ownership and immutability in generic java (oigj)

Ownership and Immutability in Generic Java (OIGJ)

Yoav Zibin+, Alex Potanin*

Paley Li*, Mahmood Ali^, and Michael Ernst$

Presenter: Yossi Gil+

+IBM *Victoria,NZ ^MIT $Washington

2/22

Ownership + Immutability

Our previous work OGJ: added Ownership to Java IGJ: added Immutability to Java

This work OIGJ: combine Ownership + Immutability The sum is greater than its parts

IGJ could not type-check existing code for creating immutable cyclic data-structures (e.g., lists, trees)

We found a non-trivial connection between ownership and immutability

3/22

Contributions

No refactoring of existing code Prototype implementation

No syntax changes (uses type-annotations in Java 7) No runtime overhead Backward compatible

Verified that Java’s collection classes are properly encapsulated (using few annotations)

Flexibility OIGJ can type-check more code than previous work:

cyclic structures, the factory and visitor design patterns Formalization

Formalized the concepts of raw/cooked immutable objects and wildcards as owner parameters

Proved soundness

4/22

Problem 1: Representation exposure

Internal representation leaks to the outside private doesn’t offer real protection!Real life example!class Class { private List signers; public List getSigners() { return this.signers; }}

http://java.sun.com/security/getSigners.htmlBug: the system thinks that code signed by one identity is signed by a different identity

Forgot to copy signers!

5/22

Solution for Representation Exposure

Ownership! Class should own the list signers No outside alias can exist Ownership can be nested: note the tree structure

Classsigners

entry1 entry2 entryN…

elem1 elem2 elemN…

X

X

X

6/22

Ownership: Owner-as-dominator

Dominators in graph theory Given: a directed rooted graph X dominates Y if any path from the root to Y

passes X Owner-as-dominator

Object graph; roots are the static variables An object cannot leak outside its owner, i.e., Any path from a root to an object passes its

owner Conclusion: No aliases to internal state

7/22

Problem 2: Unintended Modification

Modification is not explicit in the language can Map.get() modify the map? for (Object key : map.keySet()) {

map.get(key); }throws ConcurrentModificationExceptionfor the following mapnew LinkedHashMap(100, 1, true)

Reorders elements according to last-accessed (like a cache)

8/22

Solution: Immutability

Varieties of Immutability Class immutability (like String or Integer in

Java) Object immutability

The same class may have both mutable and immutable instances

Reference immutability A particular reference cannot be used to mutate its

referent (but other aliases might cause mutations)

class Student { @Immutable Date dateOfBirth; … void setTutor(@ReadOnly Student tutor) @Mutable { … }}

Method may modify the this object

Example in IGJ syntax

9/23

Objects vs. References

Objects mutable or immutable Creation of an immutable object

Raw state: Fields can be assigned Cooked state: Fields cannot be assigned

References mutable, immutable, or readonly

Challenge: Cyclic Immutability

Cooking a cyclic data-structure is complicatedMany objects must be raw simultaneously to

manipulate backward pointersThen everything must become immutable

simultaneously

OIGJ’s novel idea:Prolong the cooking phase by using

ownership informationEnables creation of immutable cyclic

structures

11/22

Cooking immutable objects

Previous work An object becomes cooked when its

constructor finishes OIGJ’s observation

An object becomes cooked when its owner’s constructor finishes

The outside world will not see this cooking phase

The complex object with its representation becomes immutable simultenously

12/22

Cooking LinkedList (1 of 2)

No refactoring – the original code must compile in OIGJ

1 : LinkedList(Collection<E> c) {2 : this();3 : Entry<E> succ = this.header, pred = succ.prev;4 : for (E e : c) {5 : Entry<E> entry = 6 : new Entry<E>(e,succ,pred);7 : // An entry is modified after it’s constructor finished8 : pred.next = entry; pred = entry;9 : }10: succ.prev = pred;11: }

1 : LinkedList(Collection<E> c) {2 : this();3 : Entry<E> succ = this.header, pred = succ.prev;4 : for (E e : c) {5 : Entry<E> entry = 6 : new Entry<E>(e,succ,pred);7 : // An entry is modified after it’s constructor finished8 : pred.next = entry; pred = entry;9 : }10: succ.prev = pred;11: }

Sun’s code is similar

13/22

Cooking LinkedList (2 of 2)

The list owns its entries Therefore, it can mutate them, even

after their constructor finished

1 : LinkedList(@ReadOnly Collection<E> c) @Raw {2 : this();3 : @This @I Entry<E> succ = this.header, pred = succ.prev;4 : for (E e : c) {5 : @This @I Entry<E> entry = 6 : new @This @I Entry<E>(e,succ,pred);7 : // An entry is modified after it’s constructor finished8 : pred.next = entry; pred = entry;9 : }10: succ.prev = pred;11: }

1 : LinkedList(@ReadOnly Collection<E> c) @Raw {2 : this();3 : @This @I Entry<E> succ = this.header, pred = succ.prev;4 : for (E e : c) {5 : @This @I Entry<E> entry = 6 : new @This @I Entry<E>(e,succ,pred);7 : // An entry is modified after it’s constructor finished8 : pred.next = entry; pred = entry;9 : }10: succ.prev = pred;11: }

Sun’s code is similar

Code in OIGJ; Annotations next slide.

14/22

Hierarchies in OIGJ

Immutability hierarchy

ReadOnly – no modification

Raw – object under construction

Ownership hierarchyWorld – anyone can

accessThis – this owns the

object

World

This

ReadOnly

Raw

Mutable

Immut

15/22

OIGJ syntax: fields (1 of 2)

Two annotations per type

1:class Foo {2: // An immutable reference to an immutable date.

@O @Immut Date imD = new @O @Immut Date ();3: // A mutable reference to a mutable date.

@O @Mutable Date mutD = new @O @Mutable Date();4: // A readonly reference to any date. Both roD and imD cannot mutate // their referent, however the referent of roD might be mutated by an // alias, whereas the referent of imD is immutable.

@O @ReadOnly Date roD = ... ? imD : mutD;5: // A date with the same owner and immutability as this

@O @I Date sameD;6: // A date owned by this; it cannot leak.

@This @I Date ownedD;7: // Anyone can access this date.

@World @I Date publicD;

1:class Foo {2: // An immutable reference to an immutable date.

@O @Immut Date imD = new @O @Immut Date ();3: // A mutable reference to a mutable date.

@O @Mutable Date mutD = new @O @Mutable Date();4: // A readonly reference to any date. Both roD and imD cannot mutate // their referent, however the referent of roD might be mutated by an // alias, whereas the referent of imD is immutable.

@O @ReadOnly Date roD = ... ? imD : mutD;5: // A date with the same owner and immutability as this

@O @I Date sameD;6: // A date owned by this; it cannot leak.

@This @I Date ownedD;7: // Anyone can access this date.

@World @I Date publicD;

16/22

OIGJ syntax: methods (2 of 2)

Method receiver’s annotation has a dual purpose: Determines if the method is applicable. Inside the method, the bound of @I is the annotation.

8 : // Can be called on any receiver; cannot mutate this. int readonlyMethod() @ReadOnly {...}9 : // Can be called only on mutable receivers; can mutate this. void mutatingMethod() @Mutable {...}10: // Constructor that can create (im)mutable objects. Foo(@O @I Date d) @Raw {11: this.sameD = d;12: this.ownedD = new @This @I Date ();13: // Illegal, because sameD came from the outside. // this.sameD.setTime(...);14: // OK, because Raw is transitive for owned fields. this.ownedD.setTime(...);15: }

8 : // Can be called on any receiver; cannot mutate this. int readonlyMethod() @ReadOnly {...}9 : // Can be called only on mutable receivers; can mutate this. void mutatingMethod() @Mutable {...}10: // Constructor that can create (im)mutable objects. Foo(@O @I Date d) @Raw {11: this.sameD = d;12: this.ownedD = new @This @I Date ();13: // Illegal, because sameD came from the outside. // this.sameD.setTime(...);14: // OK, because Raw is transitive for owned fields. this.ownedD.setTime(...);15: }

17/22

Formalization: Featherweight OIGJ

Novel idea: CookersEvery location l in the heap is of the form:

l' is the owner of ll” is the cooker of l, i.e., when the

constructor of l” finishes then l becomes cooked

We keep track of the set of ongoing constructors

Subtyping rules connect cookers and owners

lFoo<l’,Immut >lFoo<l’,Mutable> or l"

Proved soundness and type preservation

18/22

Case studies

Implementation uses the checkers frameworkOnly 1600 lines of code (but still a

prototype)Requires type annotations available in

Java 7 Java’s Collections case study

77 classes, 33K lines of code85 ownership-related annotations46 immutability-related annotations

19/22

Case studies conclusions

Verified that collections own their representation

Method clone is problematic Clone makes a shallow copy that breaks ownership Our suggestion: compiler-generated clone that

nullifies fields, and then calls a copy-constructor

20/22

Previous Work

Universes Relaxed owner-as-dominator to owner-as-

modifier ReadOnly references can be freely shared Constrains modification instead of aliasing,

i.e., only the owner can modify an object

Reference immutability: C++’s const Javari

21/22

Future work

Inferring ownership and immutability annotations

Bigger case study Extending OIGJ

owner-as-modifier uniqueness or external uniqueness

22/22

Conclusions

Ownership Immutability Generic Java (OIGJ)Simple, intuitive, smallStatic – no runtime penalties (like

generics)Backward compatible, no JVM changes

Case study proving usefulnessFormal proof of soundnessPaper submitted to OOPSLA. Links:

http://ecs.victoria.ac.nz/twiki/pub/Main/TechnicalReportSeries/

http://code.google.com/p/checker-framework/ http://code.google.com/p/ownership-immutability/

24/22

OIGJ typing rules

Ownership nesting Field access Field assignment Method invocation Method guards Raw parameter

can be used only in method guards (see the paper for all rules, such as

inner classes, covariant subtyping)

25/22

Ownership example in OGJ

This-owned fields/methods can be accessed only via this

class Class { @This List signers; // This-owned field public @This List getSigners1() { return this.signers; } public @World List getSigners2() { return new @World LinkedList(this.signers); } public void example(Class other) { this.signers = …; // Ok other.signers = …; // Illegal this.getSigners1(); // Ok other.getSigners1(); // Illegal other.getSigners2(); // Ok}

26/22

OIGJ syntax: fields (1 of 2)

Two new generic parameters were added

1:class Foo<O extends World, I extends ReadOnly> {2: // An immutable reference to an immutable date.

Date<O,Immut> imD = new Date<O,Immut>();3: // A mutable reference to a mutable date.

Date<O,Mutable> mutD = new Date<O,Mutable>();4: // A readonly reference to any date. Both roD and imD cannot mutate // their referent, however the referent of roD might be mutated by an // alias, whereas the referent of imD is immutable.

Date<O,ReadOnly> roD = ... ? imD : mutD;5: // A date with the same owner and immutability as this

Date<O,I> sameD;6: // A date owned by this; it cannot leak.

Date<This,I> ownedD;7: // Anyone can access this date.

Date<World,I> publicD;

1:class Foo<O extends World, I extends ReadOnly> {2: // An immutable reference to an immutable date.

Date<O,Immut> imD = new Date<O,Immut>();3: // A mutable reference to a mutable date.

Date<O,Mutable> mutD = new Date<O,Mutable>();4: // A readonly reference to any date. Both roD and imD cannot mutate // their referent, however the referent of roD might be mutated by an // alias, whereas the referent of imD is immutable.

Date<O,ReadOnly> roD = ... ? imD : mutD;5: // A date with the same owner and immutability as this

Date<O,I> sameD;6: // A date owned by this; it cannot leak.

Date<This,I> ownedD;7: // Anyone can access this date.

Date<World,I> publicD;

27/22

OIGJ syntax: methods (2 of 2)

Method guard <T extends U>? has a dual purpose: The method is included only if T extends U Inside the method, the bound of T is U.

8 : // Can be called on any receiver; cannot mutate this. ? int readonlyMethod(){...}9 : // Can be called only on mutable receivers; can mutate this. ? void mutatingMethod(){...}10: // Constructor that can create (im)mutable objects. ? Foo(Date<O,I> d) {11: this.sameD = d;12: this.ownedD = new Date<This,I>();13: // Illegal, because sameD came from the outside. // this.sameD.setTime(...);14: // OK, because Raw is transitive for owned fields. this.ownedD.setTime(...);15: }

8 : // Can be called on any receiver; cannot mutate this. ? int readonlyMethod(){...}9 : // Can be called only on mutable receivers; can mutate this. ? void mutatingMethod(){...}10: // Constructor that can create (im)mutable objects. ? Foo(Date<O,I> d) {11: this.sameD = d;12: this.ownedD = new Date<This,I>();13: // Illegal, because sameD came from the outside. // this.sameD.setTime(...);14: // OK, because Raw is transitive for owned fields. this.ownedD.setTime(...);15: }

28/22

LinkedList Example

1 : class Entry<O,I,E> {2 : E element;3 : Entry<O,I,E> next, prev; …4 : }5 : class LinkedList<O,I,E> {6 : Entry<This,I,E> header; …7 : ? LinkedList(8 : Collection<?,ReadOnly,E> c) {9 : this(); this.addAll(c);10: }11: ? void addAll(12: Collection<?,ReadOnly,E> c) {13: Entry<This,I,E> succ = this.header,14: pred = succ.prev;15: for (E e : c) {16: Entry<This,I,E> en =17: new Entry<This,I,E>(e,succ,pred);18: pred.next = en; pred = en; }19: succ.prev = pred;20: }

1 : class Entry<O,I,E> {2 : E element;3 : Entry<O,I,E> next, prev; …4 : }5 : class LinkedList<O,I,E> {6 : Entry<This,I,E> header; …7 : ? LinkedList(8 : Collection<?,ReadOnly,E> c) {9 : this(); this.addAll(c);10: }11: ? void addAll(12: Collection<?,ReadOnly,E> c) {13: Entry<This,I,E> succ = this.header,14: pred = succ.prev;15: for (E e : c) {16: Entry<This,I,E> en =17: new Entry<This,I,E>(e,succ,pred);18: pred.next = en; pred = en; }19: succ.prev = pred;20: }

29/22

Ownership nesting

List<This, I,Date<World,I>> l1; // Legal nestingList<World,I,Date<This, I>> l2; // Illegal!

List<This, I,Date<World,I>> l1; // Legal nestingList<World,I,Date<This, I>> l2; // Illegal!

The main owner parameter must be inside any other owner parameter.

public static Object<World,ReadOnly> alias_l2;public static Object<World,ReadOnly> alias_l2;

It’s illegal because we can store l2 in this variable:

30/22

Field access/assignment

1: class Foo<O extends World, I extends ReadOnly> {2: Date<This,I> ownedD; // this-owned field3: Date<O,I> sameD;4: ? void bar(Foo<This,I> other) {5: this.ownedD = …; // Legal: assign via this6: other.ownedD = …; // Illegal: not via this7: other.sameD = …; // Legal: not this-owned8: }

1: class Foo<O extends World, I extends ReadOnly> {2: Date<This,I> ownedD; // this-owned field3: Date<O,I> sameD;4: ? void bar(Foo<This,I> other) {5: this.ownedD = …; // Legal: assign via this6: other.ownedD = …; // Illegal: not via this7: other.sameD = …; // Legal: not this-owned8: }

this-owned fields can be accessed/assigned only via this

31/22

Field assignment

1 : class Foo<O extends World, I extends ReadOnly> {2 : Date<O,I> sameD;3 : ? void bar(4 : Foo<?,Mutable> mutableFoo,5 : Foo<?,ReadOnly> readonlyFoo,6 : Foo<?,I> rawFoo1,7 : Foo<This,I> rawFoo2) {8 : mutableFoo.sameD = …; // Legal: object is Mutable9 : readonlyFoo.sameD = …; // Illegal: object is not Raw nor Mutable10: rawFoo1.sameD = …; // Illegal: object is not this nor this-owned11: rawFoo2.sameD = …; // Legal: object is Raw and this-owned12: this.sameD = …; // Legal: object is Raw and this13: }

1 : class Foo<O extends World, I extends ReadOnly> {2 : Date<O,I> sameD;3 : ? void bar(4 : Foo<?,Mutable> mutableFoo,5 : Foo<?,ReadOnly> readonlyFoo,6 : Foo<?,I> rawFoo1,7 : Foo<This,I> rawFoo2) {8 : mutableFoo.sameD = …; // Legal: object is Mutable9 : readonlyFoo.sameD = …; // Illegal: object is not Raw nor Mutable10: rawFoo1.sameD = …; // Illegal: object is not this nor this-owned11: rawFoo2.sameD = …; // Legal: object is Raw and this-owned12: this.sameD = …; // Legal: object is Raw and this13: }

1) A field can be assigned only if the object is Raw or Mutable.

2) If it is Raw, then the object must be this or this-owned.

32/22

Method invocation

1 : class Foo<O extends World, I extends ReadOnly> {2 : Date<This,I> m1() { … } // Parameter is this-owned 3 : ? void m2() { … }4 : ? void bar(5 : Foo<?,I> rawFoo1,6 : Foo<This,I> rawFoo2) {7 : this.m1(); // Legal: object is this 8 : rawFoo2.m1(); // Illegal: object is not this9 : rawFoo1.m2(); // Illegal: object is not this nor this-owned10: rawFoo2.m2(); // Legal: both Raw and object is this-owned11: this.m2(); // Legal: both Raw and object is this12: }

1 : class Foo<O extends World, I extends ReadOnly> {2 : Date<This,I> m1() { … } // Parameter is this-owned 3 : ? void m2() { … }4 : ? void bar(5 : Foo<?,I> rawFoo1,6 : Foo<This,I> rawFoo2) {7 : this.m1(); // Legal: object is this 8 : rawFoo2.m1(); // Illegal: object is not this9 : rawFoo1.m2(); // Illegal: object is not this nor this-owned10: rawFoo2.m2(); // Legal: both Raw and object is this-owned11: this.m2(); // Legal: both Raw and object is this12: }

Method invocation is the same as field access/assignment:1)If any parameter is this-owned, then the receiver must be this.2)If the guard is Raw and the object is Raw, then the receiver must be this or this-owned.

33/22

Method guards

1: class Foo<O extends World, I extends ReadOnly> {2: ? void rawM() { … }3: ? void bar(4: Foo<?,ReadOnly> readonlyFoo,5: Foo<?,I> mutableFoo) {6: readonlyFoo.rawM(); // Illegal: ReadOnly is not a subtype of Raw // The bound of I in this method is Mutable7: mutableFoo.rawM(); // Legal: Mutable is a subtype of Raw8: this.rawM(); // Legal: Mutable is a subtype of Raw9: }

1: class Foo<O extends World, I extends ReadOnly> {2: ? void rawM() { … }3: ? void bar(4: Foo<?,ReadOnly> readonlyFoo,5: Foo<?,I> mutableFoo) {6: readonlyFoo.rawM(); // Illegal: ReadOnly is not a subtype of Raw // The bound of I in this method is Mutable7: mutableFoo.rawM(); // Legal: Mutable is a subtype of Raw8: this.rawM(); // Legal: Mutable is a subtype of Raw9: }

Guard “<T extends U>?” has a dual purpose:1)The receiver’s T must be a subtype of U.2)Inside the method, the bound of T is U.

Conditional Java (cJ) proposed method guards for Java

ownership and immutability in generic java (oigj)

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