jruby_debugger.rb

require 'java'

module JRuby
  class Debugger
    VMM = com.sun.jdi.Bootstrap.virtual_machine_manager
    
    attr_accessor :vm
    
    def initialize(options = {})
      connectors = VMM.attaching_connectors
      if options[:port]
        connector = connectors.find {|ac| ac.name =~ /Socket/}
      elsif options[:pid]
        connector = connectors.find {|ac| ac.name =~ /Process/}
      end

      args = connector.default_arguments
      for k, v in options
        args[k.to_s].value = v.to_s
      end
      
      @vm = connector.attach(args)
    end

    # Generate a histogram of all classes in the system
    def histogram
      classes = @vm.all_classes
      counts = @vm.instance_counts(classes)
      classes.zip(counts)
    end
  end
end

ruby_histogram.rb

require 'jruby_debugger'

# connect to the VM
debugr = JRuby::Debugger.new(:hostname => 'localhost', :port => 12345)
histo = debugr.histogram
# sort by count
histo.sort! {|a,b| b[1] <=> a[1]}
# filter to only user-created Ruby classes with >0 instances
histo.each do |cls,num|
  next if num == 0 || cls.name[0..4] != 'ruby.' || cls.name[5..7] == 'jit'
  puts "#{num} instances of #{cls.name[5..-1].gsub('.', '::')}"
end

tti_digger.rb

require 'jruby_debugger'

# connect to the VM
debugr = JRuby::Debugger.new(:hostname => 'localhost', :port => 12345)
tti_class = debugr.vm.classes_by_name('ruby.TZInfo.TimezoneTransitionInfo')[0]
tti_obj = tti_class.instances(1)[0]

# get the varTable array from the object
var_table_field = tti_class.field_by_name('varTable')
tti_vars = tti_obj.get_value(var_table_field)

# get the metaclass object and class reference
metaclass_field = tti_class.field_by_name('metaClass')
tti_class_obj = tti_obj.get_value(metaclass_field)
tti_class_class = tti_class_obj.reference_type

# get the variable names from the metaclass object
var_names_field = tti_class_class.field_by_name('variableNames')
var_names = tti_class_obj.get_value(var_names_field)

# splice the names and values together
table = var_names.values.zip(tti_vars.values)
puts table