JRuby versions prior to 1.6 did not support Ruby C extensions, and even in 1.6 the support is still "in development" and considered experimental. This page lists common C extensions and non-C alternatives you can use to replace them.

• RDiscount - Use Maruku (pure Ruby) or markdown_j (wrapper around a Java library)

• RMagick - Try RMagick4J (implements ImageMagick functionality in Java) or preferably use alternatives mini_magick & quick_magick. For simple resizing, cropping, greyscaling, etc look at image_voodoo.

• Unicorn - Try any one of the following JRuby-based servers: Trinidad, Mizuno, Kirk or TorqueBox.

• Thin - Thin might compile and run but is not recommended. Try any one of the following JRuby-based servers: Trinidad, Mizuno, Kirk or TorqueBox.

• Typhoeus - The C extension doesn't currently compile. There's no equivalent library for JRuby, but you might try any of the pure-Ruby HTTP clients (net/http, httpclient, etc.). There are also several Java HTTP client libraries that will work (Apache HttpClient, HttpURLConnection, etc).

• mysql - Use activerecord-jdbc-adapter instead along with jdbc-mysql.

• mysql2 - Use activerecord-jdbc-adapter instead along with jdbc-mysql.

• sqlite3 - Use activerecord-jdbc-adapter instead along with jdbc-sqlite3.

• Nokogiri - For best results, use the pure-Java version of Nokogiri (1.5 or greater).

• yajl-ruby - Try json or json_pure instead. Unfortunately there is no known equivalent JSON stream parser.

• bson_ext - bson_ext isn't used with JRuby. Instead, some native Java extensions are bundled with the bson gem.

• win32ole - Use the jruby-win32ole gem (preinstalled in JRuby's Windows installer).

Please add to this list with your findings.

Note that the JRuby-Lint gem parses the contents of the list above to use for its Ruby gem checker. In order for JRuby-Lint to use the information, please adhere to the gem_name - instructions format.