Class SyncCollection
- java.lang.Object
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- EDU.oswego.cs.dl.util.concurrent.SyncCollection
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- All Implemented Interfaces:
java.lang.Iterable
,java.util.Collection
public class SyncCollection extends java.lang.Object implements java.util.Collection
SyncCollections wrap Sync-based control around java.util.Collections. They are similar in operation to those provided by java.util.Collection.synchronizedCollection, but have several extended capabilities.The Collection interface is conceptually broken into two parts for purposes of synchronization control. The purely inspective reader operations are:
- size
- isEmpty
- toArray
- contains
- containsAll
- iterator
- add
- addAll
- remove
- clear
- removeAll
- retainAll
SyncCollections can be used with either Syncs or ReadWriteLocks. When used with single Syncs, the same lock is used as both the reader and writer lock. The SyncCollection class cannot itself guarantee that using a pair of read/write locks will always correctly protect objects, since Collection implementations are not precluded from internally performing hidden unprotected state changes within conceptually read-only operations. However, they do work with current java.util implementations. (Hopefully, implementations that do not provide this natural guarantee will be clearly documentented as such.)
This class provides a straight implementation of Collections interface. In order to conform to this interface, sync failures due to interruption do NOT result in InterruptedExceptions. Instead, upon detection of interruption,
- All mutative operations convert the interruption to an UnsupportedOperationException, while also propagating the interrupt status of the thread. Thus, unlike normal java.util.Collections, SyncCollections can transiently behave as if mutative operations are not supported.
- All read-only operations
attempt to return a result even upon interruption. In some contexts,
such results will be meaningless due to interference, but
provide best-effort status indications that can be useful during
recovery. The cumulative number of synchronization failures encountered
during such operations is accessible using method
synchronizationFailures()
. Non-zero values may indicate serious program errors.
The iterator() method returns a SyncCollectionIterator with properties and methods that are analogous to those of SyncCollection itself: hasNext and next are read-only, and remove is mutative. These methods allow fine-grained controlled access, but do NOT preclude concurrent modifications from being interleaved with traversals, which may lead to ConcurrentModificationExceptions. However, the class also supports method
unprotectedIterator
that can be used in conjunction with thereaderSync
orwriterSync
methods to perform locked traversals. For example, to protect a block of reads:Sync lock = coll.readerSync(); try { lock.acquire(); try { Iterator it = coll.unprotectedIterator(); while (it.hasNext()) System.out.println(it.next()); } finally { lock.release(); } } catch (InterruptedException ex) { ... }
If you need to protect blocks of writes, you must use some form of reentrant lock (for exampleReentrantLock
orReentrantWriterPreferenceReadWriteLock
) as the Sync for the collection in order to allow mutative methods to proceed while the current thread holds the lock. For example, you might need to hold a write lock during an initialization sequence:Collection c = new SyncCollection(new ArrayList(), new ReentrantWriterPreferenceReadWriteLock()); // ... c.writeLock().acquire(); try { for (...) { Object x = someStream.readObject(); c.add(x); // would block if writeLock not reentrant } } catch (IOException iox) { ... } finally { c.writeLock().release(); } catch (InterruptedException ex) { ... }
(It would normally be better practice here to not make the collection accessible until initialization is complete.)
This class does not specifically support use of timed synchronization through the attempt method. However, you can obtain this effect via the TimeoutSync class. For example:
Mutex lock = new Mutex(); TimeoutSync timedLock = new TimeoutSync(lock, 1000); // 1 sec timeouts Collection c = new SyncCollection(new HashSet(), timedlock);
The same can be done with read-write locks:
ReadWriteLock rwl = new WriterPreferenceReadWriteLock(); Sync rlock = new TimeoutSync(rwl.readLock(), 100); Sync wlock = new TimeoutSync(rwl.writeLock(), 100); Collection c = new SyncCollection(new HashSet(), rlock, wlock);
In addition to synchronization control, SyncCollections may be useful in any context requiring before/after methods surrounding collections. For example, you can use ObservableSync to arrange notifications on method calls to collections, as in:
class X { Collection c; static class CollectionObserver implements ObservableSync.SyncObserver { public void onAcquire(Object arg) { Collection coll = (Collection) arg; System.out.println("Starting operation on" + coll); // Other plausible responses include performing integrity // checks on the collection, updating displays, etc } public void onRelease(Object arg) { Collection coll = (Collection) arg; System.out.println("Finished operation on" + coll); } } X() { ObservableSync s = new ObservableSync(); c = new SyncCollection(new HashSet(), s); s.setNotificationArgument(c); CollectionObserver obs = new CollectionObserver(); s.attach(obs); } ... }
- See Also:
LayeredSync
,TimeoutSync
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Nested Class Summary
Nested Classes Modifier and Type Class Description class
SyncCollection.SyncCollectionIterator
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Field Summary
Fields Modifier and Type Field Description protected java.util.Collection
c_
protected Sync
rd_
protected SynchronizedLong
syncFailures_
protected Sync
wr_
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Constructor Summary
Constructors Constructor Description SyncCollection(java.util.Collection collection, ReadWriteLock rwl)
Create a new SyncCollection protecting the given collection, and using the given ReadWriteLock to control reader and writer methods.SyncCollection(java.util.Collection collection, Sync sync)
Create a new SyncCollection protecting the given collection, and using the given sync to control both reader and writer methods.SyncCollection(java.util.Collection collection, Sync readLock, Sync writeLock)
Create a new SyncCollection protecting the given collection, and using the given pair of locks to control reader and writer methods.
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Method Summary
All Methods Instance Methods Concrete Methods Modifier and Type Method Description boolean
add(java.lang.Object o)
boolean
addAll(java.util.Collection coll)
protected void
afterRead(boolean wasInterrupted)
Clean up after a reader operationprotected boolean
beforeRead()
Try to acquire sync before a reader operation; record failurevoid
clear()
boolean
contains(java.lang.Object o)
boolean
containsAll(java.util.Collection coll)
boolean
isEmpty()
java.util.Iterator
iterator()
Sync
readerSync()
Return the Sync object managing read-only operationsboolean
remove(java.lang.Object o)
boolean
removeAll(java.util.Collection coll)
boolean
retainAll(java.util.Collection coll)
int
size()
long
syncFailures()
Return the number of synchronization failures for read-only operationsjava.lang.Object[]
toArray()
java.lang.Object[]
toArray(java.lang.Object[] a)
java.util.Iterator
unprotectedIterator()
Return the base iterator of the underlying collectionSync
writerSync()
Return the Sync object managing mutative operations-
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
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Field Detail
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c_
protected final java.util.Collection c_
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rd_
protected final Sync rd_
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wr_
protected final Sync wr_
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syncFailures_
protected final SynchronizedLong syncFailures_
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Constructor Detail
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SyncCollection
public SyncCollection(java.util.Collection collection, Sync sync)
Create a new SyncCollection protecting the given collection, and using the given sync to control both reader and writer methods. Common, reasonable choices for the sync argument include Mutex, ReentrantLock, and Semaphores initialized to 1.Sample Usage
Collection c = new SyncCollection(new ArrayList(), new Mutex());
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SyncCollection
public SyncCollection(java.util.Collection collection, ReadWriteLock rwl)
Create a new SyncCollection protecting the given collection, and using the given ReadWriteLock to control reader and writer methods.Sample Usage
Collection c = new SyncCollection(new HashSet(), new WriterPreferenceReadWriteLock());
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Method Detail
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readerSync
public Sync readerSync()
Return the Sync object managing read-only operations
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writerSync
public Sync writerSync()
Return the Sync object managing mutative operations
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syncFailures
public long syncFailures()
Return the number of synchronization failures for read-only operations
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beforeRead
protected boolean beforeRead()
Try to acquire sync before a reader operation; record failure
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afterRead
protected void afterRead(boolean wasInterrupted)
Clean up after a reader operation
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size
public int size()
- Specified by:
size
in interfacejava.util.Collection
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isEmpty
public boolean isEmpty()
- Specified by:
isEmpty
in interfacejava.util.Collection
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contains
public boolean contains(java.lang.Object o)
- Specified by:
contains
in interfacejava.util.Collection
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toArray
public java.lang.Object[] toArray()
- Specified by:
toArray
in interfacejava.util.Collection
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toArray
public java.lang.Object[] toArray(java.lang.Object[] a)
- Specified by:
toArray
in interfacejava.util.Collection
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containsAll
public boolean containsAll(java.util.Collection coll)
- Specified by:
containsAll
in interfacejava.util.Collection
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add
public boolean add(java.lang.Object o)
- Specified by:
add
in interfacejava.util.Collection
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remove
public boolean remove(java.lang.Object o)
- Specified by:
remove
in interfacejava.util.Collection
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addAll
public boolean addAll(java.util.Collection coll)
- Specified by:
addAll
in interfacejava.util.Collection
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removeAll
public boolean removeAll(java.util.Collection coll)
- Specified by:
removeAll
in interfacejava.util.Collection
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retainAll
public boolean retainAll(java.util.Collection coll)
- Specified by:
retainAll
in interfacejava.util.Collection
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clear
public void clear()
- Specified by:
clear
in interfacejava.util.Collection
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unprotectedIterator
public java.util.Iterator unprotectedIterator()
Return the base iterator of the underlying collection
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iterator
public java.util.Iterator iterator()
- Specified by:
iterator
in interfacejava.util.Collection
- Specified by:
iterator
in interfacejava.lang.Iterable
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