使用案例
案例取自jdk源码
/**
* 用例样本。下面举例，在一个维护简单二维点的类中的用法风格。这个样本代码举例中一些try/catch转换
* 虽然在这里他们不是严格需要的。因为没有异常可能发生在他们的方法体中。
*/
class Point {
private double x, y;
private final StampedLock sl = new StampedLock();
void move(double deltaX, double deltaY) { // an exclusively locked method
long stamp = sl.writeLock();
try {
x += deltaX;
y += deltaY;
} finally {
sl.unlockWrite(stamp);
}
}
double distanceFromOrigin() { // A read-only method
long stamp = sl.tryOptimisticRead();
double currentX = x, currentY = y;
if (!sl.validate(stamp)) {
stamp = sl.readLock();
try {
currentX = x;
currentY = y;
} finally {
sl.unlockRead(stamp);
}
}
return Math.sqrt(currentX * currentX + currentY * currentY);
}
void moveIfAtOrigin(double newX, double newY) { // upgrade
// Could instead start with optimistic, not read mode
long stamp = sl.readLock();
try {
while (x == 0.0 && y == 0.0) {
long ws = sl.tryConvertToWriteLock(stamp);
if (ws != 0L) {
stamp = ws;
x = newX;
y = newY;
break;
}
else {
sl.unlockRead(stamp);
stamp = sl.writeLock();
}
}
} finally {
sl.unlock(stamp);
}
}
}}
源码解析
/**
* 一种基于性能的锁，有三种模式用于控制读写的访问.
* StampedLock的状态由版本和模式组成.
* 锁获取方法返回一个表示并控制对锁状态的访问的戳记；
* 这些方法的“尝试”版本(前缀加了try的方法)可以返回一个特殊的值0代理失败的获取访问。
* 锁的释放与转换方法需要戳记作为参数，如果锁的状态没有匹配则操作失败。这三种模式是：
* Writing写模式。方法 {@link #writeLock} 可能会阻塞等待排他访问，返回一个可以用于释放写锁
* {@link #unlockWrite} 的戳。{@code tryWriteLock} 的非计时与计时版本的方法也有提供。
* 当锁被获取处于写模式中时，不可以获得读锁，并且所有乐观读的校验将会失败。
*
* Reading读模式。方法 {@link #readLock} 可能会阻塞等待非排他的访问，返回一个可以用于释放读锁
* {@link #unlockRead} 的戳。{@code tryReadLock} 的非计时与计时版本也有提供
*
* Optimistic Reading乐观读模式。方法 {@link #tryOptimisticRead} 仅在锁当前不是写模式时
* 返回一个非零的戳。在获得一个戳之后如果锁还没有处于写模式，方法 {@link #validate} 返回true
* 这个模式可以被认为是作为一个弱版本的读锁，它可能被一个写者在任意时间打断。
* 对于短的只读代码段使用乐观模式通常可以减少竞争并提高吞吐量。无论如何，它的使用本质上是脆弱的。
* 乐观读部分应该仅读取字段并把他们保存在本地变量中，以便在校验之后使用。在乐观模式下读取字段
* 可能是非常的不一致，那么只能当你是足够的熟悉数据代替检查一致性
* 并且/或者重复的调用方法{@code validate()}时使用。例如, 像这些步骤典型的需求是当你首先读取
* 一个对象或数组引用，然后访问它的字段，数组元素或者方法中的一个
*
* 这个类也支持带有条件提供转换三种模式的方法。例如，方法 {@link #tryConvertToWriteLock}
* 尝试“升级”一个模式（非注解翻译：写是最高级别的隔离，所以称之为升级，名称很直白就是要尝试
* 转换成写锁模式）, 如果（1）已经处理写模式（2）处于读取模式并且没有其他的等待读，或者
* （3）在乐观模式下并且锁是有效的，则返回一个有效的写戳。这些方法的设计形式为了减少一些基于
* 重试机制设计的代码膨胀。
*
* StampedLocks的设计是作为在开发线程安全组件的内部工具使用。他们的使用依赖于他们所保护的
* 内部properties数据知识库，objects，与方法。他们是不可重入的，那么已锁的代码体不应该调用
* 其他可能尝试重新获取锁的未知方法（尽管你可以传递一个戳给其他可以使用它或转换它的方法）。
* 读锁模式的使用依赖于关联代码部分是无副作用的。未经验证的乐观读部分不能调用未知的
* 容忍潜在不一致性的方法。戳使用有限的表示，并且不是密码安全的（例如，一个有效的戳是可预测的）。
* 戳可以在持续操作（不早于）一年后循环。持有未经使用或校验的戳超过此期限可能无法正确验证。
* StampedLocks是可序列化的，但是总是会反序列化为初始的无锁状态，所以他们对于远程锁是无用的。
*
* StampedLock的调度策略并不总是优先选择读胜过写，反之亦然。所有的“try”方法都是尽力而为，
* 不一定符合调度或者公平策略。任何用于获取或者转换锁的“try”方法的零返回都不包含任何
* 锁状态的相关信息；后续的调用可能会成功。
*
* 因为它支持跨多个锁模式的协调使用，这个类没有直接实现{@link Lock} or {@link ReadWriteLock}
* 接口。无论怎样，在一个应用只需要功能集里的某些相关功能，StampedLock可以被看作
* {@link #asReadLock()}, {@link #asWriteLock()}, or {@link #asReadWriteLock()}。
*
* @since 1.8
* @author Doug Lea
*/
public class StampedLock implements java.io.Serializable {
/*
* 算法笔记:
*
* 该设计采用了序列锁的原理
* (作为linux内核的使用; 查看 Lameter's
* http://www.lameter.com/gelato2005.pdf和其他地方;查看
* Boehm's http://www.hpl.hp.com/techreports/2012/HPL-2012-68.html)
* 和排序的RW读写锁(查看 Shirako et al
* http://dl.acm.org/citation.cfm?id=2312015)
*
* 从概念上讲，锁的主要状态包含一个在写的时候为奇数和其他时候为偶数的序列数。
* 无论怎样, 这个补偿值通过读线程获得读锁时一个非零的数补偿。
* 在验证“乐观”seqlock-reader-style戳记时将忽略读计数。
* 因为对于读我们必须使用一个小的有限位数（当前是7），当读数量超过count字段时
* 使用辅助读溢出字。我们通过处理这个最大读计数器值（RBITS）以自旋锁的方式保护溢出更新
* 来实现这一点
*
* 等待器使用在AbstractQueuedSynchronizer中使用的CLH锁的修改形式
* (查看他的内部文档以获得一个完整的描述)，每一个节点被打了一个标签（mode属性）
* 作为一个读或者写。等待读的集合被分组（链接）在一个公共节点（字段cowait）下，所以
* 事实上如同一个单独节点遵循大部分的CLH机制。借助队列结构，等待节点不需要实际
* 携带序列号；我们知道每一个节点是大于它的前置节点的。这样将调度策略简化为一个主要的FIFO模式，
* 该模式合并了Phase-Fair锁原理(查看 Brandenburg &
* Anderson, especially http://www.cs.unc.edu/~bbb/diss/). 特别是，我们使用phase-fair
* anti-barging 规则: 在一个持有一个读锁的时候如果有一个读者到达，但是有一个写已经在排队，
* 这个进来的读会被压入队列等待。（这个规则对于一些复杂的的方法获取读是可靠的，但是没有它，
* 这个锁将变得很不公平。）方法释放本身不会（有时是不可能）唤醒cowaiters。这是由主线程完成的，
* 在acquireRead与acquireWrite里没有更好的做法，但是其他线程可以提供帮助（非翻译：唤醒）。
*
* 这些规则适用于实际排队中的线程。所有tryLock形式适时的尝试去获取锁不管优先规则，并且因此
* 他们可能会“闯入”。在acquire获取方法里使用随机的自旋来减少（越来越昂贵的）上下文切换
* 同时避免在多个线程之间持续的内存抖动。我们限制自旋在队列的头部。一个线程在阻塞之前
* 自旋等待直至自旋次数（每次迭代减少自旋次数的概率为50%）上限。如果，此后唤醒它去获取锁失败，
* 并且依然是（或者变成）第一个等待线程（意味着一些其他的线程闯入并且获取锁成功），
* 它逐渐增强自旋（上限为MAX_HEAD_SPINS）来减少持续输给闯入线程的可能性。
*
* 几乎所有的这些机制都是在acquireWrite与acquireRead方法中实现的，这是典型的此类代码，
* 因为操作与重试的扩展延伸依赖于本地缓存读操作集的一致性
*
* 正如Boehm论文中提到的（如上），序列验证（主要的方法为validate()）比请求普通的
* volatile读（的“状态”）需要更严格的排序规则。为了在校验之前对读操作强制指令顺序，与
* 那些校验本身还没有被强制指令顺序的情况，我们使用Unsafe.loadFence。
*
* 内存布局将锁状态与队列的指针保持在一起（通常在同一个cacheline中）。
* 这通常适用于以读为主的负载。在大多数情况下，自适应自旋CLH锁减少内存内存争用的自然趋势会
* 降低进一步扩展争用位置的动机，但是可能会受到未来改进的影响。
*/
private static final long serialVersionUID = -6001602636862214147L;
/** Number of processors, for spin control */
private static final int NCPU = Runtime.getRuntime().availableProcessors();
/** Maximum number of retries before enqueuing on acquisition */
private static final int SPINS = (NCPU > 1) ? 1 << 6 : 0;
/** Maximum number of retries before blocking at head on acquisition */
private static final int HEAD_SPINS = (NCPU > 1) ? 1 << 10 : 0;
/** Maximum number of retries before re-blocking */
private static final int MAX_HEAD_SPINS = (NCPU > 1) ? 1 << 16 : 0;
/** The period for yielding when waiting for overflow spinlock */
private static final int OVERFLOW_YIELD_RATE = 7; // must be power 2 - 1
/** The number of bits to use for reader count before overflowing */
private static final int LG_READERS = 7;
// Values for lock state and stamp operations
private static final long RUNIT = 1L;
private static final long WBIT = 1L << LG_READERS;
private static final long RBITS = WBIT - 1L;
private static final long RFULL = RBITS - 1L;
private static final long ABITS = RBITS | WBIT;
private static final long SBITS = ~RBITS; // note overlap with ABITS
// Initial value for lock state; avoid failure value zero
private static final long ORIGIN = WBIT << 1;
// Special value from cancelled acquire methods so caller can throw IE
private static final long INTERRUPTED = 1L;
// Values for node status; order matters
private static final int WAITING = -1;
private static final int CANCELLED = 1;
// Modes for nodes (int not boolean to allow arithmetic)
private static final int RMODE = 0;
private static final int WMODE = 1;
/** Wait nodes */
static final class WNode {
volatile WNode prev;
volatile WNode next;
volatile WNode cowait; // list of linked readers
volatile Thread thread; // non-null while possibly parked
volatile int status; // 0, WAITING, or CANCELLED
final int mode; // RMODE or WMODE
WNode(int m, WNode p) { mode = m; prev = p; }
}
/** Head of CLH queue */
private transient volatile WNode whead;
/** Tail (last) of CLH queue */
private transient volatile WNode wtail;
// views
transient ReadLockView readLockView;
transient WriteLockView writeLockView;
transient ReadWriteLockView readWriteLockView;
/** Lock sequence/state */
private transient volatile long state;
/** extra reader count when state read count saturated */
private transient int readerOverflow;
/**
* Creates a new lock, initially in unlocked state.
*/
public StampedLock() {
state = ORIGIN;
}
/**
* Exclusively acquires the lock, blocking if necessary
* until available.
*
* @return a stamp that can be used to unlock or convert mode
*/
public long writeLock() {
long s, next; // bypass acquireWrite in fully unlocked case only
return ((((s = state) & ABITS) == 0L &&
U.compareAndSwapLong(this, STATE, s, next = s + WBIT)) ?
next : acquireWrite(false, 0L));
}
/**
* Exclusively acquires the lock if it is immediately available.
*
* @return a stamp that can be used to unlock or convert mode,
* or zero if the lock is not available
*/
public long tryWriteLock() {
long s, next;
return ((((s = state) & ABITS) == 0L &&
U.compareAndSwapLong(this, STATE, s, next = s + WBIT)) ?
next : 0L);
}
/**
* Exclusively acquires the lock if it is available within the
* given time and the current thread has not been interrupted.
* Behavior under timeout and interruption matches that specified
* for method {@link Lock#tryLock(long,TimeUnit)}.
*
* @param time the maximum time to wait for the lock
* @param unit the time unit of the {@code time} argument
* @return a stamp that can be used to unlock or convert mode,
* or zero if the lock is not available
* @throws InterruptedException if the current thread is interrupted
* before acquiring the lock
*/
public long tryWriteLock(long time, TimeUnit unit)
throws InterruptedException {
long nanos = unit.toNanos(time);
if (!Thread.interrupted()) {
long next, deadline;
if ((next = tryWriteLock()) != 0L)
return next;
if (nanos <= 0L)
return 0L;
if ((deadline = System.nanoTime() + nanos) == 0L)
deadline = 1L;
if ((next = acquireWrite(true, deadline)) != INTERRUPTED)
return next;
}
throw new InterruptedException();
}
/**
* Exclusively acquires the lock, blocking if necessary
* until available or the current thread is interrupted.
* Behavior under interruption matches that specified
* for method {@link Lock#lockInterruptibly()}.
*
* @return a stamp that can be used to unlock or convert mode
* @throws InterruptedException if the current thread is interrupted
* before acquiring the lock
*/
public long writeLockInterruptibly() throws InterruptedException {
long next;
if (!Thread.interrupted() &&
(next = acquireWrite(true, 0L)) != INTERRUPTED)
return next;
throw new InterruptedException();
}
/**
* Non-exclusively acquires the lock, blocking if necessary
* until available.
*
* @return a stamp that can be used to unlock or convert mode
*/
public long readLock() {
long s = state, next; // bypass acquireRead on common uncontended case
return ((whead == wtail && (s & ABITS) < RFULL &&
U.compareAndSwapLong(this, STATE, s, next = s + RUNIT)) ?
next : acquireRead(false, 0L));
}
/**
* Non-exclusively acquires the lock if it is immediately available.
*
* @return a stamp that can be used to unlock or convert mode,
* or zero if the lock is not available
*/
public long tryReadLock() {
for (;;) {
long s, m, next;
if ((m = (s = state) & ABITS) == WBIT)
return 0L;
else if (m < RFULL) {
if (U.compareAndSwapLong(this, STATE, s, next = s + RUNIT))
return next;
}
else if ((next = tryIncReaderOverflow(s)) != 0L)
return next;
}
}
/**
* Non-exclusively acquires the lock if it is available within the
* given time and the current thread has not been interrupted.
* Behavior under timeout and interruption matches that specified
* for method {@link Lock#tryLock(long,TimeUnit)}.
*
* @param time the maximum time to wait for the lock
* @param unit the time unit of the {@code time} argument
* @return a stamp that can be used to unlock or convert mode,
* or zero if the lock is not available
* @throws InterruptedException if the current thread is interrupted
* before acquiring the lock
*/
public long tryReadLock(long time, TimeUnit unit)
throws InterruptedException {
long s, m, next, deadline;
long nanos = unit.toNanos(time);
if (!Thread.interrupted()) {
if ((m = (s = state) & ABITS) != WBIT) {
if (m < RFULL) {
if (U.compareAndSwapLong(this, STATE, s, next = s + RUNIT))
return next;
}
else if ((next = tryIncReaderOverflow(s)) != 0L)
return next;
}
if (nanos <= 0L)
return 0L;
if ((deadline = System.nanoTime() + nanos) == 0L)
deadline = 1L;
if ((next = acquireRead(true, deadline)) != INTERRUPTED)
return next;
}
throw new InterruptedException();
}
/**
* Non-exclusively acquires the lock, blocking if necessary
* until available or the current thread is interrupted.
* Behavior under interruption matches that specified
* for method {@link Lock#lockInterruptibly()}.
*
* @return a stamp that can be used to unlock or convert mode
* @throws InterruptedException if the current thread is interrupted
* before acquiring the lock
*/
public long readLockInterruptibly() throws InterruptedException {
long next;
if (!Thread.interrupted() &&
(next = acquireRead(true, 0L)) != INTERRUPTED)
return next;
throw new InterruptedException();
}
/**
* Returns a stamp that can later be validated, or zero
* if exclusively locked.
*
* @return a stamp, or zero if exclusively locked
*/
public long tryOptimisticRead() {
long s;
return (((s = state) & WBIT) == 0L) ? (s & SBITS) : 0L;
}
/**
* Returns true if the lock has not been exclusively acquired
* since issuance of the given stamp. Always returns false if the
* stamp is zero. Always returns true if the stamp represents a
* currently held lock. Invoking this method with a value not
* obtained from {@link #tryOptimisticRead} or a locking method
* for this lock has no defined effect or result.
*
* @param stamp a stamp
* @return {@code true} if the lock has not been exclusively acquired
* since issuance of the given stamp; else false
*/
public boolean validate(long stamp) {
U.loadFence();
return (stamp & SBITS) == (state & SBITS);
}
/**
* If the lock state matches the given stamp, releases the
* exclusive lock.
*
* @param stamp a stamp returned by a write-lock operation
* @throws IllegalMonitorStateException if the stamp does
* not match the current state of this lock
*/
public void unlockWrite(long stamp) {
WNode h;
if (state != stamp || (stamp & WBIT) == 0L)
throw new IllegalMonitorStateException();
state = (stamp += WBIT) == 0L ? ORIGIN : stamp;
if ((h = whead) != null && h.status != 0)
release(h);
}
/**
* If the lock state matches the given stamp, releases the
* non-exclusive lock.
*
* @param stamp a stamp returned by a read-lock operation
* @throws IllegalMonitorStateException if the stamp does
* not match the current state of this lock
*/
public void unlockRead(long stamp) {
long s, m; WNode h;
for (;;) {
if (((s = state) & SBITS) != (stamp & SBITS) ||
(stamp & ABITS) == 0L || (m = s & ABITS) == 0L || m == WBIT)
throw new IllegalMonitorStateException();
if (m < RFULL) {
if (U.compareAndSwapLong(this, STATE, s, s - RUNIT)) {
if (m == RUNIT && (h = whead) != null && h.status != 0)
release(h);
break;
}
}
else if (tryDecReaderOverflow(s) != 0L)
break;
}
}
/**
* If the lock state matches the given stamp, releases the
* corresponding mode of the lock.
*
* @param stamp a stamp returned by a lock operation
* @throws IllegalMonitorStateException if the stamp does
* not match the current state of this lock
*/
public void unlock(long stamp) {
long a = stamp & ABITS, m, s; WNode h;
while (((s = state) & SBITS) == (stamp & SBITS)) {
if ((m = s & ABITS) == 0L)
break;
else if (m == WBIT) {
if (a != m)
break;
state = (s += WBIT) == 0L ? ORIGIN : s;
if ((h = whead) != null && h.status != 0)
release(h);
return;
}
else if (a == 0L || a >= WBIT)
break;
else if (m < RFULL) {
if (U.compareAndSwapLong(this, STATE, s, s - RUNIT)) {
if (m == RUNIT && (h = whead) != null && h.status != 0)
release(h);
return;
}
}
else if (tryDecReaderOverflow(s) != 0L)
return;
}
throw new IllegalMonitorStateException();
}
/**
* If the lock state matches the given stamp, performs one of
* the following actions. If the stamp represents holding a write
* lock, returns it. Or, if a read lock, if the write lock is
* available, releases the read lock and returns a write stamp.
* Or, if an optimistic read, returns a write stamp only if
* immediately available. This method returns zero in all other
* cases.
*
* @param stamp a stamp
* @return a valid write stamp, or zero on failure
*/
public long tryConvertToWriteLock(long stamp) {
long a = stamp & ABITS, m, s, next;
while (((s = state) & SBITS) == (stamp & SBITS)) {
if ((m = s & ABITS) == 0L) {
if (a != 0L)
break;
if (U.compareAndSwapLong(this, STATE, s, next = s + WBIT))
return next;
}
else if (m == WBIT) {
if (a != m)
break;
return stamp;
}
else if (m == RUNIT && a != 0L) {
if (U.compareAndSwapLong(this, STATE, s,
next = s - RUNIT + WBIT))
return next;
}
else
break;
}
return 0L;
}
/**
* If the lock state matches the given stamp, performs one of
* the following actions. If the stamp represents holding a write
* lock, releases it and obtains a read lock. Or, if a read lock,
* returns it. Or, if an optimistic read, acquires a read lock and
* returns a read stamp only if immediately available. This method
* returns zero in all other cases.
*
* @param stamp a stamp
* @return a valid read stamp, or zero on failure
*/
public long tryConvertToReadLock(long stamp) {
long a = stamp & ABITS, m, s, next; WNode h;
while (((s = state) & SBITS) == (stamp & SBITS)) {
if ((m = s & ABITS) == 0L) {
if (a != 0L)
break;
else if (m < RFULL) {
if (U.compareAndSwapLong(this, STATE, s, next = s + RUNIT))
return next;
}
else if ((next = tryIncReaderOverflow(s)) != 0L)
return next;
}
else if (m == WBIT) {
if (a != m)
break;
state = next = s + (WBIT + RUNIT);
if ((h = whead) != null && h.status != 0)
release(h);
return next;
}
else if (a != 0L && a < WBIT)
return stamp;
else
break;
}
return 0L;
}
/**
* If the lock state matches the given stamp then, if the stamp
* represents holding a lock, releases it and returns an
* observation stamp. Or, if an optimistic read, returns it if
* validated. This method returns zero in all other cases, and so
* may be useful as a form of "tryUnlock".
*
* @param stamp a stamp
* @return a valid optimistic read stamp, or zero on failure
*/
public long tryConvertToOptimisticRead(long stamp) {
long a = stamp & ABITS, m, s, next; WNode h;
U.loadFence();
for (;;) {
if (((s = state) & SBITS) != (stamp & SBITS))
break;
if ((m = s & ABITS) == 0L) {
if (a != 0L)
break;
return s;
}
else if (m == WBIT) {
if (a != m)
break;
state = next = (s += WBIT) == 0L ? ORIGIN : s;
if ((h = whead) != null && h.status != 0)
release(h);
return next;
}
else if (a == 0L || a >= WBIT)
break;
else if (m < RFULL) {
if (U.compareAndSwapLong(this, STATE, s, next = s - RUNIT)) {
if (m == RUNIT && (h = whead) != null && h.status != 0)
release(h);
return next & SBITS;
}
}
else if ((next = tryDecReaderOverflow(s)) != 0L)
return next & SBITS;
}
return 0L;
}
/**
* Releases the write lock if it is held, without requiring a
* stamp value. This method may be useful for recovery after
* errors.
*
* @return {@code true} if the lock was held, else false
*/
public boolean tryUnlockWrite() {
long s; WNode h;
if (((s = state) & WBIT) != 0L) {
state = (s += WBIT) == 0L ? ORIGIN : s;
if ((h = whead) != null && h.status != 0)
release(h);
return true;
}
return false;
}
/**
* Releases one hold of the read lock if it is held, without
* requiring a stamp value. This method may be useful for recovery
* after errors.
*
* @return {@code true} if the read lock was held, else false
*/
public boolean tryUnlockRead() {
long s, m; WNode h;
while ((m = (s = state) & ABITS) != 0L && m < WBIT) {
if (m < RFULL) {
if (U.compareAndSwapLong(this, STATE, s, s - RUNIT)) {
if (m == RUNIT && (h = whead) != null && h.status != 0)
release(h);
return true;
}
}
else if (tryDecReaderOverflow(s) != 0L)
return true;
}
return false;
}
// status monitoring methods
/**
* Returns combined state-held and overflow read count for given
* state s.
*/
private int getReadLockCount(long s) {
long readers;
if ((readers = s & RBITS) >= RFULL)
readers = RFULL + readerOverflow;
return (int) readers;
}
/**
* Returns {@code true} if the lock is currently held exclusively.
*
* @return {@code true} if the lock is currently held exclusively
*/
public boolean isWriteLocked() {
return (state & WBIT) != 0L;
}
/**
* Returns {@code true} if the lock is currently held non-exclusively.
*
* @return {@code true} if the lock is currently held non-exclusively
*/
public boolean isReadLocked() {
return (state & RBITS) != 0L;
}
/**
* Queries the number of read locks held for this lock. This
* method is designed for use in monitoring system state, not for
* synchronization control.
* @return the number of read locks held
*/
public int getReadLockCount() {
return getReadLockCount(state);
}
/**
* Returns a string identifying this lock, as well as its lock
* state. The state, in brackets, includes the String {@code
* "Unlocked"} or the String {@code "Write-locked"} or the String
* {@code "Read-locks:"} followed by the current number of
* read-locks held.
*
* @return a string identifying this lock, as well as its lock state
*/
public String toString() {
long s = state;
return super.toString() +
((s & ABITS) == 0L ? "[Unlocked]" :
(s & WBIT) != 0L ? "[Write-locked]" :
"[Read-locks:" + getReadLockCount(s) + "]");
}
// views
/**
* Returns a plain {@link Lock} view of this StampedLock in which
* the {@link Lock#lock} method is mapped to {@link #readLock},
* and similarly for other methods. The returned Lock does not
* support a {@link Condition}; method {@link
* Lock#newCondition()} throws {@code
* UnsupportedOperationException}.
*
* @return the lock
*/
public Lock asReadLock() {
ReadLockView v;
return ((v = readLockView) != null ? v :
(readLockView = new ReadLockView()));
}
/**
* Returns a plain {@link Lock} view of this StampedLock in which
* the {@link Lock#lock} method is mapped to {@link #writeLock},
* and similarly for other methods. The returned Lock does not
* support a {@link Condition}; method {@link
* Lock#newCondition()} throws {@code
* UnsupportedOperationException}.
*
* @return the lock
*/
public Lock asWriteLock() {
WriteLockView v;
return ((v = writeLockView) != null ? v :
(writeLockView = new WriteLockView()));
}
/**
* Returns a {@link ReadWriteLock} view of this StampedLock in
* which the {@link ReadWriteLock#readLock()} method is mapped to
* {@link #asReadLock()}, and {@link ReadWriteLock#writeLock()} to
* {@link #asWriteLock()}.
*
* @return the lock
*/
public ReadWriteLock asReadWriteLock() {
ReadWriteLockView v;
return ((v = readWriteLockView) != null ? v :
(readWriteLockView = new ReadWriteLockView()));
}
// view classes
final class ReadLockView implements Lock {
public void lock() { readLock(); }
public void lockInterruptibly() throws InterruptedException {
readLockInterruptibly();
}
public boolean tryLock() { return tryReadLock() != 0L; }
public boolean tryLock(long time, TimeUnit unit)
throws InterruptedException {
return tryReadLock(time, unit) != 0L;
}
public void unlock() { unstampedUnlockRead(); }
public Condition newCondition() {
throw new UnsupportedOperationException();
}
}
final class WriteLockView implements Lock {
public void lock() { writeLock(); }
public void lockInterruptibly() throws InterruptedException {
writeLockInterruptibly();
}
public boolean tryLock() { return tryWriteLock() != 0L; }
public boolean tryLock(long time, TimeUnit unit)
throws InterruptedException {
return tryWriteLock(time, unit) != 0L;
}
public void unlock() { unstampedUnlockWrite(); }
public Condition newCondition() {
throw new UnsupportedOperationException();
}
}
final class ReadWriteLockView implements ReadWriteLock {
public Lock readLock() { return asReadLock(); }
public Lock writeLock() { return asWriteLock(); }
}
// Unlock methods without stamp argument checks for view classes.
// Needed because view-class lock methods throw away stamps.
final void unstampedUnlockWrite() {
WNode h; long s;
if (((s = state) & WBIT) == 0L)
throw new IllegalMonitorStateException();
state = (s += WBIT) == 0L ? ORIGIN : s;
if ((h = whead) != null && h.status != 0)
release(h);
}
final void unstampedUnlockRead() {
for (;;) {
long s, m; WNode h;
if ((m = (s = state) & ABITS) == 0L || m >= WBIT)
throw new IllegalMonitorStateException();
else if (m < RFULL) {
if (U.compareAndSwapLong(this, STATE, s, s - RUNIT)) {
if (m == RUNIT && (h = whead) != null && h.status != 0)
release(h);
break;
}
}
else if (tryDecReaderOverflow(s) != 0L)
break;
}
}
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
s.defaultReadObject();
state = ORIGIN; // reset to unlocked state
}
// internals
/**
* Tries to increment readerOverflow by first setting state
* access bits value to RBITS, indicating hold of spinlock,
* then updating, then releasing.
*
* @param s a reader overflow stamp: (s & ABITS) >= RFULL
* @return new stamp on success, else zero
*/
private long tryIncReaderOverflow(long s) {
// assert (s & ABITS) >= RFULL;
if ((s & ABITS) == RFULL) {
if (U.compareAndSwapLong(this, STATE, s, s | RBITS)) {
++readerOverflow;
state = s;
return s;
}
}
else if ((LockSupport.nextSecondarySeed() &
OVERFLOW_YIELD_RATE) == 0)
Thread.yield();
return 0L;
}
/**
* Tries to decrement readerOverflow.
*
* @param s a reader overflow stamp: (s & ABITS) >= RFULL
* @return new stamp on success, else zero
*/
private long tryDecReaderOverflow(long s) {
// assert (s & ABITS) >= RFULL;
if ((s & ABITS) == RFULL) {
if (U.compareAndSwapLong(this, STATE, s, s | RBITS)) {
int r; long next;
if ((r = readerOverflow) > 0) {
readerOverflow = r - 1;
next = s;
}
else
next = s - RUNIT;
state = next;
return next;
}
}
else if ((LockSupport.nextSecondarySeed() &
OVERFLOW_YIELD_RATE) == 0)
Thread.yield();
return 0L;
}
/**
* Wakes up the successor of h (normally whead). This is normally
* just h.next, but may require traversal from wtail if next
* pointers are lagging. This may fail to wake up an acquiring
* thread when one or more have been cancelled, but the cancel
* methods themselves provide extra safeguards to ensure liveness.
*/
private void release(WNode h) {
if (h != null) {
WNode q; Thread w;
U.compareAndSwapInt(h, WSTATUS, WAITING, 0);
if ((q = h.next) == null || q.status == CANCELLED) {
for (WNode t = wtail; t != null && t != h; t = t.prev)
if (t.status <= 0)
q = t;
}
if (q != null && (w = q.thread) != null)
U.unpark(w);
}
}
/**
* See above for explanation.
*
* @param interruptible true if should check interrupts and if so
* return INTERRUPTED
* @param deadline if nonzero, the System.nanoTime value to timeout
* at (and return zero)
* @return next state, or INTERRUPTED
*/
private long acquireWrite(boolean interruptible, long deadline) {
WNode node = null, p;
for (int spins = -1;;) { // spin while enqueuing
long m, s, ns;
if ((m = (s = state) & ABITS) == 0L) {
if (U.compareAndSwapLong(this, STATE, s, ns = s + WBIT))
return ns;
}
else if (spins < 0)
spins = (m == WBIT && wtail == whead) ? SPINS : 0;
else if (spins > 0) {
if (LockSupport.nextSecondarySeed() >= 0)
--spins;
}
else if ((p = wtail) == null) { // initialize queue
WNode hd = new WNode(WMODE, null);
if (U.compareAndSwapObject(this, WHEAD, null, hd))
wtail = hd;
}
else if (node == null)
node = new WNode(WMODE, p);
else if (node.prev != p)
node.prev = p;
else if (U.compareAndSwapObject(this, WTAIL, p, node)) {
p.next = node;
break;
}
}
for (int spins = -1;;) {
WNode h, np, pp; int ps;
if ((h = whead) == p) {
if (spins < 0)
spins = HEAD_SPINS;
else if (spins < MAX_HEAD_SPINS)
spins <<= 1;
for (int k = spins;;) { // spin at head
long s, ns;
if (((s = state) & ABITS) == 0L) {
if (U.compareAndSwapLong(this, STATE, s,
ns = s + WBIT)) {
whead = node;
node.prev = null;
return ns;
}
}
else if (LockSupport.nextSecondarySeed() >= 0 &&
--k <= 0)
break;
}
}
else if (h != null) { // help release stale waiters
WNode c; Thread w;
while ((c = h.cowait) != null) {
if (U.compareAndSwapObject(h, WCOWAIT, c, c.cowait) &&
(w = c.thread) != null)
U.unpark(w);
}
}
if (whead == h) {
if ((np = node.prev) != p) {
if (np != null)
(p = np).next = node; // stale
}
else if ((ps = p.status) == 0)
U.compareAndSwapInt(p, WSTATUS, 0, WAITING);
else if (ps == CANCELLED) {
if ((pp = p.prev) != null) {
node.prev = pp;
pp.next = node;
}
}
else {
long time; // 0 argument to park means no timeout
if (deadline == 0L)
time = 0L;
else if ((time = deadline - System.nanoTime()) <= 0L)
return cancelWaiter(node, node, false);
Thread wt = Thread.currentThread();
U.putObject(wt, PARKBLOCKER, this);
node.thread = wt;
if (p.status < 0 && (p != h || (state & ABITS) != 0L) &&
whead == h && node.prev == p)
U.park(false, time); // emulate LockSupport.park
node.thread = null;
U.putObject(wt, PARKBLOCKER, null);
if (interruptible && Thread.interrupted())
return cancelWaiter(node, node, true);
}
}
}
}
/**
* See above for explanation.
*
* @param interruptible true if should check interrupts and if so
* return INTERRUPTED
* @param deadline if nonzero, the System.nanoTime value to timeout
* at (and return zero)
* @return next state, or INTERRUPTED
*/
private long acquireRead(boolean interruptible, long deadline) {
WNode node = null, p;
for (int spins = -1;;) {
WNode h;
if ((h = whead) == (p = wtail)) {
for (long m, s, ns;;) {
if ((m = (s = state) & ABITS) < RFULL ?
U.compareAndSwapLong(this, STATE, s, ns = s + RUNIT) :
(m < WBIT && (ns = tryIncReaderOverflow(s)) != 0L))
return ns;
else if (m >= WBIT) {
if (spins > 0) {
if (LockSupport.nextSecondarySeed() >= 0)
--spins;
}
else {
if (spins == 0) {
WNode nh = whead, np = wtail;
if ((nh == h && np == p) || (h = nh) != (p = np))
break;
}
spins = SPINS;
}
}
}
}
if (p == null) { // initialize queue
WNode hd = new WNode(WMODE, null);
if (U.compareAndSwapObject(this, WHEAD, null, hd))
wtail = hd;
}
else if (node == null)
node = new WNode(RMODE, p);
else if (h == p || p.mode != RMODE) {
if (node.prev != p)
node.prev = p;
else if (U.compareAndSwapObject(this, WTAIL, p, node)) {
p.next = node;
break;
}
}
else if (!U.compareAndSwapObject(p, WCOWAIT,
node.cowait = p.cowait, node))
node.cowait = null;
else {
for (;;) {
WNode pp, c; Thread w;
if ((h = whead) != null && (c = h.cowait) != null &&
U.compareAndSwapObject(h, WCOWAIT, c, c.cowait) &&
(w = c.thread) != null) // help release
U.unpark(w);
if (h == (pp = p.prev) || h == p || pp == null) {
long m, s, ns;
do {
if ((m = (s = state) & ABITS) < RFULL ?
U.compareAndSwapLong(this, STATE, s,
ns = s + RUNIT) :
(m < WBIT &&
(ns = tryIncReaderOverflow(s)) != 0L))
return ns;
} while (m < WBIT);
}
if (whead == h && p.prev == pp) {
long time;
if (pp == null || h == p || p.status > 0) {
node = null; // throw away
break;
}
if (deadline == 0L)
time = 0L;
else if ((time = deadline - System.nanoTime()) <= 0L)
return cancelWaiter(node, p, false);
Thread wt = Thread.currentThread();
U.putObject(wt, PARKBLOCKER, this);
node.thread = wt;
if ((h != pp || (state & ABITS) == WBIT) &&
whead == h && p.prev == pp)
U.park(false, time);
node.thread = null;
U.putObject(wt, PARKBLOCKER, null);
if (interruptible && Thread.interrupted())
return cancelWaiter(node, p, true);
}
}
}
}
for (int spins = -1;;) {
WNode h, np, pp; int ps;
if ((h = whead) == p) {
if (spins < 0)
spins = HEAD_SPINS;
else if (spins < MAX_HEAD_SPINS)
spins <<= 1;
for (int k = spins;;) { // spin at head
long m, s, ns;
if ((m = (s = state) & ABITS) < RFULL ?
U.compareAndSwapLong(this, STATE, s, ns = s + RUNIT) :
(m < WBIT && (ns = tryIncReaderOverflow(s)) != 0L)) {
WNode c; Thread w;
whead = node;
node.prev = null;
while ((c = node.cowait) != null) {
if (U.compareAndSwapObject(node, WCOWAIT,
c, c.cowait) &&
(w = c.thread) != null)
U.unpark(w);
}
return ns;
}
else if (m >= WBIT &&
LockSupport.nextSecondarySeed() >= 0 && --k <= 0)
break;
}
}
else if (h != null) {
WNode c; Thread w;
while ((c = h.cowait) != null) {
if (U.compareAndSwapObject(h, WCOWAIT, c, c.cowait) &&
(w = c.thread) != null)
U.unpark(w);
}
}
if (whead == h) {
if ((np = node.prev) != p) {
if (np != null)
(p = np).next = node; // stale
}
else if ((ps = p.status) == 0)
U.compareAndSwapInt(p, WSTATUS, 0, WAITING);
else if (ps == CANCELLED) {
if ((pp = p.prev) != null) {
node.prev = pp;
pp.next = node;
}
}
else {
long time;
if (deadline == 0L)
time = 0L;
else if ((time = deadline - System.nanoTime()) <= 0L)
return cancelWaiter(node, node, false);
Thread wt = Thread.currentThread();
U.putObject(wt, PARKBLOCKER, this);
node.thread = wt;
if (p.status < 0 &&
(p != h || (state & ABITS) == WBIT) &&
whead == h && node.prev == p)
U.park(false, time);
node.thread = null;
U.putObject(wt, PARKBLOCKER, null);
if (interruptible && Thread.interrupted())
return cancelWaiter(node, node, true);
}
}
}
}
/**
* If node non-null, forces cancel status and unsplices it from
* queue if possible and wakes up any cowaiters (of the node, or
* group, as applicable), and in any case helps release current
* first waiter if lock is free. (Calling with null arguments
* serves as a conditional form of release, which is not currently
* needed but may be needed under possible future cancellation
* policies). This is a variant of cancellation methods in
* AbstractQueuedSynchronizer (see its detailed explanation in AQS
* internal documentation).
*
* @param node if nonnull, the waiter
* @param group either node or the group node is cowaiting with
* @param interrupted if already interrupted
* @return INTERRUPTED if interrupted or Thread.interrupted, else zero
*/
private long cancelWaiter(WNode node, WNode group, boolean interrupted) {
if (node != null && group != null) {
Thread w;
node.status = CANCELLED;
// unsplice cancelled nodes from group
for (WNode p = group, q; (q = p.cowait) != null;) {
if (q.status == CANCELLED) {
U.compareAndSwapObject(p, WCOWAIT, q, q.cowait);
p = group; // restart
}
else
p = q;
}
if (group == node) {
for (WNode r = group.cowait; r != null; r = r.cowait) {
if ((w = r.thread) != null)
U.unpark(w); // wake up uncancelled co-waiters
}
for (WNode pred = node.prev; pred != null; ) { // unsplice
WNode succ, pp; // find valid successor
while ((succ = node.next) == null ||
succ.status == CANCELLED) {
WNode q = null; // find successor the slow way
for (WNode t = wtail; t != null && t != node; t = t.prev)
if (t.status != CANCELLED)
q = t; // don't link if succ cancelled
if (succ == q || // ensure accurate successor
U.compareAndSwapObject(node, WNEXT,
succ, succ = q)) {
if (succ == null && node == wtail)
U.compareAndSwapObject(this, WTAIL, node, pred);
break;
}
}
if (pred.next == node) // unsplice pred link
U.compareAndSwapObject(pred, WNEXT, node, succ);
if (succ != null && (w = succ.thread) != null) {
succ.thread = null;
U.unpark(w); // wake up succ to observe new pred
}
if (pred.status != CANCELLED || (pp = pred.prev) == null)
break;
node.prev = pp; // repeat if new pred wrong/cancelled
U.compareAndSwapObject(pp, WNEXT, pred, succ);
pred = pp;
}
}
}
WNode h; // Possibly release first waiter
while ((h = whead) != null) {
long s; WNode q; // similar to release() but check eligibility
if ((q = h.next) == null || q.status == CANCELLED) {
for (WNode t = wtail; t != null && t != h; t = t.prev)
if (t.status <= 0)
q = t;
}
if (h == whead) {
if (q != null && h.status == 0 &&
((s = state) & ABITS) != WBIT && // waiter is eligible
(s == 0L || q.mode == RMODE))
release(h);
break;
}
}
return (interrupted || Thread.interrupted()) ? INTERRUPTED : 0L;
}
// Unsafe mechanics
private static final sun.misc.Unsafe U;
private static final long STATE;
private static final long WHEAD;
private static final long WTAIL;
private static final long WNEXT;
private static final long WSTATUS;
private static final long WCOWAIT;
private static final long PARKBLOCKER;
static {
try {
U = sun.misc.Unsafe.getUnsafe();
Class<?> k = StampedLock.class;
Class<?> wk = WNode.class;
STATE = U.objectFieldOffset
(k.getDeclaredField("state"));
WHEAD = U.objectFieldOffset
(k.getDeclaredField("whead"));
WTAIL = U.objectFieldOffset
(k.getDeclaredField("wtail"));
WSTATUS = U.objectFieldOffset
(wk.getDeclaredField("status"));
WNEXT = U.objectFieldOffset
(wk.getDeclaredField("next"));
WCOWAIT = U.objectFieldOffset
(wk.getDeclaredField("cowait"));
Class<?> tk = Thread.class;
PARKBLOCKER = U.objectFieldOffset
(tk.getDeclaredField("parkBlocker"));
} catch (Exception e) {
throw new Error(e);
}
}
}
