使用了synchronized，为啥还有线程安全问题？

使用了synchronized，为啥还有线程安全问题？

public class Run9 {
    static int count = 5;
    static class MyThread extends Thread{
        public MyThread(String name) {
            this.setName(name);
        }

        @Override
        synchronized public void run() {
            super.run();
            {
                count --;
                System.out.println("由 " + this.getName() + "计算，count = " + count);
            }
        }

    }

    public static void main(String[] args) {
        MyThread[] arr = new MyThread[5];
        String[] name = {"A","B","C","D","E"};
        for (int i = 0; i < 5; i++) {
            arr[i] = new MyThread(name[i]);
        }

        for (int i = 0; i < 5; i++) {
            arr[i].start();
        }
    }
}

synchronized 用于非 static 方法时，会把锁加在对象实例上，而不是类对象上。你在 main() 中实际创建了 5 个不同的 MyThread 对象实例，因此他们之间没有锁竞争关系。

public class Run9 {
    static int count = 5;

    static class MyThread extends Thread {

        public MyThread(String name) {
            this.setName(name);
        }

        @Override
        public void run() {
            synchronized (Run9.class) {
                count--;
                System.out.println("由 " + this.getName() + "计算，count = " + count);
            }
        }

    }

    public static void main(String[] args) {
        MyThread[] arr = new MyThread[5];
        String[] name = {"A", "B", "C", "D", "E"};
        for (int i = 0; i < 5; i++) {
            arr[i] = new MyThread(name[i]);
        }

        for (int i = 0; i < 5; i++) {
            arr[i].start();
        }
    }
}

count是静态变量，实例共享的，所以线程间操作会有影响

推荐使用AtomicInteger，java自带的线程安全类。
private static AtomicInteger count = new AtomicInteger(5);
count.incrementAndGet();