gems that are used in benchmark should not be defined in gemspec.

Gemfile

@@ -1,2 +1,9 @@
 source 'https://rubygems.org'
+
 gemspec
+
+group :benchmark do
+  gem 'fuzzy-string-match'
+  gem 'hotwater'
+  gem 'amatch'
+end

jaro_winkler.gemspec

@@ -1,24 +1,25 @@
 # coding: utf-8
-lib = File.expand_path("../lib", __FILE__)
+lib = File.expand_path('../lib', __FILE__)
 $LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
 require 'jaro_winkler/version'
 
 Gem::Specification.new do |spec|
-  spec.name = "jaro_winkler"
+  spec.name = 'jaro_winkler'
   spec.version = JaroWinkler::VERSION
-  spec.authors = ["Jian Weihang"]
-  spec.email = "tonytonyjan@gmail.com"
-  spec.extensions = ["ext/jaro_winkler/extconf.rb"]
-  spec.summary = "Ruby & C implementation of Jaro-Winkler distance algorithm which both support UTF-8 string."
-  spec.description = "It's a implementation of Jaro-Winkler distance algorithm, it uses C extension and will fallback to pure Ruby version in JRuby. Both implementation supports UTF-8 string."
-  spec.homepage = "https://github.com/tonytonyjan/jaro_winkler"
-  spec.license = "MIT"
-  spec.files = Dir["lib/**/*.rb", "ext/**/*.{h,c}"]
-  spec.add_development_dependency "bundler", "~> 1.7"
-  spec.add_development_dependency "rake", "~> 10.0"
-  spec.add_development_dependency "rake-compiler"
-  spec.add_development_dependency "minitest"
-  spec.add_development_dependency "fuzzy-string-match"
-  spec.add_development_dependency "hotwater"
-  spec.add_development_dependency "amatch"
+  spec.authors = ['Jian Weihang']
+  spec.email = 'tonytonyjan@gmail.com'
+  if RUBY_PLATFORM == 'java'
+    spec.platform = 'java'
+  else
+    spec.extensions = ['ext/jaro_winkler/extconf.rb']
+  end
+  spec.summary = 'Ruby & C implementation of Jaro-Winkler distance algorithm which both support UTF-8 string.'
+  spec.description = %q{It's a implementation of Jaro-Winkler distance algorithm, it uses C extension and will fallback to pure Ruby version in JRuby. Both implementation supports UTF-8 string.}
+  spec.homepage = 'https://github.com/tonytonyjan/jaro_winkler'
+  spec.license = 'MIT'
+  spec.files = Dir['lib/**/*.rb', 'ext/**/*.{h,c}']
+  spec.add_development_dependency 'bundler', '~> 1.7'
+  spec.add_development_dependency 'rake', '~> 10.0'
+  spec.add_development_dependency 'rake-compiler'
+  spec.add_development_dependency 'minitest'
 end