An implementation of
ReadWriteLock
supporting similar
semantics to
ReentrantLock
.
This class has the following properties:
- Acquisition order
This class does not impose a reader or writer preference
ordering for lock access. However, it does support an optional
fairness policy.
- Non-fair mode (default)
- When constructed as non-fair (the default), the order of entry
to the read and write lock is unspecified, subject to reentrancy
constraints. A nonfair lock that is continously contended may
indefinitely postpone one or more reader or writer threads, but
will normally have higher throughput than a fair lock.
- Fair mode
- When constructed as fair, threads contend for entry using an
approximately arrival-order policy. When the currently held lock
is released either the longest-waiting single writer thread will
be assigned the write lock, or if there is a group of reader threads
waiting longer than all waiting writer threads, that group will be
assigned the read lock.
A thread that tries to acquire a fair read lock (non-reentrantly)
will block if either the write lock is held, or there is a waiting
writer thread. The thread will not acquire the read lock until
after the oldest currently waiting writer thread has acquired and
released the write lock. Of course, if a waiting writer abandons
its wait, leaving one or more reader threads as the longest waiters
in the queue with the write lock free, then those readers will be
assigned the read lock.
A thread that tries to acquire a fair write lock (non-reentrantly)
will block unless both the read lock and write lock are free (which
implies there are no waiting threads). (Note that the non-blocking
ReentrantReadWriteLock.ReadLock.tryLock()
and ReentrantReadWriteLock.WriteLock.tryLock()
methods
do not honor this fair setting and will acquire the lock if it is
possible, regardless of waiting threads.)
- Reentrancy
This lock allows both readers and writers to reacquire read or
write locks in the style of a ReentrantLock
. Non-reentrant
readers are not allowed until all write locks held by the writing
thread have been released.
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.
- Lock downgrading
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
not possible.
- Interruption of lock acquisition
The read lock and write lock both support interruption during lock
acquisition.
Condition
support
The write lock provides a Condition
implementation that
behaves in the same way, with respect to the write lock, as the
Condition
implementation provided by
ReentrantLock.newCondition()
does for ReentrantLock
.
This Condition
can, of course, only be used with the write lock.
The read lock does not support a Condition
and
readLock().newCondition()
throws
UnsupportedOperationException
.
- Instrumentation
This class supports methods to determine whether locks
are held or contended. These methods are designed for monitoring
system state, not for synchronization control.
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.
Sample usages. Here is a code sketch showing how to exploit
reentrancy to perform lock downgrading after updating a cache (exception
handling is elided for simplicity):
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();
}
}
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.
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(); }
}
}
Implementation Notes
This lock supports a maximum of 65535 recursive write locks
and 65535 read locks. Attempts to exceed these limits result in
Error
throws from locking methods.