Something has changed in the Gemspec for bson 1.7.1, causing the non-Java version to be installed on JRuby. This in turn causes it to fail because it's missing the "mongo-1.6.5.jar" file in ext/.

My Gemfile.lock differs like this when I upgrade to 1.7.1:

• bson (1.7.0)
• bson (1.7.0-java)
• bson_ext (1.7.0)
• bson (~> 1.7.0)
  + bson (1.7.1)
  + bson_ext (1.7.1)
  + bson (~> 1.7.1)

Notice that the "-java" thing is there for 1.7.0 but not for 1.7.1.

Doing a normal "gem install bson" seems to install the 1.7.1-java version however, so you need to use Bundler to reproduce.