The supported languages were chosen because a) the languages have a lot of traction in the Java enterprise community, b) no requests were made for other languages within the Spring 2.0 development timeframe, and c) the Spring developers were most familiar with them. There is nothing stopping the inclusion of further languages though. If you want to see support for <insert your favourite dynamic language here>, you can always raise an issue on Spring's JIRA page (or implement such support yourself). Spring 2.0 introduces comprehensive support for using classes and objects that have been defined using a dynamic language (such as JRuby) with Spring. This support allows you to write any number of classes in a supported dynamic language, and have the Spring container transparently instantiate, configure and dependency inject the resulting objects. The dynamic languages currently supported are: JRuby 0.9 / 1.0 Groovy 1.0 / 1.5 BeanShell 2.0 Fully working examples of where this dynamic language support can be immediately useful are described in . Note: Only the specific versions as listed above are supported in Spring 2.5. In particular, JRuby 1.1 (which introduced many incompatible API changes) is not supported at this point of time.

This bulk of this chapter is concerned with describing the dynamic language support in detail. Before diving into all of the ins and outs of the dynamic language support, let's look at a quick example of a bean defined in a dynamic language. The dynamic language for this first bean is Groovy (the basis of this example was taken from the Spring test suite, so if you want to see equivalent examples in any of the other supported languages, take a look at the source code). Find below the Messenger interface that the Groovy bean is going to be implementing, and note that this interface is defined in plain Java. Dependent objects that are injected with a reference to the Messenger won't know that the underlying implementation is a Groovy script. Here is the definition of a class that has a dependency on the Messenger interface. Here is an implementation of the Messenger interface in Groovy. Finally, here are the bean definitions that will effect the injection of the Groovy-defined Messenger implementation into an instance of the DefaultBookingService class. To use the custom dynamic language tags to define dynamic-language-backed beans, you need to have the XML Schema preamble at the top of your Spring XML configuration file. You also need to be using a Spring ApplicationContext implementation as your IoC container. Using the dynamic-language-backed beans with a plain BeanFactory implementation is supported, but you have to manage the plumbing of the Spring internals to do so. For more information on schema-based configuration, see . ]]><!-- this is the bean definition for the Groovy-backed Messenger implementation --> ]]><!-- an otherwise normal bean that will be injected by the Groovy-backed Messenger --> ]]> The bookingService bean (a DefaultBookingService) can now use its private messenger member variable as normal because the Messenger instance that was injected into it is a Messenger instance. There is nothing special going on here, just plain Java and plain Groovy. Hopefully the above XML snippet is self-explanatory, but don't worry unduly if it isn't. Keep reading for the in-depth detail on the whys and wherefores of the above configuration.

This section describes exactly how you define Spring managed beans in any of the supported dynamic languages. Please note that this chapter does not attempt to explain the syntax and idioms of the supported dynamic languages. For example, if you want to use Groovy to write certain of the classes in your application, then the assumption is that you already know Groovy. If you need further details about the dynamic languages themselves, please consult at the end of this chapter.

The steps involved in using dynamic-language-backed beans are as follows: Write the test for the dynamic language source code (naturally) Then write the dynamic language source code itself :) Define your dynamic-language-backed beans using the appropriate <lang:language/> element in the XML configuration (you can of course define such beans programmatically using the Spring API - although you will have to consult the source code for directions on how to do this as this type of advanced configuration is not covered in this chapter). Note this is an iterative step. You will need at least one bean definition per dynamic language source file (although the same dynamic language source file can of course be referenced by multiple bean definitions). 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.

All of the configuration examples in this chapter make use of the new XML Schema support that was added in Spring 2.0. It is possible to forego the use of XML Schema and stick with the old-style DTD based validation of your Spring XML files, but then you lose out on the convenience offered by the <lang:language/> element. See the Spring test suite for examples of the older style configuration that doesn't require XML Schema-based validation (it is quite verbose and doesn't hide any of the underlying Spring implementation from you). The final step involves defining dynamic-language-backed bean definitions, one for each bean that you want to configure (this is no different from normal JavaBean configuration). However, instead of specifying the fully qualified classname of the class that is to be instantiated and configured by the container, you use the <lang:language/> element to define the dynamic language-backed bean. Each of the supported languages has a corresponding <lang:language/> element: <lang:jruby/> (JRuby) <lang:groovy/> (Groovy) <lang:bsh/> (BeanShell) The exact attributes and child elements that are available for configuration depends on exactly which language the bean has been defined in (the language-specific sections below provide the full lowdown on this).

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. 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: ]]><!-- this bean is now 'refreshable' due to the presence of the 'refresh-check-delay' attribute --><!-- switches refreshing on with 5 seconds between checks --> ]]> 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. 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. 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 ). 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.

The dynamic language support can also cater for dynamic language source files that are embedded directly in Spring bean definitions. More specifically, the <lang:inline-script/> element allows you to define dynamic language source immediately inside a Spring configuration file. An example will perhaps make the inline script feature crystal clear: package org.springframework.scripting.groovy; import org.springframework.scripting.Messenger class GroovyMessenger implements Messenger { String message } ]]> If we put to one side the issues surrounding whether it is good practice to define dynamic language source inside a Spring configuration file, the <lang:inline-script/> element can be useful in some scenarios. For instance, we might want to quickly add a Spring Validator implementation to a Spring MVC Controller. This is but a moment's work using inline source. (See for such an example.) Find below an example of defining the source for a JRuby-based bean directly in a Spring XML configuration file using the inline: notation. (Notice the use of the characters to denote a '<' character. In such a case surrounding the inline source in a <![CDATA[]]> region might be better.) require 'java' include_class 'org.springframework.scripting.Messenger' class RubyMessenger < Messenger def setMessage(message) @@message = message end def getMessage @@message end end ]]>

There is one very important thing to be aware of with regard to Spring's dynamic language support. Namely, it is not (currently) possible to supply constructor arguments to dynamic-language-backed beans (and hence constructor-injection is not available for dynamic-language-backed beans). In the interests of making this special handling of constructors and properties 100% clear, the following mixture of code and configuration will not work. ]]> <!-- this next constructor argument will *not* be injected into the GroovyMessenger --> <!-- in fact, this isn't even allowed according to the schema -->This will *not* work]]> <!-- only property values are injected into the dynamic-language-backed object --> ]]> In practice this limitation is not as significant as it first appears since setter injection is the injection style favored by the overwhelming majority of developers anyway (let's leave the discussion as to whether that is a good thing to another day).

The JRuby scripting support in Spring requires the following libraries to be on the classpath of your application. (The versions listed just happen to be the versions that the Spring team used in the development of the JRuby scripting support; you may well be able to use another version of a specific library.) jruby.jar cglib-nodep-2.1_3.jar 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). And here is the Spring XML that defines an instance of the RubyMessenger JRuby bean. ]]> 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: 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): 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 for some scenarios where you might want to use JRuby-based beans.

The Groovy scripting support in Spring requires the following libraries to be on the classpath of your application. groovy-1.5.5.jar asm-2.2.2.jar antlr-2.7.6.jar 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). Here is an implementation of the Calculator interface in Groovy. // from the file 'calculator.groovy' <-- from the file 'beans.xml' --> ]]> Lastly, here is a small application to exercise the above configuration. 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 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.

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. 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. 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 2.0 namespace support. <!-- define the GroovyObjectCustomizer just like any other bean --> ]]><!-- ... and plug it into the desired Groovy bean via the 'customizer-ref' attribute -->]]> If you are not using the Spring 2.0 namespace support, you can still use the GroovyObjectCustomizer functionality. ]]><!-- define the GroovyObjectCustomizer (as an inner bean) --> ]]>

The BeanShell scripting support in Spring requires the following libraries to be on the classpath of your application. bsh-2.0b4.jar cglib-nodep-2.1_3.jar 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). Here is the BeanShell 'implementation' (the term is used loosely here) of the Messenger interface. And here is the Spring XML that defines an 'instance' of the above 'class' (again, the term is used very loosely here). ]]> See for some scenarios where you might want to use BeanShell-based beans.

The possible scenarios where defining Spring managed beans in a scripting language would be beneficial are, of course, many and varied. This section describes two possible use cases for the dynamic language support in Spring.

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. 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 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. ]]>

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. 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 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 for a discussion of the Validator interface.) 0) { return } errors.reject("whitespace", "Cannot be composed wholly of whitespace.") } }]]>

This last section contains some bits and bobs related to the dynamic language support.

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. scope="prototype" ]]> See in for a fuller discussion of the scoping support in the Spring Framework.

Find below links to further resources about the various dynamic languages described in this chapter. The JRuby homepage The Groovy homepage The BeanShell homepage Some of the more active members of the Spring community have also added support for a number of additional dynamic languages above and beyond the ones covered in this chapter. While it is possible that such third party contributions may be added to the list of languages supported by the main Spring distribution, your best bet for seeing if your favourite scripting language is supported is the Spring Modules project.