build.gradle

apply plugin: 'java'

repositories {
    mavenCentral()
}

dependencies {
    compile 'org.jruby:jruby-complete:9.1.5.0'
    testCompile 'junit:junit:4.+'
}

sourceSets {
    main {
        java {
            srcDir '.'
            exclude 'Test*.java'
        }
    }
    test {
        java {
            srcDir '.'
            include 'Test*.java'
        }
    }
}

test {
    testLogging {
        events 'passed', 'skipped', 'failed', 'standardOut', 'standardError'
    }
}

task run(type:JavaExec) {
    main = project.hasProperty('main') ? project.getProperty('main') : 'JRubyInstance'
    classpath = sourceSets.main.runtimeClasspath
}

global_variable.rb

puts ''
puts 'Main  $LOAD_PATH:'
$LOAD_PATH.each { |path| puts '    * ' + path.to_s }
puts 'Main  $FOO : <%s>' % $FOO
# puts 'Main  $BAR : <%s>' % $BAR

puts ''
puts ' Main> Appending to $LOAD_PATH'
puts ' Main> $LOAD_PATH << \'hogehoge\''
$LOAD_PATH << 'hogehoge'
puts ' Main> Assigning to $FOO'
puts ' Main> $FOO = \'HOGE\''
$FOO = 'HOGE'
puts ''

puts 'Main  $LOAD_PATH:'
$LOAD_PATH.each { |path| puts '    * ' + path.to_s }
puts 'Main  $FOO : <%s>' % $FOO
# puts 'Main  $BAR : <%s>' % $BAR
puts ''

JRubyInstance.java

import org.jruby.embed.LocalContextScope;
import org.jruby.embed.LocalVariableBehavior;
import org.jruby.embed.ScriptingContainer;

public class JRubyInstance {
    public static void main(String[] args) {
        /*
        final ScriptingContainer container1 = new ScriptingContainer(LocalContextScope.SINGLETON);
        container1.runScriptlet("puts 'Ruby1 $LOAD_PATH:'");
        container1.runScriptlet("$LOAD_PATH.each { |path| puts '    * ' + path.to_s }");
        container1.runScriptlet("puts 'Ruby1 $FOO : <%s>' % $FOO");
        container1.runScriptlet("puts 'Ruby1 $BAR : <%s>' % $BAR");

        container1.runScriptlet("puts ''");
        container1.runScriptlet("puts 'Ruby1> Assigning to BAR'");
        container1.runScriptlet("puts 'Ruby1> $BAR = \\'BAZ\\''");
        container1.runScriptlet("$BAR = 'BAZ'");
        container1.runScriptlet("puts ''");

        container1.runScriptlet("puts 'Ruby1 $LOAD_PATH:'");
        container1.runScriptlet("$LOAD_PATH.each { |path| puts '    * ' + path.to_s }");
        container1.runScriptlet("puts 'Ruby1 $FOO : <%s>' % $FOO");
        container1.runScriptlet("puts 'Ruby1 $BAR : <%s>' % $BAR");
        */

        final String[] jrubyArgs = new String[1];
        jrubyArgs[0] = "global_variable.rb";
        org.jruby.Main.main(jrubyArgs);

        final ScriptingContainer container2 = new ScriptingContainer(LocalContextScope.SINGLETON);

        /*
        container1.runScriptlet("puts 'Ruby1 $LOAD_PATH:'");
        container1.runScriptlet("$LOAD_PATH.each { |path| puts '    * ' + path.to_s }");
        container1.runScriptlet("puts 'Ruby1 $FOO : <%s>' % $FOO");
        container1.runScriptlet("puts 'Ruby1 $BAR : <%s>' % $BAR");
        */
        container2.runScriptlet("puts 'Ruby2 $LOAD_PATH:'");
        container2.runScriptlet("$LOAD_PATH.each { |path| puts '    * ' + path.to_s }");
        container2.runScriptlet("puts 'Ruby2 $FOO : <%s>' % $FOO");
        // container2.runScriptlet("puts 'Ruby2 $BAR : <%s>' % $BAR");

        container2.runScriptlet("puts ''");
        container2.runScriptlet("puts 'Ruby2> Assigning to FOO'");
        container2.runScriptlet("puts 'Ruby2> $FOO = \\'QUX\\''");
        container2.runScriptlet("$FOO = 'QUX'");
        container2.runScriptlet("puts ''");

        /*
        container1.runScriptlet("puts 'Ruby1 $LOAD_PATH:'");
        container1.runScriptlet("$LOAD_PATH.each { |path| puts '    * ' + path.to_s }");
        container1.runScriptlet("puts 'Ruby1 FOO : <%s>' % $FOO");
        container1.runScriptlet("puts 'Ruby1 BAR : <%s>' % $BAR");
        */
        container2.runScriptlet("puts 'Ruby2 $LOAD_PATH:'");
        container2.runScriptlet("$LOAD_PATH.each { |path| puts '    * ' + path.to_s }");
        container2.runScriptlet("puts 'Ruby2 $FOO : <%s>' % $FOO");
        // container2.runScriptlet("puts 'Ruby2 $BAR : <%s>' % $BAR");
    }
}

./gradlew run
:compileJava UP-TO-DATE
:processResources UP-TO-DATE
:classes UP-TO-DATE
:run
Ruby1 FOO : <>
Ruby1 BAR : <>

Ruby1> Assigning to BAR
Ruby1> $BAR = 'BAZ'

Ruby1 FOO : <>
Ruby1 BAR : <BAZ>

Main  FOO : <>
Main  BAR : <>

 Main> Assigning to FOO
 Main> $FOO = 'HOGE'

Main  FOO : <HOGE>
Main  BAR : <>

Ruby1 FOO : <>
Ruby1 BAR : <BAZ>
Ruby2 FOO : <>
Ruby2 BAR : <BAZ>

Ruby2> Assigning to FOO
Ruby2> $FOO = 'QUX'

Ruby1 FOO : <QUX>
Ruby1 BAR : <BAZ>
Ruby2 FOO : <QUX>
Ruby2 BAR : <BAZ>

BUILD SUCCESSFUL

Total time: 4.336 secs

./gradlew run
:compileJava UP-TO-DATE
:processResources UP-TO-DATE
:classes UP-TO-DATE
:run

Main  $LOAD_PATH:
    * uri:classloader:/META-INF/jruby.home/lib/ruby/2.3/site_ruby
    * uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib
Main  $FOO : <>

 Main> Appending to $LOAD_PATH
 Main> $LOAD_PATH << 'hogehoge'
 Main> Assigning to $FOO
 Main> $FOO = 'HOGE'

Main  $LOAD_PATH:
    * uri:classloader:/META-INF/jruby.home/lib/ruby/2.3/site_ruby
    * uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib
    * hogehoge
Main  $FOO : <HOGE>

Ruby2 $LOAD_PATH:
    * uri:classloader:/META-INF/jruby.home/lib/ruby/2.3/site_ruby
    * uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib
Ruby2 $FOO : <>

Ruby2> Assigning to FOO
Ruby2> $FOO = 'QUX'

Ruby2 $LOAD_PATH:
    * uri:classloader:/META-INF/jruby.home/lib/ruby/2.3/site_ruby
    * uri:classloader:/META-INF/jruby.home/lib/ruby/stdlib
Ruby2 $FOO : <QUX>

BUILD SUCCESSFUL

Total time: 4.54 secs