jdk源码解读-并发包-Lock-ReentrantReadWriteLock（1）-整体介绍以及读锁的lock 和 unlock 解析

一.属性：

ReentrantReadWriteLock实现了接口ReadWriteLock。同时ReentrantReadWriteLock 也是基于 AbstractQueuedSynchronizer 实现的，它具有下面这些属性。

1. 获取顺序：

此类不会将读取者优先或写入者优先强加给锁访问的排序。但支持可选的公平模式。

1. 非公平模式（默认）：

当使用一个非公平模式时，读和写的锁的获得顺序不是特定的，取决于重入的约束。连续竞争的非公平锁可能无限期地推迟一个或多个reader或writer线程，但吞吐量通常要高于公平锁。

1. 公平模式：

线程利用一个近似到达顺序的策略来争夺进入。当释放当前保持的锁时，以下情况二选一：

• 可以为等待时间最长的单个writer线程分配写入锁。

• 如果有一组等待时间大于所有正在等待的writer线程的reader，将为该组分配读者锁。

对于一个试图获取公平读锁的线程：如果写锁没被释放，或有一个等待的读线程，这时这个试图获取公平读锁的线程将会被阻塞。这个线程（试图获得读锁的线程）只有在最老的等待的写线程获得并释放写锁，才能获得读锁。当然，如果一个等待的写线程放弃了它的等待，随着写锁的释放，一个或更多的读线程将会获取读锁。

对于一个试图获取公平写锁的线程： 除非读锁和写锁都是空闲的（暗示没有等待线程），不然这个线程会被阻塞。 （注意非阻塞的ReadLock的tryLock()方法和WriteLock的tryLock()方法不会遵从公平锁的设置，并且将会立即尝试获取锁，如何能获得锁，无论有没有其他等待线程都会获得锁。）

2. 重入：

此锁允许reader和writer按照 ReentrantLock 的样式重新获取读取锁或写入锁。在写入线程保持的所有写入锁都已经释放后，才允许重入reader使用读取锁。writer可以获取读取锁，但reader不能获取写入锁。

3.锁降级：

重入还允许从写入锁降级为读取锁，实现方式是：先获取写入锁，然后获取读取锁，最后释放写入锁。但是，从读取锁升级到写入锁是不可能的。

锁降级的例子：

* 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>

4.锁获取的中断：

读取锁和写入锁都支持锁获取期间的中断。

5.Condition 支持：

写入锁提供了一个 Condition 实现，对于写入锁来说，该实现的行为与ReentrantLock.newCondition() 提供的 Condition 实现对 ReentrantLock 所做的行为相同。当然，此 Condition 只能用于写入锁。读取锁不支持 Condition，readLock().newCondition() 会抛出 UnsupportedOperationException。

ReentrantReadWriteLocks能被用于提升某些集合的某些操作的并发性。特别是当集合预计会变大而且读线程比写线程多，并且操作的开销大于同步的开销，这样会体现ReentrantReadWriteLocks的价值。如下面TreeMap预计会变大而且会有大量的并发访问：

*  <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>

6.监测：

此类支持一些确定是读取锁还是写入锁的方法。这些方法设计用于监视系统状态，而不是同步控制。

从类的层次关系看，ReentrantReadWriteLock与ReentrantLock没有一点关系。

ReentrantReadWriteLock实现了接口ReadWriteLock。

ReentrantReadWriteLock通过一系列内部类和工具类AbstractQueuedSynchronizer实现读锁，写锁，以及线程的同步。

ReentrantReadWriteLock有5个内部类分别是，ReadLock,WriteLock,Sync,FairSync,

NofairSync。其中FairSync和NofairSync是Sync的子类。Sync有两个内部类分别是HoldCounter和ThreadLocalHoldCounter。

二.状态保存：

1. 保存获得读锁的线程数和写锁重入的状态

ReentrantLock用一个int变量c保存重入的次数，ReentrantReadWriteLock也有一个c变量，但是要保存获得读锁的线程数和写锁重入状态。解决方案，掰成两半：

AQS 的状态是32位（int 类型）的，辦成两份，读锁用高16位，表示持有读锁的线程数（sharedCount），写锁低16位，表示写锁的重入次数 （exclusiveCount）。状态值为 0 表示锁空闲，sharedCount不为 0 表示分配了读锁，exclusiveCount 不为 0 表示分配了写锁，sharedCount和exclusiveCount 肯定不会同时不为 0。

    abstract static class Sync extends AbstractQueuedSynchronizer {
    // 
     // 
       static final int SHARED_SHIFT   = 16;

       // 由于读锁用高位部分，所以读锁个数加1，其实是状态值加 2^16
       static final int SHARED_UNIT    = (1 << SHARED_SHIFT);

       // 写锁的可重入的最大次数、读锁允许的最大数量
       static final int MAX_COUNT      = (1 << SHARED_SHIFT) - 1;

       // 写锁的掩码，用于状态的低16位有效值
       static final int EXCLUSIVE_MASK = (1 << SHARED_SHIFT) - 1;

       // 读锁计数，当前持有读锁的线程数
    static int sharedCount(int c)    { return c >>> SHARED_SHIFT; }

    // 写锁的计数，也就是它的重入次数
    static int exclusiveCount(int c) { return c & EXCLUSIVE_MASK; }
}

2.读锁重入计数：

   abstract static class Sync extends AbstractQueuedSynchronizer {
     /**
     * 每个线程特定的 read 持有计数。存放在ThreadLocal，不需要是线程安全的。
     */
    static final class HoldCounter {
        int count = 0;

        // 使用id而不是引用是为了避免保留垃圾。注意这是个常量。
        final long tid = Thread.currentThread().getId();
    }

    /**
     * 采用继承是为了重写 initialValue 方法，这样就不用进行这样的处理：
     * 如果ThreadLocal没有当前线程的计数，则new一个，再放进ThreadLocal里。
     * 可以直接调用 get。
     * */
    static final class ThreadLocalHoldCounter
        extends ThreadLocal<HoldCounter> {
        public HoldCounter initialValue() {
            return new HoldCounter();
        }
    }

    /**
     * 保存当前线程重入读锁的次数的容器。在读锁重入次数为 0 时移除。
     */
    private transient ThreadLocalHoldCounter readHolds;

    /**
     * 最近一个成功获取读锁的线程的计数。这省却了ThreadLocal查找，
     * 通常情况下，下一个释放线程是最后一个获取线程。这不是 volatile 的，
     * 因为它仅用于试探的，线程进行缓存也是可以的
     * （因为判断是否是当前线程是通过线程id来比较的）。
     */
    private transient HoldCounter cachedHoldCounter;

    /**
     * firstReader是这样一个特殊线程：它是最后一个把 共享计数 从 0 改为 1 的
     * （在锁空闲的时候），而且从那之后还没有释放读锁的。如果不存在则为null。
     * firstReaderHoldCount 是 firstReader 的重入计数。
     *
     * firstReader 不能导致保留垃圾，因此在 tryReleaseShared 里设置为null，
     * 除非线程异常终止，没有释放读锁。
     *
     * 作用是在跟踪无竞争的读锁计数时非常便宜。
     *
     * firstReader及其计数firstReaderHoldCount是不会放入 readHolds 的。
     */
    private transient Thread firstReader = null;
    private transient int firstReaderHoldCount;

    Sync() {
        readHolds = new ThreadLocalHoldCounter();
        setState(getState()); // 确保 readHolds 的内存可见性，利用 volatile 写的内存语义。
    }
}

三.读锁lock方法操作流程和调用分析：

1.ReadLock的lock()方法的类关系图：

1. lock():

读锁发起锁资源请求

/**
 * 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);
}

2. acquireShared(1):

获取共享锁，方法tryAcquireShared()尝试获取锁资源，如果没有获得再通过doAcquireShared()不断尝试，直到获得锁资源。

 * 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);
}

3. tryAcquireShared():

尝试获得共享锁。

1. 如果有另一个线程获得了写锁还没释放，则获取失败。

2. 如果没有写锁被持有，这个线程请求是否被队列策略阻塞。如果没有被策略阻塞，尝试通过cas和更新数量去获得锁资源。主要这个方法只能处理线程第一次获得读锁资源的情况，不能处理重入的情况。重入的情况的处理延迟到完整版的获取读锁资源方法处理（fullTryAcquireShared(current)）。

3. 如果第二步中，获取读锁被队列策略阻塞或CAS尝试失败，或读锁数量饱和，会进入方法fullTryAcquireShared():

// 参数变为 unused 是因为读锁的重入计数是内部维护的。
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();

    // 这个if语句是说：持有写锁的线程可以获取读锁。
    if (exclusiveCount(c) != 0 && // 已分配了写锁
        getExclusiveOwnerThread() != current) // 且当前线程不是持有写锁的线程
        return -1;

    int r = sharedCount(c); // 取读锁计数
    if (!readerShouldBlock() && // 由子类根据其公平策略决定是否允许获取读锁
        r < MAX_COUNT &&           // 读锁数量还没达到最大值

        // 尝试获取读锁。注意读线程计数的单位是  2^16
        compareAndSetState(c, c + SHARED_UNIT)) {
         // 成功获取读锁

     // 注意下面对firstReader的处理：firstReader是不会放到readHolds里的
     // 这样，在读锁只有一个的情况下，就避免了查找readHolds。
        if (r == 0) { // 是 firstReader，计数不会放入  readHolds。
            firstReader = current;
            firstReaderHoldCount = 1;
        } else if (firstReader == current) { // firstReader 重入
            firstReaderHoldCount++;
        } else {
             // 非 firstReader 读锁重入计数更新
            HoldCounter rh = cachedHoldCounter; // 首先访问缓存
            if (rh == null || rh.tid != current.getId())
                cachedHoldCounter = rh = readHolds.get();
            else if (rh.count == 0)
                readHolds.set(rh);
            rh.count++;
        }
        return 1;
    }
    // 获取读锁失败，放到循环里重试。
    return fullTryAcquireShared(current);
}

1. fullTryAcquireShared(current)：

这个方法是会不断重试让当前线程获得读锁资源。处理了tryAcquireShared方法没有处理的cas赋值失败和重入读锁的情况。

  /**
  * 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;
        }
    }
}

1. doAcquireShared()：

step 1:addWaiter(Node.SHARED)。当 tryAcquireShared()尝试获得共享锁失败返回负数时，线程进入等待读锁的队列。

step 2:node.predecessor()。判断当前线程节点的前驱节点是否是头节点，是头结点就调用tryAcquireShared(arg)再尝试获得一次锁资源。

/**
 * Acquires in shared uninterruptible mode.
 * @param arg the acquire argument
 */
private void doAcquireShared(int arg) {
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
boolean interrupted = false;
for (;;) {
final Node p = node.predecessor();
if (p == head) {
int r = tryAcquireShared(arg);
if (r >= 0) {
                    setHeadAndPropagate(node, r);
                    p.next = null; // help GC
                    if (interrupted)
selfInterrupt();
                    failed = false;
return;
                }
            }
if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                interrupted = true;
        }
    } finally {
if (failed)
            cancelAcquire(node);
    }
}

1. addWaiter(Node mode):把当前线程包装成Node,放入队列。

 /**
 * Creates and enqueues node for current thread and given mode.
 *
 * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
 * @return the new node
 */
private Node addWaiter(Node mode) {
    Node node = new Node(Thread.currentThread(), mode);
// Try the fast path of enq; backup to full enq on failure
    Node pred = tail;
if (pred != null) {
        node.prev = pred;
if (compareAndSetTail(pred, node)) {
            pred.next = node;
return node;
        }
    }
    enq(node);
return node;
}

1. parkAndCheckInterrupt()：对于暂时不能获取读锁资源的线程进行阻塞。

/**

 * Convenience method to park and then check if interrupted
 *
 * @return {@code true} if interrupted
 */
private final boolean parkAndCheckInterrupt() {
    LockSupport.park(this);
return Thread.interrupted();
}

四.读锁unlock方法操作流程和调用分析：

0）unlock():

  /**
 * 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);
}

1. releaseShared(int arg)

/**
 * Releases in shared mode.  Implemented by unblocking one or more
 * threads if {@link #tryReleaseShared} returns true.
 *
 * @param arg the release argument.  This value is conveyed to
 *        {@link #tryReleaseShared} but is otherwise uninterpreted
 *        and can represent anything you like.
 * @return the value returned from {@link #tryReleaseShared}
 */
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {
        doReleaseShared();
return true;
    }
return false;
}

1. tryReleaseShared(int unused)

protected final boolean tryReleaseShared(int unused) {
    Thread current = Thread.currentThread();
    // 清理firstReader缓存 或 readHolds里的重入计数
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) {
            // 没释放前重入是1，就完全释放读锁
readHolds.remove();
if (count <= 0)
throw unmatchedUnlockException();
        }
        --rh.count;// 主要用于重入退出
    }
    // 循环在CAS更新状态值，主要是把读锁数量减 1
for (;;) {
int c = getState();
        //
int nextc = c - SHARED_UNIT;//SHARED_UNIT表示高位的1
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;
    }
}

3)doReleaseShared():

 * Release action for shared mode -- signals successor and ensures
 * propagation. (Note: For exclusive mode, release just amounts
 * to calling unparkSuccessor of head if it needs signal.)
 */
private void doReleaseShared() {
/*
     * Ensure that a release propagates, even if there are other
     * in-progress acquires/releases.  This proceeds in the usual
     * way of trying to unparkSuccessor of head if it needs
     * signal. But if it does not, status is set to PROPAGATE to
     * ensure that upon release, propagation continues.
     * Additionally, we must loop in case a new node is added
     * while we are doing this. Also, unlike other uses of
     * unparkSuccessor, we need to know if CAS to reset status
     * fails, if so rechecking.
     */
    for (;;) {
        Node h = head;
if (h != null && h != tail) {
int ws = h.waitStatus;
if (ws == Node.SIGNAL) {
if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
continue;            // loop to recheck cases
                unparkSuccessor(h);
            }
else if (ws == 0 &&
                     !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
continue;                // loop on failed CAS
        }
if (h == head)                   // loop if head changed
            break;
    }
}

1. unparkSuccessor(h):

 /**
     * Wakes up node's successor, if one exists.
     *
     * @param node the node
     */
    private void unparkSuccessor(Node node) {
        /*
         * If status is negative (i.e., possibly needing signal) try
         * to clear in anticipation of signalling.  It is OK if this
         * fails or if status is changed by waiting thread.
         */
        int ws = node.waitStatus;
        if (ws < 0)
            compareAndSetWaitStatus(node, ws, 0);

        /*
         * Thread to unpark is held in successor, which is normally
         * just the next node.  But if cancelled or apparently null,
         * traverse backwards from tail to find the actual
         * non-cancelled successor.
         */
        Node s = node.next;
        if (s == null || s.waitStatus > 0) {
            s = null;
            for (Node t = tail; t != null && t != node; t = t.prev)
                if (t.waitStatus <= 0)
                    s = t;
        }
        if (s != null)
            LockSupport.unpark(s.thread);
    }