1 ReentrantReadWriteLock

当读操作远远高于写操作，这时候使用 读写锁 让 读-读 可以并发，提高性能，类似于数据库中的 select … from … lock in share mode

测试阻塞

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

import com.lv.juc.util.Sleeper;
import lombok.extern.slf4j.Slf4j;

import java.util.concurrent.locks.ReentrantReadWriteLock;

/**
 * @author 晓风残月Lx
 * @date 2023/4/7 11:07
 */
@Slf4j
public class MyReadWriteLockTest01 {

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

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

    }
}

@Slf4j
class DataContainer {
    private Object data;
    private ReentrantReadWriteLock rw = new ReentrantReadWriteLock();
    private ReentrantReadWriteLock.ReadLock r = rw.readLock();
    private ReentrantReadWriteLock.WriteLock w = rw.writeLock();


    public Object read() {
        log.debug("获取读锁.....");
        r.lock();

        try {
            log.debug("读取");
            Sleeper.sleep(1);
            return data;
        } finally {
            log.debug("释放读锁");
            r.unlock();
        }
    }

}

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测试读锁-写锁相互阻塞

@Slf4j
public class MyReadWriteLockTest01 {

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

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

    }

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写锁-写锁阻塞

@Slf4j
public class MyReadWriteLockTest01 {

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

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

    }
}

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注意事项

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

    r.lock();
	try {
        // ...
        w.lock();
        try {
            // ...
        } finally{
            w.unlock();
        }
    } finally{
        r.unlock();
    }

重入时降级支持：即持有写锁的情况下获取读锁

class CachedData {
    Object data;
    // 是否有效，如果失效，需要重新计算 data
    volatile boolean cacheValid;
    final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
    void processCachedData() {
        rwl.readLock().lock();
        if (!cacheValid) {
            // 获取写锁前必须释放读锁
            rwl.readLock().unlock();
            rwl.writeLock().lock();
            try {
                // 判断是否有其它线程已经获取了写锁、更新了缓存, 避免重复更新
                if (!cacheValid) {
                    data = ...
                    cacheValid = true;
                }
                // 降级为读锁, 释放写锁, 这样能够让其它线程读取缓存
                rwl.readLock().lock();
            } finally {
                rwl.writeLock().unlock();
            }
        }
        // 自己用完数据, 释放读锁 
        try {
            use(data);
        } finally {
            rwl.readLock().unlock();
        }
    }
}

2.StampedLock

该类是JDK 8 加入，是为了进一步优化读性能，特点是在使用读锁、写锁时都必须配合【戳】使用

加解读锁

long stamp = lock.readLock();
lock.unlockRead(stamp);

加解写锁

long stamp = lock.writeLock();
lock.unlockWrite(stamp);

乐观读，StampedLock 支持 tryOptimisticRead 方法（乐观读），读取完毕后需要做一次 戳校验，如果校验通过，表示这期间确实没有写操作，数据可以安全使用，如果检验没通过，需要重新获取读锁，保证数据安全

long stamp = lock.tryOptimisticRead();
// 验戳
if(!lock.validate(stamp)) {
    // 锁升级
}

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

import com.lv.juc.util.Sleeper;
import lombok.extern.slf4j.Slf4j;

import java.util.concurrent.locks.StampedLock;

/**
 * @author 晓风残月Lx
 * @date 2023/4/14 8:56
 */
@Slf4j
public class DataContainerStamped {

    private int data;

    private final StampedLock lock = new StampedLock();

    public DataContainerStamped(int data) {
        this.data = data;
    }

    public int read(int readTime) {
        long stamp = lock.tryOptimisticRead();
        log.debug("optimistic read locking.... {}", stamp);
        Sleeper.sleep(readTime);
        if (lock.validate(stamp)) {
            log.debug("read finish .... {}, data:{}", stamp, data);
            return data;
        }

        // 锁升级 - 读锁
        log.debug("updating to read lock...{}", stamp);
        try {
            stamp = lock.readLock();
            log.debug("read lock{}", stamp);
            Sleeper.sleep(readTime);
            log.debug("read finish...{},data:{}", stamp, data);
            return data;
        } finally {
            log.debug("read unlock ... {}", stamp);
            lock.unlockRead(stamp);
        }
    }

    public void write(int newData) {
        long stamp = lock.writeLock();
        log.debug("write lock {}", stamp);
        try {
            Sleeper.sleep(2);
            this.data = newData;
        } finally {
            log.debug("write unlock ... {}", stamp);
            lock.unlockWrite(stamp);
        }
    }

}

测试读读并发

public class DataContainerStampedTest {

    public static void main(String[] args) {
        DataContainerStamped dataContainerStamped = new DataContainerStamped(1);
        new Thread(() -> {
            dataContainerStamped.read(1);
        },"t1").start();

        Sleeper.sleep(0.5);
        new Thread(() -> {
            dataContainerStamped.read(0);
        }, "t2").start();
    }
}

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测试读写优化读补加读锁

import com.lv.juc.util.Sleeper;
import lombok.extern.slf4j.Slf4j;

/**
 * @author 晓风残月Lx
 * @date 2023/4/14 9:26
 */
@Slf4j
public class DataContainerStampedTest01 {

    public static void main(String[] args) {
        DataContainerStamped dataContainerStamped = new DataContainerStamped(1);
        new Thread(() -> {
            dataContainerStamped.read(1);
        },"t1").start();

        Sleeper.sleep(0.5);
        new Thread(() -> {
            dataContainerStamped.write(1000);
        }, "t2").start();
    }
}