Synchronous Active Objects Introduce CSP's Primitives in Java

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Synchronous Active Objects Introduce CSP's
Primitives in Java
Claude Petitpierre, Switzerland
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1. One reason commercial applications are so
badly designed lies in the use of the observer
design pattern !
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2. The CSP alternation can be used instead
!3. Provided it can mix inputs and outputs !
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Content
Assessment of event driven programming -
example Synchronous calls to the rescue - same
example Impact on software engineering -
design - modeling - analysis - validation
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Example Handling a Dialog Box
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The problem

• We want to develop a module (object) to get a
  username and then a password from 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 internal functioning of
the Dialog Box module
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 module (interface)
main ADialog aDialog new ADialog (
) names aDialog.getNames (
) System.out.println ( names 0 names 1
)
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Usual Treatment with Listeners and Events
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The observer design pattern
the user types enter
text field
subject
listener
modification required by another part of the
program
event
modify
observer
update data representation
subject
The main program has disappeared !!
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Implementation with events ( main )
public class DialogMain public static void main
(String args ) dm new DialogMain (
) aDialog new ADialog ( dm ) public void
continuation ( String names ) // continue
here after the events of the dialog System.out.pri
ntln ( names 0 names 1 )
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Implementation with events ( Timer listener )
timer new Timer (5000, new ActionListener ( )
public void actionPerformed (ActionEvent e)
dm.continuation (null) )
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In the 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 () dm.continuation
(names) ) timer.start () else
names 1 name.getText ( ) timer.stop
() dm.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 () dm.continuation
(names) ) timer.start () // code
executed somewhere else else names 1
name.getText ( ) timer.stop
() dm.continuation (names) )
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Rectifier
GUI thread
main's thread
Listener getEvent()
actionPerformed() while ( ! trigger )
trigger true wait () 
notify ( ) trigger false
event
lisnr.getEvent ( )
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Waiting for Several Events
Common listener while
(!maskguards) mask X wait () 
notify ( )
reset(mask)
cl.getEvent(guards)
event
event
event
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Between the specification and the
implementationthe program control has been
inverted !The alternation makes it possible
to avoid this inversion !
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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
void send (String msg) .......... run ()
accept send
void send (String msg) ......... run ()
accept send
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Dialog Box Solution using rendezvous in an
alternation as implemented in our language
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The event triggers a pending synchronous call in
a select
active class ADialog String getNames( )
instantiateDialogWindow ( ) names 0
textField.getText ( ) label.setText
("password ") select case names 1
textField.getText ( ) case delay ( 5000
) case cancel.pressed ( ) return
names
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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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Reintroducing the main
GUI
Select
stub
skeleton
The select makes listeners unnecessary.
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Guarded outputs are indispensable !
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Debugging
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An Example
Ex1 ma ? t mb ?
Ex2 ma ? mb ? ex1.ma !
Ex3 ex1.ma ! ma ? ex2.ma !
Ex4 ex1.ma ! ex2.mb ! ex3.ma ! t
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Program Analysis
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A Deadlock
a2 select m ? a2.m !
a2.m !
a1 select m ? a2.m !
a2.m !
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State diagram obtained with our tool
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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 well known formal
  languages (CSP, CCS ) and are thus prone to
  analysis and validation
• They can use the current libraries of Java
• They are particularly well adapted to distributed
  applications
• We have a compiler and a framework with
  synchronous libraries on our Web site
  http//ltiwww.epfl.ch/sJava

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Thank you !