CSC/ECE 517 Fall 2012/ch1 1w6 pp

This page is about the mixing static and dynamic Object Oriented code. This page will mention some difficulties when these two types of object orientation are mixed and then we will give on practical example of such in JRuby. JRuby is mixing of Java and Ruby, this page will cover the different types of such mixings and examples of each of them.

Difficulties in mixing static and dynamic oo

If the static oo makes use of generics for e.g. Java, then the generic types are erased during compilation for backward compatibility. As a result for a dynamically types language like Ruby, there will be problems with type conversion. For example, if you have a Map<String,String>, it will be seen as a simple Map, and JRuby will not be able to determine the correct types using reflection.

Advantages of JRuby

Advantages of JRuby over Ruby

- With JRuby you get the best of both worlds: Ruby applications and libraries, plus Java libraries. Anthreads, JRuby supports Unicode natively
- Code can be fully compiled ahead of time or just in time.
- In addition to native you can access those libraries with Ruby syntax (or Java syntax, if you want).
- On average JRuby, runs 2 and a half times faster than Ruby, except at startup

Advantages of JRuby over Java

- Ruby is fun, beautiful, and powerful language
- Ruby has features missing from Java

   Closure (blocks) 

   Open classes 

   Meta programming 

   Duck-typing 

   Domain Specific Language (DSL)

Examples JRuby

Driving Java from Ruby

This involves calling java methods from Ruby. This includes some java functionality in a Ruby code. The final code will be run on JVM. Ruby code can load and interact with Java Libraries. JRuby will forward any option to the underlying Java runtime if you preface it with -J. As a simple example we can use an ArrayList from java into Ruby code as below:

require ‘java’

list = java.util.ArrayList.new

list << ‘List of’
list << 3 
list << :assorted_items

list.each do |item|
puts “#{item.class}: #{item}”
end

Instead of just requiring a specific class you can also require the entire package. This can be done as follows:

module JavaLangDemo
  include_package "java.lang"
  # alternately, use the #import method
  import "java.lang"
end

One has to be careful in doing this as with name conflicts in both Ruby and Java can lead to loosing the Ruby functionality. For example if a File object is created using Java’s java.io.File class, then the Ruby constant File becomes useless -

import java.io.File
newfile = File.new("file.txt")

and then using -

File.open('README', 'r') {|f| puts f.readline }

This will give the error - NoMethodError: private method `open' called for Java::JavaIo::File:Class

This situation can be avoided by including the package in the module definition and then using that module scope to create using that File object.

newfile = JavaIO::File.new("file.txt")

Ruby from Java

There are three ways for using the JRuby Interpreter from Java.

Embedding JRuby

Embed Core is the main embedding API that ships with JRuby. This API offers a great deal of interoperability. You can call a Ruby method, crunch the results in Java, and hand data back into Ruby.

JSR 223, Scripting for the Java Platform

This provides an API framework for calling scripting code from within a Java application and passing data between the application and the script. These features make it possible to combine existing scripts with Java applications and to extend a Java application with general-purpose scripts that other Java applications can also use. JSR 223 Scripting APIs are available in JDK 6 software, and by default, the APIs support the JavaScript programming language. With a little setup as described in the section JRuby Scripting Engine Setup, you can use the JSR 223 Scripting APIs with any JSR 223-compliant scripting engine such as JRuby.

Steps: Java integration with Java 6 will be using the standard scripting API (JSR223). A JRuby scripting engine already exists and is located at https://scripting.dev.java.net/. Download and unzip the collection of jars from the documents and files section of the site (jsr223-engines.tar.gz or jsr223-engines.zip). Look in the uncompressed files for the jruby/build/jruby-engine.jar file. Add this file to your classpath and then use the code below to access the engine.

import javax.script.*;

public class jrubytry {
    public static void main(String[] args) throws Exception {
        ScriptEngineManager factory = new ScriptEngineManager();

        // Create a JRuby engine.
        ScriptEngine engine = factory.getEngineByName("jruby");

        // Evaluate JRuby code from string.
        try {
            engine.eval("puts('Hello')");
        } catch (ScriptException exception) {
            exception.printStackTrace();
        }
    }
}

The Bean Scripting Framework (BSF)

This is another way to call scripting code from within a Java application.

The Bean Scripting Framework, when used with JRuby, will allow you to conveniently to pass your own Java objects to your JRuby script. You can then use these objects in JRuby, and changes will affect your Java program directly. To run a JRuby script using BSF, you must first copy theBSF.jar file into your JAVA_HOME/lib/ext/ folder. Then, try the following:

import org.jruby.Ruby.*;
import org.jruby.*;
import org.jruby.javasupport.bsf.*;
import org.apache.bsf.BSFException;
import org.apache.bsf.BSFManager;
{...}
JLabel mylabel = new JLabel();
BSFManager.registerScriptingEngine("ruby", 
                                   "org.jruby.javasupport.bsf.JRubyEngine", 
                                   new String[] { "rb" });

BSFManager manager = new BSFManager();

/* Import an object using declareBean then you can access it in JRuby with $<name> */
 
manager.declareBean("label", mylabel, JFrame.class);
manager.exec("ruby", "(java)", 1, 1, "$label.setText(\"This is a test.\")");