JRuby and RubyGems

For reasons too tangential to cover here, it was decided to port a Ruby 1.8.7 project to JRuby (>= 1.6.0). This worked well enough, except for the fatal flaw in JRuby: using gems.

To begin with, getting *any* gem with native extensions to build in JRuby requires some manual intervention.

The reason? The RVM JRuby has its Ruby include files under cext/src/include/, instead of lib/native/include like the other RVM Rubies. This appears to be a known issue.

A symlink handily solves the problem:

bash$ cd /usr/local/rvm/rubies/jruby-head
bash$ ln -s cext/src/include lib/native/include

Making this change will allow most gems with native extensions (e.q. pg, sqlite
3, etc) to be built.


The qtbindings gem, however, still fails with an error like the following:

bash$ sudo rvm jruby-head do gem install qtbindings                                

CMake Error at /usr/share/cmake-2.8/Modules/FindPackageHandleStandardArgs.cmake:91 (MESSAGE):

  Could NOT find Ruby (missing: RUBY_INCLUDE_DIR)

Call Stack (most recent call first):

  /usr/share/cmake-2.8/Modules/FindPackageHandleStandardArgs.cmake:252 (_FPHSA_FAILURE_MESSAGE)

  cmake/modules/FindRuby.cmake:249 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)

  CMakeLists.txt:18 (FIND_PACKAGE)

-- Configuring incomplete, errors occurred!

make: [build] Error 1 (ignored)

cd ext/build; make

make[1]: Entering directory `/usr/local/rvm/gems/jruby-head/gems/qtbindings-4.6.3.4/ext/build'

make[1]: *** No targets specified and no makefile found.  Stop.

make[1]: Leaving directory `/usr/local/rvm/gems/jruby-head/gems/qtbindings-4.6.3.4/ext/build'

make: *** [build] Error 2

Unpacking the gem, fixing the extconf.rb to properly find JRuby, and repacking the gem gets past these problems, but ultimately fails when compiling the actual C extension code for Qt4.

The reason? Qt4 uses heavy inspection of the stack frame in order to implement its signal/slot mechanisms, and JRuby does not provide access to this information. Until equivalent functions for rb_frame_callee() and the like are provided by JRuby, Qt4 will remain incompatible with it.

Postscript: For the curious, the following lines (or variants thereof) can be added to extconf.rb to fix the Cmake build issues:

    file.puts "-DRUBY_EXECUTABLE=/usr/local/rvm/rubies/jruby-head/bin/jruby \\"

    file.puts "-DRUBY_LIBRARY=/usr/local/rvm/rubies/jruby-head/lib/native/x86_64-Linux/libjruby-cext.so \\"

    file.puts "-DRUBY_LIB_PATH=/usr/local/rvm/rubies/jruby-head/lib/native/x86_64-Linux \\"

    file.puts "-DRUBY_INCLUDE_DIR=/usr/local/rvm/rubies/jruby-head/cext/src/include/ruby \\"

    file.puts "-DRUBY_CONFIG_INCLUDE_DIR=/usr/local/rvm/rubies/jruby-head/cext/src/include/ruby \\"

    file.puts "-DCMAKE_INSTALL_PREFIX=/usr/local/rvm/rubies/jruby-head \\"

The lines go in the 'else' block of the 'if windows' condition, where the other cmake options are defined (around line 216, or search for '-DENABLE_SMOKE').