至少完成一个线程时如何启动线程

Question

我有4条线。每个人每x秒打印给定的字母x次。任务是一次启动3个线程，在至少一个前一个线程完成时启动第四个线程。我不知道如何通知最后一个线程在适当的时间运行。

Answer 1

你可以使用信号灯。前三个线程将获取所有许可，当第一个线程释放许可时，第四个线程将获取并运行。

public static void main(String [] args) throws IOException {
    Semaphore semaphore = new Semaphore(3, true);
    
    
    Runnable a = new Runnable() {   
        @Override
        public void run() {
            try {
                semaphore.acquire();
                for (int i = 0; i < 50; i++) {
                    System.out.println("A");
                }                   
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                semaphore.release();
            }
        }
    };
    Runnable b = new Runnable() {   
        @Override
        public void run() {
            try {
                semaphore.acquire();
                for (int i = 0; i < 50; i++) {
                    System.out.println("B");
                }                   
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                semaphore.release();
            }
        }
    };
    Runnable c = new Runnable() {   
        @Override
        public void run() {
            try {
                semaphore.acquire();
                for (int i = 0; i < 50; i++) {
                    System.out.println("C");
                }                   
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                semaphore.release();
            }
        }
    };
    
    Runnable d = new Runnable() {   
        @Override
        public void run() {
            try {
                semaphore.acquire();
                System.out.println("D");
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                semaphore.release();
            }
        }
    };
    
    ExecutorService exec = Executors.newFixedThreadPool(5);
    exec.execute(a);
    exec.execute(b);
    exec.execute(c);
    exec.execute(d);
}