java - JAVA中使用信号量控制线程访问

Question

我正在尝试运行此 java 代码，但它无法正常工作。

请让我知道我做错了什么。

for 循环的 i 小于 10;。如果 i 小于 1(意味着没有循环)，则程序可以正常工作，但对于 (i 小于 n)，其中 n 大于 1，它会抛出异常

public class Main {

    public static void main(String[] args) {
        final Semaphore sem = new Semaphore(1, true);
        Thread t1 = new Thread("TA") {
            public void run() {
                try {
                    sem.acquire();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("A");
                sem.release();
            }
        };
        Thread t2 = new Thread("TB") {
            public void run() {
                try {
                    sem.acquire();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("B");
                sem.release();
            }
        };
        Thread t3 = new Thread("TC") {
            public void run() {
                try {
                    sem.acquire();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("C");
                sem.release();
            }
        };

        for (int i = 0; i < 10; i++) {
            t1.start();
            t3.start();
            t2.start();
        }

    }
}

Answer 1

您不能多次启动一个线程。因此，在第二次循环期间，第二次调用 t1.start() 时会出现异常。 javadoc 中对此进行了说明。 :

It is never legal to start a thread more than once. In particular, a thread may not be restarted once it has completed execution.

Throws: IllegalThreadStateException - if the thread was already started.

您可以使用 ExecutorService 而不是直接操作线程。它可能看起来像这样:

public static void main(String[] args) throws Exception {
    ExecutorService executor = Executors.newFixedThreadPool(3);
    final Semaphore sem = new Semaphore(1, true);
    Runnable r1 = new Runnable() {
        public void run() {
            try {
                sem.acquire();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("A");
            sem.release();
        }
    };
    Runnable r2 = new Runnable() {
        public void run() {
            try {
                sem.acquire();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("B");
            sem.release();
        }
    };
    Runnable r3 = new Runnable() {
        public void run() {
            try {
                sem.acquire();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("C");
            sem.release();
        }
    };

    for (int i = 0; i < 10; i++) {
        executor.submit(r1);
        executor.submit(r2);
        executor.submit(r3);
    }

    executor.shutdown();
}