Synchronous Active Objects Provide Robust EventDriven Applications

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Synchronous Active Objects Provide Robust
Event-Driven Applications
Claude Petitpierre, Switzerland Anton Eliëns,
The Netherlands
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1. Event driven programming provides awful
programs !2. There is at least one (good)
alternative to that style !
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Content
Assessment of event driven programming -
example (read a username and a password) Synchrono
us calls to the rescue - implementation Impact
on software engineering - design - modeling -
analysis - validation
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The Problem

• We want to get a username and then a password in
  the same field.
• The entry of the username may take any amount of
  time.
• Once the username has been typed in,
• either a password is typed in or
• the cancel button is pressed (and the demand is
  cancelled) or
• nothing happens for a predefined amount of time
  (and the demand is cancelled)

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A Specification of the Dialog
class ADialog String getNames
instantiateDialogWindow ( ) names 0
textField.getText ( ) label.setText ("password
") CHOICE names 1 textField.getText (
) OR delay ( 5000 ) OR cancel.pressed (
) return names // null if no names
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Use of the Dialog Box
main ADialog aDialog new ADialog (
) names aDialog.getNames (
) System.out.println ( names 0 names 1
)
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Implementation with Listeners
listeners
GUI elements
Application program
Instantiated from the library
Coded by the developer
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Implementation With Events ( main )
public static void main (String args ) dm
new DialogMain ( ) aDialog new ADialog ( dm
) public void continuation ( String names
) cancelButton.setEnabled (false) //
? textField.setEnabled (false) //
? System.out.println ( names 0 names 1 )
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Implementation With Events ( Timer listener )
final javax.swing.Timer timer timer new
Timer (5000, new ActionListener ( ) public
void actionPerformed (ActionEvent e)
dm.continuation (null) )
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Text Field Listener
name.addActionListener (new ActionListener ()
boolean readUsername true public void
actionPerformed (ActionEvent e) if
(readUsername) names 0 name.getText
() label.setText ("password
") readUsername false cancelButton.addAc
tionListener (new ActionListener () public
void actionPerformed (ActionEvent e)
timer.stop () de.continuation
(names) ) timer.start () else
names 1 name.getText ( ) timer.stop
() de.continuation (names) )
name.addActionListener (new ActionListener ()
boolean readUsername true public void
actionPerformed (ActionEvent e) if
(readUsername) names 0 name.getText
() label.setText ("password
") readUsername false cancelButton.addAc
tionListener (new ActionListener () public
void actionPerformed (ActionEvent e)
timer.stop () de.continuation
(names) ) timer.start () // code
executed somewhere else else names 1
name.getText ( ) timer.stop
() de.continuation (names) )
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Between the Specification and the
Implementationthe Program Control has been
Inverted
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Parallel Blocking Calls
GUI elements
the calls trigger the transitions
stub
Application program
Instantiated from the library
Coded by the developer
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Synchronous Call
main's thread
GUI thread
Listener getEvent()
actionPerformed() while ( ! trigger )
trigger true wait () 
notify ( ) trigger false
event
lisner.getEvent ( )
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Waiting for Several Events
Common listener while
(!maskguards) mask X wait () 
notify ( )
mask 0
cl.getEvent(guards)
event
event
event
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With our Proposition(identical to the spec)
class ADialog String getNames
instantiateDialogWindow () names 0
textField.read ( ) label.setText ("password
") select case names 1 textField.read (
) case delay ( 5000 ) names
null case cancel.pressed ( ) names
null return names
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Parallel Pending Calls
CHOICE names 1 textField.getText (
) OR waituntil ( now ( ) 5000 ) OR
cancel.pressed ( )
cl new CommonListener (
textField_ID, timer_ID, cancel_ID ) switch (
wait ( cl ) ) case 0 names 1
textField.getText ( ) case 1 waituntil ( now (
) 5000 ) case 2 cancel.pressed ( )
compilation
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Generalization of the concept
Is it possible to use it across a whole program,
from the accesses to the devices to the
inter-thread communications ?
Yes !
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Active Objects with Synchronous Calls

• The instantiation, references, calls, structures
  of active objects are identical to those of the
  usual (passive) objects.
• An active object has a thread executing its run
  method.It can be considered as a process.
• A synchronous active object can decide when an
  incoming call can take place. Such calls realize
  the inter-process communications.

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Object O1
Object O2
run () . . . select
case O2.send (msg) case
O3.m(x) . . .
void send (String msg) .......... run ()
accept send
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Object O1
Object O2
run () . . . select
case O2.send (msg) case
O3.m(x) . . .
void send (String msg) .......... run ()
accept send
void send (String msg) ......... run ()
accept send
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A Model for Synchronous Active Objects
method (char x, int y)
obj1.m(x)
met (int y)
hod (char x)
select case obj1.m(x) case accept
method case met
actObj.op(i)
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Multitask Kernel
kernel
List of calls and accepts
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Synchronous Calls for the GUI
public void run () kernel.await
(sButton,pressed_ID) sButton.pressed ()

kernel
class SButton extends JButton // listener
public void pressed () public SButton
( ) addActionListener (new
ActionListener() public void
actionPerformed (ActionEvent e)
accept pressed // No wait !
)
event
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Lines of Code for the Problem Above
With synchronous calls 51 LOCs
(listeners integrated in the library) With
listeners 74 LOCs With PAC pattern
(presentation-abstraction-control) 133
LOCs I found no way to provide a
good separation of the various aspects !
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Other Advantages of Synchronous Calls
The main program does not block the GUI while it
accesses remote data ! The sequence of actions
can be read directly in the code.
Disadvantages Two threads (main GUI) ?
invokeLater Performances ? enough for a GUI
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Impact upon Software Engineering
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An example ! How to use this UML diagram ?
P. Kruchten, The Rational Unified Process An
Introduction, Addison-Wesley
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Implementation with a main program
4. new session 8. PIN entered
Session manager
Card validator
2. read card
3. validate card
Card reader
1. insert card
5. "Enter PIN"
Display
9. send (validate_PIN)
Authenticator
6. get PIN
7. PIN_entered
11a. receive (PIN validity) ?
10. PIN valid ?
11b. cancel ?
Keypad
ATM Network
12. "Amount ?"
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Synchronous Objects Map onCSP, CCS, FSM, Promela

Java with synchronous objects is a specification
language as precise as Promela, for example, but
it is simultaneously an implementation
language. We are actually developing a framework
for the analysis of synchronous programs.
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Philosophers' Program
active class Phil int i //
philosopher number public Phil (int i)
this.i i run () for ()
think waituntil (now() random())
aforki.get()
afork(i1)5.get() eat waituntil
(now() random())
aforki.put()
afork(i1)5.put() Phil phil
new Phil 5
active class Fork public void get ()
public void put () run () for
() accept get accept
put Fork afork new Fork 5
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Verifying the philosophers' problem
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Conclusion

• Synchronous objects avoid the program control
  inversion introduced by event driven programming
• They make non deterministic programs as simple as
  sequential ones
• They are very close to the best known formal
  languages (CSP) and are thus prone to analysis
  and validation
• They can be implemented with the current
  libraries of Java
• They are particularly well adapted to distributed
  applications
• We have a compiler and a framework with
  synchronous libraries ( see our Web site
  http//ltiwww.epfl.ch/sJava )

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Thank you !