EDU.oswego.cs.dl.util.concurrent
Class ObservableSync

java.lang.Object
  EDU.oswego.cs.dl.util.concurrent.ObservableSync

All Implemented Interfaces:

Sync

public class ObservableSync
extends java.lang.Object
implements Sync

The ObservableSync class performs no synchronization itself, but invokes event-style messages to other observer objects upon invocation of Sync methods. These observers typically perform monitoring, logging, or other bookkeeping operations surrounding the object being managed by this Sync object.

Because ObservableSync does not itself perform any synchronization control, the attempt operation always succeeds. This class is typically used (via LayeredSync) as a wrapper around those that do perform synchronization control.

This class is based around a standard Observer design pattern. It is not hard to convert this to instead use a Listener design (as seen in AWT and JavaBeans), by defining associated EventObjects and forwarding them.

[ Introduction to this package. ]

See Also:

LayeredSync

Nested Class Summary

static interface

ObservableSync.SyncObserver Interface for objects that observe ObservableSyncs.

Field Summary

protected java.lang.Object	arg_
protected CopyOnWriteArraySet	observers_

Fields inherited from interface EDU.oswego.cs.dl.util.concurrent.Sync

ONE_CENTURY, ONE_DAY, ONE_HOUR, ONE_MINUTE, ONE_SECOND, ONE_WEEK, ONE_YEAR

Constructor Summary

ObservableSync(java.lang.Object notificationArgument)
Create an ObservableSync that uses the supplied argument for all notifications.

Method Summary

void	acquire() Wait (possibly forever) until successful passage.
void	attach(ObservableSync.SyncObserver obs) Add obs to the set of observers
boolean	attempt(long msecs) Wait at most msecs to pass; report whether passed.
void	detach(ObservableSync.SyncObserver obs) Remove obs from the set of observers.
java.lang.Object	getNotificationArgument() Return the argument used for notifications
java.util.Iterator	observers() Return an iterator that can be used to traverse through current set of observers
void	release() Potentially enable others to pass.
java.lang.Object	setNotificationArgument(java.lang.Object notificationArg) Set the argument used for notifications.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

observers_

protected final CopyOnWriteArraySet observers_

arg_

protected java.lang.Object arg_

Constructor Detail

ObservableSync

public ObservableSync(java.lang.Object notificationArgument)

Create an ObservableSync that uses the supplied argument for all notifications. The argument is typically an object that is being managed by this Sync object.

Method Detail

getNotificationArgument

public java.lang.Object getNotificationArgument()

Return the argument used for notifications

setNotificationArgument

public java.lang.Object setNotificationArgument(java.lang.Object notificationArg)

Set the argument used for notifications.

Returns:

the previous value of this argument

acquire

public void acquire()

Description copied from interface: Sync

Wait (possibly forever) until successful passage. Fail only upon interuption. Interruptions always result in `clean' failures. On failure, you can be sure that it has not been acquired, and that no corresponding release should be performed. Conversely, a normal return guarantees that the acquire was successful.

Specified by:

acquire in interface Sync

attempt

public boolean attempt(long msecs)

Description copied from interface: Sync

Wait at most msecs to pass; report whether passed.

The method has best-effort semantics: The msecs bound cannot be guaranteed to be a precise upper bound on wait time in Java. Implementations generally can only attempt to return as soon as possible after the specified bound. Also, timers in Java do not stop during garbage collection, so timeouts can occur just because a GC intervened. So, msecs arguments should be used in a coarse-grained manner. Further, implementations cannot always guarantee that this method will return at all without blocking indefinitely when used in unintended ways. For example, deadlocks may be encountered when called in an unintended context.

Specified by:

attempt in interface Sync

Parameters:

msecs - the number of milleseconds to wait. An argument less than or equal to zero means not to wait at all. However, this may still require access to a synchronization lock, which can impose unbounded delay if there is a lot of contention among threads.

Returns:

true if acquired

release

public void release()

Description copied from interface: Sync

Potentially enable others to pass.

Because release does not raise exceptions, it can be used in `finally' clauses without requiring extra embedded try/catch blocks. But keep in mind that as with any java method, implementations may still throw unchecked exceptions such as Error or NullPointerException when faced with uncontinuable errors. However, these should normally only be caught by higher-level error handlers.

Specified by:

release in interface Sync

attach

public void attach(ObservableSync.SyncObserver obs)

Add obs to the set of observers

detach

public void detach(ObservableSync.SyncObserver obs)

Remove obs from the set of observers. No effect if not in set

observers

public java.util.Iterator observers()

Return an iterator that can be used to traverse through current set of observers