【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