When there is any change to that data, initiated by one of the * subscribers, an event is generated which is dispatched to all * subscribers of the object, notifying them of that change and affecting * that change to the copy of the object maintained at each client. In * this way, both a respository of shared information and a mechanism for * asynchronous notification are made available as a fundamental * application building blocks. * *
To define what information is shared, an application creates a
* distributed object declaration which is much like a class declaration
* except that it is transformed into a proper derived class of
* DObject by a script. A declaration looks something like
* this:
*
*
* public dclass RoomObject
* {
* public String description;
* public int[] occupants;
* }
*
*
* which is converted into an actual Java class that looks like this:
*
*
* public class RoomObject extends DObject
* {
* public String getDescription ()
* {
* // ...
* }
*
* public void setDescription (String description)
* {
* // ...
* }
*
* public int[] getOccupants ()
* {
* // ...
* }
*
* public void setOccupants (int[] occupants)
* {
* // ...
* }
*
* public void setOccupantsAt (int index, int value)
* {
* // ...
* }
* }
*
*
* These method calls on the actual distributed object will result in the
* proper attribute change events being generated and dispatched.
*
* Note that distributed object fields can only be of a limited set of
* supported types. These types are:
*
*
* byte, short, int, long, float, double
* Byte, Short, Integer, Long, Float, Double, String
* byte[], short[], int[], long[], float[], double[], String[]
*
objectAvailable or to a listener callback).
*
* @see DObjectManager#subscribeToObject
*/
public void addSubscriber (Subscriber sub)
{
// only add the subscriber if they're not already there
Object[] subs = ListUtil.testAndAdd(_subs, sub);
if (subs != null) {
_subs = subs;
_scount++;
}
}
/**
* Don't call this function! Go through the distributed object manager
* instead to ensure that everything is done on the proper thread.
* This function can only safely be called directly when you know you
* are operating on the omgr thread (you are in the middle of a call
* to objectAvailable or to a listener callback).
*
* @see DObjectManager#unsubscribeFromObject
*/
public void removeSubscriber (Subscriber sub)
{
if (ListUtil.clear(_subs, sub) != null) {
// if we removed something, check to see if we just removed
// the last subscriber from our list; we also want to be sure
// that we're still active otherwise there's no need to notify
// our objmgr because we don't have one
if (--_scount == 0 && _mgr != null) {
_mgr.removedLastSubscriber(this);
}
}
}
/**
* Adds an event listener to this object. The listener will be
* notified when any events are dispatched on this object that match
* their particular listener interface.
*
*
Note that the entity adding itself as a listener should have
* obtained the object reference by subscribing to it or should be
* acting on behalf of some other entity that subscribed to the
* object, and that it must be sure to remove itself from the
* listener list (via {@link #removeListener}) when it is done because
* unsubscribing from the object (done by whatever entity subscribed
* in the first place) is not guaranteed to result in the listeners
* added through that subscription being automatically removed (in
* most cases, they definitely will not be removed).
*
* @param listener the listener to be added.
*
* @see EventListener
* @see AttributeChangeListener
* @see SetListener
* @see OidListListener
*/
public void addListener (ChangeListener listener)
{
// only add the listener if they're not already there
Object[] els = ListUtil.testAndAdd(_listeners, listener);
if (els != null) {
_listeners = els;
}
}
/**
* Removes an event listener from this object. The listener will no
* longer be notified when events are dispatched on this object.
*
* @param listener the listener to be removed.
*/
public void removeListener (ChangeListener listener)
{
ListUtil.clear(_listeners, listener);
}
/**
* At times, an entity on the server may need to ensure that events it
* has queued up have made it through the event queue and are applied
* to their respective objects before a service may safely be
* undertaken again. To make this possible, it can acquire a lock on a
* distributed object, generate the events in question and then
* release the lock (via a call to releaseLock) which
* will queue up a final event, the processing of which will release
* the lock. Thus the lock will not be released until all of the
* previously generated events have been processed. If the service is
* invoked again before that lock is released, the associated call to
* acquireLock will fail and the code can respond
* accordingly. An object may have any number of outstanding locks as
* long as they each have a unique name.
*
* @param name the name of the lock to acquire.
*
* @return true if the lock was acquired, false if the lock was not
* acquired because it has not yet been released from a previous
* acquisition.
*
* @see #releaseLock
*/
public boolean acquireLock (String name)
{
// check for the existence of the lock in the list and add it if
// it's not already there
Object[] list = ListUtil.testAndAdd(_locks, name);
if (list == null) {
// a null list means the object was already in the list
return false;
} else {
// a non-null list means the object was added
_locks = list;
return true;
}
}
/**
* Queues up an event that when processed will release the lock of the
* specified name.
*
* @see #acquireLock
*/
public void releaseLock (String name)
{
// queue up a release lock event
ReleaseLockEvent event = new ReleaseLockEvent(_oid, name);
_mgr.postEvent(event);
}
/**
* Don't call this function! It is called by a remove lock event when
* that event is processed and shouldn't be called at any other time.
* If you mean to release a lock that was acquired with
* acquireLock you should be using
* releaseLock.
*
* @see #acquireLock
* @see #releaseLock
*/
protected void clearLock (String name)
{
// clear the lock from the list
if (ListUtil.clearEqual(_locks, name) == null) {
// complain if we didn't find the lock
Log.info("Unable to clear non-existent lock [lock=" + name +
", dobj=" + this + "].");
}
}
/**
* Requests that this distributed object be destroyed. It does so by
* queueing up an object destroyed event which the server will
* validate and process.
*/
public void destroy ()
{
_mgr.postEvent(new ObjectDestroyedEvent(_oid));
}
/**
* Checks to ensure that the specified subscriber has access to this
* object. This will be called before satisfying a subscription
* request. By default objects are accessible to all subscribers, but
* certain objects may wish to implement more fine grained access
* control.
*
* @param sub the subscriber that will subscribe to this object.
*
* @return true if the subscriber has access to the object, false if
* they do not.
*/
public boolean checkPermissions (Subscriber sub)
{
return true;
}
/**
* Checks to ensure that this event which is about to be processed,
* has the appropriate permissions. By default objects accept all
* manner of events, but certain objects may wish to implement more
* fine grained access control.
*
* @param event the event that will be dispatched, object permitting.
*
* @return true if the event is valid and should be dispatched, false
* if the event fails the permissions check and should be aborted.
*/
public boolean checkPermissions (DEvent event)
{
return true;
}
/**
* Called by the distributed object manager after it has applied an
* event to this object. This dispatches an event notification to all
* of the subscribers of this object.
*
* @param event the event that was just applied.
*/
public void notifyListeners (DEvent event)
{
// if we have no listeners, we're home free
if (_listeners == null) {
return;
}
// iterate over the listener list, performing the necessary
// notifications
int llength = _listeners.length;
for (int i = 0; i < llength; i++) {
Object listener = _listeners[i];
// skip empty slots
if (listener == null) {
continue;
}
try {
// do any event specific notifications
event.notifyListener(listener);
// and notify them if they are listening for all events
if (listener instanceof EventListener) {
((EventListener)listener).eventReceived(event);
}
} catch (Exception e) {
Log.warning("Listener choked during notification " +
"[listener=" + listener +
", event=" + event + "].");
Log.logStackTrace(e);
}
}
}
/**
* Sets the named attribute to the specified value. This is only used
* by the internals of the event dispatch mechanism and should not be
* called directly by users. Use the generated attribute setter
* methods instead.
*/
public void setAttribute (String name, Object value)
throws ObjectAccessException
{
try {
// for values that contain other values (arrays and DSets), we
// need to clone them before putting them in the object
// because otherwise a subsequent event might come along and
// modify these values before the networking thread has had a
// chance to propagate this event to the clients
// i wish i could just call value.clone() but Object declares
// clone() to be inaccessible, so we must cast the values to
// their actual types to gain access to the widened clone()
// methods
if (value instanceof DSet) {
value = ((DSet)value).clone();
} else if (value instanceof int[]) {
value = ((int[])value).clone();
} else if (value instanceof String[]) {
value = ((String[])value).clone();
} else if (value instanceof byte[]) {
value = ((byte[])value).clone();
} else if (value instanceof long[]) {
value = ((long[])value).clone();
} else if (value instanceof float[]) {
value = ((float[])value).clone();
} else if (value instanceof short[]) {
value = ((short[])value).clone();
} else if (value instanceof double[]) {
value = ((double[])value).clone();
}
// now actually set the value
getClass().getField(name).set(this, value);
} catch (Exception e) {
String errmsg = "Attribute setting failure [name=" + name +
", value=" + value + "].";
throw new ObjectAccessException(errmsg, e);
}
}
/**
* Looks up the named attribute and returns a reference to it. This
* should only be used by the internals of the event dispatch
* mechanism and should not be called directly by users. Use the
* generated attribute getter methods instead.
*/
public Object getAttribute (String name)
throws ObjectAccessException
{
try {
return getClass().getField(name).get(this);
} catch (Exception e) {
String errmsg = "Attribute getting failure [name=" + name + "].";
throw new ObjectAccessException(errmsg, e);
}
}
/**
* Returns true if this object is active and registered with the
* distributed object system. If an object is created via
* DObjectManager.createObject it will be active until
* such time as it is destroyed.
*/
public boolean isActive ()
{
return _mgr != null;
}
/**
* Don't call this function! It initializes this distributed object
* with the supplied distributed object manager. This is called by the
* distributed object manager when an object is created and registered
* with the system.
*
* @see DObjectManager#createObject
*/
public void setManager (DObjectManager mgr)
{
_mgr = mgr;
}
/**
* Don't call this function. It is called by the distributed object
* manager when an object is created and registered with the system.
*
* @see DObjectManager#createObject
*/
public void setOid (int oid)
{
_oid = oid;
}
/**
* Generates a string representation of this object.
*/
public String toString ()
{
StringBuffer buf = new StringBuffer();
buf.append("[oid=").append(_oid).append(", ");
StringUtil.fieldsToString(buf, this);
return buf.append("]").toString();
}
/**
* Called by derived instances when an attribute setter method was
* called.
*/
protected void requestAttributeChange (String name, Object value)
{
// generate an attribute changed event
DEvent event = new AttributeChangedEvent(_oid, name, value);
// and dispatch it to our dobjmgr
_mgr.postEvent(event);
}
/**
* Calls by derived instances when an oid adder method was called.
*/
protected void requestOidAdd (String name, int oid)
{
// generate an object added event
DEvent event = new ObjectAddedEvent(_oid, name, oid);
// and dispatch it to our dobjmgr
_mgr.postEvent(event);
}
/**
* Calls by derived instances when an oid remover method was called.
*/
protected void requestOidRemove (String name, int oid)
{
// generate an object removed event
DEvent event = new ObjectRemovedEvent(_oid, name, oid);
// and dispatch it to our dobjmgr
_mgr.postEvent(event);
}
/**
* Calls by derived instances when a set adder method was called.
*/
protected void requestElementAdd (String name, DSet.Element elem)
{
try {
DSet set = (DSet)getAttribute(name);
// generate an element added event
DEvent event = new ElementAddedEvent(
_oid, name, elem, !set.homogenous());
// and dispatch it to our dobjmgr
_mgr.postEvent(event);
} catch (ObjectAccessException oae) {
Log.warning("Unable to request elementAdd [name=" + name +
", elem=" + elem + ", error=" + oae + "].");
}
}
/**
* Calls by derived instances when a set remover method was called.
*/
protected void requestElementRemove (String name, Object key)
{
// generate an element removed event
DEvent event = new ElementRemovedEvent(_oid, name, key);
// and dispatch it to our dobjmgr
_mgr.postEvent(event);
}
/**
* Calls by derived instances when a set updater method was called.
*/
protected void requestElementUpdate (String name, DSet.Element elem)
{
try {
DSet set = (DSet)getAttribute(name);
// generate an element updated event
DEvent event = new ElementUpdatedEvent(
_oid, name, elem, !set.homogenous());
// and dispatch it to our dobjmgr
_mgr.postEvent(event);
} catch (ObjectAccessException oae) {
Log.warning("Unable to request elementUpdate [name=" + name +
", elem=" + elem + ", error=" + oae + "].");
}
}
/** Our object id. */
protected int _oid;
/** A reference to our object manager. */
protected DObjectManager _mgr;
/** A list of outstanding locks. */
protected Object[] _locks;
/** Our subscribers list. */
protected Object[] _subs;
/** Our event listeners list. */
protected Object[] _listeners;
/** Our subscriber count. */
protected int _scount;
}