The steps involved in using dynamic-language-backed beans are as follows:

The first two steps (testing and writing your dynamic language source files) are beyond the scope of this chapter. Refer to the language specification and / or reference manual for your chosen dynamic language and crack on with developing your dynamic language source files. You will first want to read the rest of this chapter though, as Spring's dynamic language support does make some (small) assumptions about the contents of your dynamic language source files.

24.3.1.2. Refreshable beans

One of the (if not the) most compelling value adds of the dynamic language support in Spring is the 'refreshable bean' feature.

A refreshable bean is a dynamic-language-backed bean that with a small amount of configuration, a dynamic-language-backed bean can monitor changes in its underlying source file resource, and then reload itself when the dynamic language source file is changed (for example when a developer edits and saves changes to the file on the filesystem).

This allows a developer to deploy any number of dynamic language source files as part of an application, configure the Spring container to create beans backed by dynamic language source files (using the mechanisms described in this chapter), and then later, as requirements change or some other external factor comes into play, simply edit a dynamic language source file and have any change they make reflected in the bean that is backed by the changed dynamic language source file. There is no need to shut down a running application (or redeploy in the case of a web application). The dynamic-language-backed bean so amended will pick up the new state and logic from the changed dynamic language source file.

	Note
	Please note that this feature is off by default.

Let's take a look at an example to see just how easy it is to start using refreshable beans. To turn on the refreshable beans feature, you simply have to specify exactly one additional attribute on the <lang:language/> element of your bean definition. So if we stick with the example from earlier in this chapter, here's what we would change in the Spring XML configuration to effect refreshable beans:

<beans>

    <!-- this bean is now 'refreshable' due to the presence of the 'refresh-check-delay' attribute -->
    <lang:groovy id="messenger"
          refresh-check-delay="5000" <!-- switches refreshing on with 5 seconds between checks -->
          script-source="classpath:Messenger.groovy">
        <lang:property name="message" value="I Can Do The Frug" />
    </lang:groovy>

    <bean id="bookingService" class="x.y.DefaultBookingService">
        <property name="messenger" ref="messenger" />
    </bean>

</beans>

That really is all you have to do. The 'refresh-check-delay' attribute defined on the 'messenger' bean definition is the number of milliseconds after which the bean will be refreshed with any changes made to the underlying dynamic language source file. You can turn off the refresh behavior by assigning a negative value to the 'refresh-check-delay' attribute. Remember that, by default, the refresh behavior is disabled. If you don't want the refresh behavior, then simply don't define the attribute.

If we then run the following application we can exercise the refreshable feature; please do excuse the 'jumping-through-hoops-to-pause-the-execution' shenanigans in this next slice of code. The System.in.read() call is only there so that the execution of the program pauses while I (the author) go off and edit the underlying dynamic language source file so that the refresh will trigger on the dynamic-language-backed bean when the program resumes execution.

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import org.springframework.scripting.Messenger;

public final class Boot {

    public static void main(final String[] args) throws Exception {

        ApplicationContext ctx = new ClassPathXmlApplicationContext("beans.xml");
        Messenger messenger = (Messenger) ctx.getBean("messenger");
        System.out.println(messenger.getMessage());
        // pause execution while I go off and make changes to the source file...
        System.in.read();
        System.out.println(messenger.getMessage());
    }
}

Let's assume then, for the purposes of this example, that all calls to the getMessage() method of Messenger implementations have to be changed such that the message is surrounded by quotes. Below are the changes that I (the author) make to the Messenger.groovy source file when the execution of the program is paused.

package org.springframework.scripting

class GroovyMessenger implements Messenger {

    private String message = "Bingo"

    public String getMessage() {
        // change the implementation to surround the message in quotes
        return "'" + this.message + "'"
    }

    public void setMessage(String message) {
        this.message = message
    }
}

When the program executes, the output before the input pause will be I Can Do The Frug. After the change to the source file is made and saved, and the program resumes execution, the result of calling the getMessage() method on the dynamic-language-backed Messenger implementation will be 'I Can Do The Frug' (notice the inclusion of the additional quotes).

It is important to understand that changes to a script will not trigger a refresh if said changes occur within the window of the 'refresh-check-delay' value. It is equally important to understand that changes to the script are not actually 'picked up' until a method is called on the dynamic-language-backed bean. It is only when a method is called on a dynamic-language-backed bean that it checks to see if its underlying script source has changed. Any exceptions relating to refreshing the script (such as encountering a compilation error, or finding that the script file has been deleted) will result in a fatal exception being propagated to the calling code.

The refreshable bean behavior described above does not apply to dynamic language source files defined using the <lang:inline-script/> element notation (see the section entitled Section 24.3.1.3, “Inline dynamic language source files”). Additionally, it only applies to beans where changes to the underlying source file can actually be detected; for example, by code that checks the last modified date of a dynamic language source file that exists on the filesystem.

<lang:groovy id="messenger">
    <lang:property name="message" value="I Can Do The Frug" />
    <lang:inline-script>
package org.springframework.scripting.groovy;

import org.springframework.scripting.Messenger

class GroovyMessenger implements Messenger {

    @Property String message;
}
    </lang:inline-script>
</lang:groovy>

<lang:jruby id="messenger" script-interfaces="org.springframework.scripting.Messenger">
    <lang:inline-script>
require 'java'

include_class 'org.springframework.scripting.Messenger'

class RubyMessenger &lt; Messenger

 def setMessage(message)
  @@message = message
 end

 def getMessage
  @@message
 end
 
end
    </lang:inline-script>
    <lang:property name="message" value="Hello World!" />
</lang:jruby>

// from the file 'Messenger.groovy'
package org.springframework.scripting.groovy;

import org.springframework.scripting.Messenger

class GroovyMessenger implements Messenger {

    GroovyMessenger() {}

    // this constructor is not available for Constructor Injection...
    GroovyMessenger(String message) {
        this.message = message;
    }

    @Property String message;

    @Property String anotherMessage
}

<lang:groovy id="badMessenger"
    script-source="classpath:Messenger.groovy">

    <!-- this next constructor argument will *not* be injected into the GroovyMessenger -->
    <!--     in fact, this isn't even allowed according to the schema -->
    <constructor-arg value="This will *not* work" />
    
    <!-- only property values are injected into the dynamic-language-backed object -->
    <lang:property name="anotherMessage" value="Passed straight through to the dynamic-language-backed object" />

</lang>

From the JRuby homepage...

In keeping with the Spring philosophy of offering choice, Spring's dynamic language support also supports beans defined in the JRuby language. The JRuby language is (of course) based on the quite intuitive Ruby language, and has support for inline regular expressions, blocks (closures), and a whole host of other features that do make solutions for some domain problems a whole lot easier to develop.

The implementation of the JRuby dynamic language support in Spring is interesting in that what happens is this: Spring creates a JDK dynamic proxy implementing all of the interfaces that are specified in the 'script-interfaces' attribute value of the <lang:ruby> element (this is why you must supply at least one interface in the value of said attribute, and (accordingly) program to interfaces when using JRuby-backed beans).

Let's look at a fully working example of using a JRuby-based bean. Here is the JRuby implementation of the Messenger interface that was defined earlier in this chapter (for your convenience it is repeated below).

package org.springframework.scripting;

public interface Messenger {

    String getMessage();

}

require 'java'

include_class 'org.springframework.scripting.Messenger'

class RubyMessenger < Messenger

 def setMessage(message)
  @@message = message
 end

 def getMessage
  @@message
 end

end

RubyMessenger.new # this last line is not essential (but see below)

And here is the Spring XML that defines an instance of the RubyMessenger JRuby bean.

<lang:jruby id="messageService"
            script-interfaces="org.springframework.scripting.Messenger"
            script-source="classpath:RubyMessenger.rb">
    
    <lang:property name="message" value="Hello World!" />

</lang:jruby>

Take note of the last line of that JRuby source ('RubyMessenger.new'). When using JRuby in the context of Spring's dynamic language support, you are encouraged to instantiate and return a new instance of the JRuby class that you want to use as a dynamic-language-backed bean as the result of the execution of your JRuby source. You can achieve this by simply instantiating a new instance of your JRuby class on the last line of the source file like so:

require 'java'

include_class 'org.springframework.scripting.Messenger'

# class definition same as above...

# instantiate and return a new instance of the RubyMessenger class
RubyMessenger.new

If you forget to do this, it's not the end of the world; this will however result in Spring having to trawl (reflectively) through the type representation of your JRuby class looking for a class to instantiate. In the grand scheme of things this will be so fast that you'll never notice it, but it is something that can be avoided by simply having a line such as the one above as the last line of your JRuby script. If you don't supply such a line, or if Spring cannot find a JRuby class in your script to instantiate then an opaque ScriptCompilationException will be thrown immediately after the source is executed by the JRuby interpreter. The key text that identifies this as the root cause of an exception can be found immediately below (so if your Spring container throws the following exception when creating your dynamic-language-backed bean and the following text is there in the corresponding stacktrace, this will hopefully allow you to identify and then easily rectify the issue):

org.springframework.scripting.ScriptCompilationException: Compilation of JRuby script returned ''

To rectify this, simply instantiate a new instance of whichever class you want to expose as a JRuby-dynamic-language-backed bean (as shown above). Please also note that you can actually define as many classes and objects as you want in your JRuby script; what is important is that the source file as a whole must return an object (for Spring to configure).

See the section entitled Section 24.4, “Scenarios” for some scenarios where you might want to use JRuby-based beans.

From the Groovy homepage...

If you have read this chapter straight from the top, you will already have seen an example of a Groovy-dynamic-language-backed bean. Let's look at another example (again using an example from the Spring test suite).

	Note
	Groovy itself requires JDK 1.4+.

package org.springframework.scripting;

public interface Calculator {

    int add(int x, int y);
}

Here is an implementation of the Calculator interface in Groovy.

// from the file 'calculator.groovy'
package org.springframework.scripting.groovy

class GroovyCalculator implements Calculator {

    int add(int x, int y) {
        x + y
    }
}

And here is the attendant Spring configuration (XML).

<-- from the file 'beans.xml' -->
<beans>
    <lang:groovy id="calculator" script-source="classpath:calculator.groovy"/>
</beans>

Lastly, here is a small application to exercise the above configuration.

package org.springframework.scripting;

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

public class Main {

    public static void Main(String[] args) {
        ApplicationContext ctx = new ClassPathXmlApplicationContext("beans.xml");
        Calculator calc = (Calculator) ctx.getBean("calculator");
        System.out.println(calc.add(2, 8));
    }
}

The resulting output from running the above program will be (unsurprisingly) 10. (Exciting example, huh? Remember that the intent is to illustrate the concept. Please consult the dynamic language showcase project for a more complex example, or indeed the section entitled Section 24.4, “Scenarios” later in this chapter).

It is important that you do not define more than one class per Groovy source file. While this is perfectly legal in Groovy, it is (arguably) a bad practice: in the interests of a consistent approach, you should (in the opinion of this author) respect the standard Java conventions of one (public) class per source file.

That is pretty much it so far as Groovy-based beans go. Groove on!

From the BeanShell homepage...

In contrast to Groovy, BeanShell-backed bean definitions require some (small) additional configuration. The implementation of the BeanShell dynamic language support in Spring is interesting in that what happens is this: Spring creates a JDK dynamic proxy implementing all of the interfaces that are specified in the 'script-interfaces' attribute value of the <lang:bsh> element (this is why you must supply at least one interface in the value of said attribute, and (accordingly) program to interfaces when using BeanShell-backed beans). This means that every method call on a BeanShell-backed object is going through the JDK dynamic proxy invocation mechanism.

Let's look at a fully working example of using a BeanShell-based bean that implements the Messenger interface that was defined earlier in this chapter (repeated below for your convenience).

package org.springframework.scripting;

public interface Messenger {

    String getMessage();
}

Here is the BeanShell 'implementation' (the term is used loosely here) of the Messenger interface.

String message;

String getMessage() {
    return message;
}

void setMessage(String aMessage) {
    message = aMessage;
}

And here is the Spring XML that defines an 'instance' of the above 'class' (again, the term is used very loosely here).

<lang:bsh id="messageService" script-source="classpath:BshMessenger.bsh"
    script-interfaces="org.springframework.scripting.Messenger">

    <lang:property name="message" value="Hello World!" />
</lang:bsh>

See the section entitled Section 24.4, “Scenarios” for some scenarios where you might want to use BeanShell-based beans.

One group of classes that may benefit from using dynamic-language-backed beans is that of Spring MVC controllers. In pure Spring MVC applications, the navigational flow through a web application is to a large extent determined by code encapsulated within your Spring MVC controllers. As the navigational flow and other presentation layer logic of a web application needs to be updated to respond to support issues or changing business requirements, it may well be easier to effect any such required changes by editing one or more dynamic language source files and seeing those changes being immediately reflected in the state of a running application.

Remember that in the lightweight architectural model espoused by projects such as Spring, you are typically aiming to have a really thin presentation layer, with all the meaty business logic of an application being contained in the domain and service layer classes. Developing Spring MVC controllers as dynamic-language-backed beans allows you to change presentation layer logic by simply editing and saving text files; any changes to such dynamic language source files will (depending on the configuration) automatically be reflected in the beans that are backed by said dynamic language source files.

	Note
	Please note that in order to effect this automatic 'pickup' of any changes to dynamic-language-backed beans, you will have had to enable the 'refreshable beans' functionality. See the section entitle Section 24.3.1.2, “Refreshable beans” for a full treatment of this feature.

Find below an example of an org.springframework.web.servlet.mvc.Controller implemented using the Groovy dynamic language. This example is taken in part from the dynamic language support showcase project that is supplied with the Spring distribution; please do see the project in the 'samples/showcases/dynamvc/' directory of the Spring distribution.

<!-- from the file '/WEB-INF/groovy/FortuneController.groovy' -->
package org.springframework.showcase.fortune.web;

import org.springframework.showcase.fortune.service.FortuneService;
import org.springframework.showcase.fortune.domain.Fortune;
import org.springframework.web.servlet.ModelAndView;
import org.springframework.web.servlet.mvc.Controller;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class FortuneController implements Controller {

    @Property FortuneService fortuneService

    public ModelAndView handleRequest(
            HttpServletRequest request, HttpServletResponse httpServletResponse) {

        return new ModelAndView("tell", "fortune", this.fortuneService.tellFortune())
    }

}

The attendant bean definition for the above Controller might look like this:

<lang:groovy id="fortune"
             refresh-check-delay="3000"
             script-source="/WEB-INF/groovy/FortuneController.groovy">
    <lang:property name="fortuneService" ref="fortuneService"/>
</lang:groovy>

Another area of application development with Spring that may benefit from the flexibility afforded by dynamic-language-backed beans is that of validation. It may be easier to express complex validation logic using a loosely typed dynamic language (that may also have support for inline regular expressions) as opposed to regular Java.

Again, developing validators as dynamic-language-backed beans allows you to change validation logic by simply editing and saving a simple text file; any such changes will (depending on the configuration) automatically be reflected in the execution of a running application and would not require the restart of an application.

	Note
	Please note that in order to effect the automatic 'pickup' of any changes to dynamic-language-backed beans, you will have had to enable the 'refreshable beans' feature. See the section entitled Section 24.3.1.2, “Refreshable beans” for a full and detailed treatment of this feature.

Find below an example of a Spring org.springframework.validation.Validator implemented using the Groovy dynamic language. (See the section entitled Section 5.2, “Validation using Spring's Validator interface” for a discussion of the Validator interface.)

import org.springframework.validation.Validator
import org.springframework.validation.Errors
import org.springframework.beans.TestBean

public class TestBeanValidator implements Validator {

    boolean supports(Class clazz) {
        return TestBean.isAssignableFrom(clazz)
    }
    
    void validate(Object bean, Errors errors) {
        String name = bean.getName()
        if(name == null || name.trim().length == 0) {
            errors.reject("whitespace", "Cannot be composed wholly of whitespace")
        }
    }
}