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.

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 the 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 called “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:inline-script>

package org.springframework.scripting.groovy;

import org.springframework.scripting.Messenger

class GroovyMessenger implements Messenger {
    String message
}

    </lang:inline-script>
    <lang:property name="message" value="I Can Do The Frug"/>
</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;
    }

    String message

    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…​

"JRuby is an 100% pure-Java implementation of the Ruby programming language."

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 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 the attribute, and (accordingly) program to interfaces when using JRuby-backed beans).

Let us 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'

class RubyMessenger
    include org.springframework.scripting.Messenger

    def setMessage(message)
        @@message = message
    end

    def getMessage
        @@message
    end
end

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

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 is 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 Section 35.4, “Scenarios” for some scenarios where you might want to use JRuby-based beans.

From the Groovy homepage…​

"Groovy is an agile dynamic language for the Java 2 Platform that has many of the features that people like so much in languages like Python, Ruby and Smalltalk, making them available to Java developers using a Java-like syntax. "

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).

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
    }

}

<-- 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 Section 35.4, “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.

Customizing Groovy objects via a callback

The GroovyObjectCustomizer interface is a callback that allows you to hook additional creation logic into the process of creating a Groovy-backed bean. For example, implementations of this interface could invoke any required initialization method(s), or set some default property values, or specify a custom MetaClass.

public interface GroovyObjectCustomizer {

    void customize(GroovyObject goo);
}

The Spring Framework will instantiate an instance of your Groovy-backed bean, and will then pass the created GroovyObject to the specified GroovyObjectCustomizer if one has been defined. You can do whatever you like with the supplied GroovyObject reference: it is expected that the setting of a custom MetaClass is what most folks will want to do with this callback, and you can see an example of doing that below.

public final class SimpleMethodTracingCustomizer implements GroovyObjectCustomizer {

    public void customize(GroovyObject goo) {
        DelegatingMetaClass metaClass = new DelegatingMetaClass(goo.getMetaClass()) {

            public Object invokeMethod(Object object, String methodName, Object[] arguments) {
                System.out.println("Invoking '" + methodName + "'.");
                return super.invokeMethod(object, methodName, arguments);
            }
        };
        metaClass.initialize();
        goo.setMetaClass(metaClass);
    }

}

A full discussion of meta-programming in Groovy is beyond the scope of the Spring reference manual. Consult the relevant section of the Groovy reference manual, or do a search online: there are plenty of articles concerning this topic. Actually making use of a GroovyObjectCustomizer is easy if you are using the Spring namespace support.

<!-- define the GroovyObjectCustomizer just like any other bean -->
<bean id="tracingCustomizer" class="example.SimpleMethodTracingCustomizer"/>

    <!-- ... and plug it into the desired Groovy bean via the 'customizer-ref' attribute -->
    <lang:groovy id="calculator"
        script-source="classpath:org/springframework/scripting/groovy/Calculator.groovy"
        customizer-ref="tracingCustomizer"/>

If you are not using the Spring namespace support, you can still use the GroovyObjectCustomizer functionality.

<bean id="calculator" class="org.springframework.scripting.groovy.GroovyScriptFactory">
    <constructor-arg value="classpath:org/springframework/scripting/groovy/Calculator.groovy"/>
    <!-- define the GroovyObjectCustomizer (as an inner bean) -->
    <constructor-arg>
        <bean id="tracingCustomizer" class="example.SimpleMethodTracingCustomizer"/>
    </constructor-arg>
</bean>

<bean class="org.springframework.scripting.support.ScriptFactoryPostProcessor"/>

	Note
	As of Spring Framework 4.3.3, you may also specify a Groovy CompilationCustomizer (such as an ImportCustomizer) or even a full Groovy CompilerConfiguration object in the same place as Spring’s GroovyObjectCustomizer.

From the BeanShell homepage…​

"BeanShell is a small, free, embeddable Java source interpreter with dynamic language features, written in Java. BeanShell dynamically executes standard Java syntax and extends it with common scripting conveniences such as loose types, commands, and method closures like those in Perl and JavaScript."

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 the 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 Section 35.4, “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 dynamic language source files.

	Note
	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 called “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.

// 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

class FortuneController implements Controller {

    @Property FortuneService fortuneService

    ModelAndView handleRequest(HttpServletRequest request,
            HttpServletResponse httpServletResponse) {
        return new ModelAndView("tell", "fortune", this.fortuneService.tellFortune())
    }

}

<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 called “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 Section 9.2, “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

class TestBeanValidator implements Validator {

    boolean supports(Class clazz) {
        return TestBean.class.isAssignableFrom(clazz)
    }

    void validate(Object bean, Errors errors) {
        if(bean.name?.trim()?.size() > 0) {
            return
        }
        errors.reject("whitespace", "Cannot be composed wholly of whitespace.")
    }

}

It is possible to use the Spring AOP framework to advise scripted beans. The Spring AOP framework actually is unaware that a bean that is being advised might be a scripted bean, so all of the AOP use cases and functionality that you may be using or aim to use will work with scripted beans. There is just one (small) thing that you need to be aware of when advising scripted beans…​ you cannot use class-based proxies, you must use interface-based proxies.

You are of course not just limited to advising scripted beans…​ you can also write aspects themselves in a supported dynamic language and use such beans to advise other Spring beans. This really would be an advanced use of the dynamic language support though.

In case it is not immediately obvious, scripted beans can of course be scoped just like any other bean. The scope attribute on the various <lang:language/> elements allows you to control the scope of the underlying scripted bean, just as it does with a regular bean. (The default scope is singleton, just as it is with 'regular' beans.)

Find below an example of using the scope attribute to define a Groovy bean scoped as a prototype.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:lang="http://www.springframework.org/schema/lang"
    xsi:schemaLocation="
        http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/lang http://www.springframework.org/schema/lang/spring-lang.xsd">

    <lang:groovy id="messenger" script-source="classpath:Messenger.groovy" scope="prototype">
        <lang:property name="message" value="I Can Do The RoboCop"/>
    </lang:groovy>

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

</beans>

See Section 7.5, “Bean scopes” in Chapter 7, The IoC container for a fuller discussion of the scoping support in the Spring Framework.