Ownership and Immutability in Generic Java (OIGJ)

Yoav Zibin, Alex PotaninPaley Li, Mahmood Ali, and Michael D.

Ernst

Presenter: Yossi Gil

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Ownership and Immutability

Owner-as-dominator All reference-chains

from a root to an object pass via its owner

An object cannot leak beyond its owner

No aliases to internal state

Mutable object Fields can be assigned

Immutable object Raw state:

Fields can be assigned Cooked state:

Fields cannot be assigned

ReadOnly references

Ownership Immutability

How can we benefit from a combination?

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Benefits from Ownership + Immutability

Previous work 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

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

information Makes it easier to create immutable cyclic structures Many objects must be raw simultaneously to create

cyclic structures

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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

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Cooking Example: LinkedList

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

after their constructor finished

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 = new Entry<E>(e,succ,pred);6: // It’s illegal to change an entry after it’s cooked.7: pred.next = entry; pred = entry;8: }9: succ.prev = pred;10: }

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 = new Entry<E>(e,succ,pred);6: // It’s illegal to change an entry after it’s cooked.7: pred.next = entry; pred = entry;8: }9: succ.prev = pred;10: }

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Contributions

Formalization Formalized the concepts of raw/cooked objects and

wildcards as owner parameters, and proved soundness

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

cyclic structures, the factory and visitor design patterns

No refactoring of existing code Verified that Java’s collection classes are properly

encapsulated (using only 3 ownership annotations) Simplicity

Use generics to express immutability and ownership

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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

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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;

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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. <I extends ReadOnly>? int readonlyMethod(){...}9 : // Can be called only on mutable receivers; can mutate this. <I extends Mutable>? void mutatingMethod(){...}10: // Constructor that can create (im)mutable objects. <I extends Raw>? 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. <I extends ReadOnly>? int readonlyMethod(){...}9 : // Can be called only on mutable receivers; can mutate this. <I extends Mutable>? void mutatingMethod(){...}10: // Constructor that can create (im)mutable objects. <I extends Raw>? 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: }

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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 : <I extends Raw>? LinkedList(8 : Collection<?,ReadOnly,E> c) {9 : this(); this.addAll(c);10: }11: <I extends Raw>? 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 : <I extends Raw>? LinkedList(8 : Collection<?,ReadOnly,E> c) {9 : this(); this.addAll(c);10: }11: <I extends Raw>? 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: }

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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)

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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:

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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: <I extends Mutable>? 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: <I extends Mutable>? 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

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Field assignment

1 : class Foo<O extends World, I extends ReadOnly> {2 : Date<O,I> sameD;3 : <I extends Raw>? 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 : <I extends Raw>? 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.

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Method invocation

1 : class Foo<O extends World, I extends ReadOnly> {2 : Date<This,I> m1() { … } // Parameter is this-owned 3 : <I extends Raw>? void m2() { … }4 : <I extends Raw>? 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 : <I extends Raw>? void m2() { … }4 : <I extends Raw>? 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.

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Method guards

1: class Foo<O extends World, I extends ReadOnly> {2: <I extends Raw>? void rawM() { … }3: <I extends Mutable>? 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: <I extends Raw>? void rawM() { … }3: <I extends Mutable>? 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

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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

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Case studies

From Generics to AnnotationsDate<O,I> @OI Date

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

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Case studies conclusions

Verified that collections own their representation

Method get must be Mutable! new LinkedHashMap(100, 1, /*accessOrder=*/ true)

Order of iteration is the order in which its entries were last accessed

Method overriding can only make the guard less strict

Method clone is problematicClone makes a shallow copy that breaks

ownershipOur suggestion: compiler-generated clone that

nullifies fields, and then calls a copy-constructor

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Conclusions

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

generics)Backward compatible, no JVM changesHigh degree of polymorphism using

generics and safe covariant subtypingCase study proving usefulnessFormal proof of soundness

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Future work

Inferring ownership and immutability annotations

Bigger case study Extending OIGJ

owner-as-modifier uniqueness or external uniqueness

Questions?