JDK8/Java8源码在线阅读
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*
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* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
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*
* This code is distributed in the hope that it will be useful, but WITHOUT
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* accompanied this code).
*
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/*
* This file is available under and governed by the GNU General Public
* License version 2 only, as published by the Free Software Foundation.
* However, the following notice accompanied the original version of this
* file:
*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/
*/
package java.util.concurrent.locks;
import java.util.concurrent.TimeUnit;
import java.util.Collection;
/**
* An implementation of {@link ReadWriteLock} supporting similar
* semantics to {@link ReentrantLock}.
* <p>This class has the following properties:
*
* <ul>
* <li><b>Acquisition order</b>
*
* <p>This class does not impose a reader or writer preference
* ordering for lock access. However, it does support an optional
* <em>fairness</em> policy.
*
* <dl>
* <dt><b><i>Non-fair mode (default)</i></b>
* <dd>When constructed as non-fair (the default), the order of entry
* to the read and write lock is unspecified, subject to reentrancy
* constraints. A nonfair lock that is continuously contended may
* indefinitely postpone one or more reader or writer threads, but
* will normally have higher throughput than a fair lock.
*
* <dt><b><i>Fair mode</i></b>
* <dd>When constructed as fair, threads contend for entry using an
* approximately arrival-order policy. When the currently held lock
* is released, either the longest-waiting single writer thread will
* be assigned the write lock, or if there is a group of reader threads
* waiting longer than all waiting writer threads, that group will be
* assigned the read lock.
*
* <p>A thread that tries to acquire a fair read lock (non-reentrantly)
* will block if either the write lock is held, or there is a waiting
* writer thread. The thread will not acquire the read lock until
* after the oldest currently waiting writer thread has acquired and
* released the write lock. Of course, if a waiting writer abandons
* its wait, leaving one or more reader threads as the longest waiters
* in the queue with the write lock free, then those readers will be
* assigned the read lock.
*
* <p>A thread that tries to acquire a fair write lock (non-reentrantly)
* will block unless both the read lock and write lock are free (which
* implies there are no waiting threads). (Note that the non-blocking
* {@link ReadLock#tryLock()} and {@link WriteLock#tryLock()} methods
* do not honor this fair setting and will immediately acquire the lock
* if it is possible, regardless of waiting threads.)
* <p>
* </dl>
*
* <li><b>Reentrancy</b>
*
* <p>This lock allows both readers and writers to reacquire read or
* write locks in the style of a {@link ReentrantLock}. Non-reentrant
* readers are not allowed until all write locks held by the writing
* thread have been released.
*
* <p>Additionally, a writer can acquire the read lock, but not
* vice-versa. Among other applications, reentrancy can be useful
* when write locks are held during calls or callbacks to methods that
* perform reads under read locks. If a reader tries to acquire the
* write lock it will never succeed.
*
* <li><b>Lock downgrading</b>
* <p>Reentrancy also allows downgrading from the write lock to a read lock,
* by acquiring the write lock, then the read lock and then releasing the
* write lock. However, upgrading from a read lock to the write lock is
* <b>not</b> possible.
*
* <li><b>Interruption of lock acquisition</b>
* <p>The read lock and write lock both support interruption during lock
* acquisition.
*
* <li><b>{@link Condition} support</b>
* <p>The write lock provides a {@link Condition} implementation that
* behaves in the same way, with respect to the write lock, as the
* {@link Condition} implementation provided by
* {@link ReentrantLock#newCondition} does for {@link ReentrantLock}.
* This {@link Condition} can, of course, only be used with the write lock.
*
* <p>The read lock does not support a {@link Condition} and
* {@code readLock().newCondition()} throws
* {@code UnsupportedOperationException}.
*
* <li><b>Instrumentation</b>
* <p>This class supports methods to determine whether locks
* are held or contended. These methods are designed for monitoring
* system state, not for synchronization control.
* </ul>
*
* <p>Serialization of this class behaves in the same way as built-in
* locks: a deserialized lock is in the unlocked state, regardless of
* its state when serialized.
*
* <p><b>Sample usages</b>. Here is a code sketch showing how to perform
* lock downgrading after updating a cache (exception handling is
* particularly tricky when handling multiple locks in a non-nested
* fashion):
*
* <pre> {@code
* class CachedData {
* Object data;
* volatile boolean cacheValid;
* final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
*
* void processCachedData() {
* rwl.readLock().lock();
* if (!cacheValid) {
* // Must release read lock before acquiring write lock
* rwl.readLock().unlock();
* rwl.writeLock().lock();
* try {
* // Recheck state because another thread might have
* // acquired write lock and changed state before we did.
* if (!cacheValid) {
* data = ...
* cacheValid = true;
* }
* // Downgrade by acquiring read lock before releasing write lock
* rwl.readLock().lock();
* } finally {
* rwl.writeLock().unlock(); // Unlock write, still hold read
* }
* }
*
* try {
* use(data);
* } finally {
* rwl.readLock().unlock();
* }
* }
* }}</pre>
*
* ReentrantReadWriteLocks can be used to improve concurrency in some
* uses of some kinds of Collections. This is typically worthwhile
* only when the collections are expected to be large, accessed by
* more reader threads than writer threads, and entail operations with
* overhead that outweighs synchronization overhead. For example, here
* is a class using a TreeMap that is expected to be large and
* concurrently accessed.
*
* <pre> {@code
* class RWDictionary {
* private final Map<String, Data> m = new TreeMap<String, Data>();
* private final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
* private final Lock r = rwl.readLock();
* private final Lock w = rwl.writeLock();
*
* public Data get(String key) {
* r.lock();
* try { return m.get(key); }
* finally { r.unlock(); }
* }
* public String[] allKeys() {
* r.lock();
* try { return m.keySet().toArray(); }
* finally { r.unlock(); }
* }
* public Data put(String key, Data value) {
* w.lock();
* try { return m.put(key, value); }
* finally { w.unlock(); }
* }
* public void clear() {
* w.lock();
* try { m.clear(); }
* finally { w.unlock(); }
* }
* }}</pre>
*
* <h3>Implementation Notes</h3>
*
* <p>This lock supports a maximum of 65535 recursive write locks
* and 65535 read locks. Attempts to exceed these limits result in
* {@link Error} throws from locking methods.
*
* @since 1.5
* @author Doug Lea
*/
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;
/**
* Creates a new {@code ReentrantReadWriteLock} with
* default (nonfair) ordering properties.
*/
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; }
/**
* A counter for per-thread read hold counts.
* Maintained as a ThreadLocal; cached in cachedHoldCounter
*/
static final class HoldCounter {
int count = 0;
// Use id, not reference, to avoid garbage retention
final long tid = getThreadId(Thread.currentThread());
}
/**
* ThreadLocal subclass. Easiest to explicitly define for sake
* of deserialization mechanics.
*/
static final class ThreadLocalHoldCounter
extends ThreadLocal<HoldCounter> {
public HoldCounter initialValue() {
return new HoldCounter();
}
}
/**
* The number of reentrant read locks held by current thread.
* Initialized only in constructor and readObject.
* Removed whenever a thread's read hold count drops to 0.
*/
private transient ThreadLocalHoldCounter readHolds;
/**
* The hold count of the last thread to successfully acquire
* readLock. This saves ThreadLocal lookup in the common case
* where the next thread to release is the last one to
* acquire. This is non-volatile since it is just used
* as a heuristic, and would be great for threads to cache.
*
* <p>Can outlive the Thread for which it is caching the read
* hold count, but avoids garbage retention by not retaining a
* reference to the Thread.
*
* <p>Accessed via a benign data race; relies on the memory
* model's final field and out-of-thin-air guarantees.
*/
private transient HoldCounter cachedHoldCounter;
/**
* firstReader is the first thread to have acquired the read lock.
* firstReaderHoldCount is firstReader's hold count.
*
* <p>More precisely, firstReader is the unique thread that last
* changed the shared count from 0 to 1, and has not released the
* read lock since then; null if there is no such thread.
*
* <p>Cannot cause garbage retention unless the thread terminated
* without relinquishing its read locks, since tryReleaseShared
* sets it to null.
*
* <p>Accessed via a benign data race; relies on the memory
* model's out-of-thin-air guarantees for references.
*
* <p>This allows tracking of read holds for uncontended read
* locks to be very cheap.
*/
private transient Thread firstReader = null;
private transient int firstReaderHoldCount;
Sync() {
readHolds = new ThreadLocalHoldCounter();
setState(getState()); // ensures visibility of readHolds
}
/*
* Acquires and releases use the same code for fair and
* nonfair locks, but differ in whether/how they allow barging
* when queues are non-empty.
*/
/**
* Returns true if the current thread, when trying to acquire
* the read lock, and otherwise eligible to do so, should block
* because of policy for overtaking other waiting threads.
*/
abstract boolean readerShouldBlock();
/**
* Returns true if the current thread, when trying to acquire
* the write lock, and otherwise eligible to do so, should block
* because of policy for overtaking other waiting threads.
*/
abstract boolean writerShouldBlock();
/*
* Note that tryRelease and tryAcquire can be called by
* Conditions. So it is possible that their arguments contain
* both read and write holds that are all released during a
* condition wait and re-established in tryAcquire.
*/
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) {
/*
* 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;
}
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) {
/*
* 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);
}
/**
* Full version of acquire for reads, that handles CAS misses
* and reentrant reads not dealt with in tryAcquireShared.
*/
final int fullTryAcquireShared(Thread current) {
/*
* This code is in part redundant with that in
* tryAcquireShared but is simpler overall by not
* complicating tryAcquireShared with interactions between
* retries and lazily reading hold counts.
*/
HoldCounter rh = null;
for (;;) {
int c = getState();
if (exclusiveCount(c) != 0) {
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;
}
}
}
/**
* Performs tryLock for write, enabling barging in both modes.
* This is identical in effect to tryAcquire except for lack
* of calls to writerShouldBlock.
*/
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;
}
/**
* Performs tryLock for read, enabling barging in both modes.
* This is identical in effect to tryAcquireShared except for
* lack of calls to readerShouldBlock.
*/
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() {
/* As a heuristic to avoid indefinite writer starvation,
* block if the thread that momentarily appears to be head
* of queue, if one exists, is a waiting writer. This is
* only a probabilistic effect since a new reader will not
* block if there is a waiting writer behind other enabled
* readers that have not yet drained from the queue.
*/
return apparentlyFirstQueuedIsExclusive();
}
}
/**
* 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);
}
/**
* Acquires the read lock unless the current thread is
* {@linkplain Thread#interrupt interrupted}.
*
* <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 one of two things happens:
*
* <ul>
*
* <li>The read lock is acquired by the current thread; or
*
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread.
*
* </ul>
*
* <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 read 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.acquireSharedInterruptibly(1);
}
/**
* Acquires the read lock only if the write lock is not held by
* another thread at the time of invocation.
*
* <p>Acquires the read lock if the write lock is not held by
* another thread and returns immediately with the value
* {@code true}. Even when this lock has been set to use a
* fair ordering policy, a call to {@code tryLock()}
* <em>will</em> immediately acquire the read lock if it is
* available, whether or not other threads are currently
* waiting for the read 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 write lock is held by another thread then
* this method will return immediately with the value
* {@code false}.
*
* @return {@code true} if the read lock was acquired
*/
public boolean tryLock() {
return sync.tryReadLock();
}
/**
* Acquires the read lock if the write lock 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 read lock if the write lock is not held by
* another thread and returns immediately with the value
* {@code true}. 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
* 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 write 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 read 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 read lock is acquired then the value {@code true} is
* returned.
*
* <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 read 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 read lock
* @param unit the time unit of the timeout argument
* @return {@code true} if the read lock was 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.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}
/**
* Attempts to release this lock.
*
* <p>If the number of readers is now zero then the lock
* is made available for write lock attempts.
*/
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();
}
/**
* Returns a string identifying this lock, as well as its lock state.
* The state, in brackets, includes the String {@code "Read locks ="}
* followed by the number of held read locks.
*
* @return a string identifying this lock, as well as its lock state
*/
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();
}
/**
* Returns a collection containing threads that may be waiting to
* acquire the write lock. Because the actual set of threads may
* change dynamically while constructing this result, the returned
* collection is only a best-effort estimate. The elements of the
* returned collection are in no particular order. This method is
* designed to facilitate construction of subclasses that provide
* more extensive lock monitoring facilities.
*
* @return the collection of threads
*/
protected Collection<Thread> getQueuedWriterThreads() {
return sync.getExclusiveQueuedThreads();
}
/**
* Returns a collection containing threads that may be waiting to
* acquire the read lock. Because the actual set of threads may
* change dynamically while constructing this result, the returned
* collection is only a best-effort estimate. The elements of the
* returned collection are in no particular order. This method is
* designed to facilitate construction of subclasses that provide
* more extensive lock monitoring facilities.
*
* @return the collection of threads
*/
protected Collection<Thread> getQueuedReaderThreads() {
return sync.getSharedQueuedThreads();
}
/**
* Queries whether any threads are waiting to acquire the read or
* write lock. Note that because cancellations may occur at any
* time, a {@code true} return does not guarantee that any other
* thread will ever acquire a lock. This method is designed
* primarily for use in monitoring of the system state.
*
* @return {@code true} if there may be other threads waiting to
* acquire the lock
*/
public final boolean hasQueuedThreads() {
return sync.hasQueuedThreads();
}
/**
* Queries whether the given thread is waiting to acquire either
* the read or write lock. Note that because cancellations may
* occur at any time, a {@code true} return does not guarantee
* that this thread will ever acquire a lock. This method is
* designed primarily for use in monitoring of the system state.
*
* @param thread the thread
* @return {@code true} if the given thread is queued waiting for this lock
* @throws NullPointerException if the thread is null
*/
public final boolean hasQueuedThread(Thread thread) {
return sync.isQueued(thread);
}
/**
* Returns an estimate of the number of threads waiting to acquire
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