Park and unpark in unsafe

One 、 The method is as follows

public native void unpark(Object var1);

public native void park(boolean var1, long var2);

Two 、 Instructions

park： Suspends the current thread .unpark： Wake up a thread accurately .

park Parameters of , Indicates the pending expiration time , If the first one is true, Indicates absolute time , be var2 Is the absolute time value , In milliseconds . The first parameter is if false, It means relative time , be var2 Is the relative time value , The unit is nanosecond .

unpark Parameters of , Represents a thread .

A simple example ：

park(false,0) Means never to expire , Always on hold , Until awakened

long time = System.currentTimeMillis()+3000;
park(true,time + 3000) Express 3 Wake up automatically in seconds

park(false,3000000000L) Express 3 Wake up automatically in seconds

3、 ... and 、 Test examples

package com.suncy.article.article5;

import sun.misc.Unsafe;

import java.lang.reflect.Field;

public class ParkTest {
    public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException, InterruptedException {
        Field f = Unsafe.class.getDeclaredField("theUnsafe");
        f.setAccessible(true);
        Unsafe unsafe = (Unsafe) f.get(null);

        // Threads 1 Must wait for awakening 
        Thread thread1 = new Thread(() -> {
            System.out.println(" Threads 1: Perform tasks ");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
            }
            System.out.println(" Threads 1: Hang up , Wait for wakeup to continue the task ");
            unsafe.park(false, 0);
            System.out.println(" Threads 1: completion of enforcement ");
        });
        thread1.start();

        // Threads 2 Must wait for awakening 
        Thread thread2 = new Thread(() -> {
            System.out.println(" Threads 2: Perform tasks ");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
            }
            System.out.println(" Threads 2: Hang up , Wait for wakeup to continue the task ");
            unsafe.park(false, 0);
            System.out.println(" Threads 2: completion of enforcement ");
        });
        thread2.start();

        Thread.sleep(5000);
        System.out.println(" Wake up the thread 2");
        unsafe.unpark(thread2);
        Thread.sleep(1000);
        System.out.println(" Wake up the thread 1");
        unsafe.unpark(thread1);

        // Threads 3 Wake up automatically 
        Thread thread3 = new Thread(() -> {
            System.out.println(" Threads 3: Perform tasks ");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
            }
            System.out.println(" Threads 3: Hang up , Wait for the time to wake up automatically ");
            unsafe.park(false, 3000000000L);
            System.out.println(" Threads 3: completion of enforcement ");
        });
        thread3.start();

        // Threads 4 Wake up automatically 
        Thread thread4 = new Thread(() -> {
            System.out.println(" Threads 4: Perform tasks ");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
            }
            System.out.println(" Threads 4: Hang up , Wait for the time to wake up automatically ");
            long time = System.currentTimeMillis() + 3000;
            unsafe.park(true, time);
            System.out.println(" Threads 4: completion of enforcement ");
        });
        thread4.start();
    }
}

test result ：

Four 、 Purpose

1、 Use Unsafe Medium park and unpark And the last article said 《Unsafe Medium CAS》 Can complete a lock of its own , This should be a prerequisite for the foundation of concurrent programming .

author ： Small blank space
link ：https://www.jianshu.com/p/15f33406543b
source ： Simple books
The copyright belongs to the author . Commercial reprint please contact the author for authorization , Non-commercial reprint please indicate the source .