Classboxes

Classboxes: A Minimal Module Model Supporting Local Class Extension Alexandre Bergel, Stéphane Ducasse, Oscar Nierstrasz, and Roel Wuyts University of Bern (Switzerland) Université Libre de Bruxelles bergel@iam.unibe.ch 1

Outline 1. Class Extension 2. Supporting Unanticipated Changes 3. The Classbox Model 4. Local Rebinding 5. Implementation 6. Conclusion 2

Class Extension Squeak UrlPack String Url asUrl • Adding a new method or redefining one on an already existing class is a class extension • Decoupling a class definition from method definitions • Relevant: HyperJ, AspectJ, MultiJava, Smalltalk, Clos, ... 3

In Smalltalk-80 • Class extensions are global • Any application can modify any class in the system • Three consequences: • Conflicts may arise (e.g., two applications bring the same extensions) • Robustness aspect (e.g., an application may redefine a critical method) • Implicit dependencies 4

In Squeak... Morph Morph bounds ^ bounds bounds ^ self Changeset that modify Morph>>bounds Result: unrecoverable crash 5

Supporting Unanticipated Changes • Java, Modula3 provide package mechanism and no class extension • AspectJ, Smalltalk, CLOS provide class extension and weak packages • How to combine module + class extension? • Scoped changes: avoiding globality and reconciling with security 6

The Classbox Model • A classbox is a unit of scoping (it behaves as a namespace). • Within a classbox: • Classes can be defined • Classes can be imported from other classboxes • Methods can be defined on any visible class • Code can be evaluated • Local redefinitions take precedence over previous definitions 7

Local Rebinding import MorphCB EnhMorphCB class extension ... Morph Morph self paint “new repaint paint implementation” ... paint Button Button Button new paint Button new paint Button new repaint Button new repaint Flatten view within a classbox 8

Properties of the Model • From within a classbox: flattened view of the world. • Local changes are as if they would have been global. • Extending some classes does not impact their clients. 9

Visibility Bounded to a Classbox import class extension MorphCB EnhMorphCB Button Button repaint “new “original paint implementation” paint implementation” OldGUI NewGUI Button Button Multiple versions of a 10 method at the same time

Limited Class Extension Impact import class extension MorphCB MorphHackCB FillInTheBlank FillInTheBlank requestPassword requestPassword ... ... Transcript show: pass ... BankAppCB FillInTheBlank SecureApplication run Original Behavior passField := FillInTheBlank requestPassword 11 is Preserved!

Implementation • In Squeak, an open-source Smalltalk. • New method lookup semantics. • No need to modify the VM. • Uses a cache mechanism. • Cache is checked in redefined methods by adding 5 extra byte-codes. • No overhead for added method invocation • Redefined method invocation (worst case): 2.5 times 12

Conclusion • Secure module system for controlling class extensions: • Control the visibility of method addition and replacement • Support for unanticipated evolution • New method lookup semantics • Use of a cache mechanism 13

The Classbox Model • • http://www.iam.unibe.ch/~scg • or Ask google.com about classbox • Information: bergel@iam.unibe.ch 14

Import Before Inheritance Rounded DoubleBuffer DoubleBuffer GraphicCB WindowCB CB AndRoundedCB update Component Component Component ... paint paint self paint "code 2" ... update super paint ... paint Window Window "code 1" paint paint super paint ... Frame Frame Frame Frame paint Frame new update => Frame new update => Frame new update => Double Buffered Double Buffered Frame new update => Normal Frame Rounded Frame Frame Rounded Frame 16

Cache Mechanism (1/4) A Class creation 17

Cache Mechanism (2/4) A foo CM1 Method addition 18

Cache Mechanism (3/4) A CM1 foo Dispatcher CM2 Method redefinition 19

Cache Mechanism (4/4) A foo cache check CM1 Dispatcher CM2’ kept in the literal frame CM2 Method execution: A new foo 20

• Model composed of 3 classes: Classbox, ClassboxSystem and Dispatcher 21

Scope of a Method import class extension MorphCB EnhMorphCB ... Button Button self paint “new repaint paint implementation” ... paint Button new repaint Button new repaint 22

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