A short sample of BinData Ruby gem to pp a constant pool of a Java class file.

test.rb

require 'bindata'
require 'pp'

class ConstantClassInfo < BinData::Record
  uint16be :name_index
end

class ConstantFieldrefInfo < BinData::Record
  uint16be :class_index
  uint16be :name_and_type_index
end

class ConstantMethodrefInfo < BinData::Record
  uint16be :class_index
  uint16be :name_and_type_index
end

class ConstantInterfaceMethodrefInfo < BinData::Record
  uint16be :class_index
  uint16be :name_and_type_index
end

class ConstantStringInfo < BinData::Record
  uint16be :string_index
end

class ConstantIntegerInfo < BinData::Record
  uint32be :bytes
end

class ConstantFloatInfo < BinData::Record
  uint32be :bytes
end

class ConstantLongInfo < BinData::Record
  uint32be :high_bytes
  uint32be :low_bytes
end

class ConstantDoubleInfo < BinData::Record
  uint32be :high_bytes
  uint32be :low_bytes
end

class ConstantNameAndTypeInfo < BinData::Record
  uint16be :name_index
  uint16be :descriptor_index
end

class ConstantUtf8Info < BinData::Record
  uint16be :len
  string :bytes, :read_length => :len
end

class ConstantMethodHandleInfo < BinData::Record
  uint8 :reference_kind
  uint16be :reference_index
end

class ConstantMethodTypeInfo < BinData::Record
  uint16be :descriptor_index
end

class ConstantInvokeDynamicInfo < BinData::Record
  uint16be :bootstrap_method_attr_index
  uint16be :name_and_type_index
end

class CpInfo < BinData::Record
  uint8 :tag
  choice :info, :selection => :tag do
    constant_class_info 7
    constant_fieldref_info 9
    constant_methodref_info 10
    constant_interface_methodref_info 11
    constant_string_info 8
    constant_integer_info 3
    constant_float_info 4
    constant_long_info 5
    constant_double_info 6
    constant_name_and_type_info 12
    constant_utf8_info 1
    constant_method_type_info 16
    constant_invoke_dynamic_info 18
  end
end

class ClassFile < BinData::Record
  endian :big
  uint32 :magic
  uint16 :minor_version
  uint16 :major_version
  uint16 :constant_pool_count
  array :cp_info, :type => :cp_info, :initial_length => lambda { constant_pool_count - 1}
end

io = File.open("Hello.class")
c = ClassFile.read(io)
pp c