深入分析实战可重入读写锁ReentrantReadWriteLock

梦里梦外; 2024-03-17 09:38 47阅读 0赞

#### 文章目录 ####

*   *  前言
     *  加锁规则
     *  同步原理
     *  源码解析
     *  实战演示

### 前言 ###

前面我们学习了[可重入锁ReentrantLock][ReentrantLock]，可重入锁是一个排他锁，只要不是当前线程访问加锁资源都不能够进入，只能等待锁的释放。当然，这种加锁方式也有一定的适用场景。但是，如果在读多写少的情况下可重入锁ReentrantLock可能不是那么完美，比如缓存的写入和读取。今天，我们就引出可重入读写锁ReentrantReadWriteLock，其读写分离的机制，大大提升缓存场景的系统性能。

### 加锁规则 ###

可重入锁ReentrantReadWriteLock内部分为读锁和写锁，读锁与写锁的加锁规则如下：

### 同步原理 ###

可重入锁ReentrantReadWriteLock的同步机制依然是继承[抽象同步队列AQS][AQS]，其内部实现了自身的读锁与写锁的规则，覆写了AQS的一些同步标识和方法。其本质上都是使用AQS同步原理，并用AQS的阻塞队列保存阻塞线程，用AQS的STATE来表示当前锁状态，可重入本质上也是加减STATE来达到对应的目的。

### 源码解析 ###

进入 package java.util.concurrent.locks 查看可重入ReentrantReadWriteLock源码：

public class ReentrantReadWriteLock
            implements ReadWriteLock, java.io.Serializable {
        private static final long serialVersionUID = -6992448646407690164L;
        /** Inner class providing readlock */
        private final ReentrantReadWriteLock.ReadLock readerLock;
        /** Inner class providing writelock */
        private final ReentrantReadWriteLock.WriteLock writerLock;
        /** Performs all synchronization mechanics */
        final Sync sync;
    
        /**
         * Creates a new {@code ReentrantReadWriteLock} with
         * default (nonfair) ordering properties.
         */
        public ReentrantReadWriteLock() {
            this(false);
        }
    
        /**
         * Creates a new {@code ReentrantReadWriteLock} with
         * the given fairness policy.
         *
         * @param fair {@code true} if this lock should use a fair ordering policy
         */
        public ReentrantReadWriteLock(boolean fair) {
            sync = fair ? new FairSync() : new NonfairSync();
            readerLock = new ReadLock(this);
            writerLock = new WriteLock(this);
        }
    
        public ReentrantReadWriteLock.WriteLock writeLock() { return writerLock; }
        public ReentrantReadWriteLock.ReadLock  readLock()  { return readerLock; }
    }

> 如上源码所示，可重入读写锁实现了读写锁ReadWriteLock，保留读写锁的机制，增加了可重入机制。调用方可以通过传入标识实现非公平锁和公平锁来保证同步，当然可重入读写锁默认NonfairSync非公平锁同步。

继续查看源码：

//内部类继承抽象同步队列
    abstract static class Sync extends AbstractQueuedSynchronizer{
           //加解锁省略
    }
    
    //公平锁
    static final class FairSync extends Sync {
        private static final long serialVersionUID = -2274990926593161451L;
        final boolean writerShouldBlock() {
            return hasQueuedPredecessors();
        }
        final boolean readerShouldBlock() {
            return hasQueuedPredecessors();
        }
    }
    //非公平锁
    static final class NonfairSync extends Sync {
        private static final long serialVersionUID = -8159625535654395037L;
        final boolean writerShouldBlock() {
            return false; // writers can always barge
        }
        final boolean readerShouldBlock() {
            /* As a heuristic to avoid indefinite writer starvation,
             * block if the thread that momentarily appears to be head
             * of queue, if one exists, is a waiting writer.  This is
             * only a probabilistic effect since a new reader will not
             * block if there is a waiting writer behind other enabled
             * readers that have not yet drained from the queue.
             */
            return apparentlyFirstQueuedIsExclusive();
        }
    }
    
    //获取读锁
    public static class ReadLock implements Lock, java.io.Serializable {
        private static final long serialVersionUID = -5992448646407690164L;
        private final Sync sync;
    
        /**
         * Constructor for use by subclasses
         *
         * @param lock the outer lock object
         * @throws NullPointerException if the lock is null
         */
        protected ReadLock(ReentrantReadWriteLock lock) {
            sync = lock.sync;
        }
           //加解锁省略
    }
    
    //写锁
    public static class WriteLock implements Lock, java.io.Serializable {
        private static final long serialVersionUID = -4992448646407690164L;
        private final Sync sync;
    
        /**
         * Constructor for use by subclasses
         *
         * @param lock the outer lock object
         * @throws NullPointerException if the lock is null
         */
        protected WriteLock(ReentrantReadWriteLock lock) {
            sync = lock.sync;
        }
        //加解锁省略
    }

如上源码所示，可重入读写锁公平与非公平锁都集成内部类Sync，本质上还是继承至AQS。ReentrantReadWriteLock的读锁与写锁都是实现Lock,并覆写了其增加解锁的方法；对于锁机制的同步规则则是直接传入内部类Sync。

> 由此可知，可重入读写锁ReentrantReadWriteLock的加锁方法来源是Lock，同步机制来源与抽象同步队列AQS。当然对于可重入读写锁为保证读锁与写锁、写锁与写锁的互斥，覆写了AQS的同步方法，加入了满足自身规则的一些方法。

继续查看获取锁的源码：

//尝试获取共享锁
    protected final int tryAcquireShared(int unused) {
        /*
         * 1. 如果是写锁持有资源，其他线程直接返回失败
         * 2. 如果没有资源没有加写锁，则会检查资源的锁定状态，
         并尝试用cas修改state状态
         * 3. 如果第二步失败，则表示当前线程没有获取锁资格或者cas修改state失败，
         则该线程会重试。
         */
        Thread current = Thread.currentThread();
        int c = getState();
        if (exclusiveCount(c) != 0 &&
            getExclusiveOwnerThread() != current)
            return -1;
        int r = sharedCount(c);
        if (!readerShouldBlock() &&
            r < MAX_COUNT &&
            compareAndSetState(c, c + SHARED_UNIT)) {
            if (r == 0) {
                firstReader = current;
                firstReaderHoldCount = 1;
            } else if (firstReader == current) {
                firstReaderHoldCount++;
            } else {
                HoldCounter rh = cachedHoldCounter;
                if (rh == null || rh.tid != getThreadId(current))
                    cachedHoldCounter = rh = readHolds.get();
                else if (rh.count == 0)
                    readHolds.set(rh);
                rh.count++;
            }
            return 1;
        }
        return fullTryAcquireShared(current);
    }
    
    /**
     * 重试获取共享锁，相比之前的方法更为简单的验证锁和cas修改state
     */
    final int fullTryAcquireShared(Thread current) {
    
        HoldCounter rh = null;
        for (;;) {
            int c = getState();
            if (exclusiveCount(c) != 0) {
                if (getExclusiveOwnerThread() != current)
                    return -1;
                // else we hold the exclusive lock; blocking here
                // would cause deadlock.
            } else if (readerShouldBlock()) {
                // Make sure we're not acquiring read lock reentrantly
                if (firstReader == current) {
                    // assert firstReaderHoldCount > 0;
                } 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;
                }
            }
            if (sharedCount(c) == MAX_COUNT)
                throw new Error("Maximum lock count exceeded");
            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;
            }
        }
    }

如上源码所示，获取共享锁的源码主要是验证当前资源是否已经被加写锁，如果加了写锁其他线程会互斥不能获取该锁；如果没有加写锁则会进行加锁资格验证，读锁则可以共享加锁。在验证当前线程有加锁资格后，程序会用CAS修改STATE状态以保证可重入和释放锁机制的正常运行。

> 当然，如果当前线程之前是加了写锁，现在该线程对资源加读锁，那么此时当前写锁会进行降级为读锁。但是，反之当前线程加了读锁，然后该线程又对资源加写锁是不被支持的，因为这样会造成死锁。

### 实战演示 ###

虽然可重入读写锁ReentrantReadWriteLock在实际生产中使用的场景较少，但是还是有它的一席之地的。话说有需要才会存在，可重入读写锁对我们生产的缓存操作极其重要。

> 以下我将就缓存的读写进行代码演示，当然也是因为引入可重入读写锁会使得缓存操作效率更高。

/**
     * ReentrantReadWriteLockDemo
     * @author senfel
     * @version 1.0
     * @date 2023/5/22 10:48
     */
    @Slf4j
    public class ReentrantReadWriteLockDemo {
    
        private static final ReentrantReadWriteLock readWriteLock = new ReentrantReadWriteLock();
        /**
         * 用内存缓存模拟
         */
        private static final Map<String,String> cache  = new HashMap<>();
    
    
        /**
         * 获取缓存
         * @param key
         * @author senfel
         * @date 2023/5/22 14:44
         * @return java.lang.String
         */
        public static String getValue(String key){
            //加读锁获取缓存
            try{
                readWriteLock.readLock().lock();
                String str = cache.get(key);
                if(StringUtils.isNotBlank(str)){
                    return str;
                }
            }catch (Exception e){
                log.error("获取缓存异常：{}",e.getStackTrace());
            }finally {
                readWriteLock.readLock().unlock();
            }
            //没有获取到写入缓存，需要加写锁
            try {
                readWriteLock.writeLock().lock();
                //加写锁成功，再次验证是否存在缓存
                String str = cache.get(key);
                if(StringUtils.isNotBlank(str)){
                    return str;
                }else{
                    //查询数据库获取缓存，这里模拟设置一个常量标识以获取缓存
                    str = "senfel";
                    cache.put(key,str);
                    return str;
                }
            }catch (Exception e){
                log.error("获取缓存异常：{}",e.getStackTrace());
            }finally {
                readWriteLock.writeLock().unlock();
            }
            return null;
        }
        
        /**
         * 设置缓存
         * @param key
         * @author senfel
         * @date 2023/5/22 14:44
         * @return java.lang.String
         */
        public void setValue(String key,String value){
            try{
                readWriteLock.writeLock().lock();
                cache.put(key,value);
            }catch (Exception e){
                log.error("设置缓存异常：{}",e.getStackTrace());
            }finally {
                readWriteLock.writeLock().unlock();
            }
        } 
    }

[ReentrantLock]: http://t.csdn.cn/0mhLE
[AQS]: http://t.csdn.cn/zAUci