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ReentrantLock提供了标准的互斥操作，但在应用中，我们对一个资源的访问有两种方式：读和写，读操作一般不会影响数据的一致性问题。但如果我们使用ReentrantLock，则在需要在读操作的时候也独占锁，这会导致并发效率大大降低。JUC包提供了读写锁ReentrantReadWriteLock，使得读写锁分离，在上述情境下，应用读写锁相对于使用独占锁，并发性能得到较大提高。
我们先来大致了解一下ReentrantReadWriteLock的性质：
①基本性质：读锁是一个共享锁，写锁是一个独占锁。读锁能同时被多个线程获取，写锁只能被一个线程获取。读锁和写锁不能同时存在。
①重入性：一个线程可以多次重复获取读锁和写锁。
③锁降级：一个线程在已经获取写锁的情况下，可以再次获取读锁，如果线程又释放了写锁，就完成了一次锁降级。
④锁升级：ReentrantReadWriteLock不支持锁升级。一个线程在获取读锁的情况下，如果试图去获取写锁，将会导致死锁（后面会详细说明）。
⑤获取锁中断：提供了可中断的lock方法。
⑥重入数：读锁和写锁的重入上限为65535（所有线程获取的锁的总数，为什么是这个值后面会详细说明）。
⑦公平性：ReentrantReadWriteLock提供了公平&非公平两种工作模式。
ReentrantReadWriteLock实现了ReadWriteLock接口：
public interface ReadWriteLock { Lock readLock(); Lock writeLock(); }
这个接口之有两个方法，分别返回读锁和写锁。ReentrantReadWriteLock定义了两个内部类：readLock&writeLock。
ReentrantReadWriteLock提供了两种自定义的同步器：FairSync&NonfairSync：
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(); } }
他们都继承自父类同步器Sync，而他们只定义了writerShouldBlock&readerShouldBlock方法。这两个方法用在获取锁的操作中，表示要获取锁的线程需要到等待队列中，还是可以直接尝试获取。后面我们会详细分析。
在自定义的同步器Sync中，定义了锁数量的记录方式：
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; /** Returns the number of shared holds represented in count */ static int sharedCount(int c) { return c >>> SHARED_SHIFT; } /** Returns the number of exclusive holds represented in count */ static int exclusiveCount(int c) { return c & EXCLUSIVE_MASK; }
可见，ReentrantReadWriteLock用一个32位无符号数记录锁的数量，高16位记录共享锁（读锁）的数量，第16位记录独占锁（写锁）的数量，因此锁的数量上限都是65535。
源代码：
/* * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. * * * * * * * * * * * * * * * * * * * * */ /* * * * * * * Written by Doug Lea with assistance from members of JCP JSR-166 * Expert Group and released to the public domain, as explained at * http://creativecommons.org/publicdomain/zero/1.0/ */ package java.util.concurrent.locks; import java.util.concurrent.TimeUnit; import java.util.Collection; /** * An implementation of {@link ReadWriteLock} supporting similar * semantics to {@link ReentrantLock}. * <p>This class has the following properties: * * <ul> * <li><b>Acquisition order</b> * * <p>This class does not impose a reader or writer preference * ordering for lock access. However, it does support an optional * <em>fairness</em> policy. * * <dl> * <dt><b><i>Non-fair mode (default)</i></b> * <dd>When constructed as non-fair (the default), the order of entry * to the read and write lock is unspecified, subject to reentrancy * constraints. A nonfair lock that is continuously contended may * indefinitely postpone one or more reader or writer threads, but * will normally have higher throughput than a fair lock. * * <dt><b><i>Fair mode</i></b> * <dd>When constructed as fair, threads contend for entry using an * approximately arrival-order policy. When the currently held lock * is released, either the longest-waiting single writer thread will * be assigned the write lock, or if there is a group of reader threads * waiting longer than all waiting writer threads, that group will be * assigned the read lock. * * <p>A thread that tries to acquire a fair read lock (non-reentrantly) * will block if either the write lock is held, or there is a waiting * writer thread. The thread will not acquire the read lock until * after the oldest currently waiting writer thread has acquired and * released the write lock. Of course, if a waiting writer abandons * its wait, leaving one or more reader threads as the longest waiters * in the queue with the write lock free, then those readers will be * assigned the read lock. * * <p>A thread that tries to acquire a fair write lock (non-reentrantly) * will block unless both the read lock and write lock are free (which * implies there are no waiting threads). (Note that the non-blocking * {@link ReadLock#tryLock()} and {@link WriteLock#tryLock()} methods * do not honor this fair setting and will immediately acquire the lock * if it is possible, regardless of waiting threads.) * <p> * </dl> * * <li><b>Reentrancy</b> * * <p>This lock allows both readers and writers to reacquire read or * write locks in the style of a {@link ReentrantLock}. Non-reentrant * readers are not allowed until all write locks held by the writing * thread have been released. * * <p>Additionally, a writer can acquire the read lock, but not * vice-versa. Among other applications, reentrancy can be useful * when write locks are held during calls or callbacks to methods that * perform reads under read locks. If a reader tries to acquire the * write lock it will never succeed. * * <li><b>Lock downgrading</b> * <p>Reentrancy also allows downgrading from the write lock to a read lock, * by acquiring the write lock, then the read lock and then releasing the * write lock. However, upgrading from a read lock to the write lock is * <b>not</b> possible. * * <li><b>Interruption of lock acquisition</b> * <p>The read lock and write lock both support interruption during lock * acquisition. * * <li><b>{@link Condition} support</b> * <p>The write lock provides a {@link Condition} implementation that * behaves in the same way, with respect to the write lock, as the * {@link Condition} implementation provided by * {@link ReentrantLock#newCondition} does for {@link ReentrantLock}. * This {@link Condition} can, of course, only be used with the write lock. * * <p>The read lock does not support a {@link Condition} and * {@code readLock().newCondition()} throws * {@code UnsupportedOperationException}. * * <li><b>Instrumentation</b> * <p>This class supports methods to determine whether locks * are held or contended. These methods are designed for monitoring * system state, not for synchronization control. * </ul> * * <p>Serialization of this class behaves in the same way as built-in * locks: a deserialized lock is in the unlocked state, regardless of * its state when serialized. * * <p><b>Sample usages</b>. Here is a code sketch showing how to perform * lock downgrading after updating a cache (exception handling is * particularly tricky when handling multiple locks in a non-nested * fashion): * * <pre> {@code * class CachedData { * Object data; * volatile boolean cacheValid; * final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock(); * * void processCachedData() { * rwl.readLock().lock(); * if (!cacheValid) { * // Must release read lock before acquiring write lock * rwl.readLock().unlock(); * rwl.writeLock().lock(); * try { * // Recheck state because another thread might have * // acquired write lock and changed state before we did. * if (!cacheValid) { * data = ... * cacheValid = true; * } * // Downgrade by acquiring read lock before releasing write lock * rwl.readLock().lock(); * } finally { * rwl.writeLock().unlock(); // Unlock write, still hold read * } * } * * try { * use(data); * } finally { * rwl.readLock().unlock(); * } * } * }}</pre> * * ReentrantReadWriteLocks can be used to improve concurrency in some * uses of some kinds of Collections. This is typically worthwhile * only when the collections are expected to be large, accessed by * more reader threads than writer threads, and entail operations with * overhead that outweighs synchronization overhead. For example, here * is a class using a TreeMap that is expected to be large and * concurrently accessed. * * <pre> {@code * class RWDictionary { * private final Map<String, Data> m = new TreeMap<String, Data>(); * private final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock(); * private final Lock r = rwl.readLock(); * private final Lock w = rwl.writeLock(); * * public Data get(String key) { * r.lock(); * try { return m.get(key); } * finally { r.unlock(); } * } * public String[] allKeys() { * r.lock(); * try { return m.keySet().toArray(); } * finally { r.unlock(); } * } * public Data put(String key, Data value) { * w.lock(); * try { return m.put(key, value); } * finally { w.unlock(); } * } * public void clear() { * w.lock(); * try { m.clear(); } * finally { w.unlock(); } * } * }}</pre> * * <h3>Implementation Notes</h3> * * <p>This lock supports a maximum of 65535 recursive write locks * and 65535 read locks. Attempts to exceed these limits result in * {@link Error} throws from locking methods. * * @since 1.5 * @author Doug Lea */ 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; } /** * Synchronization implementation for ReentrantReadWriteLock. * Subclassed into fair and nonfair versions. */ abstract static class Sync extends AbstractQueuedSynchronizer { private static final long serialVersionUID = 6317671515068378041L; /* * Read vs write count extraction constants and functions. * Lock state is logically divided into two unsigned shorts: * The lower one representing the exclusive (writer) lock hold count, * and the upper the shared (reader) hold count. */ 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; /** Returns the number of shared holds represented in count */ static int sharedCount(int c) { return c >>> SHARED_SHIFT; } /** Returns the number of exclusive holds represented in count */ static int exclusiveCount(int c) { return c & EXCLUSIVE_MASK; } /** * A counter for per-thread read hold counts. * Maintained as a ThreadLocal; cached in cachedHoldCounter */ static final class HoldCounter { int count = 0; // Use id, not reference, to avoid garbage retention final long tid = getThreadId(Thread.currentThread()); } /** * ThreadLocal subclass. Easiest to explicitly define for sake * of deserialization mechanics. */ static final class ThreadLocalHoldCounter extends ThreadLocal<HoldCounter> { public HoldCounter initialValue() { return new HoldCounter(); } } /** * The number of reentrant read locks held by current thread. * Initialized only in constructor and readObject. * Removed whenever a thread's read hold count drops to 0. */ private transient ThreadLocalHoldCounter readHolds; /** * The hold count of the last thread to successfully acquire * readLock. This saves ThreadLocal lookup in the common case * where the next thread to release is the last one to * acquire. This is non-volatile since it is just used * as a heuristic, and would be great for threads to cache. * * <p>Can outlive the Thread for which it is caching the read * hold count, but avoids garbage retention by not retaining a * reference to the Thread. * * <p>Accessed via a benign data race; relies on the memory * model's final field and out-of-thin-air guarantees. */ private transient HoldCounter cachedHoldCounter; /** * firstReader is the first thread to have acquired the read lock. * firstReaderHoldCount is firstReader's hold count. * * <p>More precisely, firstReader is the unique thread that last * changed the shared count from 0 to 1, and has not released the * read lock since then; null if there is no such thread. * * <p>Cannot cause garbage retention unless the thread terminated * without relinquishing its read locks, since tryReleaseShared * sets it to null. * * <p>Accessed via a benign data race; relies on the memory * model's out-of-thin-air guarantees for references. * * <p>This allows tracking of read holds for uncontended read * locks to be very cheap. */ private transient Thread firstReader = null; private transient int firstReaderHoldCount; Sync() { readHolds = new ThreadLocalHoldCounter(); setState(getState()); // ensures visibility of readHolds } /* * Acquires and releases use the same code for fair and * nonfair locks, but differ in whether/how they allow barging * when queues are non-empty. */ /** * Returns true if the current thread, when trying to acquire * the read lock, and otherwise eligible to do so, should block * because of policy for overtaking other waiting threads. */ abstract boolean readerShouldBlock(); /** * Returns true if the current thread, when trying to acquire * the write lock, and otherwise eligible to do so, should block * because of policy for overtaking other waiting threads. */ abstract boolean writerShouldBlock(); /* * Note that tryRelease and tryAcquire can be called by * Conditions. So it is possible that their arguments contain * both read and write holds that are all released during a * condition wait and re-established in tryAcquire. */ 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) { /* * Walkthrough: * 1. If read count nonzero or write count nonzero * and owner is a different thread, fail. * 2. If count would saturate, fail. (This can only * happen if count is already nonzero.) * 3. Otherwise, this thread is eligible for lock if * it is either a reentrant acquire or * queue policy allows it. If so, update state * and set owner. */ 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 acquire setState(c + acquires); return true; } if (writerShouldBlock() || !compareAndSetState(c, c + acquires)) return false; 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) { /* * Walkthrough: * 1. If write lock held by another thread, fail. * 2. Otherwise, this thread is eligible for * lock wrt state, so ask if it should block * because of queue policy. If not, try * to grant by CASing state and updating count. * Note that step does not check for reentrant * acquires, which is postponed to full version * to avoid having to check hold count in * the more typical non-reentrant case. * 3. If step 2 fails either because thread * apparently not eligible or CAS fails or count * saturated, chain to version with full retry loop. */ 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); } /** * Full version of acquire for reads, that handles CAS misses * and reentrant reads not dealt with in tryAcquireShared. */ final int fullTryAcquireShared(Thread current) { /* * This code is in part redundant with that in * tryAcquireShared but is simpler overall by not * complicating tryAcquireShared with interactions between * retries and lazily reading hold counts. */ 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; } } } /** * Performs tryLock for write, enabling barging in both modes. * This is identical in effect to tryAcquire except for lack * of calls to writerShouldBlock. */ 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; } /** * Performs tryLock for read, enabling barging in both modes. * This is identical in effect to tryAcquireShared except for * lack of calls to readerShouldBlock. */ 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() { // While we must in general read state before owner, // we don't need to do so to check if current thread is owner return getExclusiveOwnerThread() == Thread.currentThread(); } // Methods relayed to outer class final ConditionObject newCondition() { return new ConditionObject(); } final Thread getOwner() { // Must read state before owner to ensure memory consistency 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; } /** * Reconstitutes the instance from a stream (that is, deserializes it). */ 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(); } } /** * Nonfair version of Sync */ 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(); } } /** * Fair version of Sync */ static final class FairSync extends Sync { private static final long serialVersionUID = -2274990926593161451L; final boolean writerShouldBlock() { return hasQueuedPredecessors(); } final boolean readerShouldBlock() { return hasQueuedPredecessors(); } } /** * The lock returned by method {@link ReentrantReadWriteLock#readLock}. */ 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; } /** * Acquires the read lock. * * <p>Acquires the read lock if the write lock is not held by * another thread and returns immediately. * * <p>If the write lock is held by another thread then * the current thread becomes disabled for thread scheduling * purposes and lies dormant until the read lock has been acquired. */ public void lock() { sync.acquireShared(1); } /** * Acquires the read lock unless the current thread is * {@linkplain Thread#interrupt interrupted}. * * <p>Acquires the read lock if the write lock is not held * by another thread and returns immediately. * * <p>If the write lock is held by another thread then the * current thread becomes disabled for thread scheduling * purposes and lies dormant until one of two things happens: * * <ul> * * <li>The read lock is acquired by the current thread; or * * <li>Some other thread {@linkplain Thread#interrupt interrupts} * the current thread. * * </ul> * * <p>If the current thread: * * <ul> * * <li>has its interrupted status set on entry to this method; or * * <li>is {@linkplain Thread#interrupt interrupted} while * acquiring the read lock, * * </ul> * * then {@link InterruptedException} is thrown and the current * thread's interrupted status is cleared. * * <p>In this implementation, as this method is an explicit * interruption point, preference is given to responding to * the interrupt over normal or reentrant acquisition of the * lock. * * @throws InterruptedException if the current thread is interrupted */ public void lockInterruptibly() throws InterruptedException { sync.acquireSharedInterruptibly(1); } /** * Acquires the read lock only if the write lock is not held by * another thread at the time of invocation. * * <p>Acquires the read lock if the write lock is not held by * another thread and returns immediately with the value * {@code true}. Even when this lock has been set to use a * fair ordering policy, a call to {@code tryLock()} * <em>will</em> immediately acquire the read lock if it is * available, whether or not other threads are currently * waiting for the read lock. This "barging" behavior * can be useful in certain circumstances, even though it * breaks fairness. If you want to honor the fairness setting * for this lock, then use {@link #tryLock(long, TimeUnit) * tryLock(0, TimeUnit.SECONDS) } which is almost equivalent * (it also detects interruption). * * <p>If the write lock is held by another thread then * this method will return immediately with the value * {@code false}. * * @return {@code true} if the read lock was acquired */ public boolean tryLock() { return sync.tryReadLock(); } /** * Acquires the read lock if the write lock is not held by * another thread within the given waiting time and the * current thread has not been {@linkplain Thread#interrupt * interrupted}. * * <p>Acquires the read lock if the write lock is not held by * another thread and returns immediately with the value * {@code true}. If this lock has been set to use a fair * ordering policy then an available lock <em>will not</em> be * acquired if any other threads are waiting for the * lock. This is in contrast to the {@link #tryLock()} * method. If you want a timed {@code tryLock} that does * permit barging on a fair lock then combine the timed and * un-timed forms together: * * <pre> {@code * if (lock.tryLock() || * lock.tryLock(timeout, unit)) { * ... * }}</pre> * * <p>If the write lock is held by another thread then the * current thread becomes disabled for thread scheduling * purposes and lies dormant until one of three things happens: * * <ul> * * <li>The read lock is acquired by the current thread; or * * <li>Some other thread {@linkplain Thread#interrupt interrupts} * the current thread; or * * <li>The specified waiting time elapses. * * </ul> * * <p>If the read lock is acquired then the value {@code true} is * returned. * * <p>If the current thread: * * <ul> * * <li>has its interrupted status set on entry to this method; or * * <li>is {@linkplain Thread#interrupt interrupted} while * acquiring the read lock, * * </ul> then {@link InterruptedException} is thrown and the * current thread's interrupted status is cleared. * * <p>If the specified waiting time elapses then the value * {@code false} is returned. If the time is less than or * equal to zero, the method will not wait at all. * * <p>In this implementation, as this method is an explicit * interruption point, preference is given to responding to * the interrupt over normal or reentrant acquisition of the * lock, and over reporting the elapse of the waiting time. * * @param timeout the time to wait for the read lock * @param unit the time unit of the timeout argument * @return {@code true} if the read lock was acquired * @throws InterruptedException if the current thread is interrupted * @throws NullPointerException if the time unit is null */ public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException { return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout)); } /** * Attempts to release this lock. * * <p>If the number of readers is now zero then the lock * is made available for write lock attempts. */ public void unlock() { sync.releaseShared(1); } /** * Throws {@code UnsupportedOperationException} because * {@code ReadLocks} do not support conditions. * * @throws UnsupportedOperationException always */ public Condition newCondition() { throw new UnsupportedOperationException(); } /** * Returns a string identifying this lock, as well as its lock state. * The state, in brackets, includes the String {@code "Read locks ="} * followed by the number of held read locks. * * @return a string identifying this lock, as well as its lock state */ public String toString() { int r = sync.getReadLockCount(); return super.toString() + "[Read locks = " + r + "]"; } } /** * The lock returned by method {@link ReentrantReadWriteLock#writeLock}. */ 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; } /** * Acquires the write lock. * * <p>Acquires the write lock if neither the read nor write lock * are held by another thread * and returns immediately, setting the write lock hold count to * one. * * <p>If the current thread already holds the write lock then the * hold count is incremented by one and the method returns * immediately. * * <p>If the lock is held by another thread then the current * thread becomes disabled for thread scheduling purposes and * lies dormant until the write lock has been acquired, at which * time the write lock hold count is set to one. */ public void lock() { sync.acquire(1); } /** * Acquires the write lock unless the current thread is * {@linkplain Thread#interrupt interrupted}. * * <p>Acquires the write lock if neither the read nor write lock * are held by another thread * and returns immediately, setting the write lock hold count to * one. * * <p>If the current thread already holds this lock then the * hold count is incremented by one and the method returns * immediately. * * <p>If the lock is held by another thread then the current * thread becomes disabled for thread scheduling purposes and * lies dormant until one of two things happens: * * <ul> * * <li>The write lock is acquired by the current thread; or * * <li>Some other thread {@linkplain Thread#interrupt interrupts} * the current thread. * * </ul> * * <p>If the write lock is acquired by the current thread then the * lock hold count is set to one. * * <p>If the current thread: * * <ul> * * <li>has its interrupted status set on entry to this method; * or * * <li>is {@linkplain Thread#interrupt interrupted} while * acquiring the write lock, * * </ul> * * then {@link InterruptedException} is thrown and the current * thread's interrupted status is cleared. * * <p>In this implementation, as this method is an explicit * interruption point, preference is given to responding to * the interrupt over normal or reentrant acquisition of the * lock. * * @throws InterruptedException if the current thread is interrupted */ public void lockInterruptibly() throws InterruptedException { sync.acquireInterruptibly(1); } /** * Acquires the write lock only if it is not held by another thread * at the time of invocation. * * <p>Acquires the write lock if neither the read nor write lock * are held by another thread * and returns immediately with the value {@code true}, * setting the write lock hold count to one. Even when this lock has * been set to use a fair ordering policy, a call to * {@code tryLock()} <em>will</em> immediately acquire the * lock if it is available, whether or not other threads are * currently waiting for the write lock. This "barging" * behavior can be useful in certain circumstances, even * though it breaks fairness. If you want to honor the * fairness setting for this lock, then use {@link * #tryLock(long, TimeUnit) tryLock(0, TimeUnit.SECONDS) } * which is almost equivalent (it also detects interruption). * * <p>If the current thread already holds this lock then the * hold count is incremented by one and the method returns * {@code true}. * * <p>If the lock is held by another thread then this method * will return immediately with the value {@code false}. * * @return {@code true} if the lock was free and was acquired * by the current thread, or the write lock was already held * by the current thread; and {@code false} otherwise. */ public boolean tryLock( ) { return sync.tryWriteLock(); } /** * Acquires the write lock if it is not held by another thread * within the given waiting time and the current thread has * not been {@linkplain Thread#interrupt interrupted}. * * <p>Acquires the write lock if neither the read nor write lock * are held by another thread * and returns immediately with the value {@code true}, * setting the write lock hold count to one. If this lock has been * set to use a fair ordering policy then an available lock * <em>will not</em> be acquired if any other threads are * waiting for the write lock. This is in contrast to the {@link * #tryLock()} method. If you want a timed {@code tryLock} * that does permit barging on a fair lock then combine the * timed and un-timed forms together: * * <pre> {@code * if (lock.tryLock() || * lock.tryLock(timeout, unit)) { * ... * }}</pre> * * <p>If the current thread already holds this lock then the * hold count is incremented by one and the method returns * {@code true}. * * <p>If the lock is held by another thread then the current * thread becomes disabled for thread scheduling purposes and * lies dormant until one of three things happens: * * <ul> * * <li>The write lock is acquired by the current thread; or * * <li>Some other thread {@linkplain Thread#interrupt interrupts} * the current thread; or * * <li>The specified waiting time elapses * * </ul> * * <p>If the write lock is acquired then the value {@code true} is * returned and the write lock hold count is set to one. * * <p>If the current thread: * * <ul> * * <li>has its interrupted status set on entry to this method; * or * * <li>is {@linkplain Thread#interrupt interrupted} while * acquiring the write lock, * * </ul> * * then {@link InterruptedException} is thrown and the current * thread's interrupted status is cleared. * * <p>If the specified waiting time elapses then the value * {@code false} is returned. If the time is less than or * equal to zero, the method will not wait at all. * * <p>In this implementation, as this method is an explicit * interruption point, preference is given to responding to * the interrupt over normal or reentrant acquisition of the * lock, and over reporting the elapse of the waiting time. * * @param timeout the time to wait for the write lock * @param unit the time unit of the timeout argument * * @return {@code true} if the lock was free and was acquired * by the current thread, or the write lock was already held by the * current thread; and {@code false} if the waiting time * elapsed before the lock could be acquired. * * @throws InterruptedException if the current thread is interrupted * @throws NullPointerException if the time unit is null */ public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException { return sync.tryAcquireNanos(1, unit.toNanos(timeout)); } /** * Attempts to release this lock. * * <p>If the current thread is the holder of this lock then * the hold count is decremented. If the hold count is now * zero then the lock is released. If the current thread is * not the holder of this lock then {@link * IllegalMonitorStateException} is thrown. * * @throws IllegalMonitorStateException if the current thread does not * hold this lock */ public void unlock() { sync.release(1); } /** * Returns a {@link Condition} instance for use with this * {@link Lock} instance. * <p>The returned {@link Condition} instance supports the same * usages as do the {@link Object} monitor methods ({@link * Object#wait() wait}, {@link Object#notify notify}, and {@link * Object#notifyAll notifyAll}) when used with the built-in * monitor lock. * * <ul> * * <li>If this write lock is not held when any {@link * Condition} method is called then an {@link * IllegalMonitorStateException} is thrown. (Read locks are * held independently of write locks, so are not checked or * affected. However it is essentially always an error to * invoke a condition waiting method when the current thread * has also acquired read locks, since other threads that * could unblock it will not be able to acquire the write * lock.) * * <li>When the condition {@linkplain Condition#await() waiting} * methods are called the write lock is released and, before * they return, the write lock is reacquired and the lock hold * count restored to what it was when the method was called. * * <li>If a thread is {@linkplain Thread#interrupt interrupted} while * waiting then the wait will terminate, an {@link * InterruptedException} will be thrown, and the thread's * interrupted status will be cleared. * * <li> Waiting threads are signalled in FIFO order. * * <li>The ordering of lock reacquisition for threads returning * from waiting methods is the same as for threads initially * acquiring the lock, which is in the default case not specified, * but for <em>fair</em> locks favors those threads that have been * waiting the longest. * * </ul> * * @return the Condition object */ public Condition newCondition() { return sync.newCondition(); } /** * Returns a string identifying this lock, as well as its lock * state. The state, in brackets includes either the String * {@code "Unlocked"} or the String {@code "Locked by"} * followed by the {@linkplain Thread#getName name} of the owning thread. * * @return a string identifying this lock, as well as its lock state */ public String toString() { Thread o = sync.getOwner(); return super.toString() + ((o == null) ? "[Unlocked]" : "[Locked by thread " + o.getName() + "]"); } /** * Queries if this write lock is held by the current thread. * Identical in effect to {@link * ReentrantReadWriteLock#isWriteLockedByCurrentThread}. * * @return {@code true} if the current thread holds this lock and * {@code false} otherwise * @since 1.6 */ public boolean isHeldByCurrentThread() { return sync.isHeldExclusively(); } /** * Queries the number of holds on this write lock by the current * thread. A thread has a hold on a lock for each lock action * that is not matched by an unlock action. Identical in effect * to {@link ReentrantReadWriteLock#getWriteHoldCount}. * * @return the number of holds on this lock by the current thread, * or zero if this lock is not held by the current thread * @since 1.6 */ public int getHoldCount() { return sync.getWriteHoldCount(); } } // Instrumentation and status /** * Returns {@code true} if this lock has fairness set true. * * @return {@code true} if this lock has fairness set true */ public final boolean isFair() { return sync instanceof FairSync; } /** * Returns the thread that currently owns the write lock, or * {@code null} if not owned. When this method is called by a * thread that is not the owner, the return value reflects a * best-effort approximation of current lock status. For example, * the owner may be momentarily {@code null} even if there are * threads trying to acquire the lock but have not yet done so. * This method is designed to facilitate construction of * subclasses that provide more extensive lock monitoring * facilities. * * @return the owner, or {@code null} if not owned */ protected Thread getOwner() { return sync.getOwner(); } /** * Queries the number of read locks held for this lock. This * method is designed for use in monitoring system state, not for * synchronization control. * @return the number of read locks held */ public int getReadLockCount() { return sync.getReadLockCount(); } /** * Queries if the write lock is held by any thread. This method is * designed for use in monitoring system state, not for * synchronization control. * * @return {@code true} if any thread holds the write lock and * {@code false} otherwise */ public boolean isWriteLocked() { return sync.isWriteLocked(); } /** * Queries if the write lock is held by the current thread. * * @return {@code true} if the current thread holds the write lock and * {@code false} otherwise */ public boolean isWriteLockedByCurrentThread() { return sync.isHeldExclusively(); } /** * Queries the number of reentrant write holds on this lock by the * current thread. A writer thread has a hold on a lock for * each lock action that is not matched by an unlock action. * * @return the number of holds on the write lock by the current thread, * or zero if the write lock is not held by the current thread */ public int getWriteHoldCount() { return sync.getWriteHoldCount(); } /** * Queries the number of reentrant read holds on this lock by the * current thread. A reader thread has a hold on a lock for * each lock action that is not matched by an unlock action. * * @return the number of holds on the read lock by the current thread, * or zero if the read lock is not held by the current thread * @since 1.6 */ public int getReadHoldCount() { return sync.getReadHoldCount(); } /** * Returns a collection containing threads that may be waiting to * acquire the write lock. Because the actual set of threads may * change dynamically while constructing this result, the returned * collection is only a best-effort estimate. The elements of the * returned collection are in no particular order. This method is * designed to facilitate construction of subclasses that provide * more extensive lock monitoring facilities. * * @return the collection of threads */ protected Collection<Thread> getQueuedWriterThreads() { return sync.getExclusiveQueuedThreads(); } /** * Returns a collection containing threads that may be waiting to * acquire the read lock. Because the actual set of threads may * change dynamically while constructing this result, the returned * collection is only a best-effort estimate. The elements of the * returned collection are in no particular order. This method is * designed to facilitate construction of subclasses that provide * more extensive lock monitoring facilities. * * @return the collection of threads */ protected Collection<Thread> getQueuedReaderThreads() { return sync.getSharedQueuedThreads(); } /** * Queries whether any threads are waiting to acquire the read or * write lock. Note that because cancellations may occur at any * time, a {@code true} return does not guarantee that any other * thread will ever acquire a lock. This method is designed * primarily for use in monitoring of the system state. * * @return {@code true} if there may be other threads waiting to * acquire the lock */ public final boolean hasQueuedThreads() { return sync.hasQueuedThreads(); } /** * Queries whether the given thread is waiting to acquire either * the read or write lock. Note that because cancellations may * occur at any time, a {@code true} return does not guarantee * that this thread will ever acquire a lock. This method is * designed primarily for use in monitoring of the system state. * * @param thread the thread * @return {@code true} if the given thread is queued waiting for this lock * @throws NullPointerException if the thread is null */ public final boolean hasQueuedThread(Thread thread) { return sync.isQueued(thread); } /** * Returns an estimate of the number of threads waiting to acquire * either the read or write lock. The value is only an estimate * because the number of threads may change dynamically while this * method traverses internal data structures. This method is * designed for use in monitoring of the system state, not for * synchronization control. * * @return the estimated number of threads waiting for this lock */ public final int getQueueLength() { return sync.getQueueLength(); } /** * Returns a collection containing threads that may be waiting to * acquire either the read or write lock. Because the actual set * of threads may change dynamically while constructing this * result, the returned collection is only a best-effort estimate. * The elements of the returned collection are in no particular * order. This method is designed to facilitate construction of * subclasses that provide more extensive monitoring facilities. * * @return the collection of threads */ protected Collection<Thread> getQueuedThreads() { return sync.getQueuedThreads(); } /** * Queries whether any threads are waiting on the given condition * associated with the write lock. Note that because timeouts and * interrupts may occur at any time, a {@code true} return does * not guarantee that a future {@code signal} will awaken any * threads. This method is designed primarily for use in * monitoring of the system state. * * @param condition the condition * @return {@code true} if there are any waiting threads * @throws IllegalMonitorStateException if this lock is not held * @throws IllegalArgumentException if the given condition is * not associated with this lock * @throws NullPointerException if the condition is null */ 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); } /** * Returns an estimate of the number of threads waiting on the * given condition associated with the write lock. Note that because * timeouts and interrupts may occur at any time, the estimate * serves only as an upper bound on the actual number of waiters. * This method is designed for use in monitoring of the system * state, not for synchronization control. * * @param condition the condition * @return the estimated number of waiting threads * @throws IllegalMonitorStateException if this lock is not held * @throws IllegalArgumentException if the given condition is * not associated with this lock * @throws NullPointerException if the condition is null */ 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); } /** * Returns a collection containing those threads that may be * waiting on the given condition associated with the write lock. * Because the actual set of threads may change dynamically while * constructing this result, the returned collection is only a * best-effort estimate. The elements of the returned collection * are in no particular order. This method is designed to * facilitate construction of subclasses that provide more * extensive condition monitoring facilities. * * @param condition the condition * @return the collection of threads * @throws IllegalMonitorStateException if this lock is not held * @throws IllegalArgumentException if the given condition is * not associated with this lock * @throws NullPointerException if the condition is null */ 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); } /** * Returns a string identifying this lock, as well as its lock state. * The state, in brackets, includes the String {@code "Write locks ="} * followed by the number of reentrantly held write locks, and the * String {@code "Read locks ="} followed by the number of held * read locks. * * @return a string identifying this lock, as well as its lock state */ 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 + "]"; } /** * Returns the thread id for the given thread. We must access * this directly rather than via method Thread.getId() because * getId() is not final, and has been known to be overridden in * ways that do not preserve unique mappings. */ static final long getThreadId(Thread thread) { return UNSAFE.getLongVolatile(thread, TID_OFFSET); } // Unsafe mechanics 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); } } }
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可以看到，相对于JUC包提供的其他锁，ReentrantReadWriteLock的代码量还是比较大的。下面，我们就来分析一下读写锁的工作过程。
一、写锁
1、lock 获取写锁
public void lock() { sync.acquire(1); }
跟我们以前分析的独占锁ReentrantLock一样，lock方法调用AQS的acquire方法：
public final void acquire(int arg) { if (!tryAcquire(arg) && acquireQueued(addWaiter(Node.EXCLUSIVE), arg)) selfInterrupt(); }
acquire方法已经在笔者之前的另一篇博文AQS源码学习笔记中详细介绍，不再赘述。我么这里重点关注自定义同步器Sync重写的tryAcquire方法：
protected final boolean tryAcquire(int acquires) { /* * Walkthrough: * 1. If read count nonzero or write count nonzero * and owner is a different thread, fail. * 2. If count would saturate, fail. (This can only * happen if count is already nonzero.) * 3. Otherwise, this thread is eligible for lock if * it is either a reentrant acquire or * queue policy allows it. If so, update state * and set owner. */ 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 acquire setState(c + acquires); return true; } if (writerShouldBlock() || !compareAndSetState(c, c + acquires)) return false; setExclusiveOwnerThread(current); return true; }
首先调用获取了一下state值，然后调用exclusiveCount方法获取当前写锁的数量。
然后做了一个判断，当c！=0时：如果w==0（即读锁的数量！=0），直接返回false。因为我们前面已经说过，读锁和写锁不能同时存在。当c！=0且W！=0的时候，有写锁存在，如果写锁不是由当前线程持有（注意，写锁是独占锁，只能由一个线程持有），直接返回false。如果是当前线程持有写锁，说明当前线程正在试图“重入”写锁。调用setState更新status值。注意，由于写锁是独占锁，因此执行到setState这一步时不可能出现竞争，因此不用调用CAS操作，直接setState即可。
注意：如果一个线程在持有读锁的情况下去申请写锁（试图锁升级），会导致思索。tryAcquire在这种情况下返回false，AQS的acquire方法会将当前线程放入等待队列去等待写锁，在获取写锁之前不会释放锁持有的读锁，而读锁和写锁不能同时存在，发生死锁，他将永远不能获取这个写锁，其他线程也不能获取写锁，但读锁可被正常获取，只是永远不能获取写锁了。
如果c==0时，说明不存在任何锁。调用writerShouldBlock方法判断一下此时线程是否应该进入等待队列。注意：公平模式&非公平模式下的writerShouldBlock是不同的，非公平模式下，writerShouldBlock方法直接返回false，这也符合非公平的语义：
final boolean writerShouldBlock() { return false; // writers can always barge }
而公平模式下，则调用方法，判断下等待队列中，当前线程之前是否有其他线程正在等待：
final boolean writerShouldBlock() { return hasQueuedPredecessors(); }
注意，如果有，那么我们当时获取status的值的时候，这些线程还没来得及更改status值（因为我们当时获取的status为0），原因可能是应为刚到，或者刚被唤醒，在自旋中，还没有成功获取锁。
public final boolean hasQueuedPredecessors() { // The correctness of this depends on head being initialized // before tail and on head.next being accurate if the current // thread is first in queue. Node t = tail; // Read fields in reverse initialization order Node h = head; Node s; return h != t && ((s = h.next) == null || s.thread != Thread.currentThread()); }
返回true必须满足两个条件：①队列非空②第一个等待线程（head.next）为空 或 不为空但不是当前线程。head.next为空的情形是：在我们获取head之后，head就被队列中下一个等待线程线程踢出队列了，next被置为空，那么踢他出去的这个线程一定不是当前线程，说明有其他线程等待在队列中。
我们回到tryAcquire方法中，当发现writerShouldBlock为true，或者writerShouldBlock为false但在CAS操作中失败时（由于这里的获取写锁不是重入，因此可能有多个线程同时竞争写锁），返回false。如果CAS成功，则调用setExclusiveOwnerThread将当前持有写锁的线程设置为当前线程。
2、release 释放写锁
public void unlock() { sync.release(1); }
与ReentrantLock一样，unlock方法调用AQS提供的release方法：
public final boolean release(int arg) { if (tryRelease(arg)) { Node h = head; if (h != null && h.waitStatus != 0) unparkSuccessor(h); return true; } return false; }
release方法已经在笔者之前的另一篇博文AQS源码学习笔记中详细介绍，不再赘述。我么这里重点关注自定义同步器Sync重写的tryRelease方法：
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; }
首先，我们需要清楚一点：tryRelease方法的返回值不是表示是否成功获取，而是表示当前释放操作完成后，剩余写锁数量是否等于0（即完成此释放后，写锁是否可用）。这与同样是可重入的ReentrantLock的tryRelease方法一样，ReentrantLock的tryRelease方法返回值的意义也是剩余写锁数量是否等于0（即完成此释放后，写锁是否可用）。
3、tryLock 获取写锁
public boolean tryLock( ) { return sync.tryWriteLock(); }
WriteLock的tryLock方法调用自定义同步器Sync的tryWriteLock方法实现：
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; }
tryWriteLock方法看上去跟tryAcquire方法真的很像。唯一的区别在于，tryWriteLock忽略的writerShouldBlock方法，即，默认调用tryLock方法的时机，就是需要我们去“抢”写锁的时机。
二、读锁
1、lock 获取读锁
public void lock() { sync.acquireShared(1); }
ReadLock的lock方法调用AQS提供的acquireShared方法来实现：
public final void acquireShared(int arg) { if (tryAcquireShared(arg) < 0) doAcquireShared(arg); }
acquireShared方法已经在笔者之前的另一篇博文AQS源码学习笔记中详细介绍，不再赘述。我们重点关注自定义同步器Sync重写的tryAcquireShared方法：
protected final int tryAcquireShared(int unused) { /* * Walkthrough: * 1. If write lock held by another thread, fail. * 2. Otherwise, this thread is eligible for * lock wrt state, so ask if it should block * because of queue policy. If not, try * to grant by CASing state and updating count. * Note that step does not check for reentrant * acquires, which is postponed to full version * to avoid having to check hold count in * the more typical non-reentrant case. * 3. If step 2 fails either because thread * apparently not eligible or CAS fails or count * saturated, chain to version with full retry loop. */ 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); }
方法首先检测了一下当前是否有其他线程持有写锁，如果是的话，直接返回-1，表示获取失败。后续AQS的acquireShared方法会将当前线程放入等待队列中。
然后方法做了这样一个判断，如果当前线程可以直接参与竞争读锁的话，就调用CAS操作将status值加一个SHARED_UNIT，注意，这里不是加1是应为status的高16位代表读锁的数量。
OK，我们必须在这里暂停一下，我们需要详细解释一下几个成员变量：
static final class HoldCounter { int count = 0; // Use id, not reference, to avoid garbage retention final long tid = getThreadId(Thread.currentThread()); }
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;
HoldCounter是一个final的内部类，有两个成员：tid&count，分别代表一个线程ID和线程对应的一个计数值。
ThreadLocalHoldCounter是一个final的内部类，它继承自ThreadLocal<HoldCounter>，它重写了initialValue方法，ThreadLocalHoldCounter对象对某一个线程第一次调用get方法是，会调用initialValue方法初始化这个线程响应的本地变量，并加入到map中。
readHolds存在的作用是：记录所有持有读锁的线程所持有读锁的数量。对于写锁来说，它是独占锁，我们可以通过status的低16位+独占写锁的线程来记录关于写锁的所有信息，即它被谁持有&被重入的数量。而读锁是一个共享锁，任何线程都可能持有它，因此，我们必须对每个线程都记录一下它所持有的共享锁（读锁）的数量。本地变量ThreadLocal来实现这个记录是非常合适的。
cachedHoldCounter是一个缓存。很多情况下，一个线程获取读锁之后要更新一下它对应的记录值（线程对应的HoldCounter对象），然后有很大可能在很短的时间内就释放掉读锁，这时候需要再次更新HoldCounter，甚至需要从readHolds中删除（如果重入的读锁都被释放掉的话），需要调用readHolds的get方法，这是有一定开销的。因此，设置cachedHoldCounter作为一个缓存，在某个线程需要这个记录值的时候，先检查cachedHoldCounter对应的线程是否是这个线程自己，如果不是的话，再熊readHolds中get出来，这提高了效率。
firsReader&firstReaderHoldCount，这两个值记录了第一个获取读锁的线程和它持有的读锁的数量（可重入的嘛），这两个值在读锁全部释放之后要清空，以便记录下一次首先获取读锁的线程和其锁数目。这两个值存在的意义是：很多时候，读锁只被一个线程获取，这时候我们规定，第一个获取读锁的线程的计数不放入readHolds中，而是单独用这两个计数值来记录，这就避免了当只有一个线程操作读锁的时候，频繁地在readHolds上读取，提高了效率。
注意区别：cachedHoldCounter提高的是一个线程获取-释放之间没有其他线程来获取或释放锁时的效率；firsReader&firstReaderHoldCount提高的是只有一个线程操作锁时的效率。
这时候我们再回到tryAcquireShared方法，当CAS操作成功后，需要去更新刚刚说过的计数值。具体细节代码已经很清楚，不再赘述。
如果CAS失败或readerShouldBlock方法返回true，我们调用fullTryAcquireShared方法继续试图获取读锁。fullTryAcquireShared方法是tryAcquireShared方法的完整版，或者叫升级版，它处理了CAS失败的情况和readerShouldBlock返回true的情况。
在分析fullTryAcquireShared方法之前，我们先来看一下readerShouldBlock方法：
在公平模式下，根据等待队列中在当前线程之前有没有等待线程来判断：
final boolean readerShouldBlock() { return hasQueuedPredecessors(); }
而在非公平模式下：
final boolean readerShouldBlock() { return apparentlyFirstQueuedIsExclusive(); }
调用了apparentlyFirstQueuedIsExclusive方法：
final boolean apparentlyFirstQueuedIsExclusive() { Node h, s; return (h = head) != null && (s = h.next) != null && !s.isShared() && s.thread != null; }
这个方法返回是否队列的head.next正在等待独占锁（写锁）。当然这个方法执行的过程中队列的形态可能发生变化。这个方法的意思是：读锁不应该让写锁始终等待。
好了，我们现在来看fullTryAcquireShared方法：
/** * Full version of acquire for reads, that handles CAS misses * and reentrant reads not dealt with in tryAcquireShared. */ final int fullTryAcquireShared(Thread current) { /* * This code is in part redundant with that in * tryAcquireShared but is simpler overall by not * complicating tryAcquireShared with interactions between * retries and lazily reading hold counts. */ 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; } } }
我们可以看到：fullTryAcquireShared方法是tryAcquireShared方法的完整版，或者叫升级版，它处理了CAS失败的情况和readerShouldBlock返回true的情况。
跟tryAcquireShared方法一样，首先检查是否有其他线程正在持有写锁，如果是，直接返回false。如果没有线程正在持有写锁，则调用readerShouldBlock检测当前线程是否应该进入等待队列。就算readerShouldBlock方法返回true，原因可能因为当前是公平模式或者队列的第一个等待线程（head.next）正在等待写锁，我们也不能直接返回false，因为返回false意味着当前线程将要进入等待队列（见AQS的acquireShared方法），原因是：①如果当前线程正在持有读锁，且这次读锁的重入被放入等待队列，万一之前队列中有线程正在等待写锁，将会导致死锁；②另一种情况是当前线程正在持有写锁，且这次读锁的“降级申请”被放入等待队列，如果队列中之前有线程正在等待锁，不论等待的是写锁还是读锁，都将导致死锁。
因此，我们需要做一个判断，如果这次申请读锁是对读锁的一次重入（因为我们已经检测过没有写锁，因此只考虑上述第①种情况），我们将不能返回false（返回false意味着进队列），而是调用CAS操作去获取读锁，如果CAS失败，则一直自旋，直到成功获取，或者可以返回false去队列的时机的到来。
我们可以这样提fullTryAcquireShared方法说句话：不是我不想进队列休息，实在是因为进队列有可能死锁，所以我才一直自旋！
注意：判断重入的时候firstReader==当前线程即说明是一次重入，因为firstReader线程释放最后一个读锁的时候会将firstReader置为null，这里还不是null，说明依然持有读锁。
另外还记得我们提过apparentlyFirstQueuedIsExclusive方法是不可靠的吗，它在检测的过程中队列结构可能被更改，head可能被踢出，方法可能因为head.next为null而返回false。而且它也只是检测第一个等待线程（head.next），如果有等待写锁的线程在后面，它也不能检测出来。不过没关系，这些都导致它返回false，返回false意味着fullTryAcquireShared可以去抢“锁”并不会影响正确性。
2、unlock 释放读锁
public void unlock() { sync.releaseShared(1); }
readLock的unlock方法调用AQS提供的releaseShared方法实现：
public final boolean releaseShared(int arg) { if (tryReleaseShared(arg)) { doReleaseShared(); return true; } return false; }
releaseShared方法已在笔者的另一篇博文AQS源码学习笔记中详细介绍，不再赘述。这里我们关注自定义同步器Sync重写的tryReleaseShared方法：
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; } }
分为三部分：①如果是firstReader，对firstReader修改；②如果不是firstReader，修改readHolds；③CAS自旋更新status值。
注意：tryReleaseShared方法的返回值如果为true，表示status为0，即已经不存在任何锁，both读锁&写锁。
3、tryLock 获取读锁
public boolean tryLock() { return sync.tryReadLock(); }
ReadLock的tryLock调用自定义同步器Sync的tryReadLock方法实现：
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; } } }
与写锁的tryWriteLock方法类似，tryReadLock同样忽略了readerShouldBlock方法，因为调用这个方法就意味着：现在是适合抢占的时机。
tryReadLock方法与tryAcquireShared方法十分类似，不同在于：当CAS失败时，tryAcquireShared方法调用fullAcquireShared处理CAS失败，而tryReadLock方法遇到CAS失败时，直接返回false，毕竟只是try嘛。
总结：
ReentrantReadWriteLock相比于其他锁，还是比较复杂的，因为他结合了共享锁和独占锁，并混合使用了他们。虽然ReentrantReadWriteLock通过精巧的设计尽量避免死锁的发生，但如果我们使用不当仍然可能发生死锁，比如我们在持有读锁的情况下去申请写，企图做锁升级。
