回顾
在上一篇 Java 并发核心浅谈 我们大概了解到了 Lock 和synchronized的共同点，再简单总结下：
-
Lock主要是自定义一个 counter，从而利用CAS对其实现原子操作，而synchronized是c++ hotspot实现的 monitor（具体的咱也没看，咱就不说） - 二者都可重入（递归，互调，循环），其本质都是维护一个可计数的 counter，在其它线程访问加锁对象时会判断 counter 是否为 0
- 理论上讲二者都是阻塞式的，因为线程在拿锁时，如果拿不到，最终的结果只能等待（前提是线程的最终目的就是要获取锁）读写锁分离成两把锁了，所以不一样
举个例子：线程 A 持有了某个对象的 monitor，其它线程在访问该对象时，发现 monitor 不为 0，所以只能阻塞挂起或者加入等待队列，等着线程 A 处理完退出后将 monitor 置为 0。在线程 A 处理任务期间，其它线程要么循环访问 monitor，要么一直阻塞等着线程 A 唤醒，再不济就真的如我所说，放弃锁的竞争，去处理别的任务。但是应该做不到去处理别的任务后，任务处理到一半，被线程 A 通知后再回去抢锁
公平锁与非公平锁
不共享 counter
// 非公平锁在第一次拿锁失败也会调用该方法
public final void acquire(int arg) {
// 没拿到锁就加入队列
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();}
// 非公平锁方法
final void lock() {
// 走来就尝试获取锁
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
acquire(1); // 上面那个方法
}
// 公平锁 Acquire 计数
protected final boolean tryAcquire(int acquires) {final Thread current = Thread.currentThread();
// 拿到计数
int c = getState();
if (c == 0) {// 公平锁会先尝试排队 非公平锁少个 !hasQueuedPredecessors() 其它代码一样
if (!hasQueuedPredecessors() &&
compareAndSetState(0, acquires)) {setExclusiveOwnerThread(current);
return true;
}
}
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;
}
/**
* @return {@code true} if there is a queued thread preceding the // 当前线程前有线程等待，则排队
* current thread, and {@code false} if the current thread
* is at the head of the queue or the queue is empty // 队列为空不用排队
* @since 1.7
*/
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;
// 当前线程处于头节点的下一个且不为空则不用排队
// 或该线程就是当前持有锁的线程，即重入锁，也不用排队
return h != t &&
((s = h.next) == null || s.thread != Thread.currentThread());
}
// 加入等待队列
final boolean acquireQueued(final Node node, int arg) {
boolean failed = true;
try {
boolean interrupted = false;
for (;;) {final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {setHead(node);
p.next = null; // help GC
failed = false;
return interrupted;
}
// 获取失败会检查节点状态
// 然后 park 线程
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {if (failed)
cancelAcquire(node);
}
}
/** waitStatus value to indicate thread has cancelled */
static final int CANCELLED = 1; // 线程取消加锁
/** waitStatus value to indicate successor's thread needs unparking */
static final int SIGNAL = -1; // 解除线程 park
/** waitStatus value to indicate thread is waiting on condition */ //
static final int CONDITION = -2; // 线程被阻塞
/**
* waitStatus value to indicate the next acquireShared should
* unconditionally propagate
*/
static final int PROPAGATE = -3; // 广播
// 官方注释
/**
* Status field, taking on only the values:
* SIGNAL: The successor of this node is (or will soon be)
* blocked (via park), so the current node must
* unpark its successor when it releases or
* cancels. To avoid races, acquire methods must
* first indicate they need a signal,
* then retry the atomic acquire, and then,
* on failure, block.
* CANCELLED: This node is cancelled due to timeout or interrupt.
* Nodes never leave this state. In particular,
* a thread with cancelled node never again blocks.
* CONDITION: This node is currently on a condition queue.
* It will not be used as a sync queue node
* until transferred, at which time the status
* will be set to 0. (Use of this value here has
* nothing to do with the other uses of the
* field, but simplifies mechanics.)
* PROPAGATE: A releaseShared should be propagated to other
* nodes. This is set (for head node only) in
* doReleaseShared to ensure propagation
* continues, even if other operations have
* since intervened.
* 0: None of the above
*
* The values are arranged numerically to simplify use.
* Non-negative values mean that a node doesn't need to
* signal. So, most code doesn't need to check for particular
* values, just for sign.
*
* The field is initialized to 0 for normal sync nodes, and
* CONDITION for condition nodes. It is modified using CAS
* (or when possible, unconditional volatile writes).
*/
volatile int waitStatus;读锁与写锁（共享锁与排他锁）
读锁：共享 counter
写锁：不共享 counter
// 读写锁和线程池的类似之处
// 高 16 位为读计数，低 16 位为写计数
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. */ // 获取读计数
static int sharedCount(int c) {return c >>> SHARED_SHIFT;}
/** Returns the number of exclusive holds represented in count. */ // 获取写计数
static int exclusiveCount(int c) {return c & EXCLUSIVE_MASK;}
/**
* A counter for per-thread read hold counts. 每个线程自己的读计数
* Maintained as a ThreadLocal; cached in cachedHoldCounter.
*/
static final class HoldCounter {
int count; // initially 0
// Use id, not reference, to avoid garbage retention
final long tid = LockSupport.getThreadId(Thread.currentThread()); // 线程 id
}
// 尝试获取读锁
protected final int tryAcquireShared(int unused) {
// ReentrantReadWriteLock ReadLock 读锁
/*
* 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();
// 如果写锁计数不为零，且当前线程不是写锁持有线程，则可以获得读锁
// 言外之意，获得写锁的线程不可以再获得读锁
// 个人认为不用判断写计数也行
if (exclusiveCount(c) != 0 &&
getExclusiveOwnerThread() != current)
return -1;
// 获得读计数
int r = sharedCount(c);
// 检查等待队列 读计数上限
if (!readerShouldBlock() &&
r < MAX_COUNT &&
// 在高 16 位更新
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 != LockSupport.getThreadId(current))
// cachedHoldCounter 每个线程自己的读计数，非共享。但是锁计数与其它读操作共享，不与写操作共享
// readHolds 为 ThreadLocalHoldCounter，继承于 ThreadLocal，存 cachedHoldCounter
cachedHoldCounter = rh = readHolds.get();
else if (rh.count == 0)
readHolds.set(rh);
rh.count++;
}
return 1;
}
// 说明在排队中，就一直遍历，直到队首，实际起作用的代码和上面代码差不多
// 大师本人也说了代码有冗余
/*
* 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.
*/
return fullTryAcquireShared(current);
}
// 获得写锁
protected final boolean tryAcquire(int acquires) {
/*
* Walkthrough:
* 1. If read count nonzero or write count nonzero
* and owner is a different thread, fail.
* 读锁不为零（读锁排除写锁，但是与读锁共享）* 写锁不为零且锁持有者不为当前线程，则获得锁失败
* 2. If count would saturate, fail. (This can only
* happen if count is already nonzero.) // 计数器已达最大值，获得锁失败
* 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. // 重入是可以的；队列策略也是可以的，会在下面解释
*/
Thread current = Thread.currentThread();
int c = getState();
// 获得写计数
int w = exclusiveCount(c);
if (c != 0) {// (Note: if c != 0 and w == 0 then shared count != 0)
// 检查所持有线程
if (w == 0 || current != getExclusiveOwnerThread())
return false;
// 检查最大计数
if (w + exclusiveCount(acquires) > MAX_COUNT)
throw new Error("Maximum lock count exceeded");
// Reentrant acquire 线程重入获得锁，直接更新计数
setState(c + acquires);
return true;
}
// 队列策略
// state 为 0，检查是否需要排队
// 针对公平锁：去排队，如果当前线程在队首或等待队列为空，则返回 false，自然会走后面的 CAS
// 否则就返回 true，则进入 return false;
// 针对非公平锁：写死为 false，直接 CAS
if (writerShouldBlock() ||
!compareAndSetState(c, c + acquires))
return false;
// 设置当前写锁持有线程
setExclusiveOwnerThread(current);
return true;
}
// 因为读锁是多个线程共享读计数，各自维护了自己的读计数，所以释放的时候比写锁释放要多些操作
protected final boolean tryReleaseShared(int unused) {Thread current = Thread.currentThread();
// 当前线程是第一读线程的操作
// firstReader 作为字段缓存，是考虑到第一次读的线程使用率高？if (firstReader == current) {
// assert firstReaderHoldCount > 0;
if (firstReaderHoldCount == 1)
firstReader = null;
else
firstReaderHoldCount--;
} else {
HoldCounter rh = cachedHoldCounter;
if (rh == null ||
rh.tid != LockSupport.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;
}
}总结一下
公平锁和非公平锁的“锁”实现是基于 CAS，公平性基于内部维护的Node 链表
读写锁，可以粗略的理解为读和写两种状态，所以这儿的设计类似线程池的状态。只不过，读计数是可以多个读线程是共享的（排除写锁），每个读的线程都会维护自己的读计数。写锁的话，独占写计数，排除一切其他线程。