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Compact Java Virtual Machine

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This is my modest implementation of a java bytecode interpreter I created, mainly for educational purposes.

Budget: 120 working hours

Projects

• CompactJVM - virtual machine
• CompactJVMLib - a library that contains a dummy implementation of several java.lang classes (such as String and Exception) and native objects (FileReader)
• SatSolver - a demo project that can be tested with the JVM (boolean satisfiability problem solver)

Virtual machine

Installation

• just execute ant compile from within each folder of each project or use NetBeans IDE

Run

Run the CompactJVM as follows:

java -jar CompactJVM.jar <app-dir> <lib-dir> <main-class> <args>

• app-dir is path to the bytecode directory of your application you want to execute
• lib-dir is path to the bytecode directory of libraries (CompactJVMLib)
• main-class is the namespace of your main class (e.g. satSovler/Main)
• args are arguments of your application

Example: executing sat solver from within dist directory of the CompactJVM project:

java -jar CompactJVM.jar ../../SatSolver/build/classes ../../CompactJVMLib/build/classes satSolver/Main ../../SatSolver/examples/cnf_3_12.txt result.txt

JVM Config

There is no configuration file. If you want to increase heap size or enable or disable particular logging, go to the compactjvm.jvm.Main.java. The code should be commented enough to give you a bigger picture.

Packages

• compactjvm.attributes - attributes providing information about exceptions, constants, methods and fields
• compactjvm.classfile - entities describing classfile such as method definitions, exception tables and field definitions
• compactjvm.cpentities - constant pool entity descriptors (primitive types, classes, interfaces, strings)
• compactjvm.definitions - enums, instruction set, attribute types
• compactjvm.exceptions - exceptions the JVM may throw (e.g. ArrayOutOfBoundsException)
• compactjvm.jvm - core of the virtual machine (stack frame, heap, logger, garbage collector)
• compactjvm.jvm.instructions - instructions and their implementation
• compactjvm.natives - implementations of several native classes (FileReader, TextReader and TextWriter)
• compactjvm.parsing - classes for parsing a class file
• compactjvm.structures - bytecode entities (primitive fields, pointers, array references, objects)

Features:

• Garbage collector - Mark&Copy algorithm, searches for objects in use that are to be copied into other half of the heap, the first one will be discared afterwards
• Exceptions - support for exception from within and outside the running application (ArithmeticException, NullPointerException,...)
• Native methods - IO operations are handled by native methods, implemented in CompactJVM
• Inheritance - simple inheritance with virtual invocation

What is not implemented

• multithreading, annotations, generics, anonymous objects, Java7+ features

Implemented instructions:

aaload, aastore, aconstnull, aload, anewarray, areturn, astore, athrow,
arraylength, baload, bipush, caload, castore, dup, fconst, getstatic,
 getfield, goto, iaload, iastore, iadd, iconst, idiv, iinc, iload, iloadN,
 imul, ineg, ireturn, istore, istoreN, isub, ifacmpeq, ifacmpne, ificmpge,
 ificmpgt, ificmple, ificmpeq, ificmpne, ifeq, ifge, ifgt, iflt, ifle, ifne,
 ifnonnull, ifnull, invokespecial, invokestatic, invokevirtual, lconstn,
 lload, lloadn, ldc, newarray, new, nop, pop, putfield, sipush

SAT Solver

This is a demonstration project that can be executed in CompactJVM

Arguments

• path to the input file with clauses
• path to the output file with results

Input file

• line begining with c contains a comment
• the first line must contain p letter, followed by 2 values: number of variables M and number of clauses N
• N lines follow, containing clauses as numbers divided by spaces. Each number 1-M represents a variable. If the number is negative, the variable is negated. At the end of each line must be '0' as an ending character.
• You can find several examples inside SatSolver/examples folder

Example:

c test 176
p 3 4
1 -2 3
-1 2 -3
1 -3
2 -3

Output file:

• contains a message whether or not is the formula satisfiable
• if the formula is satisfable, the output file will contain an interpretation that satisfies it as follows (e.g. [True True False])