【juc】读写锁ReentrantReadWriteLock

一、说明

• 1.当读操作远远高于写操作时，使用读写锁让读读可以并发，来提高性能
• 2.类似于数据库中的select … from … lock in share mode
• 3.提供一个数据容器类，内部分别使用读锁保护数据的read()方法，写锁保护数据的write()方法

二、读读不互斥

2.1 代码示例

package com.learning;

import lombok.extern.slf4j.Slf4j;

import java.util.concurrent.locks.ReentrantReadWriteLock;

@Slf4j
public class ReadWriteLockLearning {
    private Object data;
    private ReentrantReadWriteLock reentrantReadWriteLock = new ReentrantReadWriteLock();
    private ReentrantReadWriteLock.ReadLock readLock = reentrantReadWriteLock.readLock();
    private ReentrantReadWriteLock.WriteLock writeLock = reentrantReadWriteLock.writeLock();


    public Object read(){
        log.debug("获取读锁");
        readLock.lock();
        try {
            log.debug("读取");
            try {
                Thread.sleep(1000);
            }catch(Exception e){
                e.printStackTrace();
            }
            return data;
        }finally {
            log.debug("释放读锁");
            readLock.unlock();
        }
    }

    public void write(){
        log.debug("获取写锁");
        writeLock.lock();
        try{
            log.debug("写入");
        }finally {
            log.debug("释放写锁");
            writeLock.unlock();
        }
    }

    public static void main(String[] args) {
        ReadWriteLockLearning readWriteLockLearning = new ReadWriteLockLearning();
        new Thread(()->{
            readWriteLockLearning.read();
        }, "t1").start();


        new Thread(()->{
            readWriteLockLearning.read();
        }, "t2").start();
    }
}

2.2 截图示例

三、读写互斥

3.1 代码示例

package com.learning;

import lombok.extern.slf4j.Slf4j;

import java.util.concurrent.locks.ReentrantReadWriteLock;

@Slf4j
public class ReadWriteLockLearning {
    private Object data;
    private ReentrantReadWriteLock reentrantReadWriteLock = new ReentrantReadWriteLock();
    private ReentrantReadWriteLock.ReadLock readLock = reentrantReadWriteLock.readLock();
    private ReentrantReadWriteLock.WriteLock writeLock = reentrantReadWriteLock.writeLock();


    public Object read(){
        log.debug("获取读锁");
        readLock.lock();
        try {
            log.debug("读取");
            return data;
        }finally {
            log.debug("释放读锁");
            readLock.unlock();
        }
    }

    public void write(){
        log.debug("获取写锁");
        writeLock.lock();
        try{
            log.debug("写入");
            try {
                Thread.sleep(1000);
            }catch(Exception e){
                e.printStackTrace();
            }
        }finally {
            log.debug("释放写锁");
            writeLock.unlock();
        }
    }

    public static void main(String[] args) {
        ReadWriteLockLearning readWriteLockLearning = new ReadWriteLockLearning();
        new Thread(()->{
            readWriteLockLearning.read();
        }, "t1").start();

        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        new Thread(()->{
            readWriteLockLearning.write();
        }, "t2").start();
    }
}

3.2 截图示例

四、写写互斥

4.1 代码示例

package com.learning;

import lombok.extern.slf4j.Slf4j;

import java.util.concurrent.locks.ReentrantReadWriteLock;

@Slf4j
public class ReadWriteLockLearning {
    private Object data;
    private ReentrantReadWriteLock reentrantReadWriteLock = new ReentrantReadWriteLock();
    private ReentrantReadWriteLock.ReadLock readLock = reentrantReadWriteLock.readLock();
    private ReentrantReadWriteLock.WriteLock writeLock = reentrantReadWriteLock.writeLock();


    public Object read(){
        log.debug("获取读锁");
        readLock.lock();
        try {
            log.debug("读取");
            return data;
        }finally {
            log.debug("释放读锁");
            readLock.unlock();
        }
    }

    public void write(){
        log.debug("获取写锁");
        writeLock.lock();
        try{
            log.debug("写入");
            try {
                Thread.sleep(1000);
            }catch(Exception e){
                e.printStackTrace();
            }
        }finally {
            log.debug("释放写锁");
            writeLock.unlock();
        }
    }

    public static void main(String[] args) {
        ReadWriteLockLearning readWriteLockLearning = new ReadWriteLockLearning();
        new Thread(()->{
            readWriteLockLearning.write();
        }, "t1").start();

        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        new Thread(()->{
            readWriteLockLearning.write();
        }, "t2").start();
    }
}

4.2 截图示例

五、注意事项

• 1.读锁不支持条件变量
• 2.不支持重入时升级：持有读锁的情况下去获取写锁，会导致获取写锁永久等待

5.2.1 代码示例

package com.learning;

import lombok.extern.slf4j.Slf4j;

import java.util.concurrent.locks.ReentrantReadWriteLock;

@Slf4j
public class ReadWriteLockLearning2 {
    private Object data;
    private ReentrantReadWriteLock reentrantReadWriteLock = new ReentrantReadWriteLock();
    private ReentrantReadWriteLock.ReadLock readLock = reentrantReadWriteLock.readLock();
    private ReentrantReadWriteLock.WriteLock writeLock = reentrantReadWriteLock.writeLock();


    public void readWrite(){
        log.debug("获取读锁");
        readLock.lock();
        try {
            try{
                log.debug("获取写锁");
                writeLock.lock();
            }finally {
                log.debug("释放写锁");
                writeLock.unlock();
            }
        }finally {
            log.debug("释放读锁");
            readLock.unlock();
        }
    }

    public static void main(String[] args) {
        ReadWriteLockLearning2 readWriteLockLearning = new ReadWriteLockLearning2();
        readWriteLockLearning.readWrite();
    }
}

5.2.2 截图示例

• 3.支持重入时降级：持有写锁的情况下去获取读锁