JRuby Reference

require 'path/to/file.jar'

require can take a path to a jar file. It searches the JRuby $LOAD_PATH for the file.

require 'java'

A special `require 'java'` directive in your file will give you access to any bundled Java libraries (classes within your java class path). However, this will not give you access to any non-bundled libraries.

#each

JRuby supports calling the `#each` method on Enumerable objects

Name translations

Java: org.foo.department.Widget

Ruby: Java::OrgFooDepartment::Widget

For the top-level Java packages java, javax, org, and com you can type in a fully qualified class name:

java_import java.lang.System

You can use your own top-level package names by adding a def:

def edu
  Java::Edu
end

java_import edu.example.MyClass

Importing a package into a class or module

module M
 include_package "org.xxx.yyy"
 # now any class within "org.xxx.yyy" will be available within
 # this module, ex if "org.xxx.yyy.MyClass" exists
 # a = MyClass.new
end

Note that there are issues with include_package, discussed at http://jira.codehaus.org/browse/JRUBY-5558.

enums

Available as constants:

ms_constant_value = java.util.concurrent.TimeUnit::MILLISECONDS

Alternative Names and Beans Convention

You can use underscore-separated names in addition to camel-cased names. These two call the same method:

java.lang.System.currentTimeMillis
java.lang.System.current_time_millis

JRuby also translates methods following the 'beans-convention':

x.getSomething            becomes   x.something
x.setSomething(newValue)  becomes   x.something = new_value
x.isSomething             becomes   x.something?

Constructors

JavaClass.new or JavaClass.new(x,y,z) generally work as expected. If you wish to select a particular constructor by signature use reflection:

 # Get the the three-integer constructor for this class
 construct = JavaClass.java_class.constructor(Java::int, Java::int, Java::int)
 # Use the constructor
 object = cons.new_instance(0xa4, 0x00, 0x00)