wait/notify使用详解

1. 使用注意事项

1. wait/notify(All)可用于线程间(线程数量>3)通信

2. 永远在synchronized方法或对象里使用wait/notify(All),不然JVM报java.lang.IllegalMonitorStateException

3. 永远在while循环里使用wait，防止其他原因改变先前判断条件

4. 永远在线程间共享对象(生产者消费者中为缓冲区队列)上使用wait/notify(All)

5. 多线程间协作，更倾向于使用notifyAll，唤醒在该对象上等待的全部线程

2. 实现生产者消费者

public class ProducerConsumerDemo {public static void main(String[] args) {Queue<Integer> queue = new LinkedList<>();int maxSize = 10;Thread producer = new Thread(new Producer(queue, maxSize));producer.setName("Producer");Thread consumerOne = new Thread(new Consumer(queue, maxSize));consumerOne.setName("ConsumerOne");Thread consumerTwo = new Thread(new Consumer(queue, maxSize));consumerTwo.setName("ConsumerTwo");producer.start();consumerOne.start();consumerTwo.start();}
}class Producer implements Runnable {private Queue<Integer> queue;private int maxSize;public Producer(Queue<Integer> queue, int maxSize) {this.queue = queue;this.maxSize = maxSize;}@Overridepublic void run() {Random random = new Random();while (true) {synchronized (queue) {while (queue.size() == maxSize) {try {System.out.println("Queue is full, Producer " + Thread.currentThread().getName() + " waiting");queue.wait();} catch (InterruptedException e) {e.printStackTrace();}}int value = random.nextInt();System.out.println("Producer " + value);queue.add(value);queue.notifyAll();try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}}}}
}class Consumer implements Runnable {private Queue<Integer> queue;private int maxSize;public Consumer(Queue<Integer> queue, int maxSize) {this.queue = queue;this.maxSize = maxSize;}@Overridepublic void run() {while (true) {// synchronized方法或对象里使用wait/notify(All),不然JVM报java.lang.IllegalMonitorStateExceptionsynchronized (queue) {// while不可改为if,如果改为if，则可能抛出java.util.NoSuchElementException// 当前等待的线程被唤醒时，但是由于其他消费者刚好消费完，使得队列为空// 此时如果不重新判断队列为空，代码继续向下执行queue.remove势必抛出java.util.NoSuchElementException// 若放入while循环中重新判断条件，若条件不满足，线程则继续挂起，无影响while (queue.isEmpty()) {try {System.out.println("Queue is empty, Consumer " + Thread.currentThread().getName() + " waiting");queue.wait(); // 在线程间共享对象(生产者消费者中为缓冲区队列)上使用wait/notify(All)} catch (InterruptedException e) {e.printStackTrace();}}System.out.println(Thread.currentThread().getName() + " consume " + queue.remove());queue.notifyAll(); // 多线程间协作，更倾向于使用notifyAll，唤醒在该对象上等待的全部线程try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}}}}
}

执行结果：

Queue is empty, Consumer ConsumerOne waiting
Queue is empty, Consumer ConsumerTwo waiting
Producer -1203276467
Producer 626398544
Producer -108152878
Producer -705401944
ConsumerTwo consume -1203276467
ConsumerTwo consume 626398544
ConsumerTwo consume -108152878
ConsumerTwo consume -705401944
Queue is empty, Consumer ConsumerTwo waiting
Queue is empty, Consumer ConsumerOne waiting
Producer -1487489511
Producer 420807949
Producer 428506738
ConsumerOne consume -1487489511
ConsumerTwo consume 420807949
ConsumerTwo consume 428506738
Queue is empty, Consumer ConsumerTwo waiting
Queue is empty, Consumer ConsumerOne waiting
Producer 1411031303
Producer 1426589341
Producer -1307293143
Producer -970522822
Producer -2074755062
ConsumerOne consume 1411031303
ConsumerOne consume 1426589341

3. wait/nofity内部运行细节

1. 使用wait()、notify()、notifyAll()时需要先对对象加锁
2. 调用wait()方法后，线程状态由RUNNING变为WAITTING,并将当前线程放置到对象的等待队列
3. notify()、notifyAll()方法调用后，等待线程依旧不会从wait()返回，需要调用notify()、notify()的线程释放锁后，等待线程才有机会从wait()返回
4. notify()、notifyAll()方法将线程从等待队列移到同步队列(前者移1个，后者移全部),被移动的线程由WAITTING变为BLOCKED
5. 从wait()方法返回的前提是获得了调用对象的锁[这也是3)中为啥说有机会返回的原因]

4. sleep与wait的区别

sleep是Thread中定义的方法, wait是Object中定义的方法;

可以在任何地方调用线程对象的sleep方法，wait方法只能在同步代码块或同步方法中调用；

调用线程对象的sleep方法后，不释放锁，调用对象的wait方法，释放对象获得的锁