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<div class="subTitle">edu.emory.mathcs.backport.java.util.concurrent.locks</div>
<h2 title="Class ReentrantReadWriteLock" class="title">Class ReentrantReadWriteLock</h2>
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<ul class="inheritance">
<li>java.lang.Object</li>
<li>
<ul class="inheritance">
<li>edu.emory.mathcs.backport.java.util.concurrent.locks.ReentrantReadWriteLock</li>
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<dt>All Implemented Interfaces:</dt>
<dd><a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReadWriteLock.html" title="interface in edu.emory.mathcs.backport.java.util.concurrent.locks">ReadWriteLock</a>, java.io.Serializable</dd>
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<pre>public class <span class="strong">ReentrantReadWriteLock</span>
extends java.lang.Object
implements <a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReadWriteLock.html" title="interface in edu.emory.mathcs.backport.java.util.concurrent.locks">ReadWriteLock</a>, java.io.Serializable</pre>
<div class="block">An implementation of <a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReadWriteLock.html" title="interface in edu.emory.mathcs.backport.java.util.concurrent.locks"><code>ReadWriteLock</code></a> supporting similar
semantics to <a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReentrantLock.html" title="class in edu.emory.mathcs.backport.java.util.concurrent.locks"><code>ReentrantLock</code></a>.
<p>This class has the following properties:
<ul>
<li><b>Acquisition order</b>
<p>The order of entry
to the read and write lock is unspecified, subject to reentrancy
constraints. A nonfair lock that is continously contended may
indefinitely postpone one or more reader or writer threads, but
will normally have higher throughput than a fair lock.
<p>
DEPARTURE FROM java.util.concurrent: this implementation impose
a writer-preferrence and thus its acquisition order may be different
than in java.util.concurrent.
<li><b>Reentrancy</b>
<p>This lock allows both readers and writers to reacquire read or
write locks in the style of a <a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReentrantLock.html" title="class in edu.emory.mathcs.backport.java.util.concurrent.locks"><code>ReentrantLock</code></a>. 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><a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/Condition.html" title="interface in edu.emory.mathcs.backport.java.util.concurrent.locks"><code>Condition</code></a> support</b>
<p>The write lock provides a <a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/Condition.html" title="interface in edu.emory.mathcs.backport.java.util.concurrent.locks"><code>Condition</code></a> implementation that
behaves in the same way, with respect to the write lock, as the
<a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/Condition.html" title="interface in edu.emory.mathcs.backport.java.util.concurrent.locks"><code>Condition</code></a> implementation provided by
<a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReentrantLock.html#newCondition()"><code>ReentrantLock.newCondition()</code></a> does for <a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReentrantLock.html" title="class in edu.emory.mathcs.backport.java.util.concurrent.locks"><code>ReentrantLock</code></a>.
This <a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/Condition.html" title="interface in edu.emory.mathcs.backport.java.util.concurrent.locks"><code>Condition</code></a> can, of course, only be used with the write lock.
<p>The read lock does not support a <a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/Condition.html" title="interface in edu.emory.mathcs.backport.java.util.concurrent.locks"><code>Condition</code></a> and
<code>readLock().newCondition()</code> throws
<code>UnsupportedOperationException</code>.
<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 exploit
reentrancy to perform lock downgrading after updating a cache (exception
handling is elided for simplicity):
<pre>
class CachedData {
Object data;
volatile boolean cacheValid;
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();
// 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();
rwl.writeLock().unlock(); // Unlock write, still hold read
}
use(data);
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(); }
}
}</code></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
<code>Error</code> throws from locking methods.</div>
<dl><dt><span class="strong">Since:</span></dt>
<dd>1.5</dd>
<dt><span class="strong">Author:</span></dt>
<dd>Doug Lea</dd>
<dt><span class="strong">See Also:</span></dt><dd><a href="../../../../../../../../serialized-form.html#edu.emory.mathcs.backport.java.util.concurrent.locks.ReentrantReadWriteLock">Serialized Form</a></dd></dl>
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<th class="colFirst" scope="col">Modifier and Type</th>
<th class="colLast" scope="col">Class and Description</th>
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<tr class="altColor">
<td class="colFirst"><code>static class </code></td>
<td class="colLast"><code><strong><a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReentrantReadWriteLock.ReadLock.html" title="class in edu.emory.mathcs.backport.java.util.concurrent.locks">ReentrantReadWriteLock.ReadLock</a></strong></code>
<div class="block">The lock returned by method <a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReentrantReadWriteLock.html#readLock()"><code>readLock()</code></a>.</div>
</td>
</tr>
<tr class="rowColor">
<td class="colFirst"><code>static class </code></td>
<td class="colLast"><code><strong><a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReentrantReadWriteLock.WriteLock.html" title="class in edu.emory.mathcs.backport.java.util.concurrent.locks">ReentrantReadWriteLock.WriteLock</a></strong></code>
<div class="block">The lock returned by method <a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReentrantReadWriteLock.html#writeLock()"><code>writeLock()</code></a>.</div>
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<th class="colOne" scope="col">Constructor and Description</th>
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<tr class="altColor">
<td class="colOne"><code><strong><a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReentrantReadWriteLock.html#ReentrantReadWriteLock()">ReentrantReadWriteLock</a></strong>()</code>
<div class="block">Creates a new <code>ReentrantReadWriteLock</code> with
default (nonfair) ordering properties.</div>
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<tr class="altColor">
<td class="colFirst"><code>protected java.lang.Thread</code></td>
<td class="colLast"><code><strong><a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReentrantReadWriteLock.html#getOwner()">getOwner</a></strong>()</code>
<div class="block">Returns the thread that currently owns the write lock, or
<code>null</code> if not owned.</div>
</td>
</tr>
<tr class="rowColor">
<td class="colFirst"><code>int</code></td>
<td class="colLast"><code><strong><a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReentrantReadWriteLock.html#getQueueLength()">getQueueLength</a></strong>()</code>
<div class="block">Returns an estimate of the number of threads waiting to acquire
either the read or write lock.</div>
</td>
</tr>
<tr class="altColor">
<td class="colFirst"><code>int</code></td>
<td class="colLast"><code><strong><a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReentrantReadWriteLock.html#getReadHoldCount()">getReadHoldCount</a></strong>()</code>
<div class="block">Queries the number of reentrant read holds on this lock by the
current thread.</div>
</td>
</tr>
<tr class="rowColor">
<td class="colFirst"><code>int</code></td>
<td class="colLast"><code><strong><a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReentrantReadWriteLock.html#getReadLockCount()">getReadLockCount</a></strong>()</code>
<div class="block">Queries the number of read locks held for this lock.</div>
</td>
</tr>
<tr class="altColor">
<td class="colFirst"><code>int</code></td>
<td class="colLast"><code><strong><a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReentrantReadWriteLock.html#getWriteHoldCount()">getWriteHoldCount</a></strong>()</code>
<div class="block">Queries the number of reentrant write holds on this lock by the
current thread.</div>
</td>
</tr>
<tr class="rowColor">
<td class="colFirst"><code>boolean</code></td>
<td class="colLast"><code><strong><a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReentrantReadWriteLock.html#hasQueuedThreads()">hasQueuedThreads</a></strong>()</code>
<div class="block">Queries whether any threads are waiting to acquire the read or
write lock.</div>
</td>
</tr>
<tr class="altColor">
<td class="colFirst"><code>boolean</code></td>
<td class="colLast"><code><strong><a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReentrantReadWriteLock.html#isFair()">isFair</a></strong>()</code>
<div class="block">Returns <code>true</code> if this lock has fairness set true.</div>
</td>
</tr>
<tr class="rowColor">
<td class="colFirst"><code>boolean</code></td>
<td class="colLast"><code><strong><a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReentrantReadWriteLock.html#isWriteLocked()">isWriteLocked</a></strong>()</code>
<div class="block">Queries if the write lock is held by any thread.</div>
</td>
</tr>
<tr class="altColor">
<td class="colFirst"><code>boolean</code></td>
<td class="colLast"><code><strong><a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReentrantReadWriteLock.html#isWriteLockedByCurrentThread()">isWriteLockedByCurrentThread</a></strong>()</code>
<div class="block">Queries if the write lock is held by the current thread.</div>
</td>
</tr>
<tr class="rowColor">
<td class="colFirst"><code><a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/Lock.html" title="interface in edu.emory.mathcs.backport.java.util.concurrent.locks">Lock</a></code></td>
<td class="colLast"><code><strong><a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReentrantReadWriteLock.html#readLock()">readLock</a></strong>()</code>
<div class="block">Returns the lock used for reading.</div>
</td>
</tr>
<tr class="altColor">
<td class="colFirst"><code>java.lang.String</code></td>
<td class="colLast"><code><strong><a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReentrantReadWriteLock.html#toString()">toString</a></strong>()</code>
<div class="block">Returns a string identifying this lock, as well as its lock state.</div>
</td>
</tr>
<tr class="rowColor">
<td class="colFirst"><code><a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/Lock.html" title="interface in edu.emory.mathcs.backport.java.util.concurrent.locks">Lock</a></code></td>
<td class="colLast"><code><strong><a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReentrantReadWriteLock.html#writeLock()">writeLock</a></strong>()</code>
<div class="block">Returns the lock used for writing.</div>
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<code>clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait</code></li>
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<h4>ReentrantReadWriteLock</h4>
<pre>public ReentrantReadWriteLock()</pre>
<div class="block">Creates a new <code>ReentrantReadWriteLock</code> with
default (nonfair) ordering properties.</div>
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<a name="writeLock()">
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<h4>writeLock</h4>
<pre>public <a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/Lock.html" title="interface in edu.emory.mathcs.backport.java.util.concurrent.locks">Lock</a> writeLock()</pre>
<div class="block"><strong>Description copied from interface: <code><a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReadWriteLock.html#writeLock()">ReadWriteLock</a></code></strong></div>
<div class="block">Returns the lock used for writing.</div>
<dl>
<dt><strong>Specified by:</strong></dt>
<dd><code><a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReadWriteLock.html#writeLock()">writeLock</a></code> in interface <code><a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReadWriteLock.html" title="interface in edu.emory.mathcs.backport.java.util.concurrent.locks">ReadWriteLock</a></code></dd>
<dt><span class="strong">Returns:</span></dt><dd>the lock used for writing.</dd></dl>
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<a name="readLock()">
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<h4>readLock</h4>
<pre>public <a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/Lock.html" title="interface in edu.emory.mathcs.backport.java.util.concurrent.locks">Lock</a> readLock()</pre>
<div class="block"><strong>Description copied from interface: <code><a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReadWriteLock.html#readLock()">ReadWriteLock</a></code></strong></div>
<div class="block">Returns the lock used for reading.</div>
<dl>
<dt><strong>Specified by:</strong></dt>
<dd><code><a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReadWriteLock.html#readLock()">readLock</a></code> in interface <code><a href="../../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/locks/ReadWriteLock.html" title="interface in edu.emory.mathcs.backport.java.util.concurrent.locks">ReadWriteLock</a></code></dd>
<dt><span class="strong">Returns:</span></dt><dd>the lock used for reading.</dd></dl>
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<h4>isFair</h4>
<pre>public final boolean isFair()</pre>
<div class="block">Returns <code>true</code> if this lock has fairness set true.</div>
<dl><dt><span class="strong">Returns:</span></dt><dd><code>true</code> if this lock has fairness set true</dd></dl>
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<h4>getOwner</h4>
<pre>protected java.lang.Thread getOwner()</pre>
<div class="block">Returns the thread that currently owns the write lock, or
<code>null</code> 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</code> 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.</div>
<dl><dt><span class="strong">Returns:</span></dt><dd>the owner, or <code>null</code> if not owned</dd></dl>
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<h4>getReadLockCount</h4>
<pre>public int getReadLockCount()</pre>
<div class="block">Queries the number of read locks held for this lock. This
method is designed for use in monitoring system state, not for
synchronization control.</div>
<dl><dt><span class="strong">Returns:</span></dt><dd>the number of read locks held.</dd></dl>
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<h4>isWriteLocked</h4>
<pre>public boolean isWriteLocked()</pre>
<div class="block">Queries if the write lock is held by any thread. This method is
designed for use in monitoring system state, not for
synchronization control.</div>
<dl><dt><span class="strong">Returns:</span></dt><dd><code>true</code> if any thread holds the write lock and
<code>false</code> otherwise</dd></dl>
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<h4>isWriteLockedByCurrentThread</h4>
<pre>public boolean isWriteLockedByCurrentThread()</pre>
<div class="block">Queries if the write lock is held by the current thread.</div>
<dl><dt><span class="strong">Returns:</span></dt><dd><code>true</code> if the current thread holds the write lock and
<code>false</code> otherwise</dd></dl>
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<h4>getWriteHoldCount</h4>
<pre>public int getWriteHoldCount()</pre>
<div class="block">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.</div>
<dl><dt><span class="strong">Returns:</span></dt><dd>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</dd></dl>
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<pre>public int getReadHoldCount()</pre>
<div class="block">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.</div>
<dl><dt><span class="strong">Returns:</span></dt><dd>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</dd><dt><span class="strong">Since:</span></dt>
<dd>1.6</dd></dl>
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<h4>hasQueuedThreads</h4>
<pre>public final boolean hasQueuedThreads()</pre>
<div class="block">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</code> 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.</div>
<dl><dt><span class="strong">Returns:</span></dt><dd><code>true</code> if there may be other threads waiting to
acquire the lock</dd></dl>
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<h4>getQueueLength</h4>
<pre>public final int getQueueLength()</pre>
<div class="block">Returns an estimate of the number of threads waiting to acquire
either the read or write lock. The value is only an estimate
because the number of threads may change dynamically while this
method traverses internal data structures. This method is
designed for use in monitoring of the system state, not for
synchronization control.</div>
<dl><dt><span class="strong">Returns:</span></dt><dd>the estimated number of threads waiting for this lock</dd></dl>
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<h4>toString</h4>
<pre>public java.lang.String toString()</pre>
<div class="block">Returns a string identifying this lock, as well as its lock state.
The state, in brackets, includes the String <code>"Write locks ="</code>
followed by the number of reentrantly held write locks, and the
String <code>"Read locks ="</code> followed by the number of held
read locks.</div>
<dl>
<dt><strong>Overrides:</strong></dt>
<dd><code>toString</code> in class <code>java.lang.Object</code></dd>
<dt><span class="strong">Returns:</span></dt><dd>a string identifying this lock, as well as its lock state</dd></dl>
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