代码如下：
public class Demo03 {
    public static void main(String[] args) {
        **Demo03Service service1 = new Demo03Service();**
        **Demo03Service service2 = new Demo03Service();**
        **Thread threadB = new Demo03ThreadB(service1);**
        **Thread threadA = new Demo03ThreadA(service2);**
        threadB.start();
        threadA.start();
    }
}

class Demo03Service{
    private int num;
    synchronized public void add(String userName){
        if(Objects.equals("a",userName)){
            num=100;
            System.out.println("a is set");
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }else
        {
            num=200;
            System.out.println("b is set");
        }
        System.out.println("userName="+userName+", num="+num);
    }
}

class Demo03ThreadA extends Thread{
    private Demo03Service service;
    public Demo03ThreadA(Demo03Service service){
        this.service=service;
    }

    @Override
    public void run() {
        service.add("a");
    }
}

class Demo03ThreadB extends Thread{
    private Demo03Service service;
    public Demo03ThreadB(Demo03Service service){
        this.service=service;
    }

    @Override
    public void run() {
        service.add("b");
    }
}

运行结果：
图片说明
可以看出线程A在睡眠的时候，B线程也在执行，两个线程并不是同步执行，结果数据都正确。是因为线程作用于不同的对象使用多个锁，两个线程对彼此的锁都不影响。