【并发设计模式】聊聊等待唤醒机制的规范实现

在多线程编程中，其实就是分工、协作、互斥。在很多场景中，比如A执行的过程中需要同步等待另外一个线程处理的结果，这种方式下，就是一种等待唤醒的机制。本篇我们来讲述等待唤醒机制的三种实现，以及对应的应用场景。

Guarded Suspension 模式

Guarded Suspension 翻译过来就是保护性暂停。其实就是一个线程需要等待获取另外一个线程执行的结果，先把当前线程挂起，另外一个线程执行完毕之后，通知自己，结束阻塞状态，继续执行。
等待唤醒的规范实现如下：

sychronized+wait/notify/notifyAll
reentrantLock+Condition(await/singal/singalAll)
cas+park/unpark

其实底层以来的是pthread，pthread_mutex_lock/unlock pthread_cond_wait/singal。这里就不介绍了，感兴趣的朋友可以自行查阅。

解决线程之间的协作不可避免会用到阻塞唤醒机制

实际编码

syn

package com.jia.suspension;

import java.util.Objects;
import java.util.concurrent.TimeUnit;

/**
 * @author qxlx
 * @date 2023/12/30 3:13 PM
 */
public class SynTest {

    private Object obj;

    public Object read() {
        synchronized (this) {
            while (Objects.isNull(obj)) {
                try {
                    System.out.println(Thread.currentThread().getName()+  " wait-before");
                    this.wait();
                    System.out.println(Thread.currentThread().getName()+  " wait-after");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            return obj;
        }
    }

    public void write() {
        System.out.println(Thread.currentThread().getName()+ "  write");
        synchronized (this) {
            obj = new Object();
            System.out.println(Thread.currentThread().getName()+ "  notifyAll-before");
            this.notifyAll();
            System.out.println(Thread.currentThread().getName()+ "   notifyAll-after");
        }
    }


    public static void main(String[] args) throws InterruptedException {
        SynTest synTest = new SynTest();

        new Thread(()-> {
            synTest.read();
        }).start();

        new Thread(()-> {
            synTest.write();
        }).start();

        TimeUnit.SECONDS.sleep(2);
    }

}

切记不能在main线程中启动，需要单独创建两个线程去执行，否则main线程阻塞的话，程序就会阻塞不会执行下去。

conditon

package com.jia.suspension;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

/**
 * @author qxlx
 * @date 2023/12/30 3:31 PM
 */
public class ConditionTest {

    private final Lock lock = new ReentrantLock();
    private final Condition condition = lock.newCondition();
    private Object obj;

    public Object read () {
        try {
            lock.lock();
            while (obj == null) {
                System.out.println("getLock");
                condition.await();
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
        return obj;
    }

    public void write() {
        try {
            lock.lock();
            obj = new Object();
            condition.signalAll();
            System.out.println("唤醒");
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    public static void main(String[] args) throws InterruptedException {
        ConditionTest test = new ConditionTest();

        new Thread(()-> {
            test.read();
        }).start();

        new Thread(()-> {
            test.write();
        }).start();

        TimeUnit.SECONDS.sleep(1);
    }


}

LockSouport

package com.jia.suspension;

import java.util.concurrent.locks.LockSupport;

/**
 * @author qxlx
 * @date 2023/12/30 3:38 PM
 */
public class LockSupportTest {

    private Object obj;

    public Object read() {
        while (obj == null) {
            System.out.println("read-线程等待");
            LockSupport.park();
            System.out.println("read-线程唤醒");
        }
        return obj;
    }

    public void write(Thread thread) {
        obj = new Object();
        LockSupport.unpark(thread);
        System.out.println("唤醒线程");
    }

    public static void main(String[] args) {
        LockSupportTest lockSupportTest = new LockSupportTest();

        Thread thread = new Thread(() -> {
            lockSupportTest.read();
        });

        thread.start();

        Thread thread2 = new Thread(() -> {
            lockSupportTest.write(thread);
        });

        thread2.start();
    }

}