多线程学习（七）---ReentrantLock与ReentrantReadWriteLock

参考文章：

Lock锁子类了解一下

一、ReentrantLock锁

首先我们来看看ReentrantLock锁的顶部注释，来看看他的相关特性：

/**
 * A reentrant mutual exclusion {@link Lock} with the same basic
 * behavior and semantics as the implicit monitor lock accessed using
 * {@code synchronized} methods and statements, but with extended
 * capabilities.
 *

跟synchronized具有相同的功能和语义，都是互斥锁，但它具有更好的扩展性

 * 
A {@code ReentrantLock} is owned by the thread last
 * successfully locking, but not yet unlocking it. A thread invoking
 * {@code lock} will return, successfully acquiring the lock, when
 * the lock is not owned by another thread. The method will return
 * immediately if the current thread already owns the lock. This can
 * be checked using methods {@link #isHeldByCurrentThread}, and {@link
 * #getHoldCount}.
 *

通过isHeldByCurrentThread和getHoldCount可以判断当前的线程是否拥有锁

 * 
The constructor for this class accepts an optional
 * fairness parameter.  When set {@code true}, under
 * contention, locks favor granting access to the longest-waiting
 * thread.  Otherwise this lock does not guarantee any particular
 * access order.  Programs using fair locks accessed by many threads
 * may display lower overall throughput (i.e., are slower; often much
 * slower) than those using the default setting, but have smaller
 * variances in times to obtain locks and guarantee lack of
 * starvation. Note however, that fairness of locks does not guarantee
 * fairness of thread scheduling. Thus, one of many threads using a
 * fair lock may obtain it multiple times in succession while other
 * active threads are not progressing and not currently holding the
 * lock.

构造方式支持使用参数来让锁设置为公平锁，公平锁一般的吞吐量会低一点，但一定程度保证了“相对公平”

 * Also note that the untimed {@link #tryLock()} method does not
 * honor the fairness setting. It will succeed if the lock
 * is available even if other threads are waiting.
 *

不计时tryLock方法不符合公平锁的设定

 * 
It is recommended practice to always immediately
 * follow a call to {@code lock} with a {@code try} block, most
 * typically in a before/after construction such as:
 *
 *  
 {@code
 * class X {
 *   private final ReentrantLock lock = new ReentrantLock();
 *   // ...
 *
 *   public void m() {
 *     lock.lock();  // block until condition holds
 *     try {
 *       // ... method body
 *     } finally {
 *       lock.unlock()
 *     }
 *   }
 * }}
 *

应该在try代码块之前调用lock()方法

 * 
In addition to implementing the {@link Lock} interface, this
 * class defines a number of {@code public} and {@code protected}
 * methods for inspecting the state of the lock.  Some of these
 * methods are only useful for instrumentation and monitoring.
 *

Lock接口还定义了一些监听锁状态的方法

 * 
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.
 *

一个被序列化的锁是处于不锁定状态的

 * 
This lock supports a maximum of 2147483647 recursive locks by
 * the same thread. Attempts to exceed this limit result in
 * {@link Error} throws from locking methods.
 *
 * @since 1.5
 * @author Doug Lea
 */

来总结一下要点吧：

• 比synchronized更有伸缩性(灵活)
• 支持公平锁(是相对公平的)
• 使用时最标准用法是在try之前调用lock方法，在finally代码块释放锁

class X {
    private final ReentrantLock lock = new ReentrantLock();
    // ...

    public void m() { 
        lock.lock();  // block until condition holds
        try {
            // ... method body
        } finally {
            lock.unlock()
        }
    }
}

1.1 内部类

这些内部类都是AQS的子类，这就印证了我们之前所说的：AQS是ReentrantLock的基础，AQS是构建锁、同步器的框架

可以很清晰的看到，ReentrantLock锁是支持公平锁和非公平锁的

1.2 构造方法

/**
     * Creates an instance of {@code ReentrantLock}.
     * This is equivalent to using {@code ReentrantLock(false)}.
     */
    public ReentrantLock() {
        sync = new NonfairSync();
    }
    
    /**
     * Creates an instance of {@code ReentrantLock} with the
     * given fairness policy.
     *
     * @param fair {@code true} if this lock should use a fair ordering policy
     */
    public ReentrantLock(boolean fair) {
        sync = fair ? new FairSync() : new NonfairSync();
    }

默认构造的是非同步锁

1.3 非公平lock方法

 /**
     * Sync object for non-fair locks
     */
    static final class NonfairSync extends Sync {
        private static final long serialVersionUID = 7316153563782823691L;

        /**
         * Performs lock.  Try immediate barge, backing up to normal
         * acquire on failure.
         */
        final void lock() {
            if (compareAndSetState(0, 1))
                setExclusiveOwnerThread(Thread.currentThread());
            else
                acquire(1);
        }

尝试获取锁，获取失败就调用AQS的acquire方法
acquire(1)方法我们在AQS时简单看过，其中tryAcquire()是子类来实现的

        protected final boolean tryAcquire(int acquires) {
            return nonfairTryAcquire(acquires);
        }
    }

tryAcquire()方法：

/**
         * Performs non-fair tryLock.  tryAcquire is implemented in
         * subclasses, but both need nonfair try for trylock method.
         */
        final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            // 锁是空闲状态，为该线程获取锁
            if (c == 0) {
                if (compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            // 锁是重进入状态
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }

1.4 公平lock方法

/**
         * Fair version of tryAcquire.  Don't grant access unless
         * recursive call or no waiters or is first.
         */
        protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (!hasQueuedPredecessors() &&
                    compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0)
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }

多了一个条件：!hasQueuedPredecessors()
这个方法主要是判断当前线程是否位于CLH同步队列中的第一个。如果是则返回flase，否则返回true

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());
    }

1.5 unlock方法

由子类的tryRelease(arg)实现：

/**
     * Attempts to release this lock.
     *
     * 
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);
    }
    
        protected final boolean tryRelease(int releases) {
            // 减掉releases
            int c = getState() - releases;
            // 如果释放的不是持有锁的线程，抛出异常
            if (Thread.currentThread() != getExclusiveOwnerThread())
                throw new IllegalMonitorStateException();
            boolean free = false;
             // state == 0 表示已经释放完全了，其他线程可以获取同步状态了
            if (c == 0) {
                free = true;
                setExclusiveOwnerThread(null);
            }
            setState(c);
            return free;
        }

只有当同步状态彻底释放后该方法才会返回true。当state == 0 时，则将锁持有线程设置为null，free= true，表示释放成功。

二、 ReentrantReadWriteLock

我们知道synchronized内置锁和ReentrantLock都是互斥锁(一次只能有一个线程进入到临界区(被锁定的区域))

而ReentrantReadWriteLock是一个读写锁：

• 在读取数据的时候，可以多个线程同时进入到到临界区(被锁定的区域)
• 在写数据的时候，无论是读线程还是写线程都是互斥的

一般来说：我们大多数都是读取数据得多，修改数据得少。所以这个读写锁在这种场景下就很有用了！

读写锁有一个接口ReadWriteLock，定义的方法就两个：

public interface ReadWriteLock {
    /**
     * Returns the lock used for reading.
     *
     * @return the lock used for reading
     */
    Lock readLock();

    /**
     * Returns the lock used for writing.
     *
     * @return the lock used for writing
     */
    Lock writeLock();
}

看注释：

/**
 * A {@code ReadWriteLock} maintains a pair of associated {@link
 * Lock locks}, one for read-only operations and one for writing.
 * The {@link #readLock read lock} may be held simultaneously by
 * multiple reader threads, so long as there are no writers.  The
 * {@link #writeLock write lock} is exclusive.
 *

维护了一对锁，读锁可同时进入到临界区，写锁是独占的

 * 
All {@code ReadWriteLock} implementations must guarantee that
 * the memory synchronization effects of {@code writeLock} operations
 * (as specified in the {@link Lock} interface) also hold with respect
 * to the associated {@code readLock}. That is, a thread successfully
 * acquiring the read lock will see all updates made upon previous
 * release of the write lock.
 *

读线程可以看到写线程更新过的数据

 * 
A read-write lock allows for a greater level of concurrency in
 * accessing shared data than that permitted by a mutual exclusion lock.
 * It exploits the fact that while only a single thread at a time (a
 * writer thread) can modify the shared data, in many cases any
 * number of threads can concurrently read the data (hence reader
 * threads).

只有一个写进程可以修改数据（互斥）
多个读线程并发访问共享数据

 * In theory, the increase in concurrency permitted by the use of a read-write
 * lock will lead to performance improvements over the use of a mutual
 * exclusion lock. In practice this increase in concurrency will only be fully
 * realized on a multi-processor, and then only if the access patterns for
 * the shared data are suitable.
 *

理论上，这种读写锁的并发性是比互斥锁要好的，但还要看计算机的处理器和共享资源适合这种模式才行

 * 
Whether or not a read-write lock will improve performance over the use
 * of a mutual exclusion lock depends on the frequency that the data is
 * read compared to being modified, the duration of the read and write
 * operations, and the contention for the data - that is, the number of
 * threads that will try to read or write the data at the same time.
 * For example, a collection that is initially populated with data and
 * thereafter infrequently modified, while being frequently searched
 * (such as a directory of some kind) is an ideal candidate for the use of
 * a read-write lock. However, if updates become frequent then the data
 * spends most of its time being exclusively locked and there is little, if any
 * increase in concurrency. Further, if the read operations are too short
 * the overhead of the read-write lock implementation (which is inherently
 * more complex than a mutual exclusion lock) can dominate the execution
 * cost, particularly as many read-write lock implementations still serialize
 * all threads through a small section of code. Ultimately, only profiling
 * and measurement will establish whether the use of a read-write lock is
 * suitable for your application.
 *

一个集合初始化之后如果常常是查询数据、少量修改数据，那么是适合读写锁的
但是，当修改操作并发增加，那么读写锁的开销大部分又是互斥锁的了

 *
 * 
Although the basic operation of a read-write lock is straight-forward,
 * there are many policy decisions that an implementation must make, which
 * may affect the effectiveness of the read-write lock in a given application.

在实现该接口的时候，需要考虑一些策略如下

 * Examples of these policies include:
 * 

 * 
Determining whether to grant the read lock or the write lock, when
 * both readers and writers are waiting, at the time that a writer releases
 * the write lock. Writer preference is common, as writes are expected to be
 * short and infrequent. Reader preference is less common as it can lead to
 * lengthy delays for a write if the readers are frequent and long-lived as
 * expected. Fair, or "in-order" implementations are also possible.
 *
 * 
Determining whether readers that request the read lock while a
 * reader is active and a writer is waiting, are granted the read lock.
 * Preference to the reader can delay the writer indefinitely, while
 * preference to the writer can reduce the potential for concurrency.
 *
 * 
Determining whether the locks are reentrant: can a thread with the
 * write lock reacquire it? Can it acquire a read lock while holding the
 * write lock? Is the read lock itself reentrant?
 *
 * 
Can the write lock be downgraded to a read lock without allowing
 * an intervening writer? Can a read lock be upgraded to a write lock,
 * in preference to other waiting readers or writers?
 *
 * 
 * You should consider all of these things when evaluating the suitability
 * of a given implementation for your application.
 *
 * @see ReentrantReadWriteLock
 * @see Lock
 * @see ReentrantLock
 *
 * @since 1.5
 * @author Doug Lea
 */

其实大概也是说明了：在读的时候可以共享，在写的时候是互斥的

接下来我们还是来看看对应的实现类吧：

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;

注释：

/**
 * An implementation of {@link ReadWriteLock} supporting similar
 * semantics to {@link ReentrantLock}.
 * 
This class has the following properties:
 *

实现了ReadWriteLock接口，用处跟ReentrantLock类似

 * 

 * 
Acquisition order
 *
 * 
This class does not impose a reader or writer preference
 * ordering for lock access.  However, it does support an optional
 * fairness policy.
 *

这个类不会偏好于读锁或者写锁

 * 

 * 
Non-fair mode (default)
 * 
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.
 *

默认是非公平锁，吞吐量一般比公平锁高

 * 
Fair mode
 * 
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.
 *

等待时间最长的写进程将获得写锁

 * 
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.
 *

当尝试公平获取读锁时，写锁释放后，读进程才能获取锁

 * 
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.)
 * 

 * 
 *

当尝试公平获取写锁时，当前应该处于无锁的状态

 * 
Reentrancy
 *
 * 
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.
 *

可重入

 * 
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.
 *

写锁可以获取读锁，读锁不能获取写锁

 * 
Lock downgrading
 * 
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
 * not possible.
 *

写锁可以降级为读锁，读锁不能升级为写锁

 * 
Interruption of lock acquisition
 * 
The read lock and write lock both support interruption during lock
 * acquisition.
 *

读锁和写锁都支持在获取时中断

 * 
{@link Condition} support
 * 
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.
 *

写锁支持条件对象

 * 
The read lock does not support a {@link Condition} and
 * {@code readLock().newCondition()} throws
 * {@code UnsupportedOperationException}.
 *

读锁不支持条件对象

 * 
Instrumentation
 * 
This class supports methods to determine whether locks
 * are held or contended. These methods are designed for monitoring
 * system state, not for synchronization control.
 * 
 *

该类用于监听锁的状态

 * 
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.
 *
 * 
Sample usages. 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):
 *

更新缓存后降级的demo

 * 
 {@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();
 *     }
 *   }
 * }}
 *
 * 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.
 *

TreeMap使用读写锁提高并发性的Demo

 *  
 {@code
 * class RWDictionary {
 *   private final Map m = new TreeMap();
 *   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(); }
 *   }
 * }}
 *
 * 
Implementation Notes
 *
 * 
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
 */

于是我们可以总结出读写锁的一些要点了：

• 读锁不支持条件对象，写锁支持条件对象
• 读锁不能升级为写锁，写锁可以降级为读锁
• 读写锁也有公平和非公平模式
• 读锁支持多个读线程进入临界区，写锁是互斥的

2.1 ReentrantReadWriteLock内部类

ReentrantReadWriteLock比ReentrantLock锁多了两个内部类(都是Lock实现)来维护读锁和写锁，但是主体还是使用Syn：

• WriteLock
• ReadLock

2.2 读锁和写锁的状态表示

在ReentrantLock锁上使用的是state来表示同步状态(也可以表示重入的次数)，而在ReentrantReadWriteLock是这样代表读写状态的：

/*
         * 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; }

高16位代表独享
低16位代表共享

2.3 写锁的获取

主要还是调用syn的acquire(1)：

/**
         * Acquires the write lock.
         *
         * 
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.
         *
         * 
If the current thread already holds the write lock then the
         * hold count is incremented by one and the method returns
         * immediately.
         *
         * 
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);
        }

看实现类：

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.
             */
             // 1. 需要是当前线程和无锁状态才能获取写锁
             // 2. 锁饱和了也不行
             // 3. 该线程有资格才能获取写锁
            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)
                // 不是当前线程
                if (w == 0 || current != getExclusiveOwnerThread())
                    return false;
                // 饱和
                if (w + exclusiveCount(acquires) > MAX_COUNT)
                    throw new Error("Maximum lock count exceeded");
                // Reentrant acquire
                setState(c + acquires);
                return true;
            }
            // 有锁状态
            if (writerShouldBlock() ||
                !compareAndSetState(c, c + acquires))
                return false;
            setExclusiveOwnerThread(current);
            return true;
        }

2.4 读锁获取

读锁的获取调用的是acquireShared(int arg)方法：

/**
     * Acquires in shared mode, ignoring interrupts.  Implemented by
     * first invoking at least once {@link #tryAcquireShared},
     * returning on success.  Otherwise the thread is queued, possibly
     * repeatedly blocking and unblocking, invoking {@link
     * #tryAcquireShared} until success.
     *
     * @param arg the acquire argument.  This value is conveyed to
     *        {@link #tryAcquireShared} but is otherwise uninterpreted
     *        and can represent anything you like.
     */
    public final void acquireShared(int arg) {
        if (tryAcquireShared(arg) < 0)
            doAcquireShared(arg);
    }

实现类：

protected final int tryAcquireShared(int unused) {
            /*
             * Walkthrough:
             * 1. If write lock held by another thread, fail.
            另一个线程拥有写锁，失败
             * 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();
            // 存在写锁，并且不在本线程
            if (exclusiveCount(c) != 0 &&
                getExclusiveOwnerThread() != current)
                return -1;
            int r = sharedCount(c);
            // 设置读锁的状态
            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;
            }
            // 不满足条件
            return fullTryAcquireShared(current);
        }

三、 总结

AQS是ReentrantReadWriteLock和ReentrantLock的基础，因为默认的实现都是在内部类Syn中，而Syn是继承AQS的

ReentrantReadWriteLock和ReentrantLock都支持公平和非公平模式，公平模式下会去看FIFO队列线程是否是在队头，而非公平模式下是没有的

ReentrantReadWriteLock是一个读写锁，如果读的线程比写的线程要多很多的话，那可以考虑使用它。它使用state的变量高16位是读锁，低16位是写锁

写锁可以降级为读锁，读锁不能升级为写锁

写锁是互斥的，读锁是共享的