mirror of git://gcc.gnu.org/git/gcc.git
1347 lines
52 KiB
Java
1347 lines
52 KiB
Java
/*
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* Written by Doug Lea with assistance from members of JCP JSR-166
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* Expert Group and released to the public domain, as explained at
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* http://creativecommons.org/licenses/publicdomain
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*/
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package java.util.concurrent.locks;
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import java.util.concurrent.*;
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import java.util.concurrent.atomic.*;
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import java.util.*;
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/**
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* An implementation of {@link ReadWriteLock} supporting similar
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* semantics to {@link ReentrantLock}.
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* <p>This class has the following properties:
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*
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* <ul>
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* <li><b>Acquisition order</b>
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*
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* <p> This class does not impose a reader or writer preference
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* ordering for lock access. However, it does support an optional
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* <em>fairness</em> policy.
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*
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* <dl>
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* <dt><b><i>Non-fair mode (default)</i></b>
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* <dd>When constructed as non-fair (the default), the order of entry
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* to the read and write lock is unspecified, subject to reentrancy
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* constraints. A nonfair lock that is continously contended may
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* indefinitely postpone one or more reader or writer threads, but
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* will normally have higher throughput than a fair lock.
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* <p>
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*
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* <dt><b><i>Fair mode</i></b>
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* <dd> When constructed as fair, threads contend for entry using an
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* approximately arrival-order policy. When the currently held lock
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* is released either the longest-waiting single writer thread will
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* be assigned the write lock, or if there is a group of reader threads
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* waiting longer than all waiting writer threads, that group will be
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* assigned the read lock.
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*
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* <p>A thread that tries to acquire a fair read lock (non-reentrantly)
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* will block if either the write lock is held, or there is a waiting
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* writer thread. The thread will not acquire the read lock until
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* after the oldest currently waiting writer thread has acquired and
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* released the write lock. Of course, if a waiting writer abandons
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* its wait, leaving one or more reader threads as the longest waiters
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* in the queue with the write lock free, then those readers will be
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* assigned the read lock.
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*
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* <p>A thread that tries to acquire a fair write lock (non-reentrantly)
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* will block unless both the read lock and write lock are free (which
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* implies there are no waiting threads). (Note that the non-blocking
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* {@link ReadLock#tryLock()} and {@link WriteLock#tryLock()} methods
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* do not honor this fair setting and will acquire the lock if it is
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* possible, regardless of waiting threads.)
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* <p>
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* </dl>
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*
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* <li><b>Reentrancy</b>
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*
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* <p>This lock allows both readers and writers to reacquire read or
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* write locks in the style of a {@link ReentrantLock}. Non-reentrant
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* readers are not allowed until all write locks held by the writing
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* thread have been released.
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*
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* <p>Additionally, a writer can acquire the read lock, but not
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* vice-versa. Among other applications, reentrancy can be useful
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* when write locks are held during calls or callbacks to methods that
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* perform reads under read locks. If a reader tries to acquire the
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* write lock it will never succeed.
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*
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* <li><b>Lock downgrading</b>
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* <p>Reentrancy also allows downgrading from the write lock to a read lock,
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* by acquiring the write lock, then the read lock and then releasing the
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* write lock. However, upgrading from a read lock to the write lock is
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* <b>not</b> possible.
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*
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* <li><b>Interruption of lock acquisition</b>
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* <p>The read lock and write lock both support interruption during lock
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* acquisition.
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*
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* <li><b>{@link Condition} support</b>
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* <p>The write lock provides a {@link Condition} implementation that
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* behaves in the same way, with respect to the write lock, as the
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* {@link Condition} implementation provided by
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* {@link ReentrantLock#newCondition} does for {@link ReentrantLock}.
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* This {@link Condition} can, of course, only be used with the write lock.
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*
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* <p>The read lock does not support a {@link Condition} and
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* {@code readLock().newCondition()} throws
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* {@code UnsupportedOperationException}.
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*
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* <li><b>Instrumentation</b>
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* <p>This class supports methods to determine whether locks
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* are held or contended. These methods are designed for monitoring
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* system state, not for synchronization control.
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* </ul>
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*
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* <p>Serialization of this class behaves in the same way as built-in
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* locks: a deserialized lock is in the unlocked state, regardless of
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* its state when serialized.
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*
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* <p><b>Sample usages</b>. Here is a code sketch showing how to exploit
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* reentrancy to perform lock downgrading after updating a cache (exception
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* handling is elided for simplicity):
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* <pre>
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* class CachedData {
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* Object data;
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* volatile boolean cacheValid;
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* ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
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*
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* void processCachedData() {
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* rwl.readLock().lock();
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* if (!cacheValid) {
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* // Must release read lock before acquiring write lock
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* rwl.readLock().unlock();
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* rwl.writeLock().lock();
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* // Recheck state because another thread might have acquired
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* // write lock and changed state before we did.
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* if (!cacheValid) {
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* data = ...
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* cacheValid = true;
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* }
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* // Downgrade by acquiring read lock before releasing write lock
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* rwl.readLock().lock();
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* rwl.writeLock().unlock(); // Unlock write, still hold read
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* }
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*
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* use(data);
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* rwl.readLock().unlock();
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* }
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* }
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* </pre>
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*
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* ReentrantReadWriteLocks can be used to improve concurrency in some
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* uses of some kinds of Collections. This is typically worthwhile
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* only when the collections are expected to be large, accessed by
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* more reader threads than writer threads, and entail operations with
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* overhead that outweighs synchronization overhead. For example, here
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* is a class using a TreeMap that is expected to be large and
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* concurrently accessed.
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*
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* <pre>{@code
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* class RWDictionary {
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* private final Map<String, Data> m = new TreeMap<String, Data>();
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* private final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
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* private final Lock r = rwl.readLock();
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* private final Lock w = rwl.writeLock();
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*
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* public Data get(String key) {
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* r.lock();
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* try { return m.get(key); }
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* finally { r.unlock(); }
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* }
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* public String[] allKeys() {
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* r.lock();
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* try { return m.keySet().toArray(); }
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* finally { r.unlock(); }
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* }
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* public Data put(String key, Data value) {
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* w.lock();
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* try { return m.put(key, value); }
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* finally { w.unlock(); }
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* }
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* public void clear() {
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* w.lock();
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* try { m.clear(); }
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* finally { w.unlock(); }
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* }
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* }}</pre>
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*
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* <h3>Implementation Notes</h3>
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*
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* <p>This lock supports a maximum of 65535 recursive write locks
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* and 65535 read locks. Attempts to exceed these limits result in
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* {@link Error} throws from locking methods.
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*
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* @since 1.5
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* @author Doug Lea
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*
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*/
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public class ReentrantReadWriteLock implements ReadWriteLock, java.io.Serializable {
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private static final long serialVersionUID = -6992448646407690164L;
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/** Inner class providing readlock */
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private final ReentrantReadWriteLock.ReadLock readerLock;
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/** Inner class providing writelock */
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private final ReentrantReadWriteLock.WriteLock writerLock;
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/** Performs all synchronization mechanics */
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private final Sync sync;
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/**
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* Creates a new {@code ReentrantReadWriteLock} with
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* default (nonfair) ordering properties.
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*/
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public ReentrantReadWriteLock() {
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this(false);
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}
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/**
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* Creates a new {@code ReentrantReadWriteLock} with
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* the given fairness policy.
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*
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* @param fair {@code true} if this lock should use a fair ordering policy
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*/
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public ReentrantReadWriteLock(boolean fair) {
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sync = (fair)? new FairSync() : new NonfairSync();
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readerLock = new ReadLock(this);
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writerLock = new WriteLock(this);
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}
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public ReentrantReadWriteLock.WriteLock writeLock() { return writerLock; }
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public ReentrantReadWriteLock.ReadLock readLock() { return readerLock; }
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/**
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* Synchronization implementation for ReentrantReadWriteLock.
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* Subclassed into fair and nonfair versions.
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*/
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static abstract class Sync extends AbstractQueuedSynchronizer {
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private static final long serialVersionUID = 6317671515068378041L;
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/*
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* Read vs write count extraction constants and functions.
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* Lock state is logically divided into two shorts: The lower
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* one representing the exclusive (writer) lock hold count,
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* and the upper the shared (reader) hold count.
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*/
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static final int SHARED_SHIFT = 16;
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static final int SHARED_UNIT = (1 << SHARED_SHIFT);
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static final int MAX_COUNT = (1 << SHARED_SHIFT) - 1;
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static final int EXCLUSIVE_MASK = (1 << SHARED_SHIFT) - 1;
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/** Returns the number of shared holds represented in count */
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static int sharedCount(int c) { return c >>> SHARED_SHIFT; }
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/** Returns the number of exclusive holds represented in count */
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static int exclusiveCount(int c) { return c & EXCLUSIVE_MASK; }
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/**
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* A counter for per-thread read hold counts.
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* Maintained as a ThreadLocal; cached in cachedHoldCounter
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*/
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static final class HoldCounter {
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int count;
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// Use id, not reference, to avoid garbage retention
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final long tid = Thread.currentThread().getId();
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/** Decrement if positive; return previous value */
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int tryDecrement() {
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int c = count;
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if (c > 0)
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count = c - 1;
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return c;
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}
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}
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/**
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* ThreadLocal subclass. Easiest to explicitly define for sake
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* of deserialization mechanics.
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*/
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static final class ThreadLocalHoldCounter
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extends ThreadLocal<HoldCounter> {
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public HoldCounter initialValue() {
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return new HoldCounter();
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}
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}
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/**
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* The number of read locks held by current thread.
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* Initialized only in constructor and readObject.
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*/
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transient ThreadLocalHoldCounter readHolds;
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/**
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* The hold count of the last thread to successfully acquire
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* readLock. This saves ThreadLocal lookup in the common case
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* where the next thread to release is the last one to
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* acquire. This is non-volatile since it is just used
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* as a heuristic, and would be great for threads to cache.
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*/
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transient HoldCounter cachedHoldCounter;
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Sync() {
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readHolds = new ThreadLocalHoldCounter();
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setState(getState()); // ensures visibility of readHolds
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}
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/*
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* Acquires and releases use the same code for fair and
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* nonfair locks, but differ in whether/how they allow barging
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* when queues are non-empty.
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*/
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/**
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* Return true if a reader thread that is otherwise
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* eligible for lock should block because of policy
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* for overtaking other waiting threads.
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*/
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abstract boolean readerShouldBlock(Thread current);
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/**
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* Return true if a writer thread that is otherwise
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* eligible for lock should block because of policy
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* for overtaking other waiting threads.
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*/
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abstract boolean writerShouldBlock(Thread current);
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/*
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* Note that tryRelease and tryAcquire can be called by
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* Conditions. So it is possible that their arguments contain
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* both read and write holds that are all released during a
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* condition wait and re-established in tryAcquire.
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*/
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protected final boolean tryRelease(int releases) {
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int nextc = getState() - releases;
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if (Thread.currentThread() != getExclusiveOwnerThread())
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throw new IllegalMonitorStateException();
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if (exclusiveCount(nextc) == 0) {
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setExclusiveOwnerThread(null);
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setState(nextc);
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return true;
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} else {
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setState(nextc);
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return false;
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}
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}
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protected final boolean tryAcquire(int acquires) {
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/*
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* Walkthrough:
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* 1. if read count nonzero or write count nonzero
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* and owner is a different thread, fail.
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* 2. If count would saturate, fail. (This can only
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* happen if count is already nonzero.)
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* 3. Otherwise, this thread is eligible for lock if
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* it is either a reentrant acquire or
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* queue policy allows it. If so, update state
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* and set owner.
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*/
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Thread current = Thread.currentThread();
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int c = getState();
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int w = exclusiveCount(c);
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if (c != 0) {
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// (Note: if c != 0 and w == 0 then shared count != 0)
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if (w == 0 || current != getExclusiveOwnerThread())
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return false;
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if (w + exclusiveCount(acquires) > MAX_COUNT)
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throw new Error("Maximum lock count exceeded");
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}
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if ((w == 0 && writerShouldBlock(current)) ||
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!compareAndSetState(c, c + acquires))
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return false;
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setExclusiveOwnerThread(current);
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return true;
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}
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protected final boolean tryReleaseShared(int unused) {
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HoldCounter rh = cachedHoldCounter;
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Thread current = Thread.currentThread();
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if (rh == null || rh.tid != current.getId())
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rh = readHolds.get();
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if (rh.tryDecrement() <= 0)
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throw new IllegalMonitorStateException();
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for (;;) {
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int c = getState();
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int nextc = c - SHARED_UNIT;
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if (compareAndSetState(c, nextc))
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return nextc == 0;
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}
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}
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protected final int tryAcquireShared(int unused) {
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/*
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* Walkthrough:
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* 1. If write lock held by another thread, fail
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* 2. If count saturated, throw error
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* 3. Otherwise, this thread is eligible for
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* lock wrt state, so ask if it should block
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* because of queue policy. If not, try
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* to grant by CASing state and updating count.
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* Note that step does not check for reentrant
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* acquires, which is postponed to full version
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* to avoid having to check hold count in
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* the more typical non-reentrant case.
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* 4. If step 3 fails either because thread
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* apparently not eligible or CAS fails,
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* chain to version with full retry loop.
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*/
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Thread current = Thread.currentThread();
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int c = getState();
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if (exclusiveCount(c) != 0 &&
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getExclusiveOwnerThread() != current)
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return -1;
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if (sharedCount(c) == MAX_COUNT)
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throw new Error("Maximum lock count exceeded");
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if (!readerShouldBlock(current) &&
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compareAndSetState(c, c + SHARED_UNIT)) {
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HoldCounter rh = cachedHoldCounter;
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if (rh == null || rh.tid != current.getId())
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cachedHoldCounter = rh = readHolds.get();
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rh.count++;
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return 1;
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}
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return fullTryAcquireShared(current);
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}
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/**
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* Full version of acquire for reads, that handles CAS misses
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* and reentrant reads not dealt with in tryAcquireShared.
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*/
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final int fullTryAcquireShared(Thread current) {
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/*
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* This code is in part redundant with that in
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* tryAcquireShared but is simpler overall by not
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* complicating tryAcquireShared with interactions between
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* retries and lazily reading hold counts.
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*/
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HoldCounter rh = cachedHoldCounter;
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if (rh == null || rh.tid != current.getId())
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rh = readHolds.get();
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for (;;) {
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int c = getState();
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int w = exclusiveCount(c);
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if ((w != 0 && getExclusiveOwnerThread() != current) ||
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((rh.count | w) == 0 && readerShouldBlock(current)))
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return -1;
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if (sharedCount(c) == MAX_COUNT)
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throw new Error("Maximum lock count exceeded");
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if (compareAndSetState(c, c + SHARED_UNIT)) {
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cachedHoldCounter = rh; // cache for release
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rh.count++;
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return 1;
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}
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}
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}
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/**
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* Performs tryLock for write, enabling barging in both modes.
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* This is identical in effect to tryAcquire except for lack
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* of calls to writerShouldBlock
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*/
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final boolean tryWriteLock() {
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Thread current = Thread.currentThread();
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int c = getState();
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if (c != 0) {
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int w = exclusiveCount(c);
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if (w == 0 ||current != getExclusiveOwnerThread())
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return false;
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if (w == MAX_COUNT)
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throw new Error("Maximum lock count exceeded");
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}
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if (!compareAndSetState(c, c + 1))
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return false;
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setExclusiveOwnerThread(current);
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return true;
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}
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/**
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* Performs tryLock for read, enabling barging in both modes.
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* This is identical in effect to tryAcquireShared except for
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* lack of calls to readerShouldBlock
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*/
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final boolean tryReadLock() {
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Thread current = Thread.currentThread();
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for (;;) {
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int c = getState();
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if (exclusiveCount(c) != 0 &&
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getExclusiveOwnerThread() != current)
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return false;
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if (sharedCount(c) == MAX_COUNT)
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throw new Error("Maximum lock count exceeded");
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if (compareAndSetState(c, c + SHARED_UNIT)) {
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HoldCounter rh = cachedHoldCounter;
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if (rh == null || rh.tid != current.getId())
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cachedHoldCounter = rh = readHolds.get();
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rh.count++;
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return true;
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}
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}
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}
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protected final boolean isHeldExclusively() {
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// While we must in general read state before owner,
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// we don't need to do so to check if current thread is owner
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return getExclusiveOwnerThread() == Thread.currentThread();
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}
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// Methods relayed to outer class
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final ConditionObject newCondition() {
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return new ConditionObject();
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}
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final Thread getOwner() {
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// Must read state before owner to ensure memory consistency
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return ((exclusiveCount(getState()) == 0)?
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null :
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getExclusiveOwnerThread());
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}
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final int getReadLockCount() {
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return sharedCount(getState());
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}
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final boolean isWriteLocked() {
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return exclusiveCount(getState()) != 0;
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}
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final int getWriteHoldCount() {
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return isHeldExclusively() ? exclusiveCount(getState()) : 0;
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}
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final int getReadHoldCount() {
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return getReadLockCount() == 0? 0 : readHolds.get().count;
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}
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/**
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* Reconstitute this lock instance from a stream
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* @param s the stream
|
|
*/
|
|
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
|
|
*/
|
|
final static class NonfairSync extends Sync {
|
|
private static final long serialVersionUID = -8159625535654395037L;
|
|
final boolean writerShouldBlock(Thread current) {
|
|
return false; // writers can always barge
|
|
}
|
|
final boolean readerShouldBlock(Thread current) {
|
|
/* 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 probablistic 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
|
|
*/
|
|
final static class FairSync extends Sync {
|
|
private static final long serialVersionUID = -2274990926593161451L;
|
|
final boolean writerShouldBlock(Thread current) {
|
|
// only proceed if queue is empty or current thread at head
|
|
return !isFirst(current);
|
|
}
|
|
final boolean readerShouldBlock(Thread current) {
|
|
// only proceed if queue is empty or current thread at head
|
|
return !isFirst(current);
|
|
}
|
|
}
|
|
|
|
/**
|
|
* 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>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>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
|
|
* 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.
|
|
*
|
|
* @return the estimated number of threads waiting for this lock
|
|
*/
|
|
public final int getQueueLength() {
|
|
return sync.getQueueLength();
|
|
}
|
|
|
|
/**
|
|
* Returns a collection containing threads that may be waiting to
|
|
* acquire either the read or 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 monitoring facilities.
|
|
*
|
|
* @return the collection of threads
|
|
*/
|
|
protected Collection<Thread> getQueuedThreads() {
|
|
return sync.getQueuedThreads();
|
|
}
|
|
|
|
/**
|
|
* Queries whether any threads are waiting on the given condition
|
|
* associated with the write lock. Note that because timeouts and
|
|
* interrupts may occur at any time, a {@code true} return does
|
|
* not guarantee that a future {@code signal} will awaken any
|
|
* threads. This method is designed primarily for use in
|
|
* monitoring of the system state.
|
|
*
|
|
* @param condition the condition
|
|
* @return {@code true} if there are any waiting threads
|
|
* @throws IllegalMonitorStateException if this lock is not held
|
|
* @throws IllegalArgumentException if the given condition is
|
|
* not associated with this lock
|
|
* @throws NullPointerException if the condition is null
|
|
*/
|
|
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);
|
|
}
|
|
|
|
/**
|
|
* Returns an estimate of the number of threads waiting on the
|
|
* given condition associated with the write lock. Note that because
|
|
* timeouts and interrupts may occur at any time, the estimate
|
|
* serves only as an upper bound on the actual number of waiters.
|
|
* This method is designed for use in monitoring of the system
|
|
* state, not for synchronization control.
|
|
*
|
|
* @param condition the condition
|
|
* @return the estimated number of waiting threads
|
|
* @throws IllegalMonitorStateException if this lock is not held
|
|
* @throws IllegalArgumentException if the given condition is
|
|
* not associated with this lock
|
|
* @throws NullPointerException if the condition is null
|
|
*/
|
|
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);
|
|
}
|
|
|
|
/**
|
|
* Returns a collection containing those threads that may be
|
|
* waiting on the given condition associated with 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 condition monitoring facilities.
|
|
*
|
|
* @param condition the condition
|
|
* @return the collection of threads
|
|
* @throws IllegalMonitorStateException if this lock is not held
|
|
* @throws IllegalArgumentException if the given condition is
|
|
* not associated with this lock
|
|
* @throws NullPointerException if the condition is null
|
|
*/
|
|
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);
|
|
}
|
|
|
|
/**
|
|
* Returns a string identifying this lock, as well as its lock state.
|
|
* The state, in brackets, includes the String {@code "Write locks ="}
|
|
* followed by the number of reentrantly held write locks, and 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 c = sync.getCount();
|
|
int w = Sync.exclusiveCount(c);
|
|
int r = Sync.sharedCount(c);
|
|
|
|
return super.toString() +
|
|
"[Write locks = " + w + ", Read locks = " + r + "]";
|
|
}
|
|
|
|
}
|