See how ready your Ruby code is to run on JRuby.

JRuby-Lint is a simple tool that allows you to check your project code and configuration for common gotchas and issues that might make it difficult to run your code on JRuby.


JRuby-Lint requires JRuby to run. So, install JRuby first if you already haven't, then gem install jruby-lint.

Then simply run jrlint in your project to receive a report of places in your project where you should investigate further.


Here is a list of the current checks implemented:

  • Report usage of ObjectSpace.each_object and ObjectSpace._id2ref which are expensive and disabled by default
  • Report usage of Thread.critical, which is discouraged in favor of a plain Mutex.
  • Report known gems and libraries that use C extensions and try to provide known alternatives.
  • Report usage of Kernel#fork (which does not work) and Kernel#exec (which does not replace the current process).
  • Report usage of Timeout::timeout which, when used excessively tend to be slow and expensive because of native threads
  • Report behavior difference when using system('ruby'), which launches the command in-process in a new copy of the interpreter for speed


Here is a list of checks and options we'd like to implement:

  • Options to save report off to a file.
    • Text, HTML formats
  • Add in check for `` to make sure not execing ruby ...
  • Report on more threading and concurrency issues/antipatterns
    • arr.each {|x| arr.delete(x) }
  • Try to detect IO/File resource usage without blocks
  • Check .gemspec files for extensions and extconf.rb for #create_makefile and warn about compiliing C extensions
  • Check whether Rails production.rb contains config.threadsafe!
  • Detect ERB files and skip them, or...
  • Detect ERB files and pre-process them to Ruby source with Erubis
  • Detect Bundler gems that have a platforms qualifier and ignore "platforms :ruby"
  • Change to use jruby-parser
  • 1.8/1.9 parser support/configuration without having to run JRuby itself in the right mode
  • Allow use of a comment marker to suppress individual checks

Further Down the Road

  • Arbitrary method/AST search functionality
  • Code rewriter: option to change code automatically where it's feasible
  • Revive or build an isit.jruby.org site for tracking
  • Make JRuby-Lint submit results to tracking site based on lint passes and/or test suite runs