EDU.oswego.cs.dl.util.concurrent

Class SyncCollection

public class SyncCollection extends Object implements 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:

The possibly mutative writer operations (which are also the set of operations that are allowed to throw UnsupportedOperationException) are:

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,

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);
   }
   ...
 }
 

[ Introduction to this package. ]

See Also: LayeredSync

Nested Class Summary
classSyncCollection.SyncCollectionIterator
Field Summary
protected Collectionc_
protected Syncrd_
protected SynchronizedLongsyncFailures_
protected Syncwr_
Constructor Summary
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, 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 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
booleanadd(Object o)
booleanaddAll(Collection coll)
protected voidafterRead(boolean wasInterrupted)
Clean up after a reader operation *
protected booleanbeforeRead()
Try to acquire sync before a reader operation; record failure *
voidclear()
booleancontains(Object o)
booleancontainsAll(Collection coll)
booleanisEmpty()
Iteratoriterator()
SyncreaderSync()
Return the Sync object managing read-only operations
booleanremove(Object o)
booleanremoveAll(Collection coll)
booleanretainAll(Collection coll)
intsize()
longsyncFailures()
Return the number of synchronization failures for read-only operations
Object[]toArray()
Object[]toArray(Object[] a)
IteratorunprotectedIterator()
Return the base iterator of the underlying collection *
SyncwriterSync()
Return the Sync object managing mutative operations

Field Detail

c_

protected final Collection c_

rd_

protected final Sync rd_

syncFailures_

protected final SynchronizedLong syncFailures_

wr_

protected final Sync wr_

Constructor Detail

SyncCollection

public 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. 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()); 
 

SyncCollection

public SyncCollection(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());
 

SyncCollection

public 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 Detail

add

public boolean add(Object o)

addAll

public boolean addAll(Collection coll)

afterRead

protected void afterRead(boolean wasInterrupted)
Clean up after a reader operation *

beforeRead

protected boolean beforeRead()
Try to acquire sync before a reader operation; record failure *

clear

public void clear()

contains

public boolean contains(Object o)

containsAll

public boolean containsAll(Collection coll)

isEmpty

public boolean isEmpty()

iterator

public Iterator iterator()

readerSync

public Sync readerSync()
Return the Sync object managing read-only operations

remove

public boolean remove(Object o)

removeAll

public boolean removeAll(Collection coll)

retainAll

public boolean retainAll(Collection coll)

size

public int size()

syncFailures

public long syncFailures()
Return the number of synchronization failures for read-only operations

toArray

public Object[] toArray()

toArray

public Object[] toArray(Object[] a)

unprotectedIterator

public Iterator unprotectedIterator()
Return the base iterator of the underlying collection *

writerSync

public Sync writerSync()
Return the Sync object managing mutative operations