Park/Unpark

一、 基本使用

1. 正常使用

• 先park，再unpark
• park后是waiting状态，会释放锁

# 暂停当前线程
LockSupport

# 在哪个线程中使用，就暂停哪个线程
public static void park()

# 恢复一个线程
public static void unpark(Thread thread)

package com.dreamer.multithread.day04;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.LockSupport;

public class Demo01 {
    public static void main(String[] args) throws InterruptedException {
        Thread slaveThread = new Thread("slave-thread") {
            @Override
            public void run() {
                System.out.println("prepare for PARK....");
                LockSupport.park();
                System.out.println("PARK ended");
            }
        };
        slaveThread.start();

        TimeUnit.SECONDS.sleep(2);

        LockSupport.unpark(slaveThread);
    }
}

2. 先unpark后park

• 先unpark，再park，线程就不会停下来了

package com.dreamer.multithread.day04;

import java.util.concurrent.locks.LockSupport;

public class Demo01 {
    public static void main(String[] args) {
        Thread slaveThread = new Thread("slave-thread") {
            @Override
            public void run() {
                LockSupport.unpark(Thread.currentThread());
                System.out.println("prepare for PARK....");
                LockSupport.park();
                System.out.println("PARK ended");
            }
        };
        slaveThread.start();
    }
}

3. 区别

二者都会使线程进入waitset等待，都会释放锁

wait/notify是Object的方法                    park/unpark是LockSupport
wait/notify 必须和synchronized结合使用        park/unpark不必
wait/notify 顺序不能颠倒                      park/unpark可以颠倒
wait/notify 只能随机唤醒一个或者全部唤醒         park/unpark可以指定一个线程唤醒