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();
}
}
}
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();
}
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();
}
}
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();
}
}
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();
}
}
- StampedLock 不支持条件变量
- StampedLock 不支持可重入
