ReentrantReadWriteLock

一、概述

二、源码

1、实现接口

public interface ReadWriteLock {

    Lock readLock();

    Lock writeLock();
}

2、属性

//读锁
private final ReentrantReadWriteLock.ReadLock readerLock;
//写锁
private final ReentrantReadWriteLock.WriteLock writerLock;
//内部同步器
final Sync sync;

3、构造方法

public ReentrantReadWriteLock() {
    this(false);
}

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; }

4、内置同步器和锁实现

abstract static class Sync extends AbstractQueuedSynchronizer {
    private static final long serialVersionUID = 6317671515068378041L;

    //高16位为读锁占有，低16位为写锁占有
    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;

    //共享数量
    static int sharedCount(int c)    { return c >>> SHARED_SHIFT; }
    //独占数量
    static int exclusiveCount(int c) { return c & EXCLUSIVE_MASK; }

    //重入计数器，内部类，重入次数和线程
    static final class HoldCounter {
        int count = 0;
        final long tid = getThreadId(Thread.currentThread());
    }

    //重入计数器ThreadLocal
    static final class ThreadLocalHoldCounter extends ThreadLocal<HoldCounter> {
        public HoldCounter initialValue() {
            return new HoldCounter();
        }
    }

    private transient ThreadLocalHoldCounter readHolds;

    private transient HoldCounter cachedHoldCounter;

    private transient Thread firstReader = null;
    private transient int firstReaderHoldCount;

    Sync() {
        readHolds = new ThreadLocalHoldCounter();
        setState(getState());
    }

    abstract boolean readerShouldBlock();

    abstract boolean writerShouldBlock();

    protected final boolean tryRelease(int releases) {
        if (!isHeldExclusively())
            throw new IllegalMonitorStateException();
        int nextc = getState() - releases;
        boolean free = exclusiveCount(nextc) == 0;
        if (free)
            setExclusiveOwnerThread(null);
        setState(nextc);
        return free;
    }

    protected final boolean tryAcquire(int acquires) {

        Thread current = Thread.currentThread();
        int c = getState();
        int w = exclusiveCount(c);
        if (c != 0) {
            //有读锁存在，独占线程非当前线程，就返回false
            if (w == 0 || current != getExclusiveOwnerThread())
                return false;
            if (w + exclusiveCount(acquires) > MAX_COUNT)
                throw new Error("Maximum lock count exceeded");
            setState(c + acquires);
            return true;
        }
        //如果读写锁都没被获取，如果写锁竞争时需要阻塞或状态更新不成功
        if (writerShouldBlock() || !compareAndSetState(c, c + acquires))
            return false;
        //持有线程
        setExclusiveOwnerThread(current);
        return true;
    }

    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;
        }
    }

    private IllegalMonitorStateException unmatchedUnlockException() {
        return new IllegalMonitorStateException(
            "attempt to unlock read lock, not locked by current thread");
    }

    protected final int tryAcquireShared(int unused) {

        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);
    }

    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;
            }
        }
    }

    final boolean tryWriteLock() {
        Thread current = Thread.currentThread();
        int c = getState();
        if (c != 0) {
            int w = exclusiveCount(c);
            if (w == 0 || current != getExclusiveOwnerThread())
                return false;
            if (w == MAX_COUNT)
                throw new Error("Maximum lock count exceeded");
        }
        if (!compareAndSetState(c, c + 1))
            return false;
        setExclusiveOwnerThread(current);
        return true;
    }

    final boolean tryReadLock() {
        Thread current = Thread.currentThread();
        for (;;) {
            int c = getState();
            if (exclusiveCount(c) != 0 &&
                getExclusiveOwnerThread() != current)
                return false;
            int r = sharedCount(c);
            if (r == MAX_COUNT)
                throw new Error("Maximum lock count exceeded");
            if (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 true;
            }
        }
    }

    protected final boolean isHeldExclusively() {
        return getExclusiveOwnerThread() == Thread.currentThread();
    }

    final ConditionObject newCondition() {
        return new ConditionObject();
    }

    final Thread getOwner() {
        return ((exclusiveCount(getState()) == 0) ?
                null :
                getExclusiveOwnerThread());
    }

    final int getReadLockCount() {
        return sharedCount(getState());
    }

    final boolean isWriteLocked() {
        return exclusiveCount(getState()) != 0;
    }

    final int getWriteHoldCount() {
        return isHeldExclusively() ? exclusiveCount(getState()) : 0;
    }

    final int getReadHoldCount() {
        if (getReadLockCount() == 0)
            return 0;

        Thread current = Thread.currentThread();
        if (firstReader == current)
            return firstReaderHoldCount;

        HoldCounter rh = cachedHoldCounter;
        if (rh != null && rh.tid == getThreadId(current))
            return rh.count;

        int count = readHolds.get().count;
        if (count == 0) readHolds.remove();
        return count;
    }

    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        s.defaultReadObject();
        readHolds = new ThreadLocalHoldCounter();
        setState(0); // reset to unlocked state
    }

    final int getCount() { return getState(); }
}

static final class NonfairSync extends Sync {
    private static final long serialVersionUID = -8159625535654395037L;
    final boolean writerShouldBlock() {
        return false; // writers can always barge
    }
    final boolean readerShouldBlock() {
        return apparentlyFirstQueuedIsExclusive();
    }
}

static final class FairSync extends Sync {
    private static final long serialVersionUID = -2274990926593161451L;
    final boolean writerShouldBlock() {
        return hasQueuedPredecessors();
    }
    final boolean readerShouldBlock() {
        return hasQueuedPredecessors();
    }
}

public static class ReadLock implements Lock, java.io.Serializable {
    private static final long serialVersionUID = -5992448646407690164L;
    private final Sync sync;

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

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

    public void lockInterruptibly() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }
    
    public boolean tryLock() {
        return sync.tryReadLock();
    }

    public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException {
        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
    }

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

    public Condition newCondition() {
        throw new UnsupportedOperationException();
    }

    public String toString() {
        int r = sync.getReadLockCount();
        return super.toString() +
            "[Read locks = " + r + "]";
    }
}

public static class WriteLock implements Lock, java.io.Serializable {
    private static final long serialVersionUID = -4992448646407690164L;
    private final Sync sync;

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

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

    public void lockInterruptibly() throws InterruptedException {
        sync.acquireInterruptibly(1);
    }

    public boolean tryLock( ) {
        return sync.tryWriteLock();
    }

    public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException {
        return sync.tryAcquireNanos(1, unit.toNanos(timeout));
    }

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

    public Condition newCondition() {
        return sync.newCondition();
    }

    public String toString() {
        Thread o = sync.getOwner();
        return super.toString() + ((o == null) ?
                                   "[Unlocked]" :
                                   "[Locked by thread " + o.getName() + "]");
    }

    public boolean isHeldByCurrentThread() {
        return sync.isHeldExclusively();
    }

    public int getHoldCount() {
        return sync.getWriteHoldCount();
    }
}

5、一般方法

public final boolean isFair() {
    return sync instanceof FairSync;
}

protected Thread getOwner() {
    return sync.getOwner();
}

public int getReadLockCount() {
    return sync.getReadLockCount();
}

public boolean isWriteLocked() {
    return sync.isWriteLocked();
}

public boolean isWriteLockedByCurrentThread() {
    return sync.isHeldExclusively();
}

public int getWriteHoldCount() {
    return sync.getWriteHoldCount();
}

public int getReadHoldCount() {
    return sync.getReadHoldCount();
}

protected Collection<Thread> getQueuedWriterThreads() {
    return sync.getExclusiveQueuedThreads();
}

protected Collection<Thread> getQueuedReaderThreads() {
    return sync.getSharedQueuedThreads();
}

public final boolean hasQueuedThreads() {
    return sync.hasQueuedThreads();
}

public final boolean hasQueuedThread(Thread thread) {
    return sync.isQueued(thread);
}

public final int getQueueLength() {
    return sync.getQueueLength();
}


protected Collection<Thread> getQueuedThreads() {
    return sync.getQueuedThreads();
}

public boolean hasWaiters(Condition condition) {
    if (condition == null)
        throw new NullPointerException();
    if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
        throw new IllegalArgumentException("not owner");
    return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject)condition);
}

public int getWaitQueueLength(Condition condition) {
    if (condition == null)
        throw new NullPointerException();
    if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
        throw new IllegalArgumentException("not owner");
    return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject)condition);
}

protected Collection<Thread> getWaitingThreads(Condition condition) {
    if (condition == null)
        throw new NullPointerException();
    if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
        throw new IllegalArgumentException("not owner");
    return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject)condition);
}

public String toString() {
    int c = sync.getCount();
    int w = Sync.exclusiveCount(c);
    int r = Sync.sharedCount(c);

    return super.toString() +
        "[Write locks = " + w + ", Read locks = " + r + "]";
}

static final long getThreadId(Thread thread) {
    return UNSAFE.getLongVolatile(thread, TID_OFFSET);
}

private static final sun.misc.Unsafe UNSAFE;
private static final long TID_OFFSET;
static {
    try {
        UNSAFE = sun.misc.Unsafe.getUnsafe();
        Class<?> tk = Thread.class;
        TID_OFFSET = UNSAFE.objectFieldOffset
            (tk.getDeclaredField("tid"));
    } catch (Exception e) {
        throw new Error(e);
    }
}