Detect multiple occurrences of Java classes in jar files

A developer came to me for help and said that a recent upgrade to a newer Java library broke the code. The developer was getting was a class not found exception. The developer said that the old jar file was replaced with a newer jar file. I know this same library upgrade in other applications did not have any problems, so this sounded like an issue with duplicate occurrence of the same class in the classpath. Since my Ruby Programming Language book was in handy access when this developer came to me, I ended up writing a small ruby script that went through all the jar files packed in the web archive and dumped an output of the fully qualified class along with the jar file that it can be found in and a count of the number of occurrences. Here's the ruby script running on a Windows platform:

java_home="C:\\sdk\\jdk1.6.0_24\\bin"

classmaps = Hash.new(0)

Dir["*.jar"].each do |file|
  cmd = sprintf("%s\\jar  -tf %s",java_home,file)
  lines = `#{cmd}`
  lines.each do |line|
    if line =~ /.class/
      key = line.chomp
      if classmaps.key?(key) then
        old = classmaps[key]
        data = sprintf("%s,%s",old,file)
        classmaps[key] = data 
      else
        classmaps[key] = file
      end
    end
  end
end


classmaps.each do |k,v|
    tokens = v.split(",")
    printf("%s,%i,%s\n",k,tokens.size,v)
end

I then loaded the output file in csv format into Excel and sorted the occurrences in descending order and was flabbergasted to find numerous entries that look something like the following entry:

Class Name	Occurrence	Found in Jar files:
kodo/jdo/KodoExtent.class	6	mylib.jar	mylib.jar	mylib.jar	kodo-api.jar	kodo-runtime.jar	kodo.jar

Somehow, for this application team, they were able to add the same class to the same jar file multiple times. I never thought that it was possible to add the same class to the same jar file multiple times nor would I ever want to do that. When I finally confronted the application team about this, they recognize that their build process is broken and need to fix their build process. But as a quick test, the developer removed some of the duplicate classes related to the library upgrade and the problems went away. Until Java 8 is introduced with Modularity capabilities, this tool has will be a handy way for me to check duplicate classes given a list of jar files.