Monday, March 19, 2007

How to build completely dynamic example with Spring 2, Scriptlandia, Beanshell, Groovy and JRuby

Recently, I found this project: GroovyWorks;.
It tries to use together such things as Java, Groovy, Spring 2 and Struts 2. Because Struts actions are written as Groovy scripts, it is not required to restart web application for each and every change. It is possible because of dynamic nature of Groovy language.

But still, it's not completely dynamic. Small portion of the system is written in Java and for each change in Java you have to recompile and redeploy your web application. Is it possible to make it completely dynamic?

In the following lines I will explain how to build such completely dynamic code. It is standalone application, so Struts 2 is not required. But we still want to have IoC container. Spring 2 fits for our needs, especially with new struts-scripting library, that supports 3 popular languages: Groovy, JRuby and Beanshell.

1. In order to work with scripts inside Spring 2 we have to create Java classes first. Let's create animal farm that consists of animals:


// Animal.java

public interface Animal {

public void makeSound();

}

// AnimalFarm.java

import java.util.List;

public class AnimalFarm {
private List animals;

public AnimalFarm() {
System.out.println("New Animal farm has been created.");
}

public void setAnimals(List animals) {
this.animals = animals;
}

public void wakeUp() {
for(int i=0; i < animals.size(); i++) {
Animal animal = (Animal)animals.get(i);

animal.makeSound();
}
}

}


2. Now, we can implements different animals in different languages:


// Cat.groovy

class Cat implements Animal {

void makeSound() {
println "Meow!"
}

}

# Dog.rb

require "java"

include_class("Animal")

class Dog < Animal
def makeSound
puts "Bark!!!"
end
end

Dog.new

// Cow.bsh

void makeSound() {
System.out.println("Moo...");
}


3. We'll keep all required libraries in the form of maven2 dependencies file.
Because current version of spring (2.0.3) is not compatible with jruby version 0.9.8
(see bug SPR-3255),
we have to keep reference to temporary repository:
http://scriptlandia-repository.googlecode.com/svn/trunk/patches.

4. All required beans are defined inside spring file:


<!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN//EN" "http://www.springframework.org/dtd/spring-beans.dtd">

<!-- dynamo-test.xml -->

<beans>
<!-- Enables spring-scripting. -->
<bean class="org.springframework.scripting.support.ScriptFactoryPostProcessor"/>

<!-- Creates cat. -->
<bean id="cat" class="org.springframework.scripting.groovy.GroovyScriptFactory">
<constructor-arg value="file:Cat.groovy" />
</bean>

<!-- Creates dog. -->
<bean id="dog" class="org.springframework.scripting.jruby.JRubyScriptFactory">
<constructor-arg value="file:Dog.ruby" />
<constructor-arg value="Animal" />
</bean>

<!-- Creates cow. -->
<bean id="cow" class="org.springframework.scripting.bsh.BshScriptFactory">
<constructor-arg value="file:Cow.bsh" />
<constructor-arg value="Animal" />
</bean>

<!-- Creates animal farm. -->
<bean id="farm" class="AnimalFarm">
<property name="animals">
<list>
<ref bean="cat"/>
<ref bean="dog"/>
<ref bean="cow"/>
</list>
</property>
</bean>
</beans>


5. The trickiest part here is how to avoid compilation of java code, making code
completely dynamic. To achieve it, we use janino library (http://www.janino.net/)
with JavaSourceClassLoader. We load class from source file, retrieve it as array of bytes
and then add this array as a class to our class loader.

To load required classes/libraries to CLASSPATH we use Scriptlandia API (http://scriptlandia.sf.net).
The complete example is represented below:


// dynamo-test.bsh

org.sf.scriptlandia.ScriptlandiaHelper.addMavenDependencies("pom.xml");

import org.sf.scriptlandia.launcher.ScriptlandiaLauncher;
import org.codehaus.janino.*;

import org.sf.scriptlandia.util.*;
import org.codehaus.classworlds.ClassRealm;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.FileSystemXmlApplicationContext;


public class Dynamo {
private ClassRealm classRealm;
private ApplicationContext factory;

public Dynamo(String basedir, String[] classNames, String beansFile) {
ScriptlandiaLauncher launcher = ScriptlandiaLauncher.getInstance();

classRealm = launcher.getMainRealm();

register(basedir, classNames);

factory = new FileSystemXmlApplicationContext(new String[] { basedir + "/" + beansFile });
}

private void register(String basedir, String[] classNames) {
ScriptlandiaLauncher launcher = ScriptlandiaLauncher.getInstance();

ClassLoader sourceClassloader = new JavaSourceClassLoader(
launcher.getClass().getClassLoader(), // parentClassLoader
new File[] { new File(basedir) }, // optionalSourcePath
(String) null, // optionalCharacterEncoding
DebuggingInformation.NONE // debuggingInformation
);

for(int i=0; i < classNames.length; i++) {
loadClass(classNames[i], sourceClassloader);
}
}

/**
* Loads class specified by the name.
*/
private void loadClass(String name, ClassLoader classLoader) {
String[] args = new String[] { name };

Map bytecodes = ReflectionUtil.invokePrivateMethod(
classLoader,
new Object[] { name },
JavaSourceClassLoader.class,
"generateBytecodes",
new Class[] { String.class });

classRealm.addConstituent(name, bytecodes.get(name));
}

public Object getBean(String beanName) {
return factory.getBean(beanName);
}

public static void main(String[] args) throws Exception {
// Java classes that needs to be registered.
String[] classNames = new String[] {"Animal", "AnimalFarm" };
String basedir = System.getProperty("user.dir");

Dynamo dynamo = new Dynamo(basedir, classNames, "dynamo-test.xml");

AnimalFarm animalFarm = dynamo.getBean("farm");

animalFarm.wakeUp();
}

}

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