Java-synchronized 获取序列值不唯一

Question

理论上，通过synchronized修饰的方法，应该只用一条线程能访问。这样通过getSeq()得到的值应该是不重复自增的。但是测试的结果，中间会有几个重复值。这个是因为什么导致的？测试使用jdk1.6

public class ThreadTest implements Runnable {
    private static  int seq;
    public synchronized int  getSeq(){
        return seq++;
    }
    public static void main(String[] args) {
        Thread t1 = new Thread(new ThreadTest(),"t1");
        Thread t2 = new Thread(new ThreadTest(),"t2");
        t1.start();
        t2.start();
    }
    @Override
    public void run() {
        for(int i = 0;i<100000;i++){
            int s = getSeq();
            System.out.print(s+",");
        }
    }

   
}

Answer 1

在7，8行你建了两个ThreadTest， synchronized方法是根据对像(this)来上锁的，所以你有两个锁，没有起到同步的作用。

public class ThreadTest implements Runnable {
private static int seq;
private static Object lock = new Object();

public int getSeq(){
synchronized (lock) {
return seq++;
}
}
public static void main(String[] args) {
Thread t1 = new Thread(new ThreadTest(),"t1");
Thread t2 = new Thread(new ThreadTest(),"t2");
t1.start();
t2.start();
}
@Override
public void run() {
for(int i = 0; i < 100000; i++) {
int s = getSeq();
System.out.print(s+"n");
}
}

}