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参考文章：
锁分独占锁与共享锁，公平锁与非公平锁，悲观锁与乐观锁，可重入锁与不可重入锁，相关概念可查看其它文章。
Lock操作：加锁（lock），解锁（unlock），创建条件对象（newCondition）。
Condition操作：等待（await），通知（signal）。
ReentrantLock：是一个可重入锁，独占锁，由构造参数决定是公平锁还是非公平锁。
ReadWriteLock的操作就是获取读取锁与改写锁。
ReentrantReadWriteLock：它的读取锁与改写锁都是可重入锁，并且由构造参数决定是公平锁还是非公平锁。
StampedLock：是Java8增加的锁，它不支持重入，也不支持Condition。相关文章《深入理解StampedLock及其实现原理》
ReentrantLock的源码分析：
/**
* @since 1.5
* @author Doug Lea
*/
public class ReentrantLock implements Lock, java.io.Serializable {
private static final long serialVersionUID = 7373984872572414699L; private final Sync sync; public ReentrantLock() {
// 默认使用非公平锁
sync = new NonfairSync();
} public ReentrantLock(boolean fair) {
// 由参数决定使用公平锁还是非公平锁
sync = fair ? new FairSync() : new NonfairSync();
} abstract static class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = -5179523762034025860L; abstract void lock(); final boolean nonfairTryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
// 判断锁是否未被持有
if (c == 0) {
// 尝试占用锁
if (compareAndSetState(0, acquires)) {
// 占用成功后，更新锁所属线程为当前线程
setExclusiveOwnerThread(current);
// 返回占用成功
return true;
}
}
// 如果锁已被占用，判断当前线程是否持有锁
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
// 不允许锁被释放，否则视为系统错误
if (nextc < 0) // overflow
throw new Error("Maximum lock count exceeded");
// 更新锁的持有次数
setState(nextc);
// 返回占用成功
return true;
}
// 返回占用失败
return false;
} protected final boolean tryRelease(int releases) {
int c = getState() - releases;
// 不允许当前线程释放其它线程持有的锁，否则抛出异常
if (Thread.currentThread() != getExclusiveOwnerThread())
throw new IllegalMonitorStateException();
boolean free = false;
// 判断锁是否达到释放值
if (c == 0) {
// 标记锁需要释放
free = true;
// 清空锁的所属线程
setExclusiveOwnerThread(null);
}
// 更新锁的持有次数
setState(c);
// 返回锁是否达到释放值而被释放，否则未释放。
return free;
} protected final boolean isHeldExclusively() {
// 返回当前线程是否持有锁
return getExclusiveOwnerThread() == Thread.currentThread();
} final Thread getOwner() {
// 返回锁所属线程
return getState() == 0 ? null : getExclusiveOwnerThread();
} final int getHoldCount() {
// 如果当前线程未持有锁，则返回0；如果当前线程持有锁，则返回持有的次数。
return isHeldExclusively() ? getState() : 0;
} final boolean isLocked() {
// 返回锁是否已经被占用
return getState() != 0;
} final ConditionObject newCondition() {
return new ConditionObject();
}
} static final class NonfairSync extends Sync {
private static final long serialVersionUID = 7316153563782823691L; final void lock() {
// 尝试占用锁
if (compareAndSetState(0, 1))
// 占用成功后，更新锁所属线程为当前线程
setExclusiveOwnerThread(Thread.currentThread());
else
// 占用失败后，通过正常程序获取锁
acquire(1);
} protected final boolean tryAcquire(int acquires) {
return nonfairTryAcquire(acquires);
}
} static final class FairSync extends Sync {
private static final long serialVersionUID = -3000897897090466540L; final void lock() {
// 正常程序获取锁
acquire(1);
} protected final boolean tryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
// 判断锁是否未被持有
if (c == 0) {
// 如果队列为空或队头为当前线程时，就尝试占用锁
if (!hasQueuedPredecessors() && compareAndSetState(0, acquires)) {
// 占用成功后，更新锁所属线程为当前线程
setExclusiveOwnerThread(current);
// 返回占用成功
return true;
}
}
// 如果锁已被占用，判断当前线程是否持有锁
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
// 不允许锁被释放，否则视为系统错误
if (nextc < 0)
throw new Error("Maximum lock count exceeded");
// 更新锁的持有次数
setState(nextc);
// 返回占用成功
return true;
}
// 返回占用失败
return false;
}
} public void lock() {
// 获取锁
sync.lock();
} public void lockInterruptibly() throws InterruptedException {
// 获取锁，可被中断
sync.acquireInterruptibly(1);
} public boolean tryLock() {
// 强制使用非公平方式获取锁
return sync.nonfairTryAcquire(1);
} public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException {
// 尝试在指定时间内获取锁，可被中断
return sync.tryAcquireNanos(1, unit.toNanos(timeout));
} public void unlock() {
// 释放锁
sync.release(1);
} public int getHoldCount() {
// 如果当前线程未持有锁，则返回0；如果当前线程持有锁，则返回持有的次数。
return sync.getHoldCount();
} public boolean isHeldByCurrentThread() {
// 返回当前线程是否持有锁
return sync.isHeldExclusively();
} public boolean isLocked() {
// 返回锁是否已经被占用
return sync.isLocked();
} public final boolean isFair() {
// 返回是否公平锁
return sync instanceof FairSync;
} protected Thread getOwner() {
// 返回锁的所属线程
return sync.getOwner();
} public final boolean hasQueuedThreads() {
// 返回队列中是否还有线程，即是否队列不为空
return sync.hasQueuedThreads();
} public final boolean hasQueuedThread(Thread thread) {
// 返回目标线程是否在队列中
return sync.isQueued(thread);
} public final int getQueueLength() {
// 返回队列长度
return sync.getQueueLength();
} protected Collection<Thread> getQueuedThreads() {
// 返回队列中的所有线程
return sync.getQueuedThreads();
} public Condition newCondition() {
return sync.newCondition();
} public boolean hasWaiters(Condition condition) {
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
throw new IllegalArgumentException("not owner");
return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject)condition);
} public int getWaitQueueLength(Condition condition) {
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
throw new IllegalArgumentException("not owner");
return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject)condition);
} protected Collection<Thread> getWaitingThreads(Condition condition) {
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
throw new IllegalArgumentException("not owner");
return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject)condition);
} public String toString() {
Thread o = sync.getOwner();
return super.toString() + ((o == null) ? "[Unlocked]" : "[Locked by thread " + o.getName() + "]");
}
}
ReentrantReadWriteLock的源码分析：
/**
* @since 1.5
* @author Doug Lea
*/
public class ReentrantReadWriteLock implements ReadWriteLock, java.io.Serializable {
private static final long serialVersionUID = -6992448646407690164L;
private final ReentrantReadWriteLock.ReadLock readerLock;
private final ReentrantReadWriteLock.WriteLock writerLock;
final Sync sync; public ReentrantReadWriteLock() {
this(false);
} public ReentrantReadWriteLock(boolean fair) {
sync = fair ? new FairSync() : new NonfairSync();
readerLock = new ReadLock(this);
writerLock = new WriteLock(this);
} public ReentrantReadWriteLock.WriteLock writeLock() {
return writerLock;
} public ReentrantReadWriteLock.ReadLock readLock() {
return readerLock;
} abstract static class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = 6317671515068378041L; 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; static int sharedCount(int c) {
// 使用高16位存储共享锁的共享次数
return c >>> SHARED_SHIFT;
} static int exclusiveCount(int c) {
// 使用低16位存储独占锁被持有次数
return c & EXCLUSIVE_MASK;
} static final class HoldCounter {
int count = 0;
final long tid = Thread.currentThread().getId();
} static final class ThreadLocalHoldCounter extends ThreadLocal<HoldCounter> {
public HoldCounter initialValue() {
return new HoldCounter();
}
} private transient ThreadLocalHoldCounter readHolds; private transient HoldCounter cachedHoldCounter; private transient Thread firstReader = null;
private transient int firstReaderHoldCount; Sync() {
readHolds = new ThreadLocalHoldCounter();
setState(getState()); // ensures visibility of readHolds
} abstract boolean readerShouldBlock(); abstract boolean writerShouldBlock(); protected final boolean tryAcquire(int acquires) {
Thread current = Thread.currentThread();
int c = getState();
int w = exclusiveCount(c);
// 是否存在锁
if (c != 0) {
// 如果不存在独占锁，或当前线程并不是独占锁的所属线程，则返回获取失败
if (w == 0 || current != getExclusiveOwnerThread())
return false;
// 如果累加后的持有次数，达到最大值时，则系统错误
if (w + exclusiveCount(acquires) > MAX_COUNT)
throw new Error("Maximum lock count exceeded");
// 更新独占锁被当前钱程持有的次数
setState(c + acquires);
// 返回获取成功
return true;
}
// 如果是公平策略，就需要当前线程是队头，非公平策略不需要。然后尝试占用锁
if (writerShouldBlock() || !compareAndSetState(c, c + acquires))
// 返回占用失败
return false;
// 占用成功后，更新独占锁的所属线程为当前线程
setExclusiveOwnerThread(current);
// 返回占用成功
return true;
} protected final boolean tryRelease(int releases) {
// 不允许当前线程释放其它线程持有的独占锁，否则抛出异常
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
int nextc = getState() - releases;
boolean free = exclusiveCount(nextc) == 0;
// 如果独占锁的持有次数达到释放值，就清空独占锁的所属线程
if (free)
setExclusiveOwnerThread(null);
// 更新独占锁被当前钱程持有的次数
setState(nextc);
// 返回独占锁是否已经被释放
return free;
} protected final int tryAcquireShared(int unused) {
Thread current = Thread.currentThread();
int c = getState();
// 如果存在独占锁，并且该独占锁并不属于当前线程，就返回获取失败。
if (exclusiveCount(c) != 0 && getExclusiveOwnerThread() != current)
return -1;
int r = sharedCount(c);
// 如果是公平策略，就需要当前线程是队头，如果是非公平策略，就需要队头是以共享方式取锁。
// 然后还需要共享次数未达到最大值
// 如果前面条件满足后，就尝试占用锁（注：c + SHARED_UNIT表示增加1）
if (!readerShouldBlock() && r < MAX_COUNT && compareAndSetState(c, c + SHARED_UNIT)) {
// 如果首次占用共享锁，就记录首个占用共享锁的线程和占用次数
if (r == 0) {
firstReader = current;
firstReaderHoldCount = 1;
}
// 如果不是首次占用，但当前线程就是那个首次占用共享锁的线程时，就增加记录次数
else if (firstReader == current) {
firstReaderHoldCount++;
}
else {
// 从缓存或本地变量中获取计数器
HoldCounter rh = cachedHoldCounter;
if (rh == null || rh.tid != current.getId())
cachedHoldCounter = rh = readHolds.get();
else if (rh.count == 0)
readHolds.set(rh);
// 更新当前线程所占用共享锁的次数
rh.count++;
}
// 返回占用成功
return 1;
}
return fullTryAcquireShared(current);
} final int fullTryAcquireShared(Thread current) {
HoldCounter rh = null;
for (;;) {
int c = getState();
// 判断是否存在独占锁
if (exclusiveCount(c) != 0) {
// 如果该独占锁并不属于当前线程，就返回获取失败。
if (getExclusiveOwnerThread() != current)
return -1;
} else {
// 现在不存在独占锁，如果在公平策略下，当前线程不是队头，如果在非公平策略下，队头是以独占方式取锁
// 也不是首个占用共享锁的线程，也未有占用共享锁的记录在
if (readerShouldBlock() && firstReader != current) {
// 从缓存或本地变量中获取计数器
if (rh == null) {
rh = cachedHoldCounter;
if (rh == null || rh.tid != current.getId()) {
rh = readHolds.get();
if (rh.count == 0)
readHolds.remove();
}
}
// 如果记录为0，就返回占用失败
if (rh.count == 0)
return -1;
}
} // 现在：
// 存在独占锁，并且当前线程是独占锁的所属线程
// 不存在独占锁，当前线程有占用共享锁的记录在
// 不存在独占锁，非公平策略，需要队头是以共享取锁才能插队占用共享锁
// 不存在独占锁，公平策略，需要当前线程位于队头才能占用共享锁（即不能插队） // 不允许共享次数达到最大值，否则系统错误
if (sharedCount(c) == MAX_COUNT)
throw new Error("Maximum lock count exceeded");
// 尝试占用共享锁（注：c + SHARED_UNIT表示增加1）
if (compareAndSetState(c, c + SHARED_UNIT)) {
// 如果首次占用共享锁，就记录首个占用共享锁的线程和占用次数
if (sharedCount(c) == 0) {
firstReader = current;
firstReaderHoldCount = 1;
}
// 如果不是首次占用，但当前线程就是那个首次占用共享锁的线程时，就增加记录次数
else if (firstReader == current) {
firstReaderHoldCount++;
} else {
// 从缓存或本地变量中获取计数器
if (rh == null)
rh = cachedHoldCounter;
if (rh == null || rh.tid != current.getId())
rh = readHolds.get();
else if (rh.count == 0)
readHolds.set(rh);
// 更新当前线程所占用共享锁的次数
rh.count++;
// 缓存记录
cachedHoldCounter = rh; // cache for release
}
// 返回占用成功
return 1;
}
}
} protected final boolean tryReleaseShared(int unused) {
Thread current = Thread.currentThread();
// 判断当前线程是否为第一个占用共享锁的线程
if (firstReader == current) {
// 递减占用共享锁的记录
if (firstReaderHoldCount == 1)
firstReader = null;
else
firstReaderHoldCount--;
} else {
// 从缓存或本地变量中获取计数器
HoldCounter rh = cachedHoldCounter;
if (rh == null || rh.tid != current.getId())
rh = readHolds.get();
// 如果当前线程未有占用共享的记录，则异常
int count = rh.count;
if (count <= 1) {
readHolds.remove();
if (count <= 0)
throw unmatchedUnlockException();
}
// 递减占用共享锁的记录
--rh.count;
}
for (;;) {
int c = getState();
int nextc = c - SHARED_UNIT;
// 尝试递减共享锁的占用次数，如果成功，则返回共享锁的全部占用都释放。
if (compareAndSetState(c, nextc))
return nextc == 0;
}
} private IllegalMonitorStateException unmatchedUnlockException() {
return new IllegalMonitorStateException("attempt to unlock read lock, not locked by current thread");
} final boolean tryWriteLock() {
Thread current = Thread.currentThread();
int c = getState();
// 是否存在锁
if (c != 0) {
int w = exclusiveCount(c);
// 如果不存在独占锁，或当前线程不是独占锁的所属线程，则返回占用失败
if (w == 0 || current != getExclusiveOwnerThread())
return false;
// 如果当前线程持有独占锁的次数达到最大值，则系统错误
if (w == MAX_COUNT)
throw new Error("Maximum lock count exceeded");
}
// 尝试占用独占锁
if (!compareAndSetState(c, c + 1))
// 近回占用失败
return false;
// 占用成功后，更新独占锁的所属线程为当前线程
setExclusiveOwnerThread(current);
return true;
} final boolean tryReadLock() {
Thread current = Thread.currentThread();
for (;;) {
int c = getState();
// 如果存在独占锁，并且当前线程不是独占锁的所属线程，就返回占用失败
if (exclusiveCount(c) != 0 && getExclusiveOwnerThread() != current)
return false;
int r = sharedCount(c);
// 如果共享锁的占用次数达到最值，则系统错误
if (r == MAX_COUNT)
throw new Error("Maximum lock count exceeded");
// 尝试占用共享锁
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 != current.getId())
cachedHoldCounter = rh = readHolds.get();
else if (rh.count == 0)
readHolds.set(rh);
rh.count++;
}
// 返回占用成功
return true;
}
}
} protected final boolean isHeldExclusively() {
// 返回当前线程是否持有独占锁
return getExclusiveOwnerThread() == Thread.currentThread();
} final Thread getOwner() {
// 返回独占锁的所属线程
return ((exclusiveCount(getState()) == 0) ? null : getExclusiveOwnerThread());
} final int getReadLockCount() {
// 返回共享锁的占用次数
return sharedCount(getState());
} final boolean isWriteLocked() {
// 返回独占锁的占用次数
return exclusiveCount(getState()) != 0;
} final int getWriteHoldCount() {
// 如果当前线程未持有独占锁，则返回0；如果当前线程持有独占锁，则返回持有的次数。
return isHeldExclusively() ? exclusiveCount(getState()) : 0;
} final int getReadHoldCount() {
// 返回当前线程占用共享锁的次数 // 如果独占锁未被占用，则返回0
if (getReadLockCount() == 0)
return 0; // 从第一个占用共享锁记录中（缓存）获取占用次数，并返回
Thread current = Thread.currentThread();
if (firstReader == current)
return firstReaderHoldCount; // 从缓存中获取占用次数，并返回
HoldCounter rh = cachedHoldCounter;
if (rh != null && rh.tid == current.getId())
return rh.count; // 从记录中获取占用次数，并返回
int count = readHolds.get().count;
if (count == 0)
readHolds.remove();
return count;
} final int getCount() {
// 返回锁的数量
return getState();
} final ConditionObject newCondition() {
return new ConditionObject();
}
} static final class NonfairSync extends Sync {
private static final long serialVersionUID = -8159625535654395037L; final boolean writerShouldBlock() {
return false;
} final boolean readerShouldBlock() {
// 返回队头是否以独占方式获取锁
// return apparentlyFirstQueuedIsExclusive();
return false;
}
} static final class FairSync extends Sync {
private static final long serialVersionUID = -2274990926593161451L; final boolean writerShouldBlock() {
// 如果队列为空或队头为当前线程时，就返回false，否则返回true
return hasQueuedPredecessors();
} final boolean readerShouldBlock() {
// 如果队列为空或队头为当前线程时，就返回false，否则返回true
return hasQueuedPredecessors();
}
} public static class ReadLock implements Lock, java.io.Serializable {
private static final long serialVersionUID = -5992448646407690164L;
private final Sync sync; protected ReadLock(ReentrantReadWriteLock lock) {
sync = lock.sync;
} public void lock() {
sync.acquireShared(1);
} public void lockInterruptibly() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
} public boolean tryLock() {
// 尝试占用共享锁，并返回是否成功
return sync.tryReadLock();
} public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException {
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
} public void unlock() {
sync.releaseShared(1);
} public Condition newCondition() {
// 共享锁不允许创建条件对象
throw new UnsupportedOperationException();
} public String toString() {
int r = sync.getReadLockCount();
return super.toString() + "[Read locks = " + r + "]";
}
} public static class WriteLock implements Lock, java.io.Serializable {
private static final long serialVersionUID = -4992448646407690164L;
private final Sync sync; protected WriteLock(ReentrantReadWriteLock lock) {
sync = lock.sync;
} public void lock() {
sync.acquire(1);
} public void lockInterruptibly() throws InterruptedException {
sync.acquireInterruptibly(1);
} public boolean tryLock() {
// 尝试占用独占锁，并返回是否成功
return sync.tryWriteLock();
} public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException {
return sync.tryAcquireNanos(1, unit.toNanos(timeout));
} public void unlock() {
sync.release(1);
} public boolean isHeldByCurrentThread() {
// 返回当前线程是否持有独占锁
return sync.isHeldExclusively();
} public int getHoldCount() {
// 如果当前线程未持有独占锁，则返回0；如果当前线程持有独占锁，则返回持有的次数。
return sync.getWriteHoldCount();
} public Condition newCondition() {
return sync.newCondition();
} public String toString() {
Thread o = sync.getOwner();
return super.toString() + ((o == null) ? "[Unlocked]" : "[Locked by thread " + o.getName() + "]");
}
} public final boolean isFair() {
// 返回是否公平锁
return sync instanceof FairSync;
} protected Thread getOwner() {
// 返回独占锁的所属线程
return sync.getOwner();
} public int getReadLockCount() {
// 返回共享锁的占用次数
return sync.getReadLockCount();
} public boolean isWriteLocked() {
// 返回独占锁的占用次数
return sync.isWriteLocked();
} public boolean isWriteLockedByCurrentThread() {
// 返回当前线程是否持有独占锁
return sync.isHeldExclusively();
} public int getWriteHoldCount() {
// 如果当前线程未持有独占锁，则返回0；如果当前线程持有独占锁，则返回持有的次数。
return sync.getWriteHoldCount();
} public int getReadHoldCount() {
// 返回当前线程占用共享锁的次数
return sync.getReadHoldCount();
} protected Collection<Thread> getQueuedWriterThreads() {
// 返回队列中以独占模式获取锁的所有线程
return sync.getExclusiveQueuedThreads();
} protected Collection<Thread> getQueuedReaderThreads() {
// 返回队列中以共享模式获取锁的所有线程
return sync.getSharedQueuedThreads();
} public final boolean hasQueuedThreads() {
// 返回队列中是否还有线程，即是否队列不为空
return sync.hasQueuedThreads();
} public final boolean hasQueuedThread(Thread thread) {
// 返回目标线程是否在队列中
return sync.isQueued(thread);
} public final int getQueueLength() {
// 返回队列长度
return sync.getQueueLength();
} protected Collection<Thread> getQueuedThreads() {
// 返回队列中的所有线程
return sync.getQueuedThreads();
} public boolean hasWaiters(Condition condition) {
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
throw new IllegalArgumentException("not owner");
return sync
.hasWaiters((AbstractQueuedSynchronizer.ConditionObject) condition);
} public int getWaitQueueLength(Condition condition) {
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
throw new IllegalArgumentException("not owner");
return sync
.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject) condition);
} protected Collection<Thread> getWaitingThreads(Condition condition) {
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
throw new IllegalArgumentException("not owner");
return sync
.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject) condition);
} public String toString() {
int c = sync.getCount();
int w = Sync.exclusiveCount(c);
int r = Sync.sharedCount(c); return super.toString() + "[Write locks = " + w + ", Read locks = " + r + "]";
}
}
