Java Completable future 线程在方法执行后仍然存在

Question

我编写了一个小程序来检查 Completable Future 的行为。我没有覆盖公共池。

我没有找到任何关闭方法，当我最后打印活动线程数时，我发现我的线程处于活动状态。

我的问题是他们什么时候结束，如果我在生活应用程序中使用它？ 如果我在流量很大的公共 Api 中使用它，它们会创建很多线程吗？

我的示例代码

`

package rar;

import java.util.Arrays;
import java.util.List;
import java.util.Objects;
import java.util.Set;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.stream.Collectors;

public class Rar {
    public static void main(String[] args) {
        Rar r = new Rar();
        
        
          Set<Thread> threadSet = Thread.getAllStackTraces().keySet();
          System.out.println(threadSet); r.dfo(); threadSet =
          Thread.getAllStackTraces().keySet(); System.out.println("uyuuuu"+threadSet);
         
    }

    private static void doTask3() {
        for(int i=0; i<5;i++) {
            try {
                Thread.sleep(5000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.print(3);
        }

    }

    public  void dfo() {
         System.out.println("In main");      
         ExecutorService executor = Executors.newFixedThreadPool(3);     
         CompletableFuture<Void> thenCompose =CompletableFuture.allOf(
                 CompletableFuture.runAsync(() -> doTask1()),
                 CompletableFuture.runAsync(() -> doTask2(2)),
                 CompletableFuture.runAsync(() -> doTask3()));
                
        
         //executor.shutdown();    
         try {
             Set<Thread> threadSet = Thread.getAllStackTraces().keySet();
             System.out.println(threadSet);

            thenCompose.get();
        } catch (InterruptedException | ExecutionException e) {
            e.printStackTrace();
        }
         System.out.println("Exiting main");
         Set<Thread> threadSeWt = Thread.getAllStackTraces().keySet();
         System.out.println(threadSeWt);
    }
    private  void doTask2(int num) {
        for(int i=0; i<5;i++) {
            try {
                Thread.sleep(5000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.print("4");
        }

    }

    private  int doTask1() {
        for(int i=0; i<5;i++) {
            try {
                Thread.sleep(5001);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.print(1);
        }
        return 5;

    }
}

`

样本输出：

 [Thread[Finalizer,8,system], Thread[Attach Listener,5,system], Thread[Signal Dispatcher,9,system], Thread[Reference Handler,10,system], Thread[main,5,main]]
In main
[Thread[Finalizer,8,system], Thread[ForkJoinPool.commonPool-worker-1,5,main], Thread[main,5,main], Thread[ForkJoinPool.commonPool-worker-2,5,main], Thread[Attach Listener,5,system], Thread[ForkJoinPool.commonPool-worker-3,5,main], Thread[Signal Dispatcher,9,system], Thread[Reference Handler,10,system]]
431413431431431Exiting main
[Thread[Finalizer,8,system], Thread[ForkJoinPool.commonPool-worker-1,5,main], Thread[main,5,main], Thread[ForkJoinPool.commonPool-worker-2,5,main], Thread[Attach Listener,5,system], Thread[ForkJoinPool.commonPool-worker-3,5,main], Thread[Signal Dispatcher,9,system], Thread[Reference Handler,10,system]]
uyuuuu[Thread[Finalizer,8,system], Thread[ForkJoinPool.commonPool-worker-1,5,main], Thread[main,5,main], Thread[ForkJoinPool.commonPool-worker-2,5,main], Thread[Attach Listener,5,system], Thread[ForkJoinPool.commonPool-worker-3,5,main], Thread[Signal Dispatcher,9,system], Thread[Reference Handler,10,system]]

Answer 1

你的dfo()方法中的执行器没有被使用。

当您使用runAsync() 方法时，它在公共池上运行。这就是您在调试消息中看到ForkJoinPool 的原因。

默认情况下，该池的大小受“CPU 核心数 - 1”的限制。

尝试运行 20 个任务，您会发现线程数一旦达到最大值就会停止增长。

您不需要停止 ForkJoinPool 的线程。来自documentation：

它的线程在不使用期间会慢慢回收