ReadWriteLock

ReadWriteLock的作用：

只允许一个线程写入（其他线程既不能写入也不能读取）；
没有写入时，多个线程允许同时读（提高性能）。
如何声明读写锁：

	private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
    private final Lock readLock  = lock.readLock();
    private final Lock writeLock = lock.writeLock();

运行下面的代码发现：
读写锁并发的时候，读的数据并不是写入的终态数据
打印的队列的长度

package org.meituan.javalearn.thread;

import lombok.SneakyThrows;

import java.util.LinkedList;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

/**
 * @projectName: codebetter
 * @package: org.meituan.javalearn.thread
 * @className: ReadWriteQueue
 * @author: fangjiayueyuan
 * @description: TODO
 * @date: 2023/5/8 下午10:01
 * @version: 1.0
 */
public class ReadWriteQueue {
    public static void main(String[] args) throws InterruptedException {
        LinkedList<Thread> threadPools = new LinkedList<Thread>();
        final ReadWriteTaskQueue taskQueue = new ReadWriteTaskQueue();
        for(int i=0;i<199995;i++){
            threadPools.add(new Thread(new Runnable() {
                @SneakyThrows
                @Override
                public void run() {
                    System.out.println(taskQueue.getTask());
                }
            }));
        }
        Thread addThread = new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 1005; i++) {
                    taskQueue.addTask("The"+i+"th task to be closed");
                }
            }
        });
        addThread.start(); // 读写锁无法进行wait和notify???
        for(Thread thread:threadPools){
            thread.start();

        }

    }
}
class ReadWriteTaskQueue{
    private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
    private final Lock readLock  = lock.readLock();
    private final Lock writeLock = lock.writeLock();
    LinkedList<String> taskQueue = new LinkedList<String>();
    public void addTask(String task){
        writeLock.lock();
        try{
            taskQueue.add(task);
        }finally {
            writeLock.unlock();
        }
    }
    public Integer getTask() throws InterruptedException {
        readLock.lock();
        try{
            while(taskQueue.isEmpty()){
               return 0;
            }
            return taskQueue.size();


        }finally {
            readLock.unlock();
        }

    }



}

最初，写了一个类：ReadWriteTaskQueue，如下所示。队列可以实现增加任务和读取任务并打印的功能。但实际上，这个类实际上两个功能都在写，不是读写锁的应用场景。

package org.meituan.javalearn.thread;
import lombok.SneakyThrows;
import java.util.LinkedList;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

/**
 * @projectName: codebetter
 * @package: org.meituan.javalearn.thread
 * @className: ReadWriteQueue
 * @author: fangjiayueyuan
 * @description: TODO
 * @date: 2023/5/8 下午10:01
 * @version: 1.0
 */
public class ReadWriteQueue {
    public static void main(String[] args) throws InterruptedException {
        LinkedList<Thread> threadPools = new LinkedList<Thread>();
        final ReadWriteTaskQueue taskQueue = new ReadWriteTaskQueue();
        for(int i=0;i<15;i++){
            threadPools.add(new Thread(new Runnable() {
                @SneakyThrows
                @Override
                public void run() {
                    System.out.println(taskQueue.getTask());
                }
            }));
        }
        Thread addThread = new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 5; i++) {
                    taskQueue.addTask("The"+i+"th task to be closed");
                }
            }
        });
        addThread.start(); // 读写锁无法进行wait和notify???
        for(Thread thread:threadPools){
            thread.start();

        }

    }
}
class ReadWriteTaskQueue{
    private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
    private final Lock readLock  = lock.readLock();
    private final Lock writeLock = lock.writeLock();
    LinkedList<String> taskQueue = new LinkedList<String>();
    public void addTask(String task){
        writeLock.lock();
        try{
            taskQueue.add(task);
        }finally {
            writeLock.unlock();
        }
    }
    public String getTask() throws InterruptedException {
        readLock.lock();
        try{
            while(taskQueue.isEmpty()){
               return "None";
            }
            return taskQueue.remove();


        }finally {
            readLock.unlock();
        }
    }
}

运行结果

注意：ReadWriteLock看起来无法实现线程的协调，如果需要做类似于发布-订阅这种模式的消息队列，则需要通过Condition实现wait和notify来达到多线程协调的目的。

总结：

使用ReadWriteLock可以提高读取效率：
– ReadWriteLock只允许一个线程写入；
– ReadWriteLock允许多个线程在没有写入时同时读取；
– ReadWriteLock适合读多写少的场景。
缺点：
如果有线程正在读，写线程需要等待读线程释放锁后才能获取写锁，即读的过程中不允许写，这是一种悲观的读锁。

StampedLock

乐观锁的意思就是乐观地估计读的过程中大概率不会有写入，因此被称为乐观锁。悲观锁就是在读的过程中拒绝写入，显然乐观锁的并发效率更高。但有可能造成读写不一致。因此需要增加一些代码来判断读的状态。
声明StampedLock：private final StampedLock lock = new StampedLock;
读锁，需要判断读的时候有没有写入，通过盖得“章”Stamp来判断：

long stamp = lock.tryOptimisticRead();// 先尝试使用乐观锁
if(!validate(stamp)) // 通过Stamp来判断，顾名思义StampedLock 已经盖过章的lock
stamp = lock.readLock(); // 如果在读的时候进行了写入，则需要悲观锁来读一遍 由于大概率读的时候不会写，所以大概率都是走的乐观锁，因此能提高并发
lock.unlockRead(stamp);// 记得关闭读锁

写锁：

long stamp = lock.writeLock(); // 上写锁
lock.unlockWrite(stamp); // 关闭写锁

使用StampedLock对上面的ReadWriteTaskQueue进行改造如下：

class ReadWriteTaskQueue{
    private final StampedLock lock = new StampedLock();

    LinkedList<String> taskQueue = new LinkedList<String>();
    public void addTask(String task){ // 写锁
        long stamp = lock.writeLock();
        try{
            taskQueue.add(task);
        }finally {
            lock.unlockWrite(stamp);
        }
    }
    public Integer getTask() throws InterruptedException {
        long stamp = lock.tryOptimisticRead();
        if(!lock.validate(stamp)){
            try{
                stamp = lock.readLock();
                while(taskQueue.isEmpty()){
                    return 0;
                }
                return taskQueue.size();
            }finally {
                lock.unlockRead(stamp);
            }
        }
        return taskQueue.size();
    }
}