Java多线程之JUC包：ReentrantReadWriteLock源码学习笔记
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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以上是关于Java多线程之JUC包：ReentrantReadWriteLock源码学习笔记的主要内容，如果未能解决你的问题，请参考以下文章Java多线程之JUC包：ReentrantReadWriteLock源码学习笔记