Defining JVM(Java Virtual Machine)

NAME

Java::JVM::Classfile - Parse JVM Classfiles

SYNOPSIS

use Java::JVM::Classfile;

my $c = Java::JVM::Classfile->new("HelloWorld.class");
print "Class: " . $c->class . "\n";
print "Methods: " . scalar(@{$c->methods}) . "\n";

DESCRIPTION

The Java Virtual Machine (JVM) is an abstract machine which processes JVM classfiles. Such classfiles contain, broadly speaking, representations of the Java methods and member fields forming the definition of a single class, information to support the exception mechanism and a system for representing additional class attributes. The JVM itself exists primarily to load and link classfiles into the running machine on demand (performed by the Class Loader), represent those classes internally by means of a number of runtime data structures and facilitate execution (a role shared between the Execution Engine (which is responsible for execution of JVM instructions) and the Native Method Interface which allows a Java program to execute non-Java code, generally ANSI C/C++.

This Perl module reveals the information in a highly-compressed JVM classfile by representing the information as a series of objects. It is hoped that this module will eventually lead to a JVM implementation in Perl (or Parrot), or possibly a way-ahead-of-time (WAT) to Perl (or Parrot) compiler for Java.

It is important to remember that the Java classfile is highly-compressed. Classfiles are intended to be as small as possible as they are often sent across the network. This may explain the slightly odd object tree. One of the most important things to consider is the idea of a constant pool. All constants (constant strings, method names and signatures etc.) are clustered in the constant pool at the start of the classfile, and sprinkled throughout the file are references to the constant pool. The module attempts to hide this optimisation as much as possible from the user, however.

It is probably important to at least have briefly read "The JavaTM Virtual Machine Specification", http://java.sun.com/docs/books/vmspec/

METHODS


new
This is the constructor, it takes the filename of the classfile to parse and returns an object:

my $c = Java::JVM::Classfile->new("HelloWorld.class");

magic

This method returns the magic number for the classfile. All valid classfiles should have the magic number 0xCAFEBABE:

my $magic = $c->magic;

version

This method returns the version of the classfile. The version consists of a major number and a minor number. For example, "45.3" has major number 45 and minor number 3:

my $version = $c->version;

class

This method returns the name of the class that this classfile corresponds to:

my $class = $c->class;

superclass

This method returns the name of the superclass of the class that this classfile corresponds to:

my $superclass = $c->superclass;

constant_pool

This method returns the constant pool entries as an array reference. Each entry is an object. Currently undocumented.

my $constant_pool = $c->constant_pool;

access_flags

This method returns the access flags for the class as an array reference. Possible flags are:

abstract

Declared abstract; may not be instantiated
final

Declared final; no subclasses allowed
interface

Is an interface, not a class
public

Declared public; may be accessed from outside its package
super

Treat superclass methods specially when invoked by the invokespecial instruction

print "Flags: " . join(", ", @{$c->access_flags}) . "\n";

interfaces

This method returns an array reference of the interfaces defined in the classfile. Currently unimplemented:

my $interfaces = $c->interfaces;

fields

This method returns an array reference of the fields defined in the classfile. Currently unimplemented:

my $fields = $c->fields;


methods

This method returns an array reference of the methods defined in the classfile:

my $methods = $c->methods;

Each Java method is represented by an object which has the following methods: name, descriptor, access_flags and attributes. name and descriptor return the method name and descriptor. Possible access flags are:

abstract

Declared abstract; no implementation is provided
final

Declared final; may not be overridden
native

Declared native; implemented in a language other than Java
private

Declared private; accessible only within the defining class
protected

Declared protected; may be accessed within subclasses
public

Declared public; may be accessed from outside its package
static

Declared static
strict

Declared strictfp; floating-point mode is FP-strict
synchronized

Declared synchronized; invocation is wrapped in a monitor lock

Various attributes are possible, the most common being the Code attribute, where the value holds information about the Java bytecode for the method:

foreach my $method (@{$c->methods}) {
print " " . $method->name . " " . $method->descriptor;
print "\n ";
print "is " . join(", ", @{$method->access_flags});
print "\n ";
print "has attributes: ";
foreach my $att (@{$method->attributes}) {
my $name = $att->name;
my $value = $att->value;
if ($att->name eq 'Code') {
print " $name: ";
print "stack(" . $value->max_stack . ")";
print ", locals(" . $value->max_locals . ")\n";
foreach my $instruction (@{$value->code}) {
print $instruction->label . ':' if defined $instruction->label;
print "\t" . $instruction->op . "\t" . (join ", ", @{$instruction->args}) . "\n";
}
print "\n";
foreach my $att2 (@{$value->attributes}) {
my $name2 = $att2->name;
my $value2 = $att2->value;
if ($name2 eq 'LineNumberTable') {
print "\tLineNumberTable (offset, line)\n";
print "\t" . $_->offset . ", " . $_->line . "\n" foreach (@$value2);
} else {
print "!\t$name2 = $value2\n";
}
}
} else {
print "!\t$name $value\n";
}
}
print "\n";
}

Note that in the case of the Code attribute, the value contains an object which has three main methods: max_stack (the maximum depth of stack needed by the method), max_locals (the number of local variables used by the method), code (returns an arrayref of instruction objects which have op, args and label methods), and attributes. One attribute that Code can have is the LineNumberTable attributes, which has an arrayref of objects as a value. These have offset and line methods, representing a link between bytecode offset and sourcecode line.
attributes

This method returns an array reference of the attributes defined in the classfile. Attributes are common in many places in the classfile - here in particular we have the classfile attributes.

my $attributes = $c->attributes;

Attributes are represented by an object that has name and value methods:

foreach my $attribute (@{$c->attributes}) {
print " " . $attribute->name . " = " . $attribute->value . "\n";
}

Possible attributes include the SourceFile attribute, the value of which is the source file that was compiled into this classfile.