1 Javari: Adding Reference Immutability to Java Matthew Tschantz & Michael Ernst (MIT) OOPSLA’05 Presented by Tali Shragai

2 Extension to Javari2004 [Birka]  Javari: Java + (C++ const) + more… Two new keywords: readonly, mutable  OOPSLA’04: Initial Javari implementation  Experience: 160,000 code lines Better documentation Error detection and prevention Compatible with Java & existing JVMs  But: No Generics, serialization, reflection Confuses mutability & assignability  Javari 05: More keywords: assignable, romaybe Genericity, reflection, serialization Most important (?): mutability vs. assignability

3 Protecting method arguments  A library routine does not modify its arguments: static void print(readonly Date d) {... // Cannot modify d } String toString() readonly {... // Cannot modify the receiver }

4 Protecting abstract state: observer methods class Class { private Object[] signers; Object[] getSigners() { return signers; // JDK 1.1 security hole } myClass.getSigners()[0] = “Sun”;

5 Protecting abstract state: observer methods (revised) class Class { private Object[] signers; readonly Object[] getSigners() { return signers; // Fixes JDK 1.1 bug } myClass.getSigners()[0] = “Sun”; // Error

6 Reference vs. Object immutability  Reference immutability Object modification:  Readonly reference: no!  Other reference: yes. Deep: cannot modify transitively reachable state Static type checking!  Dynamic checking for mutability downcasts  Object immutability : write protect from any reference Graph temp = new Graph(); // construct the graph readonly Graph g = temp; temp = null; // further usage through read only reference…  Javari : object immutability emulated by reference immutability

7 Related work  Old trick: pass/return by value pain for large objects  C++: const, const_cast Unsound (unchecked casts) Non-transitive! No immutability-based parameterization Leads to code duplication (const methods) No support for multidimensional arrays?  JAC [Kniesel 2001]: Implementation requires rewriting the code Unsound (subtyping, arrays) Return type mutability equals receiver's

8 Protection methods LevelJavaC++Javari Level 1 assignment final Foo aconst Foo *afinal Foo a Level 2 mutability  Foo const *areadonly Foo a Level 3  const Foo const *afinal readonly Foo a … Level ∞  Transitive protection!

9 Yield protection LevelJavaC++Javari Level 1  mutable Foo *aassignable Foo a Level 2  mutable Foo a

10 Readonly readonly Date rd = new Date(); rd.year = 2005; // Compile-time error rd.incrementDay(); // Compile-time error rd = new Foo(); // OK

11 Mutability vs. Assignability final Date fd = null; readonly Date rd = null; fd = new Date(); // Error: final rd = null; // OK Date d1 = fd; // OK Date d2 = rd; // Error: wrong type MutabilityAssignability part of the typenot part of the type may abstract state mutate? can be lvalue in assignment? readonlyfinal

12 Type system Every type (mutable) T has readonly T as a supertype readonly Object readonly Date /*mutable*/ Object /*mutable*/ Date

13 Immutable classes  A class/interface can be declared immutable using readonly in its class declaration. Non-static fields & methods  readonly (default) Reference to its objects  implicitly read-only Subclasses must be immutable too! readonly class String{…} /*readonly*/ String s1 = new String(); readonly String s2 = new String(); S1 = s2;// OK S2 = s1;// OK

14 Assignability  May a reference be assigned to? class Appointment { final Date time; // Same as in Java assignable int room; } Appointment appoint; appoint.time = new Date(); // Error appoint.room = 250; // OK

15 Assignability modifiers final: May never be assigned assignable: May always be assigned (default for locals) this-assignable: May be assigned through a mutable reference May not be assigned through a readonly reference Only applicable to fields (the default) The assignability depends on the mutability of the enclosing object: “this”

16 Assignability example class Bicycle { final int id; // Never changes assignable int hashCode; // A cache /*this-assign*/ int gear; // Abstract state } /*mutable*/ Bicycle b; readonly Bicycle rb;

17 final class Bicycle { final int id; // Never changes assignable int hashCode; // A cache /*this-assign*/ int gear; // Abstract state } /*mutable*/ Bicycle b; readonly Bicycle rb; b.id = 5; // Error rb.id = 5;// Error Resolved assignability of ref.f Declared assignability of f Resolved mutability of ref mutablereadonly final unassignable

18 assignable class Bicycle { final int id; // Never changes assignable int hashCode; // A cache /*this-assign*/ int gear; // Abstract state } /*mutable*/ Bicycle b; readonly Bicycle rb; b.hashCode = 5; // OK rb.hashCode = 5; // OK Resolved assignability of ref.f Declared assignability of f Resolved mutability of ref mutablereadonly finalunassignable assignable

19 this-assignable class Bicycle { final int id; // Never changes assignable int hashCode; // A cache /*this-assign*/ int gear; // Abstract state } /*mutable*/ Bicycle b; readonly Bicycle rb; b.gear = 5; // OK rb.gear = 5; // Error Resolved assignability of ref.f Declared assignability of f Resolved mutability of ref mutablereadonly finalunassignable assignable this-assignableassignableunassignable

20 Mutability  May the object’s transitive abstract state be modified? class Date { /*this-assignable*/ int year; } /*mutable*/ Date d; readonly Date rd; d.year = 2005; // OK rd.year = 2005; // Error

21 Mutability modifiers readonly: May never be mutated mutable: May always be mutated, the default for locals this-mutable: May be mutated through mutable reference May not be mutated through readonly references Only applicable to fields (the default) The mutability depends on the mutability of the enclosing class: “this”

22 Mutability example class Account { readonly Customer owner; mutable RequestLog requests; /*this-mut*/ Balance bal; } /*mutable*/ Account a; readonly Account ra;

23 readonly class Account { readonly Customer owner; mutable RequestLog requests; /*this-mut*/ Balance bal; } /*mutable*/ Account a; readonly Account ra; a.owner.setName(“Bob”); // Error ra.owner.setName(“Bob”); // Error Mutability of ref.f Declared mutability of f Resolved mutability of ref mutablereadonly

24 mutable class Account { readonly Customer owner; mutable RequestLog requests; /*this-mut*/ Balance bal; } /*mutable*/ Account a; readonly Account ra; a.requests.add(“checkBalance”); // OK ra.requests.add(“checkBalance”); // OK Mutability of ref.f Declared mutability of f Resolved mutability of ref mutablereadonly mutable mutable excludes requests from the abstract state of the object

25 this-mutable class Account { readonly Customer owner; mutable RequestLog requests; /*this-mut*/ Balance bal; } /*mutable*/ Account a; readonly Account ra; a.balance.withdraw(1000); // OK Declared mutability of f Resolved mutability of ref mutablereadonly mutable this-mutablemutable?

26 this-mutable class Account { readonly Customer owner; mutable RequestLog requests; /*this-mut*/ Balance bal; } /*mutable*/ Account a; readonly Account ra; a.balance.withdraw(1000); // OK ra.balance.withdraw(1000); // Error Mutability of ref.f Declared mutability of f Resolved mutability of ref mutablereadonly mutable this-mutablemutablereadonly?

27 this-mutable fields reached through a readonly reference readonly, right?

28 this-mutable fields reached through a readonly reference readonly, right? NO, would result in type loophole allowing one to convert a readonly reference to a mutable reference:

29 this-mutable fields reached through a readonly reference class Student { assignable /*this-mut*/ GradeReport grades; } /*mutable*/ Student s = new Student(); readonly Student rs = s; readonly GradeReport rg; /*mutable*/ GradeReport g; rs.grades = rg; //readonly assigned to this-mutable g = s.grades; //this-mutable assigned to mutable Downcast rs s grades Non-readonly Readonly Readonly?? assignable

30 this-mutable fields reached through a readonly reference  Solution: Disallow readonly references to be assigned to a this-mutable field  this-mutable fields through readonly reference are: readonly as rvalue (may only be assigned to a readonly reference) mutable as lvalue (may only be assigned with mutable reference)  Notation:

31 Uses of parametric classes  Local variable declarations: /*mut*/ List a; // add, mutate /*mut*/ List b; // add readonly List c; // mutate readonly List d; // neither

32 Inside parametric classes  Type arguments include mutability  Library cannot write: class C { mutable T x; } C y; // Conflicting types  Library can force a mutable type using a bound: ; class C { T x; }

33 this-mutable type arguments class Device { /*this-mut*/ List drivers; } /*mutable*/ Device d; readonly Device rd; d.drivers has type /*mut*/ List /*mut*/ List dl = d.drivers; // OK

34 this-mutable type arguments class Device { /*this-mut*/ List drivers; } /*mutable*/ Device d; readonly Device rd; rd.drivers has type ? readonly List readonly List x = rd. drivers; //OK readonly List y = rd. drivers; //Error readonly List z = rd. drivers; //Error

35 Serialization  Add a readonly-ness bit to the serialized form.  De-serialization done using 2 versions of ObjectInputStream.readObject : readObjectReadonly – returns a read-only object, without checking. readObject – returns a mutable objects, but throws an exception if the readonly-ness bit is set.

36 Reducing code duplication  In Java, each class definition defines exactly one type. In Javari, by contrast, a class C actually denotes 2 type: C and readonly C.  romaybe keyword templates over methods class Conference { /*this-mutable*/ Date d;  We wish to write two (overloaded) methods: readonly Date getDate() readonly { return d;} /*mutable*/ Date getDate() /*mutable*/{ return d;}  Syntactic sugar: romaybe Date getDate() romaybe { return d;} }

37 Downcasts  Javari guarantees that a readonly reference cannot be used, directly or indirectly, to modify its referent.  Down-casting a reference from readonly to mutable triggers run- time checks, which disallow any modifications done through the casted reference.

38 Downcast example class Foo{ Date d; void setD() /*mutable*/{ d = new Date(); } Foo f1 = new Foo(); readonly Foo rf = f1; Foo f2 = (mutable)rf; f1.d = new Date();// OK f2.d = new Date();// run-time error f1.setD();// OK F2.setD();// run-time error

39 Contributions  Transitive reference immutability  Distinguishes assignability and mutability  Formal model (not shown here)  Type system for full Java 5.0 including parametric polymorphism, reflection, and serialization  Templates to reduce code duplication  Interoperable with Java  Still to come: type inference…

40 The End! Thank you for listening…