Type-safe Implementation of Java ™ Reflection Nadeem Abdul Hamid February 27, 2001 Yale University...

Type-safe Implementation of Java™ Reflection

Nadeem Abdul HamidFebruary 27, 2001 • Yale University

Advisor: Prof. Zhong Shao

(Work in progress)

2

Outline

IntroductionJava ReflectionImplementing Java ReflectionImplementing Java Reflection Safely

Java Reflection Using ITA (Intensional Type Analysis)

Conclusions and Further Work

3

Java Mobile Code Platform

Java Bytecode:Platform independent

mobile codeVerifiable for

safety/security properties

Problem:Large Trusted

Computing Base (TCB) Runtime System

Interpreter

Verifier

Class

Library

Introduction

4

A More Principled, Flexible Platform

FLINT ILLow-level CodeSophisticated Type System = Safety/Security

Fast Type Checking

Precise SemanticsMultiple Source Languages

Small TCB

JavaClass

Library

Introduction

5

Java-FLINT Current Results

Support for large subset of Java:

classes, inheritance, interfaces, privacy, mutual recursion, dynamic cast, constructors, super, static, ...

[League, Shao, Trifonov ’99, ’01]

Introduction

6

... Reflection?

ConstraintsAdhere to Java SpecificationStraightforward, efficient (?) encodingSimilar to current implementations,

but, ...

Entirely type checked

Introduction

7

Java Warm-up

class Pt extends Object {

public int x;Pt(int newx) { this.x = newx; }public void bump(Pt y) { this.x = y.x; }

}

main() {

Pt p1 = new Pt(1), p2 = new Pt(2);p1.bump(p2);p2.x = p1.x * 2;

}

Introduction

8

Outline




9

What is Reflection?

“Reflection is the ability of a program to manipulate as data something representing the state of the program during its own execution.” [Demers and Malenfant]

Reification = “Encoding execution state as data”

Reflective Power1. Introspection2. Behavioral Reflection3. Structural Reflection

Java ReflectionMostly (1)

Java Reflection

10

Who Uses Java Reflection?

JavaBeans (component architectures)Database applicationsSerialization

Mobile objects

Scripting applicationsRuntime Debugging/Inspection Tools

Jalapeno (IBM) – remote VM debugging tool

Frappé (Antony)

Java Reflection

11

Why Use Reflection?

Dynamically loaded classes

Convenience

Efficiency

Java Reflection

12

How To Use Java Reflection

Java Reflection

class Pt extends Object { int x; Pt(int newx) { this.x = newx; } void bump(Pt y) { this.x = y.x; }}

main() {

Class c = getClass(“Pt”); Field f = c.getField(“x”); Method m = c.getMethod(“bump”);

Object p = c.newInstance( [3] );// p = new Pt(3); f.get( p ); // p.x; m.invoke( p, [ p ] ); // p.bump(p); }

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Another Example

Java Reflection

main() {Class c = getClass(“Pt”);Field f = c.getField(0);Object obj = <Network.ReceiveObject>;

if (c.isInstanceOf( obj ) {

print “It’s a point!”;print “Value: “ + f.get(obj);

}

}

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Reflection API Interface

Java Reflection

class Class extends AccessibleObject {static Class forName(String name);Object newInstance();Field getField(String name);Method getMethod(String name);boolean isInstance(Object obj);

getName(), getInterfaces(), getSuperclass(),getModifiers(), getFields(), getMethods()

}

class Field extends AccessibleObject {Object get(Object obj);void set(Object obj, Object val);

getType(), getDeclaringClass(), ...}

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Reflection API Interface (cont.)

Java Reflection

class Method extends AccessibleObject {Object invoke(Object obj, Object[] args);

getReturnType(), getParameterTypes(),getExceptionTypes(), getDeclaringClass(),...

}

class Constructor;

class AccessibleObject;

class Array;

class Proxy;

...

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Subtleties

SecurityAccess Control

Overriding

InheritanceArraysPrimitive Types

Class InitializationInner & Anonymous Classes

Java Reflection

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Reflection API Summary

Representations for Class, Field, Method

check class of an objectconstruct new class instancesaccess and modify field values of an objectaccess and invoke methods of a class

Java Reflection

18

Outline




19

Java VM/Compiler Survey

Sun J2SE (Java 1.3)Kaffe VMJalapeno (IBM)OpenJIT (Japan)JavaInJava (Sun 1998)BulletTrainRivet (MIT)Marmot (Microsoft)Classpath (GNU)

Implementing Java Reflection

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At Runtime


class Pt {Object x; Pt bump(Pt); }

Libraries: Object, Class, Field, ...

JVM

Class

“Object”

fields = /

Object

vtab

class

field1

...

Class

“Class”

...

Class

“Field”

...

Class

“Pt”

fields

Field

“x”

clazz

type

offset = 16

Pt

x = ...

...new Pt(3) ...

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At Runtime: Methods


class Pt {Object x; Pt bump(Pt); }

Libraries: Object, Class, Field, ...

JVM

Object

vtab

class

field1

...

...new Pt(3) ... Pt

vtab

class

x = ...

bump

...

Method

“bump”

clazz

args/rettype

code ptr

Class

“Pt”

fields

methods ...

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Class and Field Implementation


class Field {String name;Class type;Class clazz;int offset;

Object get(Object obj) { if (clazz.isInstance(obj)) f = ((char*)obj) + offset; return (Object)f;}

}

class Class {String name;Field[] fields;Method[] methods;boolean primitive;

bool isInstance...Object newInstance..}

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Method Implementation


class Method {String name;Class clazz;CodePtr* code;Class[] argtypes;Class rettype;

Object invoke(Object obj, Object args[]) { if (clazz.isInstance(obj)) foreach args[i]

CHECK argtypes[i].isInstance(args[i]) <unroll arguments into stack frame> <and “jump” to code > return (Object)retvalue;}

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Primitive Types

class Class {...boolean primitive;

}

class Field {String name;Class type;Class clazz;int offset;

Object get(Object obj) { if (clazz.isInstance(obj)) f = ((char*)obj) + offset; return (type.primitive = TRUE ?

wrap(f) : (Object)f);}

}new

Integer( *(int*)f)

int class Integer; boolean class Boolean;

double class Double;...


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Why Native Code is Needed

isInstanceImplemented using some form of tagsSeparate topic: Dynamic loading

Selecting arbitrary offsetNot really arbitrary – checked to make sure object is valid

Unrolling arguments and applying to method code


26

The Problem(s)

Implementation is platform-specificpassing argumentsobject layout

Logic is straightforward but code is not type checked

compiler is part of the TCB- what happens if it makes a mistake


27

Outline




28

First try: Pure Java Solution

Class, Field, Method are abstractJVM loads a class file:

Generates an instance of Class, and instances of FieldFirst generates subclass of Class, subclasses of Field and then instances of those

Subclasses generated by filling in templates

Hard code checks/selection/invocation

One subclass for each class, field, method

Implementing Java Reflection Safely

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Subclassing Field

class Field_<CLASSNAME>_<FIELDNAME> extends Field{

String name = “<FIELDNAME>”;Class type = <class of field type>;Class clazz = <class of CLASSNAME>;

Object get(Object obj) { return ( (<CLASSNAME>) obj ).<FIELDNAME>;}

}

Template:

if (clazz.isInstance(obj)) f = *((char*)obj) + offset; return (Object)f;


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Filling in the template

Point.getClass().getField(“x”) (Field) new Field_Point_x();

class Point { public Integer x; }

class Field_Point_x extends Field{

String name = “x”;

Object get(Object obj) { return ( (Point) obj ).x;}

}


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Nice try,

No native codeSemantics of Java automatically enforcedIndependent of object-layout

Not part of the TCB!


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but...

No overriding privacyConsider, if x were a private field:

( (Point) obj ).x would not compileMaybe in FLINT?

Slight specification revisions neededToo much overhead?

Explosion in number of classes generated, not just class instances


33

-

Enter Intensional Type Analysis...

[HM, CWM, SST]

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Java Encoding (Intuitively)

JAVA miniFLINT

Objects new Point (x=3)

Records

Field Selection p.x (also for methods)

Field Selection

Methods class Pt { void m(int x1) {...} }

Lambda terms

Method Invocation p.m( x1 )

Self-application

Implementing Java Reflection Using ITA

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Claim

Java + Reflection miniFLINT + ITA

Field selection:(p.x) : ?

where x unknown at compile time

Method selection & invocation:p.m( p, x1 ) : ?

where m : ?, p : ?, x1 : ?


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FLINT Framework

Object representation:


,...3

],Pt[

...},{

]Pt[

x

meth1

COBJclass

vtab

New

),,(|...

),,(|...

_

_

eetypecasee

Typerec

37

Class Objects in FLINT

unit__|

Castlist)][(

of.Castlist

,...,],[

)Castlist.(:

,"":

...,

...,

][

][:,][][:

21

21

1

ObjectObjTy

Typecase

castNewCCTOR

ctr

CStringname

class

vtab

CCOBJ

CCTORNewCObjTyObjectObjTycast

DC

CC


C ...21

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newInstance

Object newInstance(Object[] args);

end

inasopen

args cargsctor][applycur

cargs,ctor,self.ctr .args.selfenewInstanc

ERRORofcase

ERROR

as

aasa

ofcase

oftypecase

Object

Typecase

__|f[]args__|

__|

)cargs(2#

))cargs(1(#f][applycur::

args

.args.cargs.f.

Rettype[]Castlist.:applycur

__|Rettypeof.Rettype

2

21

221


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Field Selection

Powerful record selectbased on first-class labels or offset where the record type maps offsets to types


endiselect

inCinasopen n

)objcast(

cast),,,(t self.offse.obj.selfget

C

nn

castCObjTy

CObjectObjTyCninoffset

xStringnameclassvtab

xCFOBJ

]),[,0,1(

)][.(..:

,"":...,...,

],[

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Recursive Types

Complication: ObjTy[C] is a recursive

type

Representation of recursive types for ITA ?


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Summary: ITA Approach

Dynamic castName equivalence

Field/Method SelectionUnrolling arguments and applying to code

Using FLINT-based IL ( ) extended with ITA


F

42

Outline




43

Further work

Formalize reflection in JavaExtend target language with ITA

Representing recursive types

Interaction with privacy/securityEspecially overriding access control

TWELF encodingFLINT VM implementation

44

Conclusion

Type-safe implementation of Java Reflection in FLINT-based IL

Conforms to Java specificationStraightforward, efficient*Captures logic of existing native implementations

Type-safe Implementation of Java ™ Reflection Nadeem Abdul Hamid February 27, 2001 Yale University...

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