StandaloneJarsAndClasses

Often you may want to generate a "standalone" artifact using JRuby plus your own Ruby code. This may be in the form of a "real" Java .class file, or it may be in the form of an executable .jar file.

Standalone Class Files

Many Java applications need to have real .class files on disk. Normally, JRuby's compiler does not produce .class files that look like typical Java classes. It is possible, however, to use the --java or --javac flags to JRuby's compiler to generate normal .class files from simple Ruby class structures, allowing them to be used from Java as though they were normal Java classes. See Generating Java Classes.

Standalone Executable Jar Files

You may also want to bundle JRuby plus your own Ruby code into a single .jar file that when run launches your application. Starting with JRuby 1.6, you can do this by changing the Main-Class for the jar file to point at org.jruby.JarBootstrapMain and adding your own jar-bootstrap.rb to the root of the jar file. This file will be loaded and launched as though it were specified on the command line.

Assuming you downloaded a package from https://www.jruby.org/download and expanded it into ~/projects/jruby, you can follow these instructions:

~/projects/jruby $ cp lib/jruby.jar myapp.jar

~/projects/jruby $ cat jar-bootstrap.rb
puts "hello"

~/projects/jruby $ jar ufe myapp.jar org.jruby.JarBootstrapMain jar-bootstrap.rb

~/projects/jruby $ java -jar myapp.jar
hello

Here, we've taken the base jruby.jar file, copied it to a new name, modified the "Main-Class" by using the -e flag to 'jar', and added our own jar-bootstrap. You can also embed additional .rb files into the jar, and the root of the jar file will be used as an implicit LOAD_PATH entry.

See also the Warbler utility for packaging up a command-line or web application as a standalone .jar or .war, including gem dependencies.