java - 无法预测所有线程何时完成执行

Question

这是我的代码

package pe.entel.ftp;

public class Myclass implements Runnable {
    public void run() {
        System.out.println(Thread.currentThread().getName());
    }
    public static void main(String[] args) {
        Myclass runnable = new Myclass();
        ThreadGroup tg1 = new ThreadGroup("Parent ThreadGroup");

        for (int i = 0; i < 100; i++) {
            Thread t1 = new Thread(tg1, runnable, i + "");
            t1.start();
            System.out.println("Thread Group Name: " + tg1.getName());

        }
        while (tg1.isDestroyed()) {
            System.out.println("yes success");
        }

        System.out.println("completed");

    }
}

而我的o/p部分是

  Thread Group Name: Parent ThreadGroup
  Thread Group Name: Parent ThreadGroup
  Thread Group Name: Parent ThreadGroup
  Thread Group Name: Parent ThreadGroup
  Thread Group Name: Parent ThreadGroup
 completed
 84 
 86
 88
 90
 92
 94
 96
 98
 95
 97
 99

这里我无法预测线程组何时完成执行。甚至，用于检查线程组是否被销毁的 while 循环也不起作用。即使在打印完成后，一些线程仍在执行。

Answer 1

ThreadGroup 未完成执行。如果你想等待所有线程完成，你必须收集它们并等待所有线程:

List<Thread> allThreads = new ArrayList<>();

for (int i = 0; i < 100; i++) {
  Thread t1 = new Thread(tg1, runnable, i + "");
        t1.start();
   allThreads.add(t1);  
}

allThreads.forEach(Thread::join);

ThreadGroup 的存在时间比它们包含的线程还要长，您可能希望在到目前为止的所有线程都完成后向 ThreadGroup 添加新线程。它们只是对线程进行分组，而不是控制它们。