本文主要是介绍3.6、wait notify join原理及应用，同步模式之保护性暂停，异步模式之生产者消费者，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
小故事 - 为什么要wait
- 由于条件不满足，小南不能继续进行计算
- 但小南如果一直占用着锁，其他人就得一直阻塞，效率太低
- 于是老王单开了一键休息室（调用wait方法），让小南到休息室（WaitSet）等着去了，但这是锁释放开，其他人可以由老王随机安排进屋
- 知道小M将烟送来，大叫一声【你的烟到了】（调用notify方法）
- 小南于是可以离开休息室，重新进入竞争锁的队列
原理
- Owner线程发现条件不满足，调用wait方法，即可进入WaitSet变为WAITING状态
- BLOCKED和WAITING的线程都处于阻塞状态，不占用CPU时间片
- BLOCKED线程会在Owner线程释放锁时唤醒
- WAITING线程会在Owner线程调用notify或notifyAll时唤醒，单唤醒后并不意味着立即获得锁，仍需要进入EntryList重新竞争
join原理
调用者轮询检查线程alive状态
t1.join()
等价于下面的代码
synchronized(t1) {// 调用者线程进入t1的WaitSet等待，直到t1运行结束while(t1.isAlive()) {t1.wait(0);}
}
API介绍
obj.wait()让进入object监视器的线程到WaitSet等待obj.notify()在object上正在WaitSet等待的线程中挑一个唤醒obj.notifyAll()让object上正在WaitSet等待的线程全部唤醒
他们都是线程之间进行协作的手段，都属于Object对象的方法，必须获得此对象的锁，才能调用这几个方法
final static Object obj = new Object();public static void main(String[] args) throws InterruptedException {new Thread(() -> {synchronized (obj) {log.debug("执行...");try {obj.wait();} catch (InterruptedException e) {e.printStackTrace();}log.debug("其他代码...");}}, "t1").start();new Thread(() -> {synchronized (obj) {log.debug("执行...");try {obj.wait();} catch (InterruptedException e) {e.printStackTrace();}log.debug("其他代码...");}}, "t2").start();TimeUnit.SECONDS.sleep(2);log.debug("唤醒obj上的其他线程");
// obj.notify();synchronized (obj) {obj.notify();
// obj.notifyAll();}}
如果没有获得对象锁，则会报错
2022/03/06-03:56:00.386 [t1] c.Test1 - 执行...
2022/03/06-03:56:00.388 [t2] c.Test1 - 执行...
2022/03/06-03:56:02.386 [main] c.Test1 - 唤醒obj上的其他线程
Exception in thread "main" java.lang.IllegalMonitorStateExceptionat java.lang.Object.notify(Native Method)at waittest.Test1.main(Test1.java:43)
notify的一种结果
2022/03/06-03:57:15.787 [t1] c.Test1 - 执行...
2022/03/06-03:57:15.789 [t2] c.Test1 - 执行...
2022/03/06-03:57:17.789 [main] c.Test1 - 唤醒obj上的其他线程
2022/03/06-03:57:17.790 [t1] c.Test1 - 其他代码...
notifyAll的结果
2022/03/06-03:57:45.530 [t1] c.Test1 - 执行...
2022/03/06-03:57:45.532 [t2] c.Test1 - 执行...
2022/03/06-03:57:47.544 [main] c.Test1 - 唤醒obj上的其他线程
2022/03/06-03:57:47.544 [t2] c.Test1 - 其他代码...
2022/03/06-03:57:47.544 [t1] c.Test1 - 其他代码...
wait()方法会释放对象的锁，进入WaitSet等待区，从而让其他线程有机会获得对象的锁。无限制等待，直到notify为止
wait(long n)有时限的等待，到n毫秒后结束等待，或是被notify
wait notify的正确姿势
开始之前先看看
sleep(long n)和wait(long n)区别
1）sleep是Thread方法，而wait是Object方法
2）sleep不需要强制和synchronized配合使用，但wait需要和synchronized一起用
3）sleep在睡眠时，不会释放对象锁的，但wait在等待的时候会释放对象锁
4）他们的状态都是TIMED_WAITING
step 1
思考下面的解决方案好不好，为什么？
final static Object room = new Object();static boolean hasCigarette = false;static boolean hasTakeout = false;public static void main(String[] args) throws InterruptedException {new Thread(() -> {synchronized (room) {log.debug("有没有烟？{}", hasCigarette);if (!hasCigarette) {log.debug("没有烟，歇会儿");try {Thread.sleep(2000);} catch (InterruptedException e) {e.printStackTrace();}}log.debug("有没有烟？{}", hasCigarette);if (hasCigarette) {log.debug("可以干活了");}}}, "t1").start();for (int i = 0; i < 5; i++)new Thread(() -> {synchronized (room) {log.debug("可以干活哦...");}}, "其他人").start();Thread.sleep(1000);new Thread(() -> {hasCigarette = true;log.debug("烟来喽");}, "送烟的").start();}
输出
2022/03/06-13:27:54.211 [t1] c.Test2 - 有没有烟？false
2022/03/06-13:27:54.213 [t1] c.Test2 - 没有烟，歇会儿
2022/03/06-13:27:55.213 [送烟的] c.Test2 - 烟来喽
2022/03/06-13:27:56.217 [t1] c.Test2 - 有没有烟？true
2022/03/06-13:27:56.217 [t1] c.Test2 - 可以干活了
2022/03/06-13:27:56.217 [其他人] c.Test2 - 可以干活哦...
2022/03/06-13:27:56.217 [其他人] c.Test2 - 可以干活哦...
2022/03/06-13:27:56.217 [其他人] c.Test2 - 可以干活哦...
2022/03/06-13:27:56.217 [其他人] c.Test2 - 可以干活哦...
2022/03/06-13:27:56.217 [其他人] c.Test2 - 可以干活哦...
- 其他干活的线程，要一直阻塞，效率太低
- t1线程必须睡足2s后才能醒来，就算烟提前送到，也无法立刻醒来
- 加了synchronized(room)后，就好比t1在里面反锁了门睡觉，烟根本没法送进门，main没加synchronized就好像main线程是翻窗户进来的
- 解决办法，使用wait-notify机制
step 2
final static Object room = new Object();static boolean hasCigarette = false;static boolean hasTakeout = false;public static void main(String[] args) throws InterruptedException {new Thread(() -> {synchronized (room) {log.debug("有没有烟？{}", hasCigarette);if (!hasCigarette) {log.debug("没有烟，歇会儿");try {room.wait(2000);} catch (InterruptedException e) {e.printStackTrace();}}log.debug("有没有烟？{}", hasCigarette);if (hasCigarette) {log.debug("可以干活了");}}}, "t1").start();for (int i = 0; i < 5; i++)new Thread(() -> {synchronized (room) {log.debug("可以干活哦...");}}, "其他人").start();Thread.sleep(1000);new Thread(() -> {synchronized (room) {hasCigarette = true;log.debug("烟来喽");room.notify();}}, "送烟的").start();}
输出
2022/03/06-13:32:21.203 [t1] c.Test3 - 有没有烟？false
2022/03/06-13:32:21.203 [t1] c.Test3 - 没有烟，歇会儿
2022/03/06-13:32:21.203 [其他人] c.Test3 - 可以干活哦...
2022/03/06-13:32:21.203 [其他人] c.Test3 - 可以干活哦...
2022/03/06-13:32:21.203 [其他人] c.Test3 - 可以干活哦...
2022/03/06-13:32:21.203 [其他人] c.Test3 - 可以干活哦...
2022/03/06-13:32:21.203 [其他人] c.Test3 - 可以干活哦...
2022/03/06-13:32:22.202 [送烟的] c.Test3 - 烟来喽
2022/03/06-13:32:22.202 [t1] c.Test3 - 有没有烟？true
2022/03/06-13:32:22.202 [t1] c.Test3 - 可以干活了
- 解决了其他线程干活的线程阻塞问题
- 但如果有其他线程也在等待条件呢？
step 3
final static Object room = new Object();static boolean hasCigarette = false;static boolean hasTakeout = false;public static void main(String[] args) throws InterruptedException {new Thread(() -> {synchronized (room) {log.debug("有没有烟？{}", hasCigarette);if (!hasCigarette) {log.debug("没有烟，歇会儿");try {room.wait();} catch (InterruptedException e) {e.printStackTrace();}}log.debug("有没有烟？{}", hasCigarette);if (hasCigarette) {log.debug("可以干活了");} else {log.debug("干活失败");}}}, "t1").start();new Thread(() -> {synchronized (room) {log.debug("外卖来了吗？{}", hasCigarette);if (!hasTakeout) {log.debug("没外卖，歇会儿");try {room.wait();} catch (InterruptedException e) {e.printStackTrace();}}log.debug("外卖来了吗？{}", hasCigarette);if (hasTakeout) {log.debug("可以干活了");} else {log.debug("干活失败");}}}, "t2").start();Thread.sleep(1000);new Thread(() -> {synchronized (room) {hasTakeout = true;log.debug("外卖来喽");room.notify();}}, "送外卖的").start();}
输出
2022/03/06-13:37:13.816 [t1] c.Test4 - 有没有烟？false
2022/03/06-13:37:13.816 [t1] c.Test4 - 没有烟，歇会儿
2022/03/06-13:37:13.816 [t2] c.Test4 - 外卖来了吗？false
2022/03/06-13:37:13.816 [t2] c.Test4 - 没外卖，歇会儿
2022/03/06-13:37:14.823 [送外卖的] c.Test4 - 外卖来喽
2022/03/06-13:37:14.823 [t1] c.Test4 - 有没有烟？false
2022/03/06-13:37:14.823 [t1] c.Test4 - 干活失败
- notify只能随机唤醒一个WaitSet中的线程，这时如果有其他线程也在等待，那么就可能唤醒不了正确的线程，称之为【虚假唤醒】
- 解决方法，改为notifyAll
step 4
new Thread(() -> {synchronized (room) {hasTakeout = true;log.debug("外卖来喽");room.notifyAll();}}, "送外卖的").start();
输出
2022/03/06-13:39:27.661 [t1] c.Test5 - 有没有烟？false
2022/03/06-13:39:27.661 [t1] c.Test5 - 没有烟，歇会儿
2022/03/06-13:39:27.661 [t2] c.Test5 - 外卖来了吗？false
2022/03/06-13:39:27.661 [t2] c.Test5 - 没外卖，歇会儿
2022/03/06-13:39:28.675 [送外卖的] c.Test5 - 外卖来喽
2022/03/06-13:39:28.675 [t2] c.Test5 - 外卖来了吗？false
2022/03/06-13:39:28.675 [t2] c.Test5 - 可以干活了
2022/03/06-13:39:28.675 [t1] c.Test5 - 有没有烟？false
2022/03/06-13:39:28.675 [t1] c.Test5 - 干活失败
- 用notifyAll仅解决某个线程的唤醒问题，但使用if + wait 判断仅有一次机会，一单条件不成立，就没看有重新判断的机会了
- 解决办法，用while + wait，当条件不成立，再次wait
step 5
将 if 改为 while
final static Object room = new Object();static boolean hasCigarette = false;static boolean hasTakeout = false;public static void main(String[] args) throws InterruptedException {new Thread(() -> {synchronized (room) {log.debug("有没有烟？{}", hasCigarette);while (!hasCigarette) {log.debug("没有烟，歇会儿");try {room.wait();} catch (InterruptedException e) {e.printStackTrace();}}log.debug("可以干活了");}}, "t1").start();new Thread(() -> {synchronized (room) {log.debug("外卖来了吗？{}", hasCigarette);while (!hasTakeout) {log.debug("没外卖，歇会儿");try {room.wait();} catch (InterruptedException e) {e.printStackTrace();}}log.debug("可以干活了");}}, "t2").start();Thread.sleep(1000);new Thread(() -> {synchronized (room) {hasTakeout = true;log.debug("外卖来喽");room.notifyAll();}}, "送外卖的").start();}
输出
2022/03/06-13:43:45.737 [t1] c.Test6 - 有没有烟？false
2022/03/06-13:43:45.737 [t1] c.Test6 - 没有烟，歇会儿
2022/03/06-13:43:45.743 [t2] c.Test6 - 外卖来了吗？false
2022/03/06-13:43:45.743 [t2] c.Test6 - 没外卖，歇会儿
2022/03/06-13:43:46.751 [送外卖的] c.Test6 - 外卖来喽
2022/03/06-13:43:46.751 [t2] c.Test6 - 可以干活了
2022/03/06-13:43:46.751 [t1] c.Test6 - 没有烟，歇会儿
同步模式之保护性暂停
1、定义
即Guarded Suspension，用在一个线程等待另一个线程的执行结果
要点
- 有一个结果需要从一个线程传递给另一个线程，让他们关联同一个GuardedObject
- 如果有结果不断从一个线程到另一个线程，那么可以使用消息队列（生产者/消费者）
- JDK中，join的实现、Future的实现，采用的就是此模式
- 因为要等待另一方的结果，因此归类到同步模式
2、实现
@Slf4j(topic = "c.GuardedSuspension")
public class GuardedSuspension {public static void main(String[] args) {GuardedObject guardedObject = new GuardedObject();new Thread(()->{try {Thread.sleep(2000);} catch (InterruptedException e) {e.printStackTrace();}Object response = new Object();log.debug("new response : {}", response);guardedObject.complete(response);}).start();log.debug("wait response");Object response = guardedObject.get();log.debug("get response : {}", response);}}class GuardedObject {private Object response;private final Object lock = new Object();public Object get(){synchronized (lock) {while (response == null) {try {lock.wait();} catch (InterruptedException e) {e.printStackTrace();}}return response;}}public void complete(Object response) {synchronized (lock) {this.response = response;lock.notifyAll();}}}
输出
2022/03/06-14:00:11.709 [main] c.TPTInturrept - wait response
2022/03/06-14:00:13.724 [Thread-0] c.TPTInturrept - new response : java.lang.Object@77f37d84
2022/03/06-14:00:13.724 [main] c.TPTInturrept - get response : java.lang.Object@77f37d84
3、带超时版的GuardedObject
@Slf4j(topic = "c.GuardedObjectV2")
class GuardedObjectV2 {private Object response;private final Object lock = new Object();public Object get(long millis) {synchronized (lock) {// 开始时间long begin = System.currentTimeMillis();// 已经过时间long timePassed = 0;while (response == null) {// 还需要等待时间long waitTime = millis - timePassed;if (waitTime <= 0) {log.debug("超时了...");break;}try {lock.wait(waitTime);} catch (InterruptedException e) {e.printStackTrace();}timePassed = System.currentTimeMillis() - begin;log.debug("timePassed:{}, object is null? {} " , timePassed, response == null);}return response;}}public void complete(Object response) {synchronized (lock) {this.response = response;log.debug("notify...");lock.notifyAll();}}}
测试，没有超时
public static void main(String[] args) {GuardedObjectV2 guardedObjectV2 = new GuardedObjectV2();new Thread(() -> {try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}guardedObjectV2.complete(null);try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}guardedObjectV2.complete(new Object());}).start();log.debug("wait response");Object response = guardedObjectV2.get(2500);log.debug("get response : {}", response);}
输出
2022/03/06-14:32:05.620 [main] c.GuardedSuspension - wait response
2022/03/06-14:32:06.630 [Thread-0] c.GuardedObjectV2 - notify...
2022/03/06-14:32:06.630 [main] c.GuardedObjectV2 - timePassed:1010, object is null? true
2022/03/06-14:32:07.634 [Thread-0] c.GuardedObjectV2 - notify...
2022/03/06-14:32:07.634 [main] c.GuardedObjectV2 - timePassed:2014, object is null? false
2022/03/06-14:32:07.634 [main] c.GuardedSuspension - get response : java.lang.Object@3f0ee7cb
测试，超时
Object response = guardedObjectV2.get(1500);
输出
2022/03/06-14:34:23.324 [main] c.GuardedSuspension - wait response
2022/03/06-14:34:24.330 [Thread-0] c.GuardedObjectV2 - notify...
2022/03/06-14:34:24.330 [main] c.GuardedObjectV2 - timePassed:995, object is null? true
2022/03/06-14:34:24.839 [main] c.GuardedObjectV2 - timePassed:1504, object is null? true
2022/03/06-14:34:24.839 [main] c.GuardedObjectV2 - 超时了...
2022/03/06-14:34:24.839 [main] c.GuardedSuspension - get response : null
2022/03/06-14:34:25.333 [Thread-0] c.GuardedObjectV2 - notify...
4、多任务版GuardedObject
途中Futures就好比居民楼一层的信箱（每个信箱有房间编号），左侧的t0，t2，t4就好比等待邮件的居民，右侧的t1，t3，t4就好比邮递员
如果需要在多个类之间使用GuardedObject对象，作为参数传递不是很方便，因此设计一个用来解耦的中间类，这样不仅能够解耦【结果等待者】和【结果消费者】，还能同时支持多个任务管理
新增id来表示GuardedObject
@Slf4j(topic = "c.GuardedObjectV3")
class GuardedObjectV3 {private int id;public GuardedObjectV3(int id) {this.id = id;}public int getId() {return id;}private Object response;public Object get(long millis) {synchronized (this) {// 开始时间long begin = System.currentTimeMillis();// 已经过时间long timePassed = 0;while (response == null) {// 还需要等待时间long waitTime = millis - timePassed;if (waitTime <= 0) {log.debug("超时了...");break;}try {this.wait(waitTime);} catch (InterruptedException e) {e.printStackTrace();}timePassed = System.currentTimeMillis() - begin;log.debug("timePassed:{}, object is null? {} ", timePassed, response == null);}return response;}}public void complete(Object response) {synchronized (this) {this.response = response;log.debug("notify...");this.notifyAll();}}}
中间解耦类
class Mailboxs {private static final Map<Integer, GuardedObjectV3> boxes = new Hashtable<>();private static int id = 0;private static synchronized int generateId() {return ++id;}public static GuardedObjectV3 getGuardedObject(int id) {return boxes.remove(id);}public static GuardedObjectV3 createGuardedObject() {GuardedObjectV3 guardedObjectV3 = new GuardedObjectV3(generateId());boxes.put(guardedObjectV3.getId(), guardedObjectV3);return guardedObjectV3;}public static Set<Integer> getIds() {return boxes.keySet();}}
业务相关类及接口
@Slf4j(topic = "c.PeopleAction")
class PeopleAction implements Runnable {@Overridepublic void run() {// 收信GuardedObjectV3 guardedObjectV3 = Mailboxs.createGuardedObject();log.debug("开售收信，id:{}", guardedObjectV3.getId());Object mail = guardedObjectV3.get(5000);log.debug("收到信了，id:{}，内容:{}", guardedObjectV3.getId(), mail);}
}
@Slf4j(topic = "c.Postman")
class Postman extends Thread {private int id;private String mail;public Postman(int id, String mail) {this.id = id;this.mail = mail;}@Overridepublic void run() {// 收信GuardedObjectV3 guardedObjectV3 = Mailboxs.getGuardedObject(id);log.debug("开售送信，id:{}，内容:{}", guardedObjectV3.getId(), mail);guardedObjectV3.complete(mail);}
}
测试
public static void main(String[] args) throws InterruptedException {for (int i = 0; i < 3; i++) {new Thread(new PeopleAction()).start();}Thread.sleep(1000);for (Integer id : Mailboxs.getIds()) {new Postman(id, "内容" + id).start();}}
输出
2022/03/06-15:02:11.244 [Thread-0] c.PeopleAction - 开售收信，id:1
2022/03/06-15:02:11.244 [Thread-1] c.PeopleAction - 开售收信，id:2
2022/03/06-15:02:11.244 [Thread-2] c.PeopleAction - 开售收信，id:3
2022/03/06-15:02:12.241 [Thread-4] c.Postman - 开售送信，id:2，内容:内容2
2022/03/06-15:02:12.241 [Thread-3] c.Postman - 开售送信，id:3，内容:内容3
2022/03/06-15:02:12.241 [Thread-4] c.GuardedObjectV3 - notify...
2022/03/06-15:02:12.241 [Thread-3] c.GuardedObjectV3 - notify...
2022/03/06-15:02:12.241 [Thread-5] c.Postman - 开售送信，id:1，内容:内容1
2022/03/06-15:02:12.241 [Thread-5] c.GuardedObjectV3 - notify...
2022/03/06-15:02:12.241 [Thread-2] c.GuardedObjectV3 - timePassed:997, object is null? false
2022/03/06-15:02:12.241 [Thread-1] c.GuardedObjectV3 - timePassed:997, object is null? false
2022/03/06-15:02:12.241 [Thread-0] c.GuardedObjectV3 - timePassed:997, object is null? false
2022/03/06-15:02:12.241 [Thread-1] c.PeopleAction - 收到信了，id:2，内容:内容2
2022/03/06-15:02:12.241 [Thread-0] c.PeopleAction - 收到信了，id:1，内容:内容1
2022/03/06-15:02:12.241 [Thread-2] c.PeopleAction - 收到信了，id:3，内容:内容3
异步模式之生产者/消费者
1、定义
要点
- 与前面的保护性暂停中的GuardedObject不同，不需要产生结果和消费结果的线程一一对应
- 消费队列可以用来平衡生产和消费的线程资源
- 生产者仅负责产生结果数据，不关心数据如何处理，而消费者专心处理数据结果
- 消息队列是有容量限制的，满时不再加入数据，空时不再消耗数据
- JDK中各种阻塞队列，采用的就是这种模式
2、实现
class Message {private int id;private Object Object;public Message(int id, java.lang.Object object) {this.id = id;Object = object;}public int getId() {return id;}public java.lang.Object getObject() {return Object;}
}@Slf4j(topic = "c.MessageQueue")
class MessageQueue {private int capacity;private final Deque<Message> queue;public MessageQueue(int capacity) {this.capacity = capacity;queue = new LinkedList<>();}public void put(Message message) {synchronized (queue) {while (queue.size() == capacity) {log.debug("队列已满,wait");try {queue.wait();} catch (InterruptedException e) {e.printStackTrace();}}log.debug("生产消息,id:{},内容:{}", message.getId(), message.getObject());queue.addLast(message);queue.notifyAll();}}public Message take() {synchronized (queue) {while (queue.isEmpty()) {log.debug("没有可用的消息,wait");try {queue.wait();} catch (InterruptedException e) {e.printStackTrace();}}Message message = queue.removeFirst();log.debug("消费消息,id:{},内容:{}", message.getId(), message.getObject());queue.notifyAll();return message;}}
}
测试
public static void main(String[] args) {MessageQueue messageQueue = new MessageQueue(2);for (int i = 0; i < 4; i++) {int finalI = i;new Thread(() -> {messageQueue.put(new Message(finalI, "内容" + finalI));}, "生产者" + i).start();}new Thread(() -> {while (true) {Message message = messageQueue.take();}}, "消费者").start();}
输出
2022/03/06-15:56:31.921 [消费者] c.MessageQueue - 没有可用的消息,wait
2022/03/06-15:56:31.921 [生产者3] c.MessageQueue - 生产消息,id:3,内容:内容3
2022/03/06-15:56:31.930 [消费者] c.MessageQueue - 消费消息,id:3,内容:内容3
2022/03/06-15:56:31.930 [消费者] c.MessageQueue - 没有可用的消息,wait
2022/03/06-15:56:31.930 [生产者1] c.MessageQueue - 生产消息,id:1,内容:内容1
2022/03/06-15:56:31.930 [生产者0] c.MessageQueue - 生产消息,id:0,内容:内容0
2022/03/06-15:56:31.930 [生产者2] c.MessageQueue - 队列已满,wait
2022/03/06-15:56:31.930 [消费者] c.MessageQueue - 消费消息,id:1,内容:内容1
2022/03/06-15:56:31.930 [消费者] c.MessageQueue - 消费消息,id:0,内容:内容0
2022/03/06-15:56:31.930 [消费者] c.MessageQueue - 没有可用的消息,wait
2022/03/06-15:56:31.930 [生产者2] c.MessageQueue - 生产消息,id:2,内容:内容2
2022/03/06-15:56:31.930 [消费者] c.MessageQueue - 消费消息,id:2,内容:内容2
2022/03/06-15:56:31.930 [消费者] c.MessageQueue - 没有可用的消息,wait
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