【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();}}