使用ReentrantLock和Condition来代替内置锁和wait(),notify(),notifyAll()

使用ReentrantLock可以替代内置锁,当使用内置锁的时候,我们可以使用wait() nitify()和notifyAll()来控制线程之间的协作,那么,当我们使用ReentrantLock的时候,我们怎么来处理线程之间的写作呢?

JDK5.0为我们提供了Condition对象来替代内置锁的 wait(),notify()和notifyAll()方法

内置锁的话,就只能有一个等待队列,所有的在某个对象上执行wait()方法的线程都会被加入到该对象的等待队列中去(线程会被挂起),需要其他的线程在同一个对象上调用notify()或者是notifyAll()方法来唤醒等待队列中的线程

而使用Condition的话,可以使用不同的等待队列,只需要使用lock.newCondition()即可定义一个Condition对象,每一个Condition对象上都会有一个等待队列(底层使用AQS),调用某个Condition对象的await()方法,就可以把当前线程加入到这个Condition对象的等待队列上

其他的线程调用同一个Condition对象的sinal()或者是signalAll()方法则会唤醒等待队列上的线程,使其能够继续执行

我们以一个现实中的例子来说明若何使用ReentrantLock和Condition如何替代synchronized和wait(),notify(),notifyAll():

我们模拟两个线程,一个线程执行登录操作,该登录操作会阻塞,然后等待另外一个线程将其唤醒(类似扫描登录的场景,页面会阻塞,等待扫码和确认,然后页面才会跳转)

首先是使用内置锁的例子:

package com.jiaoyiping.baseproject.condition;

import java.io.Serializable;
import java.util.UUID;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.CountDownLatch;

/**
 * Created with Intellij IDEA
 *
 * @author: jiaoyiping
 * Mail: [email protected]
 * Date: 2019/04/12
 * Time: 15:29
 * To change this template use File | Settings | Editor | File and Code Templates
 */

//使用内置锁来实现的等待/通知模型

public class LoginServiceUseInnerLock {
    private ConcurrentHashMap<String, Result> loginMap = new ConcurrentHashMap<>();

    public static void main(String[] args) throws InterruptedException {
        CountDownLatch countDownLatch = new CountDownLatch(2);
        LoginServiceUseInnerLock loginService = new LoginServiceUseInnerLock();
        String uuid = UUID.randomUUID().toString();
        System.out.println("[" + Thread.currentThread().getName() + "] 使用的UUID是: " + uuid);
        new Thread(() -> {
            loginService.login(uuid, 20_000);
            countDownLatch.countDown();
        }, "登录线程").start();
        Thread.sleep(2_000);
        new Thread(() -> {
            loginService.confirm(uuid);
            countDownLatch.countDown();
        }, "确认线程").start();
        countDownLatch.await();
        System.out.println("[" + Thread.currentThread().getName() + "] 两个线程都执行完毕了");

    }


    public void login(String code, int timeout) {
        Result result = new Result();
        result.setMessage("超时");
        loginMap.put(code, result);
        synchronized (result) {
            try {
                //超时的话,会自动返回,程序继续
                System.out.println("[" + Thread.currentThread().getName() + "] 登录线程挂起");
                result.wait(timeout);
                System.out.println("[" + Thread.currentThread().getName() + "] 登录线程继续执行,得到的结果是:" + result.getMessage());
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                loginMap.remove(code);
            }
        }
    }

    public void confirm(String code) {
        assert code != null;
        Result result = loginMap.get(code);
        if (result == null) {
            System.out.println("[" + Thread.currentThread().getName() + "] 请求不存在或者已经过期");
            return;
        }
        result.setMessage("成功");
        synchronized (result) {
            //唤醒等待队列上的线程
            System.out.println("[" + Thread.currentThread().getName() + "] 确认线程开始唤醒阻塞的线程");
            result.notify();
        }

    }

    class Result implements Serializable {
        private static final long serialVersionUID = -4279280559711939661L;
        String message;

        public String getMessage() {
            return message;
        }

        public void setMessage(String message) {
            this.message = message;
        }

        public Result() {
        }
        public Result(String message) {
            this.message = message;
        }
    }

使用内置锁的时候,我们把random生成的key和一个自己定义的Result对象放置到ConcurrentHashMap中去,登录线程调用 Result对象的wait(timeout) 方法将当前线程挂起,并加入到Result对象的等待队列上去

确认线程根据key值,找到对应的Result对象,设置好message,然后调用Result对象的notify()方法唤醒等待队列上的线程,登录线程得以继续执行

那我们如何使用ReentrantLock和Condition来重写这个例子:

package com.jiaoyiping.baseproject.condition;

import java.io.Serializable;
import java.util.UUID;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

/**
 * Created with Intellij IDEA
 *
 * @author: jiaoyiping
 * Mail: [email protected]
 * Date: 2019/04/12
 * Time: 14:56
 * To change this template use File | Settings | Editor | File and Code Templates
 */

//使用ReentrantLock和Condition来实现的等待/通知模型

public class LoginServiceUseCondition {

    private ReentrantLock lock = new ReentrantLock();
    ConcurrentHashMap<String, Result> conditions = new ConcurrentHashMap<>();

    public static void main(String[] args) throws InterruptedException {
        LoginServiceUseCondition loginService = new LoginServiceUseCondition();
        String uuid = UUID.randomUUID().toString();
        System.out.println("[" + Thread.currentThread().getName() + "] 使用的UUID是:" + uuid);
        CountDownLatch countDownLatch = new CountDownLatch(2);
        new Thread(() -> {
            loginService.login(uuid, 30_000);
            countDownLatch.countDown();
        }, "登录线程").start();
        Thread.sleep(5_000);
        new Thread(() -> {
            try {
                Thread.sleep(3_000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            loginService.confirm(uuid);
            countDownLatch.countDown();
        }, "确认线程").start();
        countDownLatch.await();
        System.out.println("[" + Thread.currentThread().getName() + "] 两个线程都执行完毕了,退出");
    }


    /**
     * 过了超时时间之后,锁会自动释放
     *
     * @param code
     * @param timeout
     */
    public void login(String code, int timeout) {
        assert code != null;
        try {
            lock.tryLock(timeout, TimeUnit.MILLISECONDS);
            Condition condition = lock.newCondition();
            Result result = new Result("超时", condition);
            conditions.put(code, result);
            System.out.println("[" + Thread.currentThread().getName() + "] login()的请求开始阻塞");
            condition.await();
            System.out.println("[" + Thread.currentThread().getName() + "] 结束等待,继续执行,拿到的结果是" + result.getMessage());

        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }


    }

    //确认线程(拿这个UUID,去找到对应的Condition,唤醒上边的等待队列,并把Condition对象移除掉)

    public void confirm(String code) {
        assert code != null;
        Result result = conditions.get(code);
        Condition condition = result.getCondition();
        if (condition != null) {
            try {
                System.out.println("[" + Thread.currentThread().getName() + "] 找到对应的Condition对象,将其等待队列中的线程唤醒");
                lock.lock();
                result.setMessage("成功");
                condition.signal();
                conditions.remove(code);
            } finally {
                lock.unlock();
            }
        }
    }

    class Result implements Serializable {
        String message;
        final Condition condition;

        public Result(String message, Condition condition) {
            this.message = message;
            this.condition = condition;
        }

        public Result(Condition condition) {
            this.condition = condition;
        }

        public String getMessage() {
            return message;
        }

        public void setMessage(String message) {
            this.message = message;
        }

        public Condition getCondition() {
            return condition;
        }
    }

}    

上边的例子说明了怎么使用ReentrantLock和Condition来代替内置锁和wait(),notify(),notifyAll()

下边的一个来自jdk中的例子,演示了如何使用同一个ReentrantLock上的多个等待队列的情况

来自JDK文档中的示例(我稍加改造,加上了main方法和一些日志):

package com.jiaoyiping.baseproject.condition;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.stream.IntStream;

/**
 * Created with Intellij IDEA
 *
 * @author: jiaoyiping
 * Mail: [email protected]
 * Date: 2019/04/12
 * Time: 21:17
 * To change this template use File | Settings | Editor | File and Code Templates
 */

public class BoundedBuffer {
    final Lock lock = new ReentrantLock();
    final Condition notFull = lock.newCondition();
    final Condition notEmpty = lock.newCondition();

    final Object[] items = new Object[10];
    int putptr, takeptr, count;

    public static void main(String[] args) throws InterruptedException {
        BoundedBuffer boundedBuffer = new BoundedBuffer();
        CountDownLatch countDownLatch = new CountDownLatch(40);
        //分别启动20个put线程和20个take线程

        IntStream.rangeClosed(1, 20).forEach(i -> {
            new Thread(() -> {
                try {
                    boundedBuffer.put(new Object());
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } finally {
                    countDownLatch.countDown();
                }
            }, "put线程 - " + i).start();
        });

        IntStream.rangeClosed(1, 20).forEach(i -> {
            new Thread(() -> {
                try {
                    boundedBuffer.take();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } finally {
                    countDownLatch.countDown();
                }
            }, "take线程-" + i).start();
        });

        countDownLatch.await();
        System.out.println("[" + Thread.currentThread().getName() + "] 所有线程都执行完毕,退出");
    }

    public void put(Object x) throws InterruptedException {
        lock.lock();
        try {
            //put的线程,当队列满的时候挂起
            while (count == items.length) {
                System.out.println("[" + Thread.currentThread().getName() + "] 线程挂起");
                notFull.await();
            }
            Thread.sleep(1_000);
            items[putptr] = x;
            if (++putptr == items.length) {
                putptr = 0;
            }
            ++count;
            System.out.println("[" + Thread.currentThread().getName() + "] 执行完毕写操作,唤醒take线程");
            notEmpty.signal();
        } finally {
            lock.unlock();
        }
    }

    public Object take() throws InterruptedException {
        lock.lock();
        try {
            //take的线程,当队列为空的时候,挂起
            while (count == 0) {
                System.out.println("[" + Thread.currentThread().getName() + "] 线程挂起");
                notEmpty.await();
            }
            Thread.sleep(1_000);
            Object x = items[takeptr];
            if (++takeptr == items.length) {
                takeptr = 0;
            }
            --count;
            System.out.println("[" + Thread.currentThread().getName() + "] 执行完毕读操作,唤醒put线程");
            notFull.signal();
            return x;
        } finally {
            lock.unlock();
        }
    }
}    

我们看到,以上代码中,使用到了两个Condition:notFull和notEmpty,都是通过lock对象的newCondition()方法得来的

items被放满之后,put的线程会在notFull的等待队列上进行等待(执行了notFull.await()方法) put线程执行完操作之后,会调用 notEmpty.signal()来试图唤醒在notEmpty上等待的线程(也就是给take线程发了一个信号,告诉它,items不是空的了,你可以过来take了)

当item空了之后,take线程会在notEmpty的等待队列上进行等待(执行了notEmpty的await()方法) 当take线程执行完操作之后,会调用notFull.signal()来唤醒在notFull上等待的线程(也就是给put线程发一个信号,告诉它,items不满了,你可以进行put操作了)

和内置方法类似,在调用await(),signal(),signalAll()等方法的时候,也必须要获得锁,也就是必须在 lock.lock()和lock.unlock()代码块儿之间才能调用这些方法,否则就会抛出IllegalMonitorStateException