线程间通信 Object/wait(),notify() 和 Lock/Condition/await(),signal()

Posted 2020-11-26 myseries

 

基本前提知识：

一：Object/wait(), notify(), notifyAll()

　　1：wait() 方法暂停当前线程，并立即释放对象锁;

　　2：notify()/notifyAll() 方法唤醒其他等待该对象锁的线程,并在执行完同步代码块中的后续步骤后，释放对象锁

　　3：notify()和notifyAll()的区别在于：

    　　notify只会唤醒其中一个线程，
    　   notifyAll则会唤醒全部线程。
    
　　　 至于notify会唤醒哪个线程，是由线程调度器决定的。

例子：

public class TestWaitAndnotify {

    public static void main(String[] args) {
        demo2();
    }
    
    public static void demo2 () {
        final Object lock = new Object();
        Thread A = new Thread(new Runnable(){

            @Override
            public void run() {
                System.out.println("INFO: A 等待锁 ");
                synchronized (lock) {
                    System.out.println("INFO: A 得到了锁 lock");
                    System.out.println("A1");
                    try {
                        System.out.println("INFO: A 准备进入等待状态，放弃锁 lock 的控制权 ");
                        lock.wait();//挂起线程A 放弃锁 lock 的控制权
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println("INFO: 有人唤醒了 A, A 重新获得锁 lock");
                    System.out.println("A2");
                    System.out.println("A3");
                }
            }
        });
        
        Thread B = new Thread(new Runnable() {

            @Override
            public void run() {
                 System.out.println("INFO: B 等待锁 ");
                synchronized (lock) {
                    System.out.println("INFO: B 得到了锁 lock");
                    System.out.println("B1");
                    System.out.println("B2");
                    System.out.println("B3");
                    System.out.println("INFO: B 打印完毕，调用 notify 方法 ");
                    lock.notify(); // notify()方法唤醒正在等待lock锁的线程A
                    System.out.println("线程 B do notify method 完毕");
                }
            }
        });
        
        A.start();
        B.start();
    } 
}

输出：

 

生产者消费者例子：

import java.util.LinkedList;
import java.util.Queue;

public class ProducerAndConsumer {
    
    private final int MAX_LEN = 3;
    private Queue<Integer> queue = new LinkedList<Integer>();
    
    class Producer extends Thread {
        @Override
        public void run() {
            producer();
        }
        
        private void producer() {
            while(true) {
                synchronized (queue) {
                    if(queue.size() == MAX_LEN) {
                        System.out.println("当前队列满");
                        queue.notify();
                        try {
                            queue.wait();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    queue.add(1);
                    queue.notify();
                    try {
                        Thread.sleep(500);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println("生产者又生产一条任务，当前队列长度为" + queue.size());
                    
                }
            }
        }
    }
    
    class Consumer extends Thread {
        @Override
        public void run() {
            consumer();
        }
        private void consumer() {
            while(true) {
                synchronized (queue) {
                    if(queue.size() == 0) {
                        System.out.println("当前队列为空");
                        queue.notify();
                        try {
                            queue.wait();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                        
                    }
                    queue.poll();
                    queue.notify();
                    try {
                        Thread.sleep(500);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println("消费者消费了一条任务，当前队列长度为" + queue.size());
                }
            }
        }
    }
    
    public static void main(String[] args) {
        ProducerAndConsumer pac = new ProducerAndConsumer();
        
        Producer producer = pac.new Producer();
        Consumer consumer = pac.new Consumer();
        producer.start();
        consumer.start();
    }
}

 

一：Lock/condition/await(), signal(), signalAll()

import java.util.LinkedList;
import java.util.Queue;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class ProducerAndConsumer1 {
    
    private final int MAX_LEN = 3;
    private Queue<Integer> queue = new LinkedList<Integer>();

    final Lock lock = new ReentrantLock();
    final Condition producerSignal = lock.newCondition();
    final Condition consumerSignal = lock.newCondition();

    class Producer extends Thread {
        @Override
        public void run() {
            producer();
        }
        
        private void producer() {
            while(true) {
                lock.lock();
                try {
                    if(queue.size() == MAX_LEN) {
                        System.out.println("当前队列满");
                        try {
                            consumerSignal.signal();
                            producerSignal.await();    
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    queue.add(1);
                    consumerSignal.signal();
                    try {
                        Thread.sleep(500);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println("生产者生产了一条任务，当前队列长度为" + queue.size());
                    
                } finally {
                    lock.unlock();
                }
            }

        }
    }
    
    class Consumer extends Thread {
        @Override
        public void run() {
            consumer();
        }
        
        private void consumer() {
            while(true) {
                lock.lock();
                try {
                    if(queue.size() == 0) {
                        System.out.println("当前队列为空");
                        try {
                            producerSignal.signal();
                            consumerSignal.await();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    queue.poll();
                    producerSignal.signal();
                    try {
                        Thread.sleep(500);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println("消费者消费了一条任务，当前队列长度为" + queue.size());
                    
                } finally {
                    lock.unlock();
                }
            }

        }
    }
    
    public static void main(String[] args) {
        ProducerAndConsumer1 pac = new ProducerAndConsumer1();
        
        Producer producer = pac.new Producer();
        Consumer consumer = pac.new Consumer();
        producer.start();
        consumer.start();
    }

}

分析： 这里共享的变量是queue，生产者消费者都需要修改这个queue，为了保证安全，需要一个个的来，用lock,lock()就保证了每次只有一个线程来操作queue；

　　　在生产和消费的时候，需要关心queue.size()的大小，如果满了，就不能生产，这里用了producerSignal.await()，让生产者放弃锁，去等待消费者去消费，consumerSignal.signal()去唤醒消费者去消费，如果空了，就不能消费，　这里用consumerSignal.await()，让消费者放弃锁，去等待生产者去生产，producerSignal.signal()去唤醒生产者去生产。