示例代码：
public class ReadWriteLockTest {
ReadWriteLock readWriteLock = new ReentrantReadWriteLock( );
private Integer data=0;
public void read() {
readWriteLock.readLock().lock();
System.out.println(Thread.currentThread().getName()+" read");
try {
Thread.sleep( (long) (Math.random()*1000) );
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName()+" read over");
readWriteLock.readLock().unlock();
}
public void write(Integer data){
readWriteLock.writeLock().lock();
System.out.println(Thread.currentThread().getName()+" write");
try {
Thread.sleep( (long) (Math.random()*1000) );
} catch (InterruptedException e) {
e.printStackTrace();
}
this.data=data;
System.out.println(Thread.currentThread().getName()+" write over");
readWriteLock.writeLock().unlock();
}
public static void main(String[] args) throws Exception {
final ReadWriteLockTest test = new ReadWriteLockTest();
for (int i = 0; i <10 ; i++) {
new Thread( new Runnable() {
@Override
public void run() {
test.read();
}
} ).start();
new Thread( new Runnable() {
@Override
public void run() {
test.write(new Random().nextInt( 10 ) );
}
} ).start();
}
}
}
打印输出：
image.png
可以看出，读锁是共享锁。一个线程在读没结束时，其他读线程也可以读。
ReadWriteLock
public interface ReadWriteLock {
Lock readLock();
Lock writeLock();
}
读写锁ReadWriteLock接口包含读锁(共享锁)和写锁(排它锁)
锁升级：由读锁升级为写锁(会造成死锁，所以ReadWriteLock实现类不支持)
锁降级：由写锁降级为读锁
ReentrantReadWriteLock
package java.util.concurrent.locks;
public class ReentrantReadWriteLock
implements ReadWriteLock, java.io.Serializable {
private static final long serialVersionUID = -6992448646407690164L;
/** Inner class providing readlock */
private final ReentrantReadWriteLock.ReadLock readerLock;
/** Inner class providing writelock */
private final ReentrantReadWriteLock.WriteLock writerLock;
/** Performs all synchronization mechanics */
final Sync sync;
public ReentrantReadWriteLock() {
this(false);
}
/**
* Creates a new {@code ReentrantReadWriteLock} with
* the given fairness policy.
*
* @param fair {@code true} if this lock should use a fair ordering policy
*/
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; }
/**
* Synchronization implementation for ReentrantReadWriteLock.
* Subclassed into fair and nonfair versions.
*/
abstract static class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = 6317671515068378041L;
/*
* Read vs write count extraction constants and functions.
* Lock state is logically divided into two unsigned shorts:
* The lower one representing the exclusive (writer) lock hold count,
* and the upper the shared (reader) hold count.
*/
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 */
static int sharedCount(int c) { return c >>> SHARED_SHIFT; }
/** Returns the number of exclusive holds represented in count */
static int exclusiveCount(int c) { return c & EXCLUSIVE_MASK; }
static final class HoldCounter {
int count = 0;
// Use id, not reference, to avoid garbage retention
final long tid = getThreadId(Thread.currentThread());
}
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 tryRelease(int releases) {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
int nextc = getState() - releases;
boolean free = exclusiveCount(nextc) == 0;
if (free)
setExclusiveOwnerThread(null);
setState(nextc);
return free;
}
protected final boolean tryAcquire(int acquires) {
Thread current = Thread.currentThread();
int c = getState();
int w = exclusiveCount(c);
if (c != 0) {
// (Note: if c != 0 and w == 0 then shared count != 0)
if (w == 0 || current != getExclusiveOwnerThread())
return false;
if (w + exclusiveCount(acquires) > MAX_COUNT)
throw new Error("Maximum lock count exceeded");
// Reentrant acquire
setState(c + acquires);
return true;
}
if (writerShouldBlock() ||
!compareAndSetState(c, c + acquires))
return false;
setExclusiveOwnerThread(current);
return true;
}
protected final boolean tryReleaseShared(int unused) {
Thread current = Thread.currentThread();
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) {
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;
}
}
private IllegalMonitorStateException unmatchedUnlockException() {
return new IllegalMonitorStateException(
"attempt to unlock read lock, not locked by current thread");
}
protected final int tryAcquireShared(int unused) {
Thread current = Thread.currentThread();
int c = getState();
if (exclusiveCount(c) != 0 &&
getExclusiveOwnerThread() != current)
return -1;
int r = sharedCount(c);
if (!readerShouldBlock() &&
r < MAX_COUNT &&
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 1;
}
return fullTryAcquireShared(current);
}
final int fullTryAcquireShared(Thread current) {
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;
}
}
}
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;
}
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;
}
}
}
protected final boolean isHeldExclusively() {
// While we must in general read state before owner,
// we don't need to do so to check if current thread is owner
return getExclusiveOwnerThread() == Thread.currentThread();
}
// Methods relayed to outer class
final ConditionObject newCondition() {
return new ConditionObject();
}
final Thread getOwner() {
// Must read state before owner to ensure memory consistency
return ((exclusiveCount(getState()) == 0) ?
null :
getExclusiveOwnerThread());
}
final int getReadLockCount() {
return sharedCount(getState());
}
final boolean isWriteLocked() {
return exclusiveCount(getState()) != 0;
}
final int getWriteHoldCount() {
return isHeldExclusively() ? exclusiveCount(getState()) : 0;
}
final int getReadHoldCount() {
if (getReadLockCount() == 0)
return 0;
Thread current = Thread.currentThread();
if (firstReader == current)
return firstReaderHoldCount;
HoldCounter rh = cachedHoldCounter;
if (rh != null && rh.tid == getThreadId(current))
return rh.count;
int count = readHolds.get().count;
if (count == 0) readHolds.remove();
return count;
}
/**
* Reconstitutes the instance from a stream (that is, deserializes it).
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
s.defaultReadObject();
readHolds = new ThreadLocalHoldCounter();
setState(0); // reset to unlocked state
}
final int getCount() { return getState(); }
}
/**
* Nonfair version of Sync
*/
static final class NonfairSync extends Sync {
private static final long serialVersionUID = -8159625535654395037L;
final boolean writerShouldBlock() {
return false; // writers can always barge
}
final boolean readerShouldBlock() {
return apparentlyFirstQueuedIsExclusive();
}
}
/**
* Fair version of Sync
*/
static final class FairSync extends Sync {
private static final long serialVersionUID = -2274990926593161451L;
final boolean writerShouldBlock() {
return hasQueuedPredecessors();
}
final boolean readerShouldBlock() {
return hasQueuedPredecessors();
}
}
/**
* The lock returned by method {@link ReentrantReadWriteLock#readLock}.
*/
public static class ReadLock implements Lock, java.io.Serializable {
private static final long serialVersionUID = -5992448646407690164L;
private final Sync sync;
/**
* Constructor for use by subclasses
*
* @param lock the outer lock object
* @throws NullPointerException if the lock is null
*/
protected ReadLock(ReentrantReadWriteLock lock) {
sync = lock.sync;
}
/**
* 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);
}
public void lockInterruptibly() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
public boolean tryLock() {
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);
}
/**
* Throws {@code UnsupportedOperationException} because
* {@code ReadLocks} do not support conditions.
*
* @throws UnsupportedOperationException always
*/
public Condition newCondition() {
throw new UnsupportedOperationException();
}
public String toString() {
int r = sync.getReadLockCount();
return super.toString() +
"[Read locks = " + r + "]";
}
}
/**
* The lock returned by method {@link ReentrantReadWriteLock#writeLock}.
*/
public static class WriteLock implements Lock, java.io.Serializable {
private static final long serialVersionUID = -4992448646407690164L;
private final Sync sync;
/**
* Constructor for use by subclasses
*
* @param lock the outer lock object
* @throws NullPointerException if the lock is null
*/
protected WriteLock(ReentrantReadWriteLock lock) {
sync = lock.sync;
}
/**
* Acquires the write lock.
*
* <p>Acquires the write lock if neither the read nor write lock
* are held by another thread
* and returns immediately, setting the write lock hold count to
* one.
*
* <p>If the current thread already holds the write lock then the
* hold count is incremented by one and the method returns
* immediately.
*
* <p>If the lock is held by another thread then the current
* thread becomes disabled for thread scheduling purposes and
* lies dormant until the write lock has been acquired, at which
* time the write lock hold count is set to one.
*/
public void lock() {
sync.acquire(1);
}
/**
* Acquires the write lock unless the current thread is
* {@linkplain Thread#interrupt interrupted}.
*
* <p>Acquires the write lock if neither the read nor write lock
* are held by another thread
* and returns immediately, setting the write lock hold count to
* one.
*
* <p>If the current thread already holds this lock then the
* hold count is incremented by one and the method returns
* immediately.
*
* <p>If the lock is held by another thread then the current
* thread becomes disabled for thread scheduling purposes and
* lies dormant until one of two things happens:
*
* <ul>
*
* <li>The write lock is acquired by the current thread; or
*
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread.
*
* </ul>
*
* <p>If the write lock is acquired by the current thread then the
* lock hold count is set to one.
*
* <p>If the current thread:
*
* <ul>
*
* <li>has its interrupted status set on entry to this method;
* or
*
* <li>is {@linkplain Thread#interrupt interrupted} while
* acquiring the write lock,
*
* </ul>
*
* then {@link InterruptedException} is thrown and the current
* thread's interrupted status is cleared.
*
* <p>In this implementation, as this method is an explicit
* interruption point, preference is given to responding to
* the interrupt over normal or reentrant acquisition of the
* lock.
*
* @throws InterruptedException if the current thread is interrupted
*/
public void lockInterruptibly() throws InterruptedException {
sync.acquireInterruptibly(1);
}
/**
* Acquires the write lock only if it is not held by another thread
* at the time of invocation.
*
* <p>Acquires the write lock if neither the read nor write lock
* are held by another thread
* and returns immediately with the value {@code true},
* setting the write lock hold count to one. Even when this lock has
* been set to use a fair ordering policy, a call to
* {@code tryLock()} <em>will</em> immediately acquire the
* lock if it is available, whether or not other threads are
* currently waiting for the write lock. This "barging"
* behavior can be useful in certain circumstances, even
* though it breaks fairness. If you want to honor the
* fairness setting for this lock, then use {@link
* #tryLock(long, TimeUnit) tryLock(0, TimeUnit.SECONDS) }
* which is almost equivalent (it also detects interruption).
*
* <p>If the current thread already holds this lock then the
* hold count is incremented by one and the method returns
* {@code true}.
*
* <p>If the lock is held by another thread then this method
* will return immediately with the value {@code false}.
*
* @return {@code true} if the lock was free and was acquired
* by the current thread, or the write lock was already held
* by the current thread; and {@code false} otherwise.
*/
public boolean tryLock( ) {
return sync.tryWriteLock();
}
/**
* Acquires the write lock if it is not held by another thread
* within the given waiting time and the current thread has
* not been {@linkplain Thread#interrupt interrupted}.
*
* <p>Acquires the write lock if neither the read nor write lock
* are held by another thread
* and returns immediately with the value {@code true},
* setting the write lock hold count to one. If this lock has been
* set to use a fair ordering policy then an available lock
* <em>will not</em> be acquired if any other threads are
* waiting for the write lock. This is in contrast to the {@link
* #tryLock()} method. If you want a timed {@code tryLock}
* that does permit barging on a fair lock then combine the
* timed and un-timed forms together:
*
* <pre> {@code
* if (lock.tryLock() ||
* lock.tryLock(timeout, unit)) {
* ...
* }}</pre>
*
* <p>If the current thread already holds this lock then the
* hold count is incremented by one and the method returns
* {@code true}.
*
* <p>If the lock is held by another thread then the current
* thread becomes disabled for thread scheduling purposes and
* lies dormant until one of three things happens:
*
* <ul>
*
* <li>The write lock is acquired by the current thread; or
*
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread; or
*
* <li>The specified waiting time elapses
*
* </ul>
*
* <p>If the write lock is acquired then the value {@code true} is
* returned and the write lock hold count is set to one.
*
* <p>If the current thread:
*
* <ul>
*
* <li>has its interrupted status set on entry to this method;
* or
*
* <li>is {@linkplain Thread#interrupt interrupted} while
* acquiring the write lock,
*
* </ul>
*
* then {@link InterruptedException} is thrown and the current
* thread's interrupted status is cleared.
*
* <p>If the specified waiting time elapses then the value
* {@code false} is returned. If the time is less than or
* equal to zero, the method will not wait at all.
*
* <p>In this implementation, as this method is an explicit
* interruption point, preference is given to responding to
* the interrupt over normal or reentrant acquisition of the
* lock, and over reporting the elapse of the waiting time.
*
* @param timeout the time to wait for the write lock
* @param unit the time unit of the timeout argument
*
* @return {@code true} if the lock was free and was acquired
* by the current thread, or the write lock was already held by the
* current thread; and {@code false} if the waiting time
* elapsed before the lock could be acquired.
*
* @throws InterruptedException if the current thread is interrupted
* @throws NullPointerException if the time unit is null
*/
public boolean tryLock(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireNanos(1, unit.toNanos(timeout));
}
/**
* Attempts to release this lock.
*
* <p>If the current thread is the holder of this lock then
* the hold count is decremented. If the hold count is now
* zero then the lock is released. If the current thread is
* not the holder of this lock then {@link
* IllegalMonitorStateException} is thrown.
*
* @throws IllegalMonitorStateException if the current thread does not
* hold this lock
*/
public void unlock() {
sync.release(1);
}
/**
* Returns a {@link Condition} instance for use with this
* {@link Lock} instance.
* <p>The returned {@link Condition} instance supports the same
* usages as do the {@link Object} monitor methods ({@link
* Object#wait() wait}, {@link Object#notify notify}, and {@link
* Object#notifyAll notifyAll}) when used with the built-in
* monitor lock.
*
* <ul>
*
* <li>If this write lock is not held when any {@link
* Condition} method is called then an {@link
* IllegalMonitorStateException} is thrown. (Read locks are
* held independently of write locks, so are not checked or
* affected. However it is essentially always an error to
* invoke a condition waiting method when the current thread
* has also acquired read locks, since other threads that
* could unblock it will not be able to acquire the write
* lock.)
*
* <li>When the condition {@linkplain Condition#await() waiting}
* methods are called the write lock is released and, before
* they return, the write lock is reacquired and the lock hold
* count restored to what it was when the method was called.
*
* <li>If a thread is {@linkplain Thread#interrupt interrupted} while
* waiting then the wait will terminate, an {@link
* InterruptedException} will be thrown, and the thread's
* interrupted status will be cleared.
*
* <li> Waiting threads are signalled in FIFO order.
*
* <li>The ordering of lock reacquisition for threads returning
* from waiting methods is the same as for threads initially
* acquiring the lock, which is in the default case not specified,
* but for <em>fair</em> locks favors those threads that have been
* waiting the longest.
*
* </ul>
*
* @return the Condition object
*/
public Condition newCondition() {
return sync.newCondition();
}
/**
* Returns a string identifying this lock, as well as its lock
* state. The state, in brackets includes either the String
* {@code "Unlocked"} or the String {@code "Locked by"}
* followed by the {@linkplain Thread#getName name} of the owning thread.
*
* @return a string identifying this lock, as well as its lock state
*/
public String toString() {
Thread o = sync.getOwner();
return super.toString() + ((o == null) ?
"[Unlocked]" :
"[Locked by thread " + o.getName() + "]");
}
/**
* Queries if this write lock is held by the current thread.
* Identical in effect to {@link
* ReentrantReadWriteLock#isWriteLockedByCurrentThread}.
*
* @return {@code true} if the current thread holds this lock and
* {@code false} otherwise
* @since 1.6
*/
public boolean isHeldByCurrentThread() {
return sync.isHeldExclusively();
}
/**
* Queries the number of holds on this write lock by the current
* thread. A thread has a hold on a lock for each lock action
* that is not matched by an unlock action. Identical in effect
* to {@link ReentrantReadWriteLock#getWriteHoldCount}.
*
* @return the number of holds on this lock by the current thread,
* or zero if this lock is not held by the current thread
* @since 1.6
*/
public int getHoldCount() {
return sync.getWriteHoldCount();
}
}
// Instrumentation and status
/**
* Returns {@code true} if this lock has fairness set true.
*
* @return {@code true} if this lock has fairness set true
*/
public final boolean isFair() {
return sync instanceof FairSync;
}
/**
* Returns the thread that currently owns the write lock, or
* {@code null} if not owned. When this method is called by a
* thread that is not the owner, the return value reflects a
* best-effort approximation of current lock status. For example,
* the owner may be momentarily {@code null} even if there are
* threads trying to acquire the lock but have not yet done so.
* This method is designed to facilitate construction of
* subclasses that provide more extensive lock monitoring
* facilities.
*
* @return the owner, or {@code null} if not owned
*/
protected Thread getOwner() {
return sync.getOwner();
}
/**
* 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 sync.getReadLockCount();
}
/**
* Queries if the write lock is held by any thread. This method is
* designed for use in monitoring system state, not for
* synchronization control.
*
* @return {@code true} if any thread holds the write lock and
* {@code false} otherwise
*/
public boolean isWriteLocked() {
return sync.isWriteLocked();
}
/**
* Queries if the write lock is held by the current thread.
*
* @return {@code true} if the current thread holds the write lock and
* {@code false} otherwise
*/
public boolean isWriteLockedByCurrentThread() {
return sync.isHeldExclusively();
}
/**
* Queries the number of reentrant write holds on this lock by the
* current thread. A writer thread has a hold on a lock for
* each lock action that is not matched by an unlock action.
*
* @return the number of holds on the write lock by the current thread,
* or zero if the write lock is not held by the current thread
*/
public int getWriteHoldCount() {
return sync.getWriteHoldCount();
}
/**
* Queries the number of reentrant read holds on this lock by the
* current thread. A reader thread has a hold on a lock for
* each lock action that is not matched by an unlock action.
*
* @return the number of holds on the read lock by the current thread,
* or zero if the read lock is not held by the current thread
* @since 1.6
*/
public int getReadHoldCount() {
return sync.getReadHoldCount();
}
protected Collection<Thread> getQueuedWriterThreads() {
return sync.getExclusiveQueuedThreads();
}
protected Collection<Thread> getQueuedReaderThreads() {
return sync.getSharedQueuedThreads();
}
public final boolean hasQueuedThreads() {
return sync.hasQueuedThreads();
}
public final boolean hasQueuedThread(Thread thread) {
return sync.isQueued(thread);
}
public final int getQueueLength() {
return sync.getQueueLength();
}
protected Collection<Thread> getQueuedThreads() {
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 + "]";
}
static final long getThreadId(Thread thread) {
return UNSAFE.getLongVolatile(thread, TID_OFFSET);
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE;
private static final long TID_OFFSET;
static {
try {
UNSAFE = sun.misc.Unsafe.getUnsafe();
Class<?> tk = Thread.class;
TID_OFFSET = UNSAFE.objectFieldOffset
(tk.getDeclaredField("tid"));
} catch (Exception e) {
throw new Error(e);
}
}
}
可重入读写锁ReentrantReadWriteLock实现类ReadWriteLock。
包含ReadLock(读锁)和WriteLock(写锁)，包含实现了AQS的子类及其其他属性变量
读锁加锁
lock
ReentrantReadWriteLock.ReadLock#lock
image.png
acquireShared
AbstractQueuedSynchronizer#acquireShared
image.png
tryAcquireShared
AbstractQueuedSynchronizer#tryAcquireShared
image.png
tryAcquireShared
ReentrantReadWriteLock.Sync#tryAcquireShared
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();
if (exclusiveCount(c) != 0 &&
getExclusiveOwnerThread() != current)
return -1;
int r = sharedCount(c);
if (!readerShouldBlock() &&
r < MAX_COUNT &&
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 1;
}
return fullTryAcquireShared(current);
}
获取当前线程current和状态值c。
根据状态值c调用exclusiveCount(c)获取排它锁值(低16位)。sharedCount()方法获取共享值(高16位)
image.png
exclusiveCount(c)表示写锁。如果有写锁则exclusiveCount(c)!=0，且当前线程不等于持有锁的线程，会返回-1，这样当前线程会调用doAcquireShared方法加入队列。如果没有写锁则exclusiveCount(c)=0，会继续往下执行得到r=sharedCount(c)，读锁次数。
调用readerShouldBlock()方法
image.png
AbstractQueuedSynchronizer#hasQueuedPredecessors
image.png
调用readerShouldBlock()方法，判断AQS队列是否有值。----此时，如果另一个线程对写锁加锁，则会添加到队列，不会往下执行。会调用fullTryAcquireShared()方法自旋再次获取
如果AQS队列为空，则对高16位(读锁+1)。
获取的r重入次数，如果是第一次，则r=0，假如只有t1线程加读锁，会把当前线程t1赋值给firstReader，firstReaderHoldCount的值赋值为1。
else if (firstReader == current)判断
如果这个线程t1再次加读锁，则firstReaderHoldCount+1。
所以，firstReader表示第一个加读锁的线程。firstReaderHoldCount表示第一个线程进入的次数。
如果是另一个线程t2加读锁，第一次cachedHoldCounter的值为null。会通过readHolds.get()方法获取HoldCounter rh
image.png
ThreadLocal#setInitialValue
image.png
ThreadLocal#createMap
image.png
得到HoldCounter值之后赋值给cachedHoldCounter和rh，对rh的count+1，如果当前线程t2再次进入rh的count直接+1
如果t3线程加读锁，则rh.tid不等于current当前线程，需要重新通过readHolds.get()获取。如果t2线程再次进入cachedHoldCounter的值是上次的HoldCounter，也会从readHolds.get()方法获取当前线程的HoldCounter缓存值。
所以，cachedHoldCounter表示，最后执行线程的HoldCounter值。count表示所属线程重入次数
加入队列和队列尝试加锁
调用AQS方法
AbstractQueuedSynchronizer#addWaiter
AbstractQueuedSynchronizer#acquireQueued
读锁解锁
调用ReentrantReadWriteLock.readLock().unlock()方法
unlock
ReentrantReadWriteLock.ReadLock#unlock
image.png
releaseShared
AbstractQueuedSynchronizer#releaseShared
image.png
tryReleaseShared
ReentrantReadWriteLock.Sync#tryReleaseShared
protected final boolean tryReleaseShared(int unused) {
Thread current = Thread.currentThread();
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) {
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;
}
}
解锁过程：就是根据当前线程加读锁次数，调用tryReleaseShared一次当前线程rh.count减一。最后通过判断nextc 是否等于0返回是否要唤醒下一个线程
doReleaseShared
AbstractQueuedSynchronizer#doReleaseShared
image.png
判断头结点h.waitStatus值等于-1，CAS操作头结点的waitStatus成功(说明头结点waitStatus是-1，有下一个节点)并唤醒下一个线程。
unparkSuccessor
unparkSuccessor()调用AQS方法
AbstractQueuedSynchronizer#unparkSuccessor
写锁加锁
ReentrantReadWriteLock.WriteLock#lock
image.png
acquire
AbstractQueuedSynchronizer#acquire
image.png
tryAcquire
ReentrantReadWriteLock.Sync#tryAcquire
protected 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);
if (c != 0) {
// (Note: if c != 0 and w == 0 then shared count != 0)
if (w == 0 || current != getExclusiveOwnerThread())
return false;
if (w + exclusiveCount(acquires) > MAX_COUNT)
throw new Error("Maximum lock count exceeded");
// Reentrant acquire
setState(c + acquires);
return true;
}
if (writerShouldBlock() ||
!compareAndSetState(c, c + acquires))
return false;
setExclusiveOwnerThread(current);
return true;
}
执行流程：
获取当前执行线程赋值给current，获取状态值赋值给c。根据c值得到排它锁(低16位)的值赋值给w。
如果可用状态c等于0，表示没有任何读锁、写锁线程占有。
调用writerShouldBlock()方法，判断AQS队列是否有等待的线程节点。如果有则当前线程加入队列，如果AQS中没有等待线程，CAS设置c值会成功。exclusiveOwnerThread设置当前执行线程。返回true不会添加到线程
如果可用状态c不等于0，
如果w=0说明，c没有写锁只有读锁，则会把当前线程加入AQS队列(这里可以表明，读锁不能升级为写锁)
w不等于0，表示肯定有写锁，如果当前线程不等于持有锁线程则加入AQS队列。如果当前线程等于exclusiveOwnerThread值，重入则状态值加1
加入队列和队列尝试加锁
调用AQS方法
AbstractQueuedSynchronizer#addWaiter
AbstractQueuedSynchronizer#acquireQueued
写锁解锁
WriteLock#unlock
java.util.concurrent.locks.ReentrantReadWriteLock.WriteLock#unlock
image.png
release
AbstractQueuedSynchronizer#release
image.png
tryRelease
ReentrantReadWriteLock.Sync#tryRelease
image.png
获取状态值state，判断排它锁的低16位是否等于0，并返回是否需要唤醒下一个线程
总结：
可重入的读写锁ReentrantReadWriteLock类的数据结构，包括读锁、写锁、实现AQS的内部类，其他属性。
加锁过程中什么情况下会加入AQS队列：
读锁加锁，写锁不等于0且不是当前线程(写锁不等于0且是当前线程可以加读锁----锁降级)
写锁加锁，读锁不等于0且写锁等于0(锁升级)、写锁不等于0且不是当前线程、状态值等于0且队列不为空且当前线程不等于AQS第二个线程
可重入的读写锁ReentrantReadWriteLock对重入线程加读锁解读锁的处理维护的属性值
1、firstReader 第一个加读锁的线程
2、firstReaderHoldCount 第一个线程加读锁的次数
3、cachedHoldCounter 最后执行线程的readHolds值
4、readHolds 包含当前线程id和count次数
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