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A Framework for Implementing Pluggable Type Systems

JavaCOP. A Framework for Implementing Pluggable Type Systems. Shane Arkus Todd Millstein University of California. Chris Andreae James Noble Victoria University of Wellington. OOPSLA 2006, Oregon, Portland. Types are good …. Type-checkers help the programmer

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A Framework for Implementing Pluggable Type Systems

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  1. JavaCOP A Framework for ImplementingPluggable Type Systems Shane Arkus Todd Millstein University of California Chris Andreae James Noble Victoria Universityof Wellington OOPSLA 2006, Oregon, Portland

  2. Types are good… • Type-checkers help the programmer • But, they don’t always help enough! • Existing type systems are not always sufficient: • Non-null references • Read-only references • Confined types • Style checking • …

  3. …But not all the time • Type systems are not always relevant: • Particularly restrictive systems • Domain-specific or limited applicability • Conflicting type systems • Too many systems exist to use at once: @Nonnull @Readonly @Borrowed @LockedBy(“this”) @TransactionAttribute(MANDATORY) @ReadEffect(“foo”) @WriteEffect(“bar”) public String<Owner> fetchString(@Nonnull Integer<World> which){...

  4. Pluggable Types • Bracha [2004] introduced “Pluggable Types” : • Type systems should be optional • No effect on the runtime semantics • Type annotations not mandatory in syntax • Type systems should be pluggable • Multiple type systems for different needs • JAVACOP • Express these constraints • Check and enforce the constraints on programs

  5. Running Example – motivation • protection of access to variables, methods, classes is enabled • No mechanism to ensure that references to protected object do not escape • Security example Returns a reference to the actual array! private Identity[] signers; public Identity[] getSigners() { return signers; }

  6. Confined types prevents leaks of sensitive object references from outside a certain protection domain.

  7. Confined Class Declaration • Confined types can not be public public @confined class C { … }; + rule ConfinedClassDef(ClassDef c){ where(confined(c)){ require(!c.getSym().isPublic ()): error(c, "Confined class may not be public"); } c.java:4: Confined class may not be public public@confined class C ^ 1 error

  8. JavaCOP System - Overview JavaCOP Rules JavaCOP.Compiler Compiled constraints (Java class) Errors andWarnings JavaCOP.Framework javaccompiler Compiled Program JavaProgram

  9. JavaCOP and Javac javac compiler JavaCOP rules AST of the program Java program with new types

  10. AST Node types Tree Field or method selection, i.e., o.field or o.method(); + type : Type Instance creation, like new World(“Hello”); + pos : int Method call, like meth(args); Select MethodDef NewClass + selected : Tree + name Assign Apply ClassDef + lhs : Tree + meth : Tree + rhs : Tree + args …

  11. JavaCOP Object Model • An abstraction of javac AST • Hides irrelevant structures • Augmented with typing and information on program entities: • Type nodes • Symbol nodes • Env structs • Globals …

  12. The Rule Language rules declares requirements Compiled constraints JavaCOP.Compiler Errors andWarnings JavaCOP.Framework javaccompiler Compiled Program JavaProgram • JavaCOP.Language source file is a set of rules • Each rule • applies to a particular kind of AST node • an advice on a static joinpoint in the AST traversal program • Has a set of constraints • If fail, error/warning is emitted • Expressive language • Pattern matching, quantifiers, conditionals…

  13. Conditional Constraints • If a method is public or protected and is confined, it must be in a confined class MethodDef node type rule ConfinedMethodDef(MethodDef m){ where((m.isPublic() || m.isProtected()) && confined(m.type().restype)){ require(confined(m.enclClass())): error(m, "Confined non private method return type in unconfined class”) }} } Condition Primitive constraint Failure clause

  14. Pattern Matching rules • Conditions on structural patterns. • Example – Anonymity preserving • method calls with an (explicit or implicit) receiver of this are either to static methods or to other anonymous methods. rule AnonCallsAnon(Apply a){ where(anonymous(env.enclMethod ){ where(a.meth => [this.*] || a.meth => [*]){ require(a.meth.isStatic || anonymous(a.meth)): error(i, "’this ’ may not be used to "+ "call non -anonymous methods."); }}}

  15. Quantification Rules • Constrain properties of collection of objects • Lists, AST subtrees • Similar to Java 5 enhanced for loop • Example- confined class as extension • A confined class may not extend an unconfined type. rule ConfinedClassDef2(ClassDef c){ where(confined(c)){ forall(Type s : c.supertypes ()){ require(confined(s) || s == globals.objectType): error(c, "A confined class may not extend " + "an unconfined superclass");}}}

  16. Subtype Rules • Subtyping is fundamental to Java’s type system • Subtype Rules allow declaring subtyping relations • Example- a value of confined type cannot be viewed at an unconfined supertype • Applies to AST nodes change the type of an expression • Assignment, parameter passing, result returning, type casts. • Passed: • type of the expression • supertype it must conform to • AST node where the test occurs rule checkConfined(a <: b @ t){ where(confined(a)){ require(confined(b)): error(t, "Confined type "+a +" may not be treated as a subtype of" +"unconfined type "+b); }}

  17. Predicates Declarations • Describe complex properties without repeating. • Readable • Reusable • Invoked by the bodies of rules and other predicates declare anonymous(Tree t){ require(t.holdsSymbol && t.sym.hasAnnotation("Anon")); }

  18. Disjunctive Constraints using Predicates Can have multiple predicates with same signature. Prolog like The predicate holds if any of its definitions hold Example- definition of Confined declare confined(Type t){ require(t.isArray()); require(confined(t.elemtype)); } declare confined(Type t){ require(!t.isArray()); require(t.hasAnnotation("Confined")); } declare confined(Type t){ require(!t.isArray()); exists(Type p : t.allparams()){ require(confined(p)); }} rule ConfinedClassDef(ClassDef c){ where(confined(c)){ require(!c.getSym().isPublic ()): error(c, "Confined class may not be public");

  19. Failure Clauses Three parts error/warning Position in source Message to report If a statement fails, JavaCOP searches for the nearest enclosing failure clause and executes it. require(a){ require(b); require(c): error(pos1 , "error - c has failed"); }: warning(globals.NOPOS , "warning - a or b have failed")

  20. The JavaCop Framework JavaCOP Rules JavaCOP. Compiler Compiled constraints (Java class) Errors andWarnings JavaCOP. Framework javaccompiler Augmented AST Rest of Compiler Type Checking JavaProgram AST Parser Compiled Program • Augmented Sun javac compiler • Passes over each class after type checking • Defines JavaCOP API methods • Can be used directly • Can serve as a base for high-level systems • JavaCOP rule language

  21. The JavaCOP Framework Implementation • Checks source code, not byte code • More information available • Generic types • Many annotations • Compiles each .java file separately • Modular design, like javac • Extract interface information from bytecode • Performance • Less than twice the compile time • Insulates compiled rulesets from compiler implementation • Port to other compilers without changing the language • Implemented in Java

  22. The JavaCOP Compiler JavaCOP Rules JavaCOP.Compiler Parse syntax tree Compiled constraints Errors andWarnings Generate Java source JavaCOP.Framework javaccompiler Compiled Program JavaProgram • Translates rules into java code • Source–level transformation • New code • Extends interface exposed by Framework. • Compiled into bytecode using regular Java compiler • Implemented in Haskell

  23. Applications Successfully implemented several existing systems: • Type systems: • Confined Types [Vitek & Bokowski 1999] • Scoped Types for Real-Time Scoped Java [Zhao et al. 2004] • Race Condition Detection [Flanagan & Freund 2000] • Generic Ownership [Potanin et al. 2006] • Javari’s notion of reference immutability [Tschantz & Ernst 2005] • Other program constraints: • An EJB3.0 verifier • Two rulesets from the PMD Java style checker • Identifying “Degenerate Class” micro-patterns [Gil & Maman 2005] • Collecting structural information for OO metrics [Chidamber & Kemerer 1991]

  24. Contribution The first practical framework for pluggable types. • Rule Language for Pluggable Types • Clear, expressive, easy to use • Implementation • Integrated into standard development process • Rule language compiler • Validation • Demonstrated on a range of type systems and other constraints • Useful for programmers • Choose type systems as needed • Useful for researchers • Experiment easily with novel type systems

  25. Limitations JavaCOP doesn’t support: • Whole program analysis • Integrated with Java’s modular compilation. • Dynamic checks • Necessary for downcasts • Flow sensitivity

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