ReadWriteLock读写锁分为读锁和写锁,读锁是共享锁即多个读线程可以同时读,但是如果有线程在进行写操作,则所有读线程都阻塞等待了。读写锁相对于synchronized来说,是进一步优化了,synchronized保证了两个线程不能同时执行一段代码,而读写锁则具体分析了一下,如果都是读操作的线程,则不用进行阻塞。
写锁是排他锁,只要有线程在读,则不能进行写操作;有线程在写,当然也不能再让其他写线程获得写锁。
读写锁有两种实现,一种是ReentrantReadWriteLock类,一种是StampedLock类。下面是应用的示例代码。
1、 ReentrantReadWriteLock
通过ReentrantReadWriteLock获取读锁和写锁对象,读操作的代码加读锁,写操作的代码加写锁。lock()和unlock()。
package basic.aqs.readWriteLock;
import java.util.Random;
import java.util.concurrent.atomic.LongAdder;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
public class TestReadWriteLock {
static Lock lock = new ReentrantLock();
private static int value;
static ReadWriteLock readWriteLock = new ReentrantReadWriteLock();
static Lock readLock = readWriteLock.readLock();
static Lock writeLock = readWriteLock.writeLock();
public static void read(Lock lock) {
try {
lock.lock();
Thread.sleep(1000);
System.out.println("read over!");
//模拟读取操作
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public static void write(Lock lock, int v) {
try {
lock.lock();
Thread.sleep(1000);
value = v;
System.out.println("write over!");
//模拟写操作
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public static void main(String[] args) {
//Runnable readR = ()-> read(lock);
//Runnable writeR = ()->write(lock, new Random().nextInt());
Runnable readR = ()-> read(readLock);
Runnable writeR = ()->write(writeLock, new Random().nextInt());
for(int i=0; i<18; i++) new Thread(readR).start();
for(int i=0; i<2; i++) new Thread(writeR).start();
}
}
2、StampedLock
StampedLock分为三种锁,读锁、写锁、乐观读锁。
(1)进入悲观读锁前先看下有没有进入写模式(说白了就是有没有已经获取了悲观写锁)
(2)如果其他线程已经获取了悲观写锁,那么就只能老老实实的获取悲观读锁(这种情况相当于退化成了读写锁)
(3)如果其他线程没有获取悲观写锁,那么就不用获取悲观读锁了,减少了一次获取悲观读锁的消耗和避免了因为读锁导致写锁阻塞的问题,直接返回读的数据即可(必须再tryOptimisticRead和validate之间获取好数据,否则数据可能会不一致了,试想如果过了validate再获取数据,这时数据可能被修改并且读操作也没有任何保护措施)
package basic.aqs.readWriteLock;
import java.util.concurrent.locks.StampedLock;
public class TestStampedLock {
class Point {
private double x, y;
private final StampedLock sl = new StampedLock();
void move(double deltaX, double deltaY) { // an exclusively locked method
long stamp = sl.writeLock();
try {
x += deltaX;
y += deltaY;
} finally {
sl.unlockWrite(stamp);
}
}
//下面看看乐观读锁案例
double distanceFromOrigin() { // A read-only method
long stamp = sl.tryOptimisticRead(); //获得一个乐观读锁
double currentX = x, currentY = y; //将两个字段读入本地局部变量
if (!sl.validate(stamp)) { //检查发出乐观读锁后同时是否有其他写锁发生?
stamp = sl.readLock(); //如果没有,我们再次获得一个读悲观锁
try {
currentX = x; // 将两个字段读入本地局部变量
currentY = y; // 将两个字段读入本地局部变量
} finally {
sl.unlockRead(stamp);
}
}
return Math.sqrt(currentX * currentX + currentY * currentY);
}
//下面是悲观读锁案例
void moveIfAtOrigin(double newX, double newY) { // upgrade
// Could instead start with optimistic, not read mode
long stamp = sl.readLock();
try {
while (x == 0.0 && y == 0.0) { //循环,检查当前状态是否符合
long ws = sl.tryConvertToWriteLock(stamp); //将读锁转为写锁
if (ws != 0L) { //这是确认转为写锁是否成功
stamp = ws; //如果成功 替换票据
x = newX; //进行状态改变
y = newY; //进行状态改变
break;
} else { //如果不能成功转换为写锁
sl.unlockRead(stamp); //我们显式释放读锁
stamp = sl.writeLock(); //显式直接进行写锁 然后再通过循环再试
}
}
} finally {
sl.unlock(stamp); //释放读锁或写锁
}
}
}
}