Transcript Collaborative Applets - Florida State University

Collaborative Applets
Gordon Erlebacher
School of Computational Science &
Information Technology
FSU
7/24/2002
Pervasive Technology Labs, Indiana
University
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Caveats
I am describing my work since July 16, so
many concepts are still “rough”
Many of my explanations are simplified
Not all my descriptions will be technically
accurate, but they reflect my best
understanding at this time
7/24/2002
Pervasive Technology Labs, Indiana
University
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Problem
Multiple users wish to share a java applet
One user drives the applet: the publisher
Other users see what the first user does:
the subscribers
Users should be able to “collaborate”
– Exchange information
– See what each other is doing
– Take control of full or partial applet
7/24/2002
Pervasive Technology Labs, Indiana
University
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Approaches
Rewrite AWT to handle remote
communication
Capture system events and transfer them
to subscribers (lots of info)
Capture higher level events (ActionEvents,
etc.)
Send bitmaps (shared displays)
– Reduced user interactivity
7/24/2002
Pervasive Technology Labs, Indiana
University
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Collaborative Applets/Applications
NAWT (Rewrote AWT)
CollAWT (Rewrote AWT)
RAWT (Remote AWT from IBM)
– Client-server
JETS
– Users must use JET API in their applets
Jasmine (2001)
– Applets run in outside framework. No changes to
applet required (source not available)
– System seems to do what I require
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Pervasive Technology Labs, Indiana
University
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Collaborative Frameworks
Too many to mention
– Tango (Fox)
– Habenera
– Etc.
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Pervasive Technology Labs, Indiana
University
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Imposed Restrictions
Do not wish to rewrite AWT and Swing
classes
Users should not have to modify existing
applets (source may not be available)
Any modification to existing applets should
be minimal
Changes to applets should be sufficiently
robust to allow for automation
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Pervasive Technology Labs, Indiana
University
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User Interface Components
AWT (depends strongly on underlying
windowing system)
– Components, containers, panels, Frames,
Buttons, Textfields, Scrollbars, etc.
Swing (lightweight components)
– Written in Java
– More portable across systems
– Slower than AWT
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Pervasive Technology Labs, Indiana
University
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Publish/Subscribe
Publisher
JMS Server
Subscriber 1
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Subscriber n
Pervasive Technology Labs, Indiana
University
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Applets
isPublisher (0 or 1)
passed to applet
through <applet> tag
Publisher
Applet
Narada Server
Subscriber
Applet 1
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Subscriber
Applet n
Pervasive Technology Labs, Indiana
University
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Simple examples: AWT
GraphLayout
– Publisher
http://amur.ucs.indiana.edu/erlebach/applets/graphLayout/publish.html
– Subscriber
http://amur.ucs.indiana.edu/erlebach/applets/graphLayout/publish.html
Molecule
– Publisher
http://amur.ucs.indiana.edu/erlebach/applets/molecule/publish.html
– Subscriber
http://amur.ucs.indiana.edu/erlebach/applets/molecule/subscribe.html
7/24/2002
Pervasive Technology Labs, Indiana
University
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Event models in Java
Events have a source
Events have one or more destinations:
listeners
Listeners (event destinations) are set by
the event sources
– E.g.: a user-defined button states that a panel
in another window is listening to the action of
this button.
7/24/2002
Pervasive Technology Labs, Indiana
University
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Event Models
Top level button receives an event
The event is not transmitted to panel
below the button (Java 1.x, x > 0)
Event: time, (x,y), action, command (text
field), etc.
Event: serializable
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Pervasive Technology Labs, Indiana
University
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Life of an Event
System level events (mouse clicks, mouse
press, mouse drag, keyboard)
– Very simple: x,y, button state, key combination
Events occur over a widget
Widget translates even into EventObject or one
of its subclasses
– ActionEvent, MouseEvent, ItemEvent, etc.
– These events have increased information:
– Time of occurrence, number of clicks, command
associated with event (disable, press, highlight, etc.)
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Pervasive Technology Labs, Indiana
University
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AWT Event Model
AWTEvent
Peer Events
System Event
(mouse click)
System Queue
Native Button
Java Button
peer object
Java Button
Listener 1
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Pervasive Technology Labs, Indiana
University
Listener n
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Lightweight Component
Event Model
System Event
(mouse click)
Lightweight
Button
AWTEvent
Peer Events
Listener 1
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System queue is not used
to process peer events
System queue used to
send mouse clicks, paint
events, etc.
Lightweight components
paint themselves
Listener n
Pervasive Technology Labs, Indiana
University
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Approaches
Use Robots
– Allows application to post a system event
– System event is identical to event generated
by a mouse click or a key stroke
– Java components respond accordingly
Disadvantages of robots
– Too many events generated
– Cannot take advantage of event merging
(managed by Java System Queue)
7/24/2002
Pervasive Technology Labs, Indiana
University
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Approaches
Better Approach
– Send serialized EventObjects (root of event
hierarchy) across the network
– Recipient will transmit these events to their
destinations
Post these events to the system event queue
– Problem: event sources are not updated
E.g. text fields, selection items, etc.
– Solution (not satisfactory):
Update sources based on event and event content
Takes valuable computer time
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Pervasive Technology Labs, Indiana
University
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Other issues
EventObject contains source of event
Event source is transient: identical objects on
different platforms cannot be checked for
equality
Solution:
–
–
–
–
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Create hash tables: to each object associate an index
To each index, associate an object
Assumption: collaborating applets are identical
Send object ID along with event
Pervasive Technology Labs, Indiana
University
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How to transfer objects
(HashMap)
Object
Object ID
(1,2,…,n)
Object ID
(1,2,…,n)
Object
Hash tables on all applets have the same number of
components
– Assumes that components are not created dynamically
– I currently assume that all components have been created by
the end of the init() method
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Pervasive Technology Labs, Indiana
University
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Publish/Subscribe
Use Narada: Developed by Shrideep
Pallickara
Narada implements the JMS (Java
Messaging Service) interface
One applet is the publisher
Other applets are the subscribers
7/24/2002
Pervasive Technology Labs, Indiana
University
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Approach
Publisher sends message to server
– Serialized event + metadata (object id)
– Alternatively: byte data
Subscriber receives package from server
– Unserializes the event, sets the event source
(AWTEvent) based on the object reference obtained
from the hash table (the id is known)
– Post the message to the system queue
– Listeners receive the message and act upon it (in
exactly the same way as the publisher)
– Low level routine (jms.processEvents) checks the
source of the event and updates it accordingly
(buttons, text fields, etc.)
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Pervasive Technology Labs, Indiana
University
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JMS Messages
Byte Messages
– Arbitrary combination of bytes, ints, longs, etc.
BytesMessage msg = session.createBytesMessage();
msg.writeInt(10);
msg.writeFloat(14.7);
Object Messages
– Send the serialized object
ObjectMessage msg = session.createObjectMessage();
msg.writeObject(new ActionEvent(………));
Meta Data
– Attach string metadata
msg.setStringProperty(“eventValue”, (new Integer(value)).toString());
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Pervasive Technology Labs, Indiana
University
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Object Messages
Event e
Object o = e.getSource()
int id = hash(o)
Message m;
m.setObject(o);
m.setStringProperty(“id”, String(id)
Message m
Event e = (Event) m.getObject()
objID = m.getStringProerty(“id”)
Object obj = hash(objID)
e.setSource(obj)
SystemEventQueue.post(e)
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Pervasive Technology Labs, Indiana
University
Narada
Pseudocode
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Byte Messages
Event e
int id = hash(o)
Message m;
m.writeInt(id)
Narada
Message m
id = m.readInt()
Object obj = hash(objID)
Event e = new xxxEvent(…)
SystemEventQueue.post(e)
Pseudocode
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Pervasive Technology Labs, Indiana
University
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Event Serialization
Most classes are serializable
Only non-transient fields are serialized
Events:
– Object source (not serialized)
– Time stamp
– Modifiers (shift, meta key, etc.)
– ID (MOUSE_CLICK, MOUSE_PRESS, etc.)
– Etc.
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Pervasive Technology Labs, Indiana
University
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Performance
Object Messages
– Cost of serialization (publisher) and deserialization
(subscriber)
– Conversions between strings and ints
– Simple: format built into java Objects
Byte Messages
–
–
–
–
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Fast, no conversions
Subscriber and publisher must agree on user format
Portability is reduced in exchanged for efficiency
Still not as fast as raw sockets
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University
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Swing Hierarchy (subset)
Container
JComponent
doClick()
AbstractButton
JButton
JList
JToggleButton
JPanel
JCheckBox
JSlider
JRadioButton
JScrollPane
JTabbedPane
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Pervasive Technology Labs, Indiana
University
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Swing Applets (byte messages)
ActionEvent
– AbstractButton (objID)
ListSelectionEvent
– JList (objID, selectedIndex)
ChangeEvent
– JTabbedPane (objID, selectedIndex)
– JSlider (objID, value)
MouseEvent, MouseMotionEvent
(objID,x,y,modifiers,eventID)
– JPanel
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Pervasive Technology Labs, Indiana
University
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Listeners
AbstractButton
– ActionListener
JList
– ListSelectionListener
JSlider
– ChangeListener
JTabbedPane
– ChangeListener
JPanel:
– MouseListener, MouseMotionListener
7/24/2002
Pervasive Technology Labs, Indiana
University
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Message Arrival
javax.jms.MessageListener interface
– Implement
onMessage()
Check message object source in following
order:
– Abstract button
– JList
– JSlider
– JTabbedPane
– JPanel (for mouse events)
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Pervasive Technology Labs, Indiana
University
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Changes to existing Applet
jmsApplet jms;
naradaInitialize();
public void naradaInitialize()
try {
jms = new jmsApplet("amur.ucs.indiana.edu",
3045, isPublisher, this);
} catch (Exception e) {
e.printStackTrace();
}
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Pervasive Technology Labs, Indiana
University
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Demonstration
Publisher
- http://amur.ucs.indiana.edu/erlebach/applets/
webmap1/publish.html
Subscriber
- http://amur.ucs.indiana.edu/erlebach/applets/
webmap1/subscribe.html
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Pervasive Technology Labs, Indiana
University
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Extensions
Allow each button to be a publisher or a
subscriber
Integrate the system within a larger framework
(OKC, Anabas, Globus)
Implement floor control (allow anybody to
become a subscriber)
Floor control override by the moderator
Use JXTA for collaborative applet resource
discovery
Allow non-collaborative applets to be
automatically integrated within the framework
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Pervasive Technology Labs, Indiana
University
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TODO
How to reduce number of messages sent
(mouse moves and drags)
How to update source widgets in a more
generic fashion
How to deal with swing
What should be strategy for topic naming
Applet resource discovery mechanism
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Pervasive Technology Labs, Indiana
University
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TODO
Benchmark the system
– Use robot extension of AWT to generate
synthetic events at controlled intervals on the
publisher and measure response time of
subscribers
– Identify performance bottlenecks
Narada (JMS)
Serialization (Object Messages)
Byte encoding (Byte Messages)
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Pervasive Technology Labs, Indiana
University
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Acknowledgments
I would like to thank
Bryan Carpenter, Marlon Pierce, Shrideep
Pallickara
for enlightning discussions on various
Java and web technologies and for
increasing my proficiency with the Java
Language
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Pervasive Technology Labs, Indiana
University
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