Java多线程之ReentrantReadWriteLock读写锁

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这里就JUC包中的ReentrantReadWriteLock读写锁做相关介绍

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概述

前面介绍了ReentrantLock可重入锁，但其存在明显的弊端。对于读场景而言，实际完全可以允许多个线程同时访问，而不必使用独占锁来进行并发保护。故ReentrantReadWriteLock读写锁应运而生。其内部维护了两个锁——读锁、写锁。前者为共享锁，后者则为互斥锁。具体而言，读锁可以被多个线程同时获取，而写锁只能被一个线程获取；同时读锁、写锁之间也是互斥的，即一旦某个线程获取到了读锁，则其他线程不可以同时获得写锁。反之同理。具体地，读写锁支持公平、非公平锁两种实现方式，默认为非公平锁。在锁的获取方面，其与ReentrantLock可重入锁类似。即支持lock()、lockInterruptibly()阻塞式获取，也支持tryLock()、tryLock(long timeout, TimeUnit unit)实现非阻塞式获取。但tryLock()方法会破坏公平性，即使是一个公平的读写锁实例。故为了保证公平性，可使用支持超时的tryLock方法，同时将超时时间设为0即可——tryLock(0, TimeUnit.SECONDS)。而在条件变量Condition方面，仅写锁支持

实践

读写测试

现在分别就读读、写写、读写场景进行实践验证。示例代码如下所示

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/**
 * ReentrantReadWriteLock Test 1: 读写测试
 */
@FixMethodOrder(MethodSorters.NAME_ASCENDING)
public class ReentrantReadWriteLockTest1 {

    private static DateTimeFormatter formatter = DateTimeFormatter.ofPattern("HH:mm:ss.SSS");

    private static ExecutorService threadPool = Executors.newFixedThreadPool(10);

    private static ReentrantReadWriteLock lock = new ReentrantReadWriteLock();

    /**
     * 测试: 读锁为共享锁
     */
    @Test
    public void test1() {
        System.out.println("\n---------------------- Test 1 ----------------------");
        for (int i=0; i<3; i++) {
            Runnable task = new ReadTask( lock, "读任务 #"+i );
            threadPool.execute(  task );
        }
        // 主线程等待所有任务执行完毕
        try{ Thread.sleep( 10*1000 ); } catch (Exception e) {}
    }

    /**
     * 测试: 写锁为互斥锁
     */
    @Test
    public void test2() {
        System.out.println("\n---------------------- Test 2 ----------------------");
        for (int i=0; i<3; i++) {
            Runnable task = new WriteTask( lock, "写任务 #"+i );
            threadPool.execute(  task );
        }
        // 主线程等待所有任务执行完毕
        try{ Thread.sleep( 10*1000 ); } catch (Exception e) {}
    }

    /**
     * 测试: 读写互斥
     */
    @Test
    public void test3() {
        System.out.println("\n---------------------- Test 3 ----------------------");
        for (int i=0; i<8; i++) {
            Runnable task = null;
            Boolean isReadTask = RandomUtils.nextBoolean();
            if( isReadTask ) {
                task = new ReadTask( lock, "读任务 #"+i );
            } else {
                task = new WriteTask( lock, "写任务 #"+i );
            }
            threadPool.execute( task );
        }
        // 主线程等待所有任务执行完毕
        try{ Thread.sleep( 50*1000 ); } catch (Exception e) {}
    }

    /**
     * 打印信息
     * @param msg
     */
    public static void info(String msg) {
        String time = formatter.format(LocalTime.now());
        String thread = Thread.currentThread().getName();
        String log = "["+time+"] "+ " <"+ thread +"> " + msg;
        System.out.println(log);
    }

    @AllArgsConstructor
    private static class ReadTask implements Runnable{

        private ReentrantReadWriteLock lock;

        private String name;

        @Override
        public void run() {
            // 获取读锁
            ReentrantReadWriteLock.ReadLock readLock = lock.readLock();
            readLock.lock();
            info(name+ ": 成功获取读锁");
            try {
                // 模拟业务耗时
                Thread.sleep(RandomUtils.nextLong(1000, 3000));
            } catch (Exception e) {
                System.out.println( "Happen Exception: " + e.getMessage());
            } finally {
                info(name+ ": 释放读锁");
                readLock.unlock();
            }
        }
    }

    @AllArgsConstructor
    private static class WriteTask implements Runnable{

        private ReentrantReadWriteLock lock;

        private String name;

        @Override
        public void run() {
            // 获取写锁
            ReentrantReadWriteLock.WriteLock writeLock = lock.writeLock();
            writeLock.lock();
            info(name+ ": 成功获取写锁");
            try {
                // 模拟业务耗时
                Thread.sleep(RandomUtils.nextLong(1000, 3000));
            } catch (Exception e) {
                System.out.println( "Happen Exception: " + e.getMessage());
            } finally {
                info(name+ ": 释放写锁");
                writeLock.unlock();
            }
        }
    }
}

从Test1、Test2的测试结果可以证明读锁是共享锁、写锁是互斥锁

figure 1.jpeg

从Test3的测试结果可以看出读写之间是互斥的

figure 2.jpeg

可重入性

ReentrantReadWriteLock同样是可重入的。当一个线程获取到读锁(或写锁)后，可以继续获取相应类型的锁。示例代码如下所示

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/**
 * ReentrantReadWriteLock Test 2: 可重入性
 */
@FixMethodOrder(MethodSorters.NAME_ASCENDING)
public class ReentrantReadWriteLockTest2 {

    private static DateTimeFormatter formatter = DateTimeFormatter.ofPattern("HH:mm:ss.SSS");

    private static ExecutorService threadPool = Executors.newFixedThreadPool(10);

    private static ReentrantReadWriteLock lock = new ReentrantReadWriteLock();

    /**
     * 测试: 读锁具有可重入性
     */
    @Test
    public void test1() {
        System.out.println("\n---------------------- Test 1 ----------------------\n");
        for (int i=0; i<2; i++) {
            Runnable runnable = new Task("Task"+i, lock.readLock(), lock.readLock());
            threadPool.execute( runnable );
        }
        // 主线程等待所有任务执行完毕
        try{ Thread.sleep( 120*1000 ); } catch (Exception e) {}
    }

    /**
     * 测试: 写锁具有可重入性
     */
    @Test
    public void test2() {
        System.out.println("\n---------------------- Test 2 ----------------------\n");
        for (int i=0; i<2; i++) {
            Runnable runnable = new Task("Task"+i, lock.writeLock(), lock.writeLock());
            threadPool.execute( runnable );
        }
        // 主线程等待所有任务执行完毕
        try{ Thread.sleep( 120*1000 ); } catch (Exception e) {}
    }

    /**
     * 打印信息
     * @param msg
     */
    public static void info(String msg) {
        String time = formatter.format(LocalTime.now());
        String thread = Thread.currentThread().getName();
        //String log = "["+time+"] "+ " <"+ thread +"> " + msg;
        String log = "["+time+"] " + msg;
        System.out.println(log);
    }


    @AllArgsConstructor
    private static class Task implements Runnable{
        private String name;

        private Lock firstLock;

        private Lock secondLock;

        @Override
        public void run() {
            firstLock.lock();
            info(name+ ": 成功获取锁 firstLock");
            try {
                // 模拟业务耗时
                Thread.sleep(RandomUtils.nextLong(1000, 3000));
                methodA();
            } catch (Exception e) {
                System.out.println( "Happen Exception: " + e.getMessage());
            } finally {
                info(name+ ": 释放锁 firstLock");
                firstLock.unlock();
            }
        }

        private void methodA() {
            secondLock.lock();
            info(name+ ": 成功获取锁 secondLock");
            try {
                // 模拟业务耗时
                Thread.sleep(RandomUtils.nextLong(1000, 3000));
            } catch (Exception e) {
                System.out.println( "Happen Exception: " + e.getMessage());
            } finally {
                info(name+ ": 释放锁 secondLock");
                secondLock.unlock();
            }
        }
    }

}

测试结果，如下所示

figure 3.jpeg

锁升级、降级

所谓锁升级指的是读锁升级为写锁。当一个线程先获取到读锁再去申请写锁，显然ReentrantReadWriteLock是不支持的。理由也很简单，读锁是可以多个线程同时持有的。若其中的一个线程能够进行锁升级，成功获得写锁。显然与我们之前的所说的读写互斥相违背。因为其在获得写锁的同时，其他线程依然持有读锁

反之，ReentrantReadWriteLock是支持锁降级的，即写锁降级为读锁。当一个线程在获得写锁后，依然可以获得读锁。这个时候当其释放写锁，则将只持有读锁，即完成了锁降级过程。锁降级的场景也很常见，假设存在一个先写后读的方法。共计耗时5s。其中前1秒用于写操作、后4秒用于读操作。最简单的思路是对该方法从开始到结束全部使用写锁进行保护。但其实该方法后4秒的读操作完全没有必要使用写锁进行保护。因为这样会阻塞其他线程读锁的获取，效率较低。而如果通过写锁、读锁分别对前1秒、后4秒的操作进行控制，即先获取写锁、再释放写锁，然后获取读锁、再释放读锁的方案。则有可能导致并发问题，具体表现在执行该方法过程中，刚释放写锁、准备获取读锁时，其他线程恰好获取到了写锁并对数据进行了更新。而锁降级则为此场景提供了新的解决思路及方案。其一方面保证了安全，读锁在写锁释放前获取，另一方面保证了高效，因为读锁是共享的

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/**
 * ReentrantReadWriteLock Test 2: 锁升级、锁降级
 */
@FixMethodOrder(MethodSorters.NAME_ASCENDING)
public class ReentrantReadWriteLockTest2 {

    private static DateTimeFormatter formatter = DateTimeFormatter.ofPattern("HH:mm:ss.SSS");

    private static ExecutorService threadPool = Executors.newFixedThreadPool(10);

    private static ReentrantReadWriteLock lock = new ReentrantReadWriteLock();

    /**
     * 测试: 锁升级
     */
    @Test
    public void test3() {
        System.out.println("\n---------------------- Test 3 ----------------------\n");
        Runnable runnable1 = new Task("Task", lock.readLock(), lock.writeLock());
        threadPool.execute( runnable1 );
        // 主线程等待所有任务执行完毕
        try{ Thread.sleep( 120*1000 ); } catch (Exception e) {}
    }

    /**
     * 测试: 锁降级
     */
    @Test
    public void test4() {
        System.out.println("\n---------------------- test4 ----------------------\n");

        Runnable runnable = () -> {
            try {
                // 1. 获取写锁
                lock.writeLock().lock();
                info("成功获取写锁");
                // 2. 获取读锁
                lock.readLock().lock();
                info("成功获取读锁");
            } catch (Exception e) {
                System.out.println( "Happen Exception: " + e.getMessage());
            } finally {
                // 3. 释放写锁
                lock.writeLock().unlock();
                info("释放写锁");
            }
        };
        threadPool.execute( runnable );
        // 延时等待, 保证写锁已经释放, 而仅仅持有读锁
        try{ Thread.sleep( 10*1000 ); } catch (Exception e) {}

        for (int i=0; i<2; i++) {
            Runnable runnable2 = new Task("Task"+i, lock.readLock(), lock.readLock());
            threadPool.execute( runnable2 );
        }

        // 主线程等待所有任务执行完毕
        try{ Thread.sleep( 120*1000 ); } catch (Exception e) {}
    }
}

从Test 3的测试结果可以看出。由于不支持锁升级，故其在持有读锁的条件下尝试获取写锁会被一直阻塞下去

figure 4.png

从Test 4的测试结果可以看出锁降级是可行的。这里为了便于演示，故runnable一直未释放其持有的读锁。实际应用中需要将其释放掉

figure 5.jpeg

实现原理

基本结构

ReentrantReadWriteLock读写锁的实现过程同样依赖于AQS，其是对AQS中共享锁、互斥锁的应用。在构建ReentrantReadWriteLock读写锁实例过程中，一方面，其会创建AQS实现类Sync的实例，其中Sync根据公平性与否又可细分为NonfairSync、FairSync这两个子类。这两个子类通过实现Sync中的readerShouldBlock、writerShouldBlock抽象方法来保障公平与否这一特性；另一方面，还会相应地创建ReadLock、WriteLock实例，并通过持有Sync实例来进行对AQS的调用。而且在ReentrantReadWriteLock读写锁的实现中，其将AQS的state字段分为两部分来使用。具体地，state字段的高16位表示获取到读锁的次数；state字段的低16位表示获取到写锁的次数

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public class ReentrantReadWriteLock implements ReadWriteLock, java.io.Serializable {

    // 读锁变量
    private final ReentrantReadWriteLock.ReadLock readerLock;

    // 写锁变量
    private final ReentrantReadWriteLock.WriteLock writerLock;

    // AQS实现类变量
    final Sync sync;
    
    // 获取读锁
    public ReentrantReadWriteLock.WriteLock writeLock() {
        return writerLock; 
    }
    
    // 获取写锁
    public ReentrantReadWriteLock.ReadLock readLock() {
       return readerLock; 
    }

    /********************* 构造器 *********************/
    public ReentrantReadWriteLock() {
        this(false);
    }

    public ReentrantReadWriteLock(boolean fair) {
        sync = fair ? new FairSync() : new NonfairSync();
        readerLock = new ReadLock(this);
        writerLock = new WriteLock(this);
    }
    /************************************************/
    
    
    // 内部类: AQS实现类
    abstract static class Sync extends AbstractQueuedSynchronizer {
        
        static final int SHARED_SHIFT   = 16;
        static final int SHARED_UNIT    = (1 << SHARED_SHIFT);
        static final int MAX_COUNT      = (1 << SHARED_SHIFT) - 1;
        static final int EXCLUSIVE_MASK = (1 << SHARED_SHIFT) - 1;

        // AQS的state字段的高16位, 表示获取到读锁的次数
        static int sharedCount(int c) {
            return c >>> SHARED_SHIFT; 
        }
        
        // AQS的state字段的低16位, 表示获取到写锁的次数
        static int exclusiveCount(int c) { 
            return c & EXCLUSIVE_MASK; 
        }

        abstract boolean readerShouldBlock();

        abstract boolean writerShouldBlock();
    }

    // 内部类：非公平的Sync子类
    static final class NonfairSync extends Sync {
        
        final boolean writerShouldBlock() {
            ...
        }

        final boolean readerShouldBlock() {
            ... 
        }
    }

    // 内部类：公平的Sync子类
    static final class FairSync extends Sync {

        final boolean writerShouldBlock() {
            ...
        }

        final boolean readerShouldBlock() {
            ...
        }
    }

    // 内部类: 读锁
    public static class ReadLock implements Lock, java.io.Serializable {
        
        private final Sync sync;

        protected ReadLock(ReentrantReadWriteLock lock) {
            sync = lock.sync;
        }    
    }

    // 内部类: 写锁
    public static class WriteLock implements Lock, java.io.Serializable {
        
        private final Sync sync;

        protected WriteLock(ReentrantReadWriteLock lock) {
            sync = lock.sync;
        }
    }
}

写锁

ReentrantReadWriteLock中的写锁是对AQS中互斥锁的使用。其使用方式是通过writeLock()获取写锁实例，然后分别通过写锁的lock()、unlock()方法进行加锁、解锁操作

在调用加锁lock()方法时，其首先会调用AQS的acquire()方法。而在Sync类中则提供了tryAcquire方法的实现。如果其返回true则加锁操作结束，否则其将会进入AQS的阻塞队列。同时为了支持公平、非公平两种实现版本，Sync类中定义了writerShouldBlock抽象方法，用于判断当前线程是否可以直接通过CAS获取锁。然后通过Sync的子类NonfairSync、FairSync来实现该方法。具体地，在NonfairSync类中，writerShouldBlock方法会直接返回false。即直接利用CAS获取锁；而在FairSync类中，writerShouldBlock方法需要调用AQS的hasQueuedPredecessors方法，即如果AQS阻塞队列中如果没有其他线程在排队才可以通过CAS获取锁

类似地，在调用解锁unlock()方法时，其首先会调用AQS的release()方法。而在Sync类中则提供了tryRelease方法的实现。如果返回true则说明锁已经完全被释放了，需要AQS唤醒队列中的其他线程

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public class ReentrantReadWriteLock implements ReadWriteLock, java.io.Serializable {

    // 写锁变量
    private final ReentrantReadWriteLock.WriteLock writerLock;

    // AQS实现类变量
    final Sync sync;
        
    // 获取写锁
    public ReentrantReadWriteLock.WriteLock writeLock() {
        return writerLock; 
    }

    abstract static class Sync extends AbstractQueuedSynchronizer {
       
        abstract boolean writerShouldBlock();
        
        // 尝试获取AQS的独占锁
        protected final boolean tryAcquire(int acquires) {
            Thread current = Thread.currentThread();
            // AQS的state字段值
            int c = getState();
            //  获取到写锁的次数
            int w = exclusiveCount(c);
            // c不为0, 说明读锁或写锁已经被某线程获取了
            if (c != 0) {
                // 要么 w为0, 说明已经有线程获取了读锁, 故返回false
                // 要么 w不为0 即已经有线程获取了写锁, 但持锁线程并不是当前线程, 同样返回false
                if (w == 0 || current != getExclusiveOwnerThread())
                    return false;
                if (w + exclusiveCount(acquires) > MAX_COUNT)
                    throw new Error("Maximum lock count exceeded");
                // w不为0 且 持锁线程数是当前线程, 即此处重入写锁
                setState(c + acquires);
                return true;
            }
       
            // c=0, 说明锁未被任何线程持有
            // 根据是否需要保障公平性, 来确定下一步是否直接通过CAS获取锁
            if (writerShouldBlock() ||
                !compareAndSetState(c, c + acquires))
                // 需要排队 或 加锁失败, 返回false
                return false;
            // 加锁成功
            setExclusiveOwnerThread(current);
            return true;
        }

        protected final boolean tryRelease(int releases) {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            int nextc = getState() - releases;
            // 锁被完全释放了, 故需要返回true
            boolean free = exclusiveCount(nextc) == 0;
            if (free)
                setExclusiveOwnerThread(null);
            setState(nextc);
            return free;
        }
    }

    // 内部类：非公平的Sync子类
    static final class NonfairSync extends Sync {
        
        final boolean writerShouldBlock() {
            // 总是返回false, 即允许不排队, 直接获取锁
            return false;
        }

    }

    // 内部类：公平的Sync子类
    static final class FairSync extends Sync {
        
        final boolean writerShouldBlock() {  
            // 判断AQS队列中是否有其他线程在排队
            return hasQueuedPredecessors();
        }

    }

    // 内部类: 写锁
    public static class WriteLock implements Lock, java.io.Serializable {

        private final Sync sync;

        protected WriteLock(ReentrantReadWriteLock lock) {
            sync = lock.sync;
        }

        public void lock() {
            sync.acquire(1);
        }

        public void unlock() {
            sync.release(1);
        }
    }
}

...

public abstract class AbstractQueuedSynchronizer extends AbstractOwnableSynchronizer implements java.io.Serializable {    
    // 获取AQS的独占锁
    public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }

    // 需要子类去实现
    protected boolean tryAcquire(int arg) {
        throw new UnsupportedOperationException();
    }

    // 释放AQS的独占锁
    public final boolean release(int arg) {
        if (tryRelease(arg)) {
            Node h = head;
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);
            return true;
        }
        return false;
    }

    // 需要子类去实现
    protected boolean tryRelease(int arg) {
        throw new UnsupportedOperationException();
    }
}

读锁

ReentrantReadWriteLock中的读锁是对AQS中共享锁的使用。其使用方式是通过readLock()获取读锁实例，然后分别通过读锁的lock()、unlock()方法进行加锁、解锁操作

在调用加锁lock()方法时，其首先会调用AQS的acquireShared()方法。而在Sync类中则提供了tryAcquireShared方法的实现。如果返回值小于0则进入AQS的阻塞队列，否则加锁操作结束。同时为了支持公平、非公平两种实现版本，Sync类中定义了readerShouldBlock抽象方法，用于判断当前线程是否可以直接通过CAS获取锁。然后通过Sync的子类NonfairSync、FairSync来实现该方法。具体地，在NonfairSync类中，readerShouldBlock方法会调用AQS的apparentlyFirstQueuedIsExclusive方法来判断AQS阻塞队列中排队的第一个节点是不是获取写锁的，如果是则放弃本次CAS操作；而在FairSync类中，readerShouldBlock方法同样需要调用AQS的hasQueuedPredecessors方法，即如果AQS阻塞队列中如果没有其他线程在排队，本次才尝试通过CAS获取锁

类似地，在调用解锁unlock()方法时，其首先会调用AQS的releaseShared()方法。而在Sync类中则提供了tryReleaseShared方法的实现。如果返回true则说明锁已经完全被释放了，需要AQS进一步唤醒队列中的其他线程

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public class ReentrantReadWriteLock implements ReadWriteLock, java.io.Serializable {

    // 写锁变量
    private final ReentrantReadWriteLock.ReadLock readerLock;

    // AQS实现类变量
    final Sync sync;
        
    // 获取读锁
    public ReentrantReadWriteLock.ReadLock readLock()  {
        return readerLock; 
    }

    abstract static class Sync extends AbstractQueuedSynchronizer {
       
        abstract boolean readerShouldBlock();
        
        protected final int tryAcquireShared(int unused) {
            Thread current = Thread.currentThread();
            int c = getState();
            // 写锁被其他线程持有
            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);
        }

        protected final boolean tryReleaseShared(int unused) {
            Thread current = Thread.currentThread();
            if (firstReader == current) {
                // assert firstReaderHoldCount > 0;
                if (firstReaderHoldCount == 1)
                    firstReader = null;
                else
                    firstReaderHoldCount--;
            } else {
                HoldCounter rh = cachedHoldCounter;
                if (rh == null || rh.tid != getThreadId(current))
                    rh = readHolds.get();
                int count = rh.count;
                if (count <= 1) {
                    readHolds.remove();
                    if (count <= 0)
                        throw unmatchedUnlockException();
                }
                --rh.count;
            }
            for (;;) {
                int c = getState();
                int nextc = c - SHARED_UNIT;
                if (compareAndSetState(c, nextc))
                    // Releasing the read lock has no effect on readers,
                    // but it may allow waiting writers to proceed if
                    // both read and write locks are now free.
                    return nextc == 0;
            }
        }
    }

    // 内部类：非公平的Sync子类
    static final class NonfairSync extends Sync {
        final boolean readerShouldBlock() {
            return apparentlyFirstQueuedIsExclusive();
        }

    }

    // 内部类：公平的Sync子类
    static final class FairSync extends Sync {
        final boolean readerShouldBlock() {
            return hasQueuedPredecessors();
        }
    }

    // 内部类: 读锁
    public static class ReadLock implements Lock, java.io.Serializable {

        private final Sync sync;

        protected ReadLock(ReentrantReadWriteLock lock) {
            sync = lock.sync;
        }

        public void lock() {
            sync.acquireShared(1);
        }

        public void unlock() {
            sync.releaseShared(1);
        }
    }
}

...

public abstract class AbstractQueuedSynchronizer extends AbstractOwnableSynchronizer implements java.io.Serializable {    
    // 获取AQS的共享锁
    public final void acquireShared(int arg) {
        if (tryAcquireShared(arg) < 0)
            doAcquireShared(arg);
    }

    // 需要子类去实现
    protected int tryAcquireShared(int arg) {
        throw new UnsupportedOperationException();
    }

    // 释放AQS的共享锁
    public final boolean releaseShared(int arg) {
        if (tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }

    // 需要子类去实现
    protected boolean tryReleaseShared(int arg) {
        throw new UnsupportedOperationException();
    }
}

参考文献

  1. Java并发编程之美 翟陆续、薛宾田著
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