10 读写锁ReentrantReadWriteLock

1 介绍

为什么要使用读写锁？

        需要高并发读取和较低并发写入的应用程序，降低锁的粒度，提高系统性能

使用场景：

        读多写少的共享资源

        缓存管理：读 >> 写，控制多个线程同时读缓存，需要刷新or修改操作时才使用写锁

        数据库连接池：多个线程从池中获取连接（读操作），只有一个线程可以设置连接到池中（写操作）

        文件读写

        数据结构的并发访问

2 使用

import java.util.concurrent.locks.ReentrantReadWriteLock;public class SharedResource {private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock();private final ReentrantReadWriteLock.ReadLock readLock = lock.readLock();private final ReentrantReadWriteLock.WriteLock writeLock = lock.writeLock();public void readFromResource() {readLock.lock(); // 获取读锁try {// 执行读取共享资源的操作} finally {readLock.unlock(); // 释放读锁}}public void writeToResource() {writeLock.lock(); // 获取写锁try {// 执行写入共享资源的操作} finally {writeLock.unlock(); // 释放写锁}}
}

3 原理分析

读写锁两个状态，读状态、写状态

但AQS中只有一个state，如何记录两种状态？

        高低位；int4个字节，共32位，采用高16位控制读，低16位控制写

00000000 00000000 00000000 00000000

加锁时如何判断读锁、写锁？

        高16位>0，表示有读锁（sharedCount()）

        低16位>0，表示有写锁（exclusiveCount()）

如何实现可重入？

        写锁只有一个线程独占，重入则低16位+1即可

        写锁有多个线程持有，如何记录？ThreadLocal线程私有

4 读锁源码

读锁：tryAcquireShared()、tryReleaseShared()；读读共享

protected final int tryAcquireShared(int unused) {Thread current = Thread.currentThread();int c = getState();//是否其它线程占用排它锁，如果是则不允许获取共享锁if (exclusiveCount(c) != 0 &&getExclusiveOwnerThread() != current)return -1;//共享锁被获取的数量int r = sharedCount(c);//获取共享锁//1 未阻塞//2 不超最大读计数//3 设置共享锁成功if (!readerShouldBlock() &&r < MAX_COUNT &&compareAndSetState(c, c + SHARED_UNIT)) {//第一次读if (r == 0) {firstReader = current;firstReaderHoldCount = 1;//重入} else if (firstReader == current) {firstReaderHoldCount++;} else {//其它线程读HoldCounter rh = cachedHoldCounter;if (rh == null || rh.tid != getThreadId(current))//记录每个线程的重入次数cachedHoldCounter = rh = readHolds.get();else if (rh.count == 0)readHolds.set(rh);rh.count++;}return 1;}//若不满足上述条件，则执行方法内的获取共享锁逻辑return fullTryAcquireShared(current);}

final int fullTryAcquireShared(Thread current) {HoldCounter rh = null;//1 自旋 获取共享锁or失败for (;;) {int c = getState();//2 若存在写锁，且不是当前线程持有的，不允许获取共享锁if (exclusiveCount(c) != 0) {if (getExclusiveOwnerThread() != current)return -1;//3 读线程阻塞} else if (readerShouldBlock()) {if (firstReader == current) {} else {if (rh == null) {rh = cachedHoldCounter;if (rh == null || rh.tid != getThreadId(current)) {rh = readHolds.get();if (rh.count == 0)readHolds.remove();}}if (rh.count == 0)return -1;}}// 4 不允许再申请共享锁if (sharedCount(c) == MAX_COUNT)throw new Error("Maximum lock count exceeded");// 5 尝试获取锁if (compareAndSetState(c, c + SHARED_UNIT)) {if (sharedCount(c) == 0) {firstReader = current;firstReaderHoldCount = 1;} else if (firstReader == current) {firstReaderHoldCount++;} else {if (rh == null)rh = cachedHoldCounter;if (rh == null || rh.tid != getThreadId(current))rh = readHolds.get();else if (rh.count == 0)readHolds.set(rh);rh.count++;cachedHoldCounter = rh; // cache for release}return 1;}}}

5 写锁源码

写锁：tryAcquire()、tryRelease()；写写互斥，读写互斥

protected final boolean tryAcquire(int acquires) {Thread current = Thread.currentThread();int c = getState();int w = exclusiveCount(c);if (c != 0) {// (Note: if c != 0 and w == 0 then shared count != 0)if (w == 0 || current != getExclusiveOwnerThread())return false;if (w + exclusiveCount(acquires) > MAX_COUNT)throw new Error("Maximum lock count exceeded");// Reentrant acquiresetState(c + acquires);return true;}if (writerShouldBlock() ||!compareAndSetState(c, c + acquires))return false;setExclusiveOwnerThread(current);return true;}