synchronized同步块的同步性

class ObjectService {
    public void serviceMethodA(){
        try{
            synchronized (this){
                System.out.println("A begin time = " + System.currentTimeMillis());
                Thread.sleep(2000);
                System.out.println("A end time = " + System.currentTimeMillis());
            }
        }catch (InterruptedException e){
            e.printStackTrace();
        }
    }

    public void serviceMethodB(){
        synchronized (this){
            System.out.println("B begin time = " + System.currentTimeMillis() );
            System.out.println("B end time = " + System.currentTimeMillis());
        }
    }
}

class ThreadA extends  Thread {
    private  ObjectService service;

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

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

class ThreadB extends  Thread {
    private  ObjectService service;

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

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

public class Run {
    public static void main(String[] args) {
        ObjectService service = new ObjectService();
        ThreadA a = new ThreadA(service);
        a.setName("a");
        a.start();
        ThreadB b = new ThreadB(service);
        b.setName("b");
        b.start();
    }
}

结果：

A begin time = 1458139127424
A end time = 1458139129425
B begin time = 1458139129425
B end time = 1458139129425

即时线程B想调用MethodB，但是因为线程A获取了对象锁，线程B只能等待

1. 对其他synchronized同步方法或synchronized（this）同步代码块调用呈阻塞状态
2. 同一时间只有一个线程可以执行synchronized同步方法或同步代码块中的代码