Class SyncCollection
- All Implemented Interfaces:
Iterable
,Collection
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 the readerSync
or
writerSync
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 example
ReentrantLock
or ReentrantWriterPreferenceReadWriteLock
) 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:
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Nested Class Summary
Nested Classes -
Field Summary
FieldsModifier and TypeFieldDescriptionprotected final Collection
protected final Sync
protected final SynchronizedLong
protected final Sync
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Constructor Summary
ConstructorsConstructorDescriptionSyncCollection
(Collection collection, ReadWriteLock rwl) Create a new SyncCollection protecting the given collection, and using the given ReadWriteLock to control reader and writer methods.SyncCollection
(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
(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. -
Method Summary
Modifier and TypeMethodDescriptionboolean
boolean
addAll
(Collection coll) protected void
afterRead
(boolean wasInterrupted) Clean up after a reader operationprotected boolean
Try to acquire sync before a reader operation; record failurevoid
clear()
boolean
boolean
containsAll
(Collection coll) boolean
isEmpty()
iterator()
Return the Sync object managing read-only operationsboolean
boolean
removeAll
(Collection coll) boolean
retainAll
(Collection coll) int
size()
long
Return the number of synchronization failures for read-only operationsObject[]
toArray()
Object[]
Return the base iterator of the underlying collectionReturn the Sync object managing mutative operationsMethods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
Methods inherited from interface java.util.Collection
equals, hashCode, parallelStream, removeIf, spliterator, stream, toArray
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Field Details
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c_
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rd_
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wr_
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syncFailures_
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Constructor Details
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SyncCollection
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
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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SyncCollection
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 Details
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readerSync
Return the Sync object managing read-only operations -
writerSync
Return the Sync object managing mutative operations -
syncFailures
public long syncFailures()Return the number of synchronization failures for read-only operations -
beforeRead
protected boolean beforeRead()Try to acquire sync before a reader operation; record failure -
afterRead
protected void afterRead(boolean wasInterrupted) Clean up after a reader operation -
size
public int size()- Specified by:
size
in interfaceCollection
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isEmpty
public boolean isEmpty()- Specified by:
isEmpty
in interfaceCollection
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contains
- Specified by:
contains
in interfaceCollection
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toArray
- Specified by:
toArray
in interfaceCollection
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toArray
- Specified by:
toArray
in interfaceCollection
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containsAll
- Specified by:
containsAll
in interfaceCollection
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add
- Specified by:
add
in interfaceCollection
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remove
- Specified by:
remove
in interfaceCollection
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addAll
- Specified by:
addAll
in interfaceCollection
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removeAll
- Specified by:
removeAll
in interfaceCollection
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retainAll
- Specified by:
retainAll
in interfaceCollection
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clear
public void clear()- Specified by:
clear
in interfaceCollection
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unprotectedIterator
Return the base iterator of the underlying collection -
iterator
- Specified by:
iterator
in interfaceCollection
- Specified by:
iterator
in interfaceIterable
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