一个线程在执行同步代码块什么时候确定锁是哪个对象，求大神解释

Question

 public class Test {

    public static void main(String[] args)throws Exception {
        Test test = new Test();
        MyService service = test.new MyService();
        ThreadA threadA = test.new ThreadA(service);
        threadA.setName("threadA");
        ThreadB threadB = test.new ThreadB(service);
        threadB.setName("threadB");
        threadA.start();
        //Thread.sleep(50);
        threadB.start();


    }
    public class MyService{
        private String lock = "123";
        public void testMethod(){
            try{
                synchronized(lock){
                    System.out.println(Thread.currentThread().getName() + "begain " + System.currentTimeMillis());
                    lock="456";
                    Thread.sleep(2000);
                    System.out.println(Thread.currentThread().getName() + "end" +System.currentTimeMillis());
                }
            }catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
    public class ThreadA extends Thread{
        private MyService service;
        public ThreadA(MyService service) {
            this.service = service;
        }
        @Override
        public void run() {
            service.testMethod();
        }
    }
    public class ThreadB extends Thread{
        private MyService service;
        public ThreadB(MyService service) {
            this.service = service;
        }
        @Override
        public void run() {
            service.testMethod();
        }
    }
}

问题是这样的，我在看书时没想明白，就是执行sleep（50）这句话后两个线程就不同步了，去掉sleep（50）后线程同步，我想了一下这关系到线程什么时候确定synchronized内的锁对象，求大神解释！！！！

Answer 1

你的锁是一个String对象 lock，在synchronized代码段里面你把lock对象重新赋值了，在java中String有一个特别的特性，每次赋值其实是产生一个新的String对象。执行了sleep（50）后，导致里threadA先运行了，并且是执行过了对lock字符串的赋值，threadB后运行，这个时候赋值过后的lock是一个新的对象，没有锁，所以threadB可以执行这个代码段。你把lock="456";这行去掉，就算执行了sleep（50）两个线程还是同步的。