narya/src/java/com/threerings/presents/dobj/DObject.java

//
// $Id$
//
// Narya library - tools for developing networked games
// Copyright (C) 2002-2007 Three Rings Design, Inc., All Rights Reserved
// http://www.threerings.net/code/narya/
//
// This library is free software; you can redistribute it and/or modify it
// under the terms of the GNU Lesser General Public License as published
// by the Free Software Foundation; either version 2.1 of the License, or
// (at your option) any later version.
//
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
// Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public
// License along with this library; if not, write to the Free Software
// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA

package com.threerings.presents.dobj;

import java.lang.reflect.Field;
import java.util.Arrays;
import java.util.Comparator;
import java.util.Map;

import com.google.common.collect.Maps;

import com.samskivert.util.ListUtil;
import com.samskivert.util.StringUtil;

import com.threerings.io.Streamable;

import com.threerings.presents.net.Transport;

import static com.threerings.presents.Log.log;

/**
 * The distributed object forms the foundation of the Presents system. All information shared among
 * users of the system is done via distributed objects. A distributed object has a set of
 * listeners. These listeners have access to the object or a proxy of the object and therefore have
 * access to the data stored in the object's members at all times.
 *
 * <p> Additionally, an object has a set of subscribers. Subscribers manage the lifespan of the
 * object; while a subscriber is subscribed, the listeners registered with an object will be
 * notified of events. When the subscriber unsubscribes, the object becomes non-live and the
 * listeners are no longer notified. <em>Note:</em> on the server, object subscription is merely a
 * formality as all objects remain live all the time, so <em>do not</em> depend on event
 * notifications ceasing when a subscriber has relinquished its subscription. Always unregister all
 * listeners when they no longer need to hear from an object.
 *
 * <p> When there is any change to the the object's fields data, an event is generated which is
 * dispatched to all listeners of the object, notifying them of that change and effecting that
 * change to the copy of the object maintained at each client. In this way, both a repository of
 * shared information and a mechanism for asynchronous notification are made available as a
 * fundamental application building blocks.
 *
 * <p> 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 <code>DObject</code> by a script. A declaration looks something like this:
 *
 * <pre>
 * public dclass RoomObject
 * {
 *     public String description;
 *     public int[] occupants;
 * }
 * </pre>
 *
 * which is converted into an actual Java class that looks like this:
 *
 * <pre>
 * 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)
 *     {
 *         // ...
 *     }
 * }
 * </pre>
 *
 * These method calls on the actual distributed object will result in the proper attribute change
 * events being generated and dispatched.
 *
 * <p> Note that distributed object fields can be any of the following set of primitive types:
 *
 * <code><pre>
 * boolean, byte, short, int, long, float, double
 * Boolean, Byte, Short, Integer, Long, Float, Double, String
 * boolean[], byte[], short[], int[], long[], float[], double[], String[]
 * </pre></code>
 *
 * Fields of type {@link Streamable} can also be used.
 */
public class DObject
    implements Streamable
{
    public DObject ()
    {
        _fields = _ftable.get(getClass());
        if (_fields == null) {
            _fields = getClass().getFields();
            Arrays.sort(_fields, FIELD_COMP);
            _ftable.put(getClass(), _fields);
        }
    }

    /**
     * Returns the object id of this object. All objects in the system have a unique object id.
     */
    public int getOid ()
    {
        return _oid;
    }

    /**
     * Returns the dobject manager under the auspices of which this object operates. This could be
     * <code>null</code> if the object is not active.
     */
    public DObjectManager getManager ()
    {
        return _omgr;
    }

    /**
     * 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
     * <code>objectAvailable</code> 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.testAndAddRef(_subs, sub);
        if (subs != null) {
//             Log.info("Adding subscriber " + which() + ": " + sub + ".");
            _subs = subs;
            _scount++;

        } else {
            log.warning("Refusing subscriber that's already in the list", "dobj", which(),
                        "subscriber", sub, new Exception());
        }
    }

    /**
     * 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
     * <code>objectAvailable</code> or to a listener callback).
     *
     * @see DObjectManager#unsubscribeFromObject
     */
    public void removeSubscriber (Subscriber<?> sub)
    {
        if (ListUtil.clearRef(_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 && _omgr != null) {
                _omgr.removedLastSubscriber(this, _deathWish);
            }
        }
    }

    /**
     * Instructs this object to request to have a fork stuck in it when its last subscriber is
     * removed.
     */
    public void setDestroyOnLastSubscriberRemoved (boolean deathWish)
    {
        _deathWish = deathWish;
    }

    /**
     * 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.
     *
     * <p> 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, <em>and</em> 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.testAndAddRef(_listeners, listener);
        if (els != null) {
            _listeners = els;
        } else {
            log.warning("Refusing repeat listener registration [dobj=" + which() +
                        ", list=" + listener + "]", new Exception());
        }
    }

    /**
     * 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.clearRef(_listeners, listener);
    }

    /**
     * Provides this object with an entity that can be used to validate subscription requests and
     * events before they are processed. The access controller is handy for ensuring that clients
     * are behaving as expected and for preventing impermissible modifications or event dispatches
     * on a distributed object.
     */
    public void setAccessController (AccessController controller)
    {
        _controller = controller;
    }

    /**
     * Returns a reference to the access controller in use by this object or null if none has been
     * configured.
     */
    public AccessController getAccessController ()
    {
        return _controller;
    }

    /**
     * Get the DSet with the specified name.
     */
    public final <T extends DSet.Entry> DSet<T> getSet (String setName)
    {
        try {
            @SuppressWarnings("unchecked") DSet<T> casted = (DSet<T>)getField(setName).get(this);
            return casted;
        } catch (Exception e) {
            throw new IllegalArgumentException("No such set: " + setName);
        }
    }

    /**
     * Request to have the specified item added to the specified DSet.
     */
    public <T extends DSet.Entry> void addToSet (String setName, T entry)
    {
        requestEntryAdd(setName, getSet(setName), entry);
    }

    /**
     * Request to have the specified item updated in the specified DSet.
     */
    public void updateSet (String setName, DSet.Entry entry)
    {
        requestEntryUpdate(setName, getSet(setName), entry);
    }

    /**
     * Request to have the specified key removed from the specified DSet.
     */
    public void removeFromSet (String setName, Comparable<?> key)
    {
        requestEntryRemove(setName, getSet(setName), key);
    }

    /**
     * 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
     * <code>releaseLock</code>) 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 <code>acquireLock</code> 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
        postEvent(new ReleaseLockEvent(_oid, name));
    }

    /**
     * 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 <code>acquireLock</code> you should be using <code>releaseLock</code>.
     *
     * @see #acquireLock
     * @see #releaseLock
     */
    protected void clearLock (String name)
    {
        // clear the lock from the list
        if (ListUtil.clear(_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 ()
    {
        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. If an {@link AccessController} has been
     * specified for this object, it will be used to determine whether or not to allow the
     * subscription request. If no controller is set, the subscription will be allowed.
     *
     * @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)
    {
        if (_controller != null) {
            return _controller.allowSubscribe(this, sub);
        } else {
            return true;
        }
    }

    /**
     * Checks to ensure that this event which is about to be processed, has the appropriate
     * permissions. If an {@link AccessController} has been specified for this object, it will be
     * used to determine whether or not to allow the even dispatch. If no controller is set, all
     * events are allowed.
     *
     * @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)
    {
        if (_controller != null) {
            return _controller.allowDispatch(this, event);
        } else {
            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 listeners registered with this object.
     *
     * @param event the event that was just applied.
     */
    public void notifyListeners (DEvent event)
    {
        if (_listeners == null) {
            return;
        }

        int llength = _listeners.length;
        for (int i = 0; i < llength; i++) {
            Object listener = _listeners[i];
            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 [list=" + listener +
                            ", event=" + event + "].", e);
            }
        }
    }

    /**
     * Called by the distributed object manager after it has applied an event to this object. This
     * dispatches an event notification to all of the proxy listeners registered with this object.
     *
     * @param event the event that was just applied.
     */
    public void notifyProxies (DEvent event)
    {
        if (_subs == null || event.isPrivate()) {
            return;
        }

        for (Object sub : _subs) {
            try {
                if (sub != null && sub instanceof ProxySubscriber) {
                    ((ProxySubscriber)sub).eventReceived(event);
                }
            } catch (Exception e) {
                log.warning("Proxy choked during notification", "sub", sub, "event", event, e);
            }
        }
    }

    /**
     * Requests that the specified attribute be changed to the specified value. Normally the
     * generated setter methods should be used but in rare cases a caller may wish to update
     * distributed fields in a generic manner.
     */
    public void changeAttribute (String name, Object value)
        throws ObjectAccessException
    {
        try {
            Field f = getField(name);
            requestAttributeChange(name, value, f.get(this));
            f.set(this, value);

        } catch (Exception e) {
            String errmsg = "changeAttribute() failure [name=" + name + ", value=" + value +
                ", vclass=" + value.getClass().getName() + "].";
            throw new ObjectAccessException(errmsg, 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 {
            getField(name).set(this, value);

        } catch (Exception e) {
            String errmsg = "setAttribute() failure [name=" + name + ", value=" + value +
                ", vclass=" + value.getClass().getName() + "].";
            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 getField(name).get(this);
        } catch (Exception e) {
            throw new ObjectAccessException("getAttribute() failure [name=" + name + "].", e);
        }
    }

    /**
     * Posts a message event on this distributed object.
     */
    public void postMessage (String name, Object... args)
    {
        postMessage(Transport.DEFAULT, name, args);
    }

    /**
     * Posts a message event on this distributed object.
     *
     * @param transport a hint as to the type of transport desired for the message.
     */
    public void postMessage (Transport transport, String name, Object... args)
    {
        postEvent(new MessageEvent(_oid, name, args, transport));
    }

    /**
     * Posts the specified event either to our dobject manager or to the compound event for which
     * we are currently transacting.
     */
    public void postEvent (DEvent event)
    {
        if (_tevent != null) {
            _tevent.postEvent(event);

        } else if (_omgr != null) {
            _omgr.postEvent(event);

        } else {
            log.info("Dropping event for non- or no longer managed object [oid=" + getOid() +
                     ", class=" + getClass().getName() + ", event=" + event + "].");
        }
    }

    /**
     * Returns true if this object is active and registered with the distributed object system. If
     * an object is created via <code>DObjectManager.createObject</code> it will be active until
     * such time as it is destroyed.
     */
    public final boolean isActive ()
    {
        return _omgr != 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 RootDObjectManager#registerObject(DObject)
     */
    public void setManager (DObjectManager omgr)
    {
        _omgr = omgr;
    }

    /**
     * Don't call this function. It is called by the distributed object manager when an object is
     * created and registered with the system.
     *
     * @see RootDObjectManager#registerObject(DObject)
     */
    public void setOid (int oid)
    {
        _oid = oid;
    }

    /**
     * Generates a concise string representation of this object.
     */
    public String which ()
    {
        StringBuilder buf = new StringBuilder();
        which(buf);
        return buf.toString();
    }

    @Override // from Object
    public String toString ()
    {
        StringBuilder buf = new StringBuilder();
        toString(buf);
        return buf.append("]").toString();
    }

    /**
     * Used to briefly describe this distributed object.
     */
    protected void which (StringBuilder buf)
    {
        buf.append(StringUtil.shortClassName(this));
        buf.append(":").append(_oid);
    }

    /**
     * Generates a string representation of this object.
     */
    protected void toString (StringBuilder buf)
    {
        StringUtil.fieldsToString(buf, this, "\n");
        if (buf.length() > 0) {
            buf.insert(0, "\n");
        }
        buf.insert(0, _oid);
        buf.insert(0, "[oid=");
    }

    /**
     * Begins a transaction on this distributed object. In some situations, it is desirable to
     * cause multiple changes to distributed object fields in one unified operation. Starting a
     * transaction causes all subsequent field modifications to be stored in a single compound
     * event which can then be committed, dispatching and applying all included events in a single
     * group. Additionally, the events are dispatched over the network in a single unit which can
     * significantly enhance network efficiency.
     *
     * <p> When the transaction is complete, the caller must call {@link #commitTransaction} or
     * {@link CompoundEvent#commit} to commit the transaction and release the object back to its
     * normal non-transacting state. If the caller decides not to commit their transaction, they
     * must call {@link #cancelTransaction} or {@link CompoundEvent#cancel} to cancel the
     * transaction. Failure to do so will cause the pooch to be totally screwed.
     *
     * <p> Note: like all other distributed object operations, transactions are not thread safe. It
     * is expected that a single thread will handle all distributed object operations and that
     * thread will begin and complete a transaction before giving up control to unknown code which
     * might try to operate on the transacting distributed object.
     *
     * <p> Note also: if the object is already engaged in a transaction, a transaction participant
     * count will be incremented to note that an additional call to {@link #commitTransaction} is
     * required before the transaction should actually be committed. Thus <em>every</em> call to
     * {@link #startTransaction} must be accompanied by a call to either {@link #commitTransaction}
     * or {@link #cancelTransaction}. Additionally, if any transaction participant cancels the
     * transaction, the entire transaction is cancelled for all participants, regardless of whether
     * the other participants attempted to commit the transaction.
     */
    public void startTransaction ()
    {
        if (_tevent != null) {
            _tcount++;
        } else {
            _tevent = new CompoundEvent(this, _omgr);
        }
    }

    /**
     * Commits the transaction in which this distributed object is involved.
     *
     * @see CompoundEvent#commit
     */
    public void commitTransaction ()
    {
        if (_tevent == null) {
            String errmsg = "Cannot commit: not involved in a transaction [dobj=" + this + "]";
            throw new IllegalStateException(errmsg);
        }

        // if we are nested, we decrement our nesting count rather than committing the transaction
        if (_tcount > 0) {
            _tcount--;

        } else {
            // we may actually be doing our final commit after someone already cancelled this
            // transaction, so we need to perform the appropriate action at this point
            if (_tcancelled) {
                _tevent.cancel();
            } else {
                _tevent.commit();
            }
        }
    }

    /**
     * Returns true if this object is in the middle of a transaction or false if it is not.
     */
    public boolean inTransaction ()
    {
        return (_tevent != null);
    }

    /**
     * Cancels the transaction in which this distributed object is involved.
     *
     * @see CompoundEvent#cancel
     */
    public void cancelTransaction ()
    {
        if (_tevent == null) {
            String errmsg = "Cannot cancel: not involved in a transaction [dobj=" + this + "]";
            throw new IllegalStateException(errmsg);
        }

        // if we're in a nested transaction, make a note that it is to be cancelled when all
        // parties commit and decrement the nest count
        if (_tcount > 0) {
            _tcancelled = true;
            _tcount--;

        } else {
            _tevent.cancel();
        }
    }

    /**
     * Removes this object from participation in any transaction in which it might be taking part.
     */
    protected void clearTransaction ()
    {
        // sanity check
        if (_tcount != 0) {
            log.warning("Transaction cleared with non-zero nesting count", "dobj", this);
            _tcount = 0;
        }

        // clear our transaction state
        _tevent = null;
        _tcancelled = false;
    }

    /**
     * Called by derived instances when an attribute setter method was called.
     */
    protected void requestAttributeChange (String name, Object value, Object oldValue)
    {
        requestAttributeChange(name, value, oldValue, Transport.DEFAULT);
    }

    /**
     * Called by derived instances when an attribute setter method was called.
     */
    protected void requestAttributeChange (
        String name, Object value, Object oldValue, Transport transport)
    {
        // dispatch an attribute changed event
        postEvent(new AttributeChangedEvent(_oid, name, value, oldValue, transport));
    }

    /**
     * Called by derived instances when an element updater method was called.
     */
    protected void requestElementUpdate (String name, int index, Object value, Object oldValue)
    {
        requestElementUpdate(name, index, value, oldValue, Transport.DEFAULT);
    }

    /**
     * Called by derived instances when an element updater method was called.
     */
    protected void requestElementUpdate (
        String name, int index, Object value, Object oldValue, Transport transport)
    {
        // dispatch an attribute changed event
        postEvent(new ElementUpdatedEvent(
                      _oid, name, value, oldValue, index, transport));
    }

    /**
     * Calls by derived instances when an oid adder method was called.
     */
    protected void requestOidAdd (String name, int oid)
    {
        // dispatch an object added event
        postEvent(new ObjectAddedEvent(_oid, name, oid));
    }

    /**
     * Calls by derived instances when an oid remover method was called.
     */
    protected void requestOidRemove (String name, int oid)
    {
        // dispatch an object removed event
        postEvent(new ObjectRemovedEvent(_oid, name, oid));
    }

    /**
     * Calls by derived instances when a set adder method was called.
     */
    protected <T extends DSet.Entry> void requestEntryAdd (String name, DSet<T> set, T entry)
    {
        // if we're on the authoritative server, we update the set immediately
        boolean alreadyApplied = false;
        if (_omgr != null && _omgr.isManager(this)) {
            set.add(entry);
            alreadyApplied = true;
        }
        // dispatch an entry added event
        postEvent(new EntryAddedEvent<T>(_oid, name, entry, alreadyApplied));
    }

    /**
     * Calls by derived instances when a set remover method was called.
     */
    protected <T extends DSet.Entry> void requestEntryRemove (
        String name, DSet<T> set, Comparable<?> key)
    {
        // if we're on the authoritative server, we update the set immediately
        T oldEntry = null;
        if (_omgr != null && _omgr.isManager(this)) {
            oldEntry = set.removeKey(key);
            if (oldEntry == null) {
                log.warning("Requested to remove non-element", "set", name, "key", key,
                            new Exception());
            }
        }
        // dispatch an entry removed event
        postEvent(new EntryRemovedEvent<T>(_oid, name, key, oldEntry));
    }

    /**
     * Calls by derived instances when a set updater method was called.
     */
    protected <T extends DSet.Entry> void requestEntryUpdate (String name, DSet<T> set, T entry)
    {
        requestEntryUpdate(name, set, entry, Transport.DEFAULT);
    }

    /**
     * Calls by derived instances when a set updater method was called.
     */
    protected <T extends DSet.Entry> void requestEntryUpdate (
        String name, DSet<T> set, T entry, Transport transport)
    {
        // if we're on the authoritative server, we update the set immediately
        T oldEntry = null;
        if (_omgr != null && _omgr.isManager(this)) {
            oldEntry = set.update(entry);
            if (oldEntry == null) {
                log.warning("Set update had no old entry", "name", name, "entry", entry,
                            new Exception());
            }
        }
        // dispatch an entry updated event
        postEvent(new EntryUpdatedEvent<T>(_oid, name, entry, oldEntry, transport));
    }

    /**
     * Returns the {@link Field} with the specified name or null if there is none such.
     */
    protected final Field getField (String name)
    {
        int low = 0, high = _fields.length-1;
        while (low <= high) {
            int mid = (low + high) >> 1;
            Field midVal = _fields[mid];
            int cmp = midVal.getName().compareTo(name);
            if (cmp < 0) {
                low = mid + 1;
            } else if (cmp > 0) {
                high = mid - 1;
            } else {
                return midVal; // key found
            }
        }
        return null; // key not found.
    }

    /** Our object id. */
    protected int _oid;

    /** An array of our fields, sorted for efficient lookup. */
    protected transient Field[] _fields;

    /** A reference to our object manager. */
    protected transient DObjectManager _omgr;

    /** The entity that tells us if an event or subscription request should be allowed. */
    protected transient AccessController _controller;

    /** A list of outstanding locks. */
    protected transient Object[] _locks;

    /** Our subscribers list. */
    protected transient Object[] _subs;

    /** Our event listeners list. */
    protected transient Object[] _listeners;

    /** Our subscriber count. */
    protected transient int _scount;

    /** The compound event associated with our transaction, if we're currently in a transaction. */
    protected transient CompoundEvent _tevent;

    /** The nesting depth of our current transaction. */
    protected transient int _tcount;

    /** Whether or not our nested transaction has been cancelled. */
    protected transient boolean _tcancelled;

    /** Indicates whether we want to be destroyed when our last subscriber is removed. */
    protected transient boolean _deathWish = false;

    /** Maintains a mapping of sorted field arrays for each distributed object class. */
    protected static Map<Class<?>, Field[]> _ftable = Maps.newHashMap();

    /** Used to sort and search {@link #_fields}. */
    protected static final Comparator<Field> FIELD_COMP = new Comparator<Field>() {
        public int compare (Field f1, Field f2) {
            return f1.getName().compareTo(f2.getName());
        }
    };
}