public class ReentrantReadWriteLock implements ReadWriteLock {
    /** 讀鎖 */
    private final ReentrantReadWriteLock.ReadLock readerLock;
    /** 寫鎖 */
    private final ReentrantReadWriteLock.WriteLock writerLock;
    /** 持有的AQS子類對象 */
    final Sync sync;

    abstract static class Sync extends AbstractQueuedSynchronizer {}

    static final class NonfairSync extends Sync {}

    static final class FairSync extends Sync {}

    public static class ReadLock implements Lock {}

    public static class WriteLock implements Lock {}
  
    //默認非公平
    public ReentrantReadWriteLock() {
        this(false);
    }

    public ReentrantReadWriteLock(boolean fair) {
        sync = fair ? new FairSync() : new NonfairSync();
        readerLock = new ReadLock(this);
        writerLock = new WriteLock(this);
    }

    public static class ReadLock implements Lock {
    	private final Sync sync;
        protected ReadLock(ReentrantReadWriteLock lock) {
            sync = lock.sync;
        }
    }

    public static class WriteLock implements Lock {
    	private final Sync sync;
        protected WriteLock(ReentrantReadWriteLock lock) {
            sync = lock.sync;
        }
    }

}

abstract static class Sync extends AbstractQueuedSynchronizer {
  	//繼承的一個int的成員變量，將其拆分為高16位和低16位
    //private volatile int state;
    static final int SHARED_SHIFT   = 16;
  	//讀一次，鎖增加的值
    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;

    //讀的數(shù)量
    static int sharedCount(int c)    { return c >>> SHARED_SHIFT; }
    //寫的數(shù)量
    static int exclusiveCount(int c) { return c & EXCLUSIVE_MASK; }
}

static final class HoldCounter {
    int count = 0;
    // 使用線程id，而不是線程的引用。這樣可以防止垃圾不被回收
    final long tid = getThreadId(Thread.currentThread());
}

static final class ThreadLocalHoldCounter
    extends ThreadLocal<HoldCounter> {
    public HoldCounter initialValue() {
        return new HoldCounter();
    }
}
//使用的ThreadLocal
private transient ThreadLocalHoldCounter readHolds;
//一個緩存
private transient HoldCounter cachedHoldCounter;
//組合起來相當于一個緩存
private transient Thread firstReader = null;
private transient int firstReaderHoldCount;

//ReadLock
public void lock() {
    sync.acquireShared(1);
}  
//AQS
public final void acquireShared(int arg) {
    if (tryAcquireShared(arg) < 0)
        doAcquireShared(arg);
} 
//Sync
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();
  	// 如果寫的數(shù)量不是0，且寫線程不是當前線程，失敗
    if (exclusiveCount(c) != 0 &&
        getExclusiveOwnerThread() != current)
        return -1;
  	// 獲取讀的個數(shù)
    int r = sharedCount(c);
  	// 如果當前線程想要讀，沒有被堵塞
  	// 當前讀的數(shù)量未超過最大允許的讀的個數(shù)
  	// CAS執(zhí)行成功
    if (!readerShouldBlock() &&
        r < MAX_COUNT &&
        compareAndSetState(c, c + SHARED_UNIT)) {
      	// 第一次讀，修改firstReader和firstReaderHoldCount 
        if (r == 0) {
            firstReader = current;
            firstReaderHoldCount = 1;
          // 如果當前線程正好是firstReader
        } else if (firstReader == current) {
            firstReaderHoldCount++;
          // 其他情況，經過一系列處理后，使得rh為當前線程的HoldCounter
          // 對rh的記數(shù)加一
        } else {
            HoldCounter rh = cachedHoldCounter;
          	// 如果cached為null或者不是當前線程
            if (rh == null || rh.tid != getThreadId(current))
              	// 從readHolds中get，并修改cached
                cachedHoldCounter = rh = readHolds.get();
          	// 如果cached不是null，但記數(shù)為null
          	// 這種情況表示當前線程的HoldCounter已經被刪除，即為null，
          	// 但cached仍然保留著null之前的那個HoldCounter，
          	// 為了方便，直接將cached設置給ThreadLocal即可
            else if (rh.count == 0)
                readHolds.set(rh);
          	//執(zhí)行到這里，rh表示當前線程的HoldCounter，記數(shù)加1
            rh.count++;
        }
        return 1;
    }
  	// 前面未返回結果，執(zhí)行第三步
    return fullTryAcquireShared(current);
}

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) {
          	// 存在其他線程持有寫鎖，返回-1
            if (getExclusiveOwnerThread() != current)
                return -1;
            // else we hold the exclusive lock; blocking here
            // would cause deadlock.
          //沒有寫鎖，且該線程排在其他線程后面，應該被堵塞
          //如果已經持有讀鎖，此次獲取是重入，可以執(zhí)行else if 之后的操作；
          //否則，會被堵塞，返回-1。
        } else if (readerShouldBlock()) {
            // Make sure we're not acquiring read lock reentrantly
          	//檢查firstReader
            if (firstReader == current) {
                // assert firstReaderHoldCount > 0;
            } else {
                if (rh == null) {
                    rh = cachedHoldCounter;
                    if (rh == null || rh.tid != getThreadId(current)) {
                      	//執(zhí)行到下一步rh是cached或者readHolds.get()，檢查rh
                        rh = readHolds.get();
                      	//在get時，如果不存在，會產生一個新的HoldCounter
                      	//記數(shù)為0表示不是重入鎖，會刪除讓其重新為null
                        if (rh.count == 0)
                            readHolds.remove();
                    }
                }
              	//返回失敗
                if (rh.count == 0)
                    return -1;
            }
        }
      	//達到最大值，不允許繼續(xù)增加
        if (sharedCount(c) == MAX_COUNT)
            throw new Error("Maximum lock count exceeded");
      	//和2.1.1中相似
        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;
        }
    }
}

// FairSync
final boolean readerShouldBlock() {
    return hasQueuedPredecessors();
}
// AQS
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());
}

// NonfairSync
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();
}
// AQS
final boolean apparentlyFirstQueuedIsExclusive() {
    Node h, s;
    return (h = head) != null &&
        (s = h.next)  != null &&
        !s.isShared()         &&
        s.thread != null;
}

//ReadLock
public boolean tryLock() {
    return sync.tryReadLock();
}
//Sync
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;
        }
    }
}

// ReadLockpublic void unlock() {    sync.releaseShared(1);}// AQSpublic final boolean releaseShared(int arg) {    if (tryReleaseShared(arg)) {        doReleaseShared();        return true;    }    return false;}// Syncprotected final boolean tryReleaseShared(int unused) {    Thread current = Thread.currentThread();  	// 先檢查 firstReader是否是當前線程    if (firstReader == current) {        // assert firstReaderHoldCount > 0;        if (firstReaderHoldCount == 1)            firstReader = null;        else            firstReaderHoldCount--;      //否則，處理 cached/readHolds中的HoldCounter    } 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;    }  	//自旋修改 state    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.          	//只有讀寫鎖均釋放干凈，才返回true            return nextc == 0;    }}

//WriteLockpublic void lock() {    sync.acquire(1);}//AQSpublic final void acquire(int arg) {    if (!tryAcquire(arg) &&        acquireQueued(addWaiter(Node.EXCLUSIVE), arg))        selfInterrupt();}//Syncprotected 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);  	//c分為兩部分，寫和讀    if (c != 0) {        // (Note: if c != 0 and w == 0 then shared count != 0)      	// c非0，w是0，則讀記數(shù)非0 || 獨占的寫線程不是當前線程      	// 返回 false        if (w == 0 || current != getExclusiveOwnerThread())            return false;        if (w + exclusiveCount(acquires) > MAX_COUNT)            throw new Error("Maximum lock count exceeded");        // Reentrant acquire      	// 重入的情況        setState(c + acquires);        return true;    }  	// 寫應該被堵塞或者CAS失敗，返回false    if (writerShouldBlock() ||        !compareAndSetState(c, c + acquires))        return false;  	// 非重入，在CAS成功后，設定獨占寫線程為當前線程，返回true    setExclusiveOwnerThread(current);    return true;}

//FairSyncfinal boolean writerShouldBlock() {    return hasQueuedPredecessors();}//NonfairSyncfinal boolean writerShouldBlock() {    return false; // writers can always barge}

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;}

//WriteLockpublic void unlock() {    sync.release(1);}//AQSpublic final boolean release(int arg) {    if (tryRelease(arg)) {        Node h = head;        if (h != null && h.waitStatus != 0)            unparkSuccessor(h);        return true;    }    return false;}//Syncprotected final boolean tryRelease(int releases) {  	// 首先檢查當前線程是否持有寫鎖    if (!isHeldExclusively())        throw new IllegalMonitorStateException();    int nextc = getState() - releases;  	// 根據修改后的寫記數(shù)來確定free    boolean free = exclusiveCount(nextc) == 0;  	// 此時，寫鎖完全釋放，設定寫獨占線程為null    if (free)        setExclusiveOwnerThread(null);    setState(nextc);  	// 返回 free    return free;}