Seminarium on component based software engineering
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Seminarium on Component-based Software Engineering. Callbacks and object re-entrance. Marcello Bonsangue LIACS – Leiden University Fall 2004. Libraries. A library is a collection of procedures It is always the client to call the library

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Seminarium on Component-based Software Engineering

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Seminarium onComponent-based Software Engineering

Callbacks and

object re-entrance

Marcello Bonsangue

LIACS – Leiden University

Fall 2004


  • A library is a collection of procedures

  • It is always the client to call the library

  • Library operations run to completion before returning control

    Only states before and after a call are relevant to clients, library intermediate states are not observable

Seminarium CBSE


  • A callback is a procedure that can be registered to a library so that the library can call it back

    • Example: registration of client code to be called when library catch user events

  • They are a mechanism for extensibility

  • Callbacks operate on the intermediate state of a library!!

Seminarium CBSE

Callback: message sequence chart




Client installs callback

User calls library

Library invokes callback

Callback queries library

Callback returns



Library returns

Seminarium CBSE

A Directory service example

  • The directory service supports callbacks to notify clients about changes in the managed directory

  • A client can use this mechanism to implement a visual display of the directory content.

Seminarium CBSE

LIBRARY Directory


Notifier=PROCEDURE(IN name:Name);

PROCEDURE ThisFile(n:Name): Files.File;

(*pre n <>“” *)

(*post result=file named n OR (result=NIL and no such file *)

PROCEDURE addEntry(n:Name,f:Files.File);

(* pre n<>“” and f<>NIL *)

(* post ThisFile(n)=f *)

PROCEDURE RemoveEntry(n:Name);

(* pre n<>”” *)

(* post ThisFile(n)=NIL *)

PROCEDURE RegisterNotifier(n:Notifier);

(* pre n<>NIL *)

(* post n registered, will be called on AddEntry and RemoveEntry *)

PROCEDURE UnregisterNotifier(n:Notifier);

(* pre n<>NIL *)

(* post n unregistered, will not be called anymore *);

DirectoryDisplay (V 1.0)

IMPORT Directory;

PROCEDURE Notifer(IN n:DirectoryName);


IF Directory.ThisFile(n) = NIL THEN

(*entry under name n has been removed –

delete n in display *)


(* entry has been added under name n – include n in display *)

END Notifier;




The Directory service details

Seminarium CBSE

The Directory contract ambiguity

  • What happens on redefinition of an entry?

    • Notifier is not called …

      • Redefine uses thisfile (that does not call notifier)

    • …called once …

      • Redefine uses addEntry (that calls notifiers)

    • …twice

      • Redefine uses removeEntry and addEntry (and both call notifier)

  • The contract should specify who call notifier

Seminarium CBSE

The Directory contract ambiguity

  • Is the notifier called before or after the directory is updated?

    • It is only sensible to call it after the update.

  • It should be in the contract

Seminarium CBSE

LIBRARY Directory

PROCEDURE addEntry(n:Name,f:Files.File);


Locate entry named n

If not found append a new entry to the files list in this directory

Else replace old binding with new file

Call all registered notifiers

END AddEntry;

DirectoryDisplay (V 1.1)

IMPORT Directory;

PROCEDURE Notifier(IN n:Directory.Name);


IF Directory.ThisFile(n) = NIL THEN

(* entry under name n has been removed –

delete n in display *)


IF n = “untitled” THEN



(* entry has been added under name n include in display *)



END Notifier;




A new Directory display client

Seminarium CBSE

Broken contract

  • Request of a user to create file “untitled”

  • Notifier() is called by the directory

  • File is removed by Notifier()

  • The AddEntry() post condition


    is broken











Notifier returns

RemoveEntry returns

Notifier returns

AddEntry returns

Seminarium CBSE

What is the problem

  • The contract of the library does not says that notifiers do not have to change the file passed

    • Difficult to express as it is a transitive problem

  • Solutions:

    • Incorporates histories of interactions in specifications

      • Undecidable or very complex

    • Use state test functions in specifications

      • Test whether a particular operations is available

Seminarium CBSE

Example: Java security manager use test function to protect critical services from being called by untrusted services.

LIBRARY Directory


(* pre true *)

(* post a notifier call is in progress *)

PROCEDURE AddEntry(IN n:name:Name;f:Files.file();

(* pre (not InNotifier()and n<>”” and f<>NIL *)

(* post ThisFile(n) = f *)

PROCEDURE RemoveEntry(IN n:Name);

(* pre not InNotifier() *)

(* post ThisFile(n)=NIL *)

Test functions

Seminarium CBSE

Callbacks in objects

  • We can structure an application in layers. Object can references other objects on different layers:

    • Above the referenced object

      • Method invocation = regular call

    • Below the referenced object

      • Method invocation = up-call

  • Every method invoked as an up-call is a possible callback.

Seminarium CBSE

A text processing example

  • A text model is a sequence of characters that supports random access to read,insert and delete characters

  • Text observers can register with the model and will be notified whenever a character is inserted or deleted

  • A text viewer extends an observer and provides visualization

Seminarium CBSE


method write(int pos,char ch);

//[len:int;txt:array of char;

//pre len=this.length();

(all i:0<=i<len:txt[i] and

len< this.max() and 0<=pos<=len

//post this.length() = len +1

and (all i:0<=i<[i])

and = ch

and (all i: pos<=i<=this.length():[i-1]

method delete(int pos);

// delete char at pos pos and shift all successive char back to one

method max(): int;

//pre true

//post result=maximum length this text instance can have

method length():int

//pre true

//post 0<=result<=this.max() and result=length of text

method read(int pos):char;

//pre 0<=pos<=this.length()

//post result=character at position pos

method register(TextObserver x);

method unregister(TextObserver x);

The TextModel

Seminarium CBSE

INTERFACE TextObserver

method insertNotification(int pos);

//pre character at position pos has just been inserted

method deleteNotification(int pos);

//pre character that was at position pos has been deleted


method text():TextModel

//pre true

//post result <> null

method caretPos():int

//pre true

//post 0<=result<=this.text().length()

method type(char ch)

//pre caret:=this.caretPos():this.text().length() < this.text().max()

//post this.caretPos()=caret+1 and post(this.text().write(caret,ch))

method setCaret(int pos);

//pre 0<=pos<=this.text().length()

//post this.caretPos()=pos

method posFromCoord(int x,int y);

//pre (x,y) is valid screen coordinate

//post result=x,y-coordinates corresponding to text position pos

method insertNotification(int pos);

//post display updated and this.caretPos()=pos+1

method deleteNotification(int pos);

//post display updated and this.caretPos()=pos

The TextObserver and TextView

Seminarium CBSE

Inserting a character

Text Model

Text View

  • Any callback?





(Remove caret mark)

(Update text display)

(Redisplay caret mark)

Seminarium CBSE

Inserting a character (revised)

  • It reflects better the situation:

    TextView depends on the state of TextModel

    • See method Type

Text View

Text Model





(Remove caret mark)

(Update text display)

(Redisplay caret mark)

Seminarium CBSE

Inter-object consistency

  • Control is transferred to other objects while still updating. In the example:

    • Display calls TextView’s map methods

      • But they can be called by other objects too!

    • Multiple observers could be registered

      • Which one gains control of display?

  • Consistency can be achieved by a disciplined programming

    • semaphores

    • synchronized methods

    • monitors

Seminarium CBSE

Intra-object consistency

  • The real problem: Re-entrance

  • Re-entrance = an object method is invoked while another is still executing.

  • Methods have the right to change the state of their object, but what if it is called on an object that is not in a consistent state?

Seminarium CBSE

Class Repository

mystore :Store

max = 100

method register(store)

mystore:= store

method add (elm)

if store.check(max) then

// code for adding elm to repository

Object re-entrance

Class Store

var size:Int := 0

rep :Repository

method insert(elm int);

size:= size+1


method check (max)

if size < max return true

method notify(repository:Repository):Store

rep:= repository









(Redisplay caret mark)

Seminarium CBSE

Re-entrance (continued)

  • MyStore in in inconsistent state when the message check(max) is received

  • Obvious solution:

    • Switch the command lines

      size:= size+1


      in the code of method insert(elm int)

  • It requires inspection of code, but components must be independently verifiable based on their contracts at interfaces

Seminarium CBSE

Self-interference & re-entrance

  • Leads to inconsistent object state

  • Re-entrance is necessary

    • Recursive calls (direct or indirect)

  • As for callbacks it can be addressed by:

    • Introducing test functions for clients

    • Adding time-stamps to the state

    • Allowing histories of communication in specification

    • Not using method invocation

Seminarium CBSE

Multithreading and re-entrance

  • Locking doesn’t solve the re-entrance problem

  • When strict re-entrance is solved, but leads to self-inflicted deadlocks

  • When relaxed(as in Java), suffers by re-entrance as in single threaded systems

Seminarium CBSE


  • Alternative to specification contracts

  • Histories are sequences of messages

  • Restricting the permissible traces we can get valid state transitions

  • Permissible traces are hard to be checked dynamically

Seminarium CBSE

Without method invocation

  • Explicit dependencies between state and events

    • Remember UML state machine



book on the shelf

book on loan by obj


Seminarium CBSE

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