* Concrete subclasses must implement the {@link #doDestroy()} method instead of
* {@link Destroyable#destroy()}; the behavior of that method should match
* exactly the behavior desired of destroy().
*
* Note that if a {@link DestroyFailedException} occurs when the timeout
* expires, it will not be reported.
*
* @see Destroyable
*/
public abstract class ExpirableObject
implements Destroyable
{
/**
* The default timeout, used in the default constructor.
*/
public static final long DEFAULT_TIMEOUT = 3600000L;
/**
* The timer that expires instances.
*/
private static final Timer EXPIRER = new Timer(true);
/**
* A reference to the task that will destroy this object when the timeout
* expires.
*/
private final Destroyer destroyer;
/**
* Create a new expirable object that will expire after one hour.
*/
protected ExpirableObject()
{
this(DEFAULT_TIMEOUT);
}
/**
* Create a new expirable object that will expire after the specified timeout.
*
* @param delay The delay before expiration.
* @throws IllegalArgumentException If delay is negative, or if
* delay + System.currentTimeMillis() is negative.
*/
protected ExpirableObject(final long delay)
{
destroyer = new Destroyer(this);
EXPIRER.schedule(destroyer, delay);
}
/**
* Destroys this object. This method calls {@link #doDestroy}, then, if no
* exception is thrown, cancels the task that would destroy this object when
* the timeout is reached.
*
* @throws DestroyFailedException If this operation fails.
*/
public final void destroy() throws DestroyFailedException
{
doDestroy();
destroyer.cancel();
}
/**
* Subclasses must implement this method instead of the {@link
* Destroyable#destroy()} method.
*
* @throws DestroyFailedException If this operation fails.
*/
protected abstract void doDestroy() throws DestroyFailedException;
/**
* The task that destroys the target when the timeout elapses.
*/
private final class Destroyer
extends TimerTask
{
private final ExpirableObject target;
Destroyer(final ExpirableObject target)
{
super();
this.target = target;
}
public void run()
{
try
{
if (! target.isDestroyed())
target.doDestroy();
}
catch (DestroyFailedException dfe)
{
}
}
}
}