java在配置tomcat仅使用1个线程时，在使用completablefuture进行异步处理时，它是如何使用另一个线程的？

Question

我有一个端点使用CompletableFuture进行异步处理，并且我已经将嵌入式tomcat配置为只有一个线程，如下所示：

server.tomcat.max-threads=1

我的终点如下：

@RequestMapping(path = "/asyncCompletable", method = RequestMethod.GET)
public CompletableFuture<String> getValueAsyncUsingCompletableFuture() {
    log.info("Request received");
    CompletableFuture<String> completableFuture
            = CompletableFuture.supplyAsync(this::processRequest);
    log.info("Servlet thread released");
    return completableFuture;
}

在浏览器中多次（例如同时3次）点击端点时，控制台日志如下：

19:20:19.234 [http-nio-9191-exec-1] Request received 
19:20:19.234 [http-nio-9191-exec-1] Servlet thread released 
19:20:19.234 [ForkJoinPool.commonPool-worker-0] Start processing request 
19:20:19.839 [http-nio-9191-exec-1] Request received 
19:20:19.859 [http-nio-9191-exec-1] Servlet thread released 
19:20:19.859 [ForkJoinPool.commonPool-worker-1] Start processing request 
19:20:20.595 [http-nio-9191-exec-1] Request received 
19:20:20.596 [http-nio-9191-exec-1] Servlet thread released 
19:20:20.596 [ForkJoinPool.commonPool-worker-2] Start processing request 
19:20:24.235 [ForkJoinPool.commonPool-worker-0] Completed processing request 
19:20:24.235 [ForkJoinPool.commonPool-worker-0] Start reversing string 
19:20:24.235 [ForkJoinPool.commonPool-worker-0] Completed reversing string 
19:20:24.860 [ForkJoinPool.commonPool-worker-1] Completed processing request 
19:20:24.860 [ForkJoinPool.commonPool-worker-1] Start reversing string 
19:20:24.860 [ForkJoinPool.commonPool-worker-1] Completed reversing string 
19:20:25.596 [ForkJoinPool.commonPool-worker-2] Completed processing request 
19:20:25.597 [ForkJoinPool.commonPool-worker-2] Start reversing string 

如您所见，由于我已将tomcat配置为在其线程池中只有1个线程，因此对于所有3个请求，它都使用http-nio-9191-exec-1，但由于我使用CompletableFuture，它使用不同的线程即（，例如ForkJoinPool.commonPool-worker-2）来处理异步任务。 从何处使用新线程？因为我在tomcat线程池中只有一个线程可用

Answer 1

{a1}方法文档说明：

Returns a new CompletableFuture that is asynchronously completed by a task running in the ForkJoinPool.commonPool() with the value obtained by calling the given Supplier.

公共池由JVM创建，如ForkJoinPool API doc中所述：

A static commonPool() is available and appropriate for most applications. The common pool is used by any ForkJoinTask that is not explicitly submitted to a specified pool. Using the common pool normally reduces resource usage (its threads are slowly reclaimed during periods of non-use, and reinstated upon subsequent use).

由于此池不是由Tomcat创建的，因此最大线程限制不适用