問題描述

CompletableFuture cf1 = CompletableFuture.supplyAsync(() -> { System.out.println('enter into completableFuture()'); try {TimeUnit.SECONDS.sleep(1); } catch (InterruptedException e) {e.printStackTrace(); } System.out.println('start to out of completableFuture()'); return 'a';});System.out.println('do something else');cf1.thenApply(v -> v + ' b').thenAcceptAsync(v ->System.out.println(v));System.out.println('finalize...');//注釋最后一行，無法得到預期結果//TimeUnit.SECONDS.sleep(10);

得到引結果為：

do something elseenter into completableFuture()finalize...start to out of completableFuture()a b

以上代碼如果注釋掉最后一行，無法得到預期結果。

為什么一定要顯式的讓程序sleep10秒呢？

問題解答

見CompletableFuture.supplyAsync的javadoc：

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.

而ForkJoinPool.commonPool()的javadoc：

Returns the common pool instance. This pool is statically constructed; its run state is unaffected by attempts to shutdown or shutdownNow. However this pool and any ongoing processing are automatically terminated upon program System.exit. Any program that relies on asynchronous task processing to complete before program termination should invoke commonPool().awaitQuiescence, before exit.

如果你把最后的sleep改成ForkJoinPool.commonPool().awaitQuiescence(2, TimeUnit.SECONDS);也能達到你預期結果

搜索一下：守護線程當線程中只剩下守護線程時JVM就會退出，反之還有任意一個用戶線程在，JVM都不會退出。我們可以猜測CompletableFuture.supplyAsync啟動了一個守護線程，實際上CompletableFuture內部默認使用ForkJoinPool，該線程池初始化一個線程工廠類：

defaultForkJoinWorkerThreadFactory = new DefaultForkJoinWorkerThreadFactory();

查看他的的實現，每次都是創建守護進程。至于為什么一定要主線程sleep就很好理解。