browser(chrome) simultaneously. The point is as Tomcat is Servlet Container and it has it's own Servlet Thread Pool, thus hitting same endpoint url from different tabs in browser wouldn't necessarily use same main-thread. see my console log below:

8:07:07.334 [http-nio-9191-exec-8] Request received
18:07:07.335 [http-nio-9191-exec-8] Servlet thread released
18:07:07.335 [ForkJoinPool.commonPool-worker-3] Start processing request
18:07:08.262 [http-nio-9191-exec-7] Request received
18:07:08.262 [http-nio-9191-exec-7] Servlet thread released
18:07:08.262 [ForkJoinPool.commonPool-worker-4] Start processing request
18:07:08.860 [http-nio-9191-exec-9] Request received
18:07:08.861 [http-nio-9191-exec-9] Servlet thread released
18:07:08.861 [ForkJoinPool.commonPool-worker-5] Start processing request
18:07:09.376 [http-nio-9191-exec-10] Request received
18:07:09.377 [http-nio-9191-exec-10] Servlet thread released

As you see it is using different threads for each browser tab for the same URL and not reusing the released thread for further calls. What I mean is that it is not reusing the released thread (for example http-nio-9191-exec-7) for next incoming request. Why is that?

Answer 1

A thread pool is what it states - a pool. So there's no guarantees in a pool to re-address the same thread to a task nor would it be any helpful. The pool just return the next available thread in the pool. But what does "next" mean? This is depending on the pool implementation - and this should be abstracted from the user. Just because it's not directly reassigned the next request doesn't mean it will never be reused. The pool decides how to do this. This depends in the pool's strategy. So there might be some margin or threshold in time or some queue in the Servlet pool.

You could try if behaviour changes when you reconfigure Tomcat for evaluation purpose:

<Executor name="tomcatThreadPool" namePrefix="catalina-exec-" maxThreads="150" minSpareThreads="0"/>

But this will not be helpful performance wise and is not suggested. I wouldn't mess with the Servlet Thread Pool mechanics unless there's good reasons to do so. A pool is for handing over that control for good reasons.