Download this.readFloat()

Multiple Code Inheritance in
Java
Maria Cutumisu
Supervisors: Duane Szafron, Paul Lu
Outline
•
•
•
•
•
•
Overview
Motivation
Implementation
Validation
Syntax Support for Compilation
Conclusions
2
Type Separation Mechanisms in
Java
Type
Interface
Class
We want separation of types.
Interface-Type
Code-Type
Data-Type
C++
MI (class)
MI (class)
MI (class)
Java
MI (interface)
SI (class)
SI (class)
Our Java
MI (interface)
MI (code-type)
*We do not support multiple data inheritance.
SI* (class)
3
Code Promotion Measurements Methods
DataInput
……
readFloat()
DataInputStream
DataOutput
……
readFloat()
……
writeFloat()
RandomAccessFile
……
writeFloat()
DataOutputStream
Class
Identical
Abstract
Abstract
and
Super
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%
Total
Method
Decrease
4
Basic Implementation - I
DataOutput
DataInput
readFloat()
readInt()
RandomAccessFile
VMT
MT
DataInput
readInt
RandomAccessFile
…
12 readFloat()
13 readInt()
Interface
MT
RandomAccessFile
readInt
Class
Code
Class Loader Changed.
Code is now
in the interface!
Code
Code was
in the class!
5
Dispatch Scenarios - Ambiguities
We detect ambiguities
at load-time.
InterfaceA x;
x = new ClassB();
x.alpha();
Scenario 3
Scenario 4
InterfaceA alpha()
InterfaceA alpha()
InterfaceB alpha()
InterfaceC alpha()
ClassB
InterfaceB alpha()
ClassA
ClassB
6
Basic Implementation - II
writeFloat()
hypothetical () in DataInput
DataOutput output;
DataInput input;
output = new RAF();
input = new RAF();
output.writeFloat();
this.readFloat();
input.readFloat();
DataOutput
DataInput
readFloat()
readInt()
RandomAccessFile
invokeinterface
invokevirtual
invokevirtual
invokeinterface
invokeinterface
7
Super Call Implementation
New syntax proposed for
super calls to interfaces
Bytecode
proposed
super(InterfaceA).alpha();
Bytecode previously
generated for super calls
invokespecial
#superclass/alpha()
invokemulti-super
#InterfaceA/alpha()
Execution-time change.
Bytecode
used
compiler script
invokeinterface
#InterfaceA/alpha()
8
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.
9
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.
10
Validation - MI Programs
Multiple Inheritance
Test Program
java.io
library
Sun JVM
output
11
Validation - MI Programs
Multiple Inheritance
Test Program
Tests the super
call mechanism
java.io
library
Uses the re-factored
java.io library
Our JVM
output

12
Adding Code in Interfaces
interface DataInput{
public float readFloat()
throws IOException;
/*MI_CODE
{
return
abstract class DataInput{
public float readFloat()
throws IOException
Script {
return
Float.intBitsToFloat(readInt());
Float.intBitsToFloat(readInt());
}
MI_CODE*/
}
}
;
}
javac
javac
DataInput.class
jasper
Script
DataInput.class
jasper
DataInput_MI.j
DataInput.j
DataInput.j
jasmin
DataInput.class
13
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.
14
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.
15
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
16
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];
//REMOVED
args_size = imb.args_size;
//ADDED
optop -= args_size;
…
if (pc[3] == 255)
//ADDED
mb = interface.MT[offset];
//ADDED
goto callmethod;
//ADDED
end if
//ADDED
…
end case
17