锁对象的改变

package extthread;
import myservice.MyService;
public class ThreadA extends Thread {
	private MyService service;
	public ThreadA(MyService service) {
		super();
		this.service = service;
	}
	@Override
	public void run() {
		service.testMethod();
	}
}

package extthread;
import myservice.MyService;
public class ThreadB extends Thread {
	private MyService service;
	public ThreadB(MyService service) {
		super();
		this.service = service;
	}
	@Override
	public void run() {
		service.testMethod();
	}
}

package myservice;
public class MyService {
	private String lock = "123";
	public void testMethod() {
		try {
			synchronized (lock) {
				System.out.println(Thread.currentThread().getName() + " begin "
						+ System.currentTimeMillis());
				lock = "456";
				Thread.sleep(2000);
				System.out.println(Thread.currentThread().getName() + "   end "
						+ System.currentTimeMillis());
			}
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}
}

package test.run;
import myservice.MyService;
import extthread.ThreadA;
import extthread.ThreadB;
public class Run1 {
	public static void main(String[] args) throws InterruptedException {
		MyService service = new MyService();
		ThreadA a = new ThreadA(service);
		a.setName("A");
		ThreadB b = new ThreadB(service);
		b.setName("B");
		a.start();
		//Thread.sleep(50);// 存在50毫秒
		b.start();
	}
}

结果为（同步效果）

A begin 1533742467664
A   end 1533742469664
B begin 1533742469664
B   end 1533742471664

把注释去掉

A begin 1533742520047
B begin 1533742520097
A   end 1533742522047
B   end 1533742522097