一 同一类的方法都用 synchronized 修饰

1 代码

package concurrent;

import java.util.concurrent.TimeUnit;

public class ThisMonitor {
    public synchronized void method1() {
        System.out.println(Thread.currentThread().getName() + " enter to method1");
        try {
            TimeUnit.SECONDS.sleep(10);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(Thread.currentThread().getName() + " exit to method1");
    }

    public synchronized void method2() {
        System.out.println(Thread.currentThread().getName() + " enter to method2");
        try {
            TimeUnit.SECONDS.sleep(10);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(Thread.currentThread().getName() + " exit to method2");
    }

    public static void main(String[] args) {
        ThisMonitor thisMonitor = new ThisMonitor();
        new Thread(thisMonitor::method1, "T1").start();
        new Thread(thisMonitor::method2, "T2").start();
    }
}

2 测试

T1 enter to method1
T1 exit to method1
T2 enter to method2
T2 exit to method2

二 同一类的方法分别用 synchronized 修饰和 synchronized 代码块修饰

1 代码

package concurrent;

import java.util.concurrent.TimeUnit;

public class ThisMonitor1 {
    public synchronized void method1() {
        System.out.println(Thread.currentThread().getName() + " enter to method1");
        try {
            TimeUnit.SECONDS.sleep(10);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(Thread.currentThread().getName() + " exit to method1");
    }

    public void method2() {
        synchronized (this) {
            System.out.println(Thread.currentThread().getName() + " enter to method2");
            try {
                TimeUnit.SECONDS.sleep(10);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() + " exit to method2");
        }
    }

    public static void main(String[] args) {
        ThisMonitor1 thisMonitor = new ThisMonitor1();
        new Thread(thisMonitor::method1, "T1").start();
        new Thread(thisMonitor::method2, "T2").start();
    }
}

2 测试

T1 enter to method1
T1 exit to method1
T2 enter to method2
T2 exit to method2

三 结论

上面两组代码执行的效果是一样的，使用 synchronized 关键字同步类的不同实例方法，争抢的是同一个 monitor 的 lock，而与之关联的引用则是 ThisMonitor 实例的引用。