Multiple Code Inheritance in Java

Maria Cutumisu

Supervisors: Duane Szafron, Paul Lu

2

Outline

• Overview• Motivation• Implementation• Validation• Syntax Support for Compilation• Conclusions

3

MI (class) MI (class) MI (class)

MI (interface) SI (class) SI (class)

MI (interface) MI (code-type) SI* (class)

We want separation of types.

*We do not support multiple data inheritance.

C++

Java

Our Java

Interface-Type Code-Type Data-Type

Interface Class

Type

Type Separation Mechanisms in Java

4

Code Promotion Measurements - Methods

Class Identical AbstractAbstract

and Super

TotalMethod

Decrease

DataInputStream 4/19 8/19 0/19 12/19 63%

DataOutputStream 2/17 0/17 6*/17 2+6*/17 12%

RandomAccessFile 6/45 8/45 6/45 20/45 44%

RandomAccessFile

DataOutput

DataOutputStreamDataInputStream

DataInput

……readFloat() ……readFloat() ……writeFloat() ……writeFloat()

5

Basic Implementation - I

DataInput

RandomAccessFile

readFloat()readInt()

DataOutput

readIntreadInt

MT DataInput

MT RandomAccessFile

Interface Class

Code is nowin the interface!

Code wasin the class!

RandomAccessFile VMT

12 readFloat()

13 readInt()

…

Code Code

Class Loader Changed.

6

Dispatch Scenarios - Ambiguities

InterfaceB

ClassB

alpha() InterfaceC alpha()

Scenario 3

InterfaceB

ClassB

alpha()

InterfaceA alpha()

ClassA

Scenario 4

InterfaceA alpha()

InterfaceA x;x = new ClassB();x.alpha();

We detect ambiguities at load-time.

7

Basic Implementation - II

DataInput

RandomAccessFile

readFloat()readInt()

DataOutputwriteFloat()

DataOutput output;DataInput input;output = new RAF();input = new RAF();output.writeFloat();this.readFloat();input.readFloat();

hypothetical () in DataInput

invokeinterfaceinvokevirtual invokeinterfaceinvokevirtual invokeinterface

8

Super Call Implementation

invokespecial #superclass/alpha()

super(InterfaceA).alpha();

invokemulti-super #InterfaceA/alpha()

invokeinterface #InterfaceA/alpha()

compiler script

Bytecode previouslygenerated for super calls

Bytecodeproposed

Bytecodeused

New syntax proposed for super calls to interfaces

Execution-time change.

9

Summary of Changes to the JVM

• Small and localized changes: class loader algorithm (load-time) - 11 lines of

pseudo-code. execution of invokeinterface_quick

(execution-time) - 5 lines of code.

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Validation

• Semantics and performance are preserved for single inheritance programs. The following single inheritance Java programs ran correctly with our JVM: javac, jasper, and javap.

• Multiple inheritance programs show correct answers with the re-factored java.io library.

• Traditional super calls generate the same results under both JVMs.

11

Validation - MI Programs

java.io library Sun JVM

Multiple InheritanceTest Program

output

12

Validation - MI Programs

java.io library Our JVM

Multiple InheritanceTest Program

output

Tests the supercall mechanism

Uses the re-factored java.io library

13

Adding Code in Interfaces

Script

interface DataInput{public float readFloat() throws IOException;/*MI_CODE{return Float.intBitsToFloat(readInt());}MI_CODE*/}

abstract class DataInput{public float readFloat() throws IOException

{return Float.intBitsToFloat(readInt());};}

javac

DataInput.class

jasper

javac

jasper

DataInput.class

DataInput.j DataInput_MI.j

DataInput.j

DataInput.class

Script

jasmin

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Conclusions - I

• The first implementation of multiple code inheritance in Java is provided by modifying Sun’s JVM. It is based on the novel concept of adding code to interfaces.

• Facilitates code re-use, supports separation of inheritance concepts, and improves clarity and expressiveness of implementation.

• Only small and localized changes are made to the JVM.

• Java syntax and compiler are not changed. A set of scripts allows a programmer to add code in interfaces.

15

Conclusions - II

• We defined a super call mechanism for super calls to interfaces, resembling the one in C++.

• Single inheritance programs achieve the same performance as with the original JVM.

• Single and multiple inheritance programs run correctly with our JVM, for both our basic and super call mechanism implementations.

• Multiple code inheritance measurements show significant code decrease.

16

Load-time Change

if (imb.code <> null) //code in interface

currentmb = class.vmt[vmtIndex];

if (currentmb.code == null) //no code in MT

class.vmt[vmtIndex] = imb; //point VMT to imb

else //potential code ambiguity

if (!currentmb.class.imt.contains(imb) &&

!imb.class.imt.contains(currentmb))

throw ambiguous method exception

end if

end if

end if

17

Execution-time Change

case opc_invokeinterface_quick:imb = constant_pool[GET_INDEX(pc+1)].mb;interface = imb.class;offset = imb.offset;…//args_size = pc[3]; //REMOVEDargs_size = imb.args_size; //ADDEDoptop -= args_size;…if (pc[3] == 255) //ADDED

mb = interface.MT[offset]; //ADDEDgoto callmethod; //ADDED

end if //ADDED…

end case