Perl script that can determine the version of a java class file

java_class_version.pl

#!/usr/bin/env perl

use warnings;
use strict;

use Pod::Usage;

=head1 NAME

java_class_version.pl - determines the version of a java class file

=head1 USAGE

java_class_version.pl [class file ...]

Example:

 $ java_class_version.pl *.class
 AbstractFacesServlet.class 6 (1.6) (50.0)
 AbstractFacesServlet$InnerFacesContext.class 6 (1.6) (50.0)
 DocumentServlet.class 6 (1.6) (50.0)

=head1 OPTIONS

No options. But if you call the script without any class file names it
will display its manual page.

=head1 DESCRIPTION

This script can tell you the version of a java class file. This can
come in handy if you want to deploy some third party jar on an older
platform. You don't need to do a try-and-error, but can determine
compability upfront.

The class file format is descibed in [ClassFile.doc]. The important
parts are the first eight bytes, that contain a magic number, the
minor and the major version of the file. It looks like this for a java
5 class file:

 +----+----+----+----+----+----+----+----+
 | ca | fe | ba | be | 00 | 00 | 00 | 31 |
 +----+----+----+----+----+----+----+----+
 |   magcic number   |  minor  |  major  |
 +-------------------+---------+---------+

The integers are always stored in big-endian order, also know as
network order.

What java platform supports which class file version is a little fuzzy
for the first 1.0 and 1.1 releases. But
[source_target_class_file_version] has a nice summary, that at least
is simple for all later releases.

=for text
=encoding utf-8
=end

=head1 AUTHOR

Søren Lund, C<< <soren at lund.org> >>

=head1 SEE ALSO

=over

=item [ClassFile.doc]

http://java.sun.com/docs/books/jvms/second_edition/html/ClassFile.doc.html

=item [source_target_class_file_version]

http://blogs.sun.com/darcy/entry/source_target_class_file_version

=back

=head1 COPYRIGHT

Copyright (C) 2012 Søren Lund

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; version 2 dated June, 1991 or at your option
any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

A copy of the GNU General Public License is available in the source tree;
if not, write to the Free Software Foundation, Inc.,
59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.

=cut

my %java_platforms = (
    "45.3" => "1.1",
    "46.0" => "1.2",
    "47.0" => "1.3",
    "48.0" => "1.4",
    "49.0" => "5 (1.5)",
    "50.0" => "6 (1.6)",
    "51.0" => "7 (1.7)"
   );

pod2usage(-verbose => 2) if $#ARGV < 0;

foreach my $arg (@ARGV) {
    open my $file, '<', $arg or die $!;
    my $magic = read_int($file, 4);
    if ($magic != 0xcafebabe) {
        printf STDERR "%s is not a valid Java class.\n", $arg;
    } else {
        my $minor = read_int($file, 2);
        my $major = read_int($file, 2);
        my $version = sprintf("%d.%d", $major, $minor);
        printf "%s %s (%s)\n", $arg, $java_platforms{$version}, $version;
    }
    close $file;
}

sub read_int {
    my ($file, $size) = (@_);
    my $data;
    read $file, $data, $size;
    return unpack($size==2?'n':'N',$data);
}