synchronized加到静态方法上或者synchronized(class)代码块是给Class类上锁。Class锁对类的所有对象都其作用。例如：

1、创建一个Service类

package com.iie;

/**
 * Created by bo on 2018/3/29.
 */
public class Service {

    synchronized public static void printA() {
        try {
            System.out.println("线程名称为：" + Thread.currentThread().getName() + "在：" + System.currentTimeMillis() + "进入printA");
            Thread.sleep(3000);
            System.out.println("线程名称为：" + Thread.currentThread().getName() + "在：" + System.currentTimeMillis() + "离开printA");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    synchronized public static void printB() {
        try {
            System.out.println("线程名称为：" + Thread.currentThread().getName() + "在：" + System.currentTimeMillis() + "进入printB");
            Thread.sleep(3000);
            System.out.println("线程名称为：" + Thread.currentThread().getName() + "在：" + System.currentTimeMillis() + "离开printB");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

}

2、创建线程ThreadA调用printA（）方法，线程ThreadB调用printB（）方法。

package com.iie;

/**
 * Created by bo on 2018/3/29.
 */
public class ThreadA extends Thread {
    private Service service;

    public ThreadA(Service service) {
        this.service = service;
    }

    @Override
    public void run() {
        service.printA();
    }
}

package com.iie;

/**
 * Created by bo on 2018/3/29.
 */
public class ThreadB extends Thread {
    private Service service;

    public ThreadB(Service service) {
        this.service = service;
    }

    @Override
    public void run() {
        service.printB();
    }

}

3、创建main线程，运行。

package com.iie;

/**
 * Created by bo on 2018/3/26.
 */
public class Run {

    public static void main(String[] args) throws InterruptedException {
        Service serviceA = new Service();
        Service serviceB = new Service();
        ThreadA threadA = new ThreadA(serviceA);
        threadA.setName("a");
        ThreadB threadB = new ThreadB(serviceB);
        threadB.setName("b");

        threadA.start();
        threadB.start();
    }

}

执行结果：

可以看到，虽然创建了两个对象，但是还是同步执行，即Class锁对所有的对象实例都起作用。