Call Java from JRuby


-See here


(Premise) JRuby can be used ・ java can be compiled

  1. Write Java code (file name is
  2. Compile and prepare JavaCallTest.class
  3. Within JRuby require'java'; java_import'JavaCallTest'
  4. Call appropriately

The following is a specific example.

Java code

public class JavaCallTest{
    public static String str1(){
        return "str1";

    public String str2() {
        return "str2";

JRuby code


require 'java'
java_import 'JavaCallTest'

p JavaCallTest.str1
  jct1 =
rescue => e
  puts e.class
  puts e.message
  puts e.backtrace

jct2 =
p jct2.str2
  p JavaCallTest.str2
rescue => e
  puts e.class
  puts e.message
  puts e.backtrace


$ jruby CallJava1.rb 
undefined method `str1' for #<Java::Default::JavaCallTest:0x553a3d88>
Did you mean?  str2
CallJava1.rb:7:in `<main>'
undefined method `str2' for Java::Default::JavaCallTest:Class
Did you mean?  str1
org/jruby/ `method_missing'
CallJava1.rb:17:in `<main>'


It seems that the names correspond as follows

--Java static sod is a Ruby class method --Java non-static sod is a Ruby instance method

It was hard because my knowledge of Java was 0

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