我需要有权访问相同数据的线程同时执行，而不会相互干扰，因此使用 Thread.join() 我一直在尝试同步方法。问题是，我看不到任何变化，它不断给我相同的结果，我以前使用它们。我甚至不知道我到底做错了什么，同步方法应该阻止其他同步方法在完成之前执行，对吗？希望你能告诉我发生了什么事。

public class ThreadSync{
    public static void main(String[] args) throws InterruptedException {
         //if execute properly  
         //output can't be other than 0
         while(true) {
         ChangeValue counter = new ChangeValue();
         Threads t  = new Threads(counter,"up");
         Threads t2 = new Threads(counter,"down");
         Threads t3 = new Threads(counter,"print");
         t.start();
         t2.start();
         t3.start();
         }
    }
}
class Threads extends Thread{
    Threads(ChangeValue obj, String act){
        counter = obj;
        action = act;
    }
    @Override
    public void run() {
        switch(action) {
        case ("up"):    counter.up();    break;
        case ("down"):  counter.down();  break;
        case ("print"): counter.print(); break;     
        }
    }
    ChangeValue counter;
    String action;
}
class ChangeValue{  
    public synchronized void up()    { value++; }
    public synchronized void down()  { value--; }
    public synchronized void print() { System.out.println(value); }
    public int value = 0;
}