Download Java Layers Language Presentation

Java Layers
Language Support for Stepwise
Refinement
Rich Cardone, IBM Research & UT at Austin
Calvin Lin, University of Texas at Austin
Problem


Software development and maintenance is expensive

Difficult

Takes a long time
Assemble applications from off-the-shelf components
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Mix and match features to create applications
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Plug and unplug components to change applications
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Reuse is Key
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Separation of concerns
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One application feature per component
Flexible composition
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Presentation Overview


Part I – Motivation

Mixins

Stepwise Refinement

Drawbacks of Mixins
Part II – Java Layers
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Java Layers Overview
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Two Language Features
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An OO Problem
Car
Box
House
Lockable
Car
Lockable
Box
Lockable
House
lock(), unlock()
Problem: Lockable code replicated 3 times
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An OO Solution
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Use same lockable code for all 3 classes
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Encapsulate lockable code in a class
Subtype Car, Box, House with new class
Mixin Class
class Lockable<T> extends T {
lock(){…}
unlock(){…}
}
[Bracha90]
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Mixed-In Classes
<T>
Car
Box
House
Lockable<T>
Lockable<Car>
Lockable<Box>
Lockable<House>
Lockable code reused 3 times
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Mixins
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Types with parameterized supertypes
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Depend on type parameters
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More precisely: Parametric Polymorphism
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An OO mechanism for code reuse
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Apply same code to unrelated classes
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Work with single inheritance
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Mixins & Software Components
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Question
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If so, then mixins must support:
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Can we use mixins to build applications out of
reusable components?
Separation of concerns
Flexible composition
Let’s look at an example application
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Example: Music Server
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Variation can occur on many axes:
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Client interface
{getSong, putSong, eraseCopyright, hideBritney, …}
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Server execution strategy
{single threaded, thread-spawning, thread pool, …}
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Transport type
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Fault tolerance
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Server discovery
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…
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Music Application Instances
Simple
NoBritney
Base
Base
Base
GetSong
GetSong
EraseCopyright
PutSong
PutSong
GetSong
HideBritney
ThreadSpawn
leaf-types
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Thief
…
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Application Assembly is Easy
class Simple extends
PutSong<GetSong<Base>> {…}
class NoBritney extends
HideBritney<PutSong<GetSong<Base>>> {…}
class Thief extends
ThreadSpawn<GetSong<EraseCopyright<
Base>>> {…}
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Base Class
class Base {
static public class Client {…}
static public class Server {
void dispatchLoop(){for(;;) dispatch(readRequest());}
void dispatch(Req req){errorUnknownReq(req);}
…
}
}
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GetSong Mixin
constraint
nested mixins
class GetSong<T extends Base> extends T {
static public class Client extends T.Client {
void getSong(…){…} }
static public class Server extends T.Server {
void dispatch(Req req){
if (req.name.equals(“getSong”)) processGetSong(req);
else super.dispatch(req);
}
…
}
}
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Other Mixins
class EraseCopyright<T extends Base> extends T {
static public class Client extends T.Client {
void eraseCopyright(…){…}
} …
}
class ThreadSpawn<T extends Base> extends T {
static public class Server extends T.Server {
void dispatchLoop(){…}
} …
}
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Stepwise Program Refinement
class Thief extends
ThreadSpawn<GetSong<EraseCopyright<Base>>> {…}
Client
Server
Layers
Base
EraseCopyright
GetSong
ThreadSpawn
[Batory92]
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Drawbacks of Mixins
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Superclass initialization
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Runtime efficiency
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Leaf-type references
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Composition validation
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Semantic validity
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Syntactic correctness
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Recap
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Software components imply reuse
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Mixins reuse OO code
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Mixins build applications incrementally
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Stepwise program refinement
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Nested types encapsulate features
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Feature mixing and matching
Mixins have usability & efficiency drawbacks
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Part II – Java Layers
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Java Layers Overview
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Two JL Language Features
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Status
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Conclusion
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Goal of Java Layers
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Increase software reuse to reduce
development and maintenance costs
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Use layered, stepwise program refinement
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Encapsulate features in mixins classes
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Compose features through type instantiation
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JL’s Foundation
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Java + Constrained Parametric Polymorphism (CPP)
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There are several proposals for adding CPP to Java
[Agesen97, Bokowski98, Bracha98, Cartwright98, Myers97, Solorzano98]
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JL is a heterogeneous implementation of CPP
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Conventional syntax and semantics
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Parametric classes and interfaces
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Mixins
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The JL Language
JL is a parametric Java plus 4 features:
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Deep conformance
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Static virtual typing
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Semantic checking
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Constructor propagation
All language extensions are designed to support
stepwise refinement
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JL Compiler Support
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Class hierarchy optimization
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Remove design-time layering from runtime code
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Inline calls to superclass methods w/same signature
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Collapse class hierarchy into a single class
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The Need for Deep Conformance
Question:
class Parent {
class Inner {…}
}
Does Child contain a nested
class named Inner?
class Child
extends Parent {…}

Java supports shallow type checking
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Answer: Maybe
Interfaces and classes
JL adds support for deep type checking
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Supertypes are checked for required nested types
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Deep Conformance
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Deep Conformance supports stepwise refinement
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Enforces structural conformance at all nesting depths
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Subtypes can safely refer to nested types in their supertypes
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Feature composition is enhanced by added type precision
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Deep Conformance Example
class HideBritney<T extends Base deeply> extends deeply T {
static public class Client extends T.Client {…}
static public class Server extends T.Server {…} }
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Type parameter T binds to classes that:

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Extend Base
Contain a nested Client class that extends Base.Client
Contain a nested Server class that extends Base.Server
HideBritney contains all the public nested types of T
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Compiler generates missing nested types if necessary
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Deep Conformance Syntax
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Deeply modifier for implements and extends clauses
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Different meaning in constraint and inheritance clauses
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Operates on public nested types by default
Propagate modifier for non-public nested types
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Enables selective deep type checking
Use in parameterized and non-parameterized types
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A Use of Virtual Types
class Node {Node next;}
class Node
{virtual Node; Node next;}
class DoubleNode
extends Node
{DoubleNode prev;}
class DoubleNode extends Node
{typedef Node as DoubleNode;
DoubleNode prev;}

In DoubleNode:
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In DoubleNode:
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next is type Node
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next is type DoubleNode
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prev is type DoubleNode
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prev is type DoubleNode
[Thorup97]
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Virtual Types

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The automatic adaptation of types through inheritance.
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Virtual types change through subtyping
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A child class can change the type of its parent
Benefits of Virtual Typing
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Greater type precision
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Better type checking
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Less manual typecasting
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Genericity (Beta)
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JL’s This Virtual Type
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This pseudo-type is like the “type of this.”
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Static binding
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Used in parametric types only
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Bound at instantiation time
Enhances JL’s expressiveness
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Allows the instantiated leaf-type to be expressed
within the mixins being composed to define that
leaf-type.
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This Example
Base
class ClientFactory<T extends Base deeply>
extends T deeply {
GetSong
ClientFactory
PutSong
static public class Client
extends T.Client
{
static Client
This clientFactory()
clientFactory()
{return new This();}
Client();}
} … }
leaf-type
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Work Completed
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Implemented JL prototype
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Compared JL to OO Frameworks
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Reengineered Schmidt’s ACE
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ICSE 2001 paper
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Future Work
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Develop new JL compiler
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Implement language described here
Build a family of related applications
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Compare JL and OO approaches
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Related Work
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GenVoca – Batory92-00, Smaragdakis98-99
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Parametric Polymorphism – Agesen97, Bokowski98,
Bracha90, Bracha98, Cartwright98, Myers97, Solorzano98
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Virtual Types – Bruce97-98, Madsen89, Thorup97,
Thorup99, Torgerson98
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Semantic Checking – Batory95, Perry89-93
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Programming Paradigms – Danforth98, Gamma94,
Harrison93, Johnson91, Kiczales97, Schmidt98, Tarr99
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Conclusion
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
JL extends Java to improve reusability
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Promotes stepwise program refinement
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Assembles applications from reusable parts
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Builds on parametric polymorphism
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Adds a small number of language features
Is this approach practical for application programming?
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THE END
Think Layers
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