面試題 線程安全

面試題，是否線程安全？結果是多少？

public class Te {
    private static volatile int ii = 0;
    public synchronized static void add() {
        ii++;
    }

    private static class Plus implements Runnable {
        @Override
        public void run() {
            while (ii < 10) {
                System.out.println(Thread.currentThread().getName() + " - " + ii);
                add();
            }
        }
    }

    public static void main(String[] args) throws InterruptedException {
        Thread[] threads = new Thread[10];
        for (int i = 0; i < 10; i++) {
            threads[i] = new Thread(new Plus());
            threads[i].start();
        }
        Thread.sleep(1000);
        System.out.println("結果:" + ii);
    }
}

答案：不是線程安全，上面的只保證了add方法是線程安全，但調用的時候拿到的ii值可能一樣，所以會多加

public class Te {
    private static volatile int ii = 0;

    public static void add() {
        ii++;
    }

    private static class Plus implements Runnable {
        @Override
        public void run() {
            synchronized (Te.class) {
                while (ii < 10) {
                    System.out.println(Thread.currentThread().getName() + " - " + ii);
                    add();
                }
            }
        }
    }

    public static void main(String[] args) throws InterruptedException {
        Thread[] threads = new Thread[10];
        for (int i = 0; i < 10; i++) {
            threads[i] = new Thread(new Plus());
            threads[i].start();
        }
        Thread.sleep(1000);
        System.out.println("結果:" + ii);
    }
}

這上面使用的是類鎖，在所有線程中共用一把鎖，所以就可以作到線程安全了。