GvTagsWiki: Documentation

Why GvTags?

While building web applications with Java I always felt the need for a language which is more agile then Java, but integrates well into the Java ecosystem.

When I checked out Groovy, I felt that this is exactly what I was looking for. The integration into Java is seamless, but the language has a lot of characteristics of scripting languages.

Unfortunately Groovy did not have a good support for building web applications, so I started GvTags.

(August 2004, tof)

Example for GvTags

This is an excerpt from examples/primenumbers.jsp in the GvTags release and shows how GvTags can be used:

 <!-- Output calculated primes -->
 <gvt:if test='inputValid' >
     <gvt:gstring>Calculated primes up to ${max}:</gv:gstring>
     <br />
     <gvt:for var='isPrime' in='primeField' index='i' >
         <gvt:if test='isPrime'>
             <gv:out expr='i' />  
         </gvt:if>
     </gvt:for>
 </gvt:if>

Quickstart

See the Quick Guide how to install the all-in-one release and start working with GvTags.

Guide to features of tag library of GvTags

The GvTags tag library makes it possible to use the Groovy scripting language in JavaServer Pages (JSP) or in GTL pages.

You can use Groovy expressions in tags like if tag, for tag, while tag, switch tag
You can excute Groovy code in the code tag
You can use GStrings through the gstring tag, output expression results through the out tag or use a formatter in the format tag
You can create instances of Groovy classes by the bean tag
Handling of request parameters is simplified by the formbean tag for form data and the actionbean tag for actions according to the MVC pattern
Actions can also be handled by a controller servlet ActionServlet
The generic tag enables the use of Groovy expressions in any HTML, XML tag
The tags are very close to the Groovy programming language like: if tag, else tag, for tag, while tag, switch tag
External Groovy classes will be automatically loaded and compiled. These file can be placed in your application directory or in WEB-INF/groovy
The Groovy engine inside of GvTags detects file changes and recompiles Groovy files when needed.
Since GvTags and Groovy classes are dynamically compiled there execution is fast after the first call. Also it is possible to statically compile Groovy files using groovyc for fast execution of deployed applications

Tag Library Documentation

To learn more about all Tags defined in the library see Tags!

MVC pattern

Here is a discription how to implement the MVC pattern with GvTags and how to use the actionbean tag or the ActionServlet: ActionServlet and MVC

TagTemplateEngine

The TagTemplateEngine of GvTags can be used to create any text based documents from Groovy, from Java, as an ant task or from the command line, see TagTemplateEngine. It uses the GvTags Template Language (GTL) to create documents.

Calling GvTags from the command line

With the all-in-one release GvTags can simply be called from the command line, see: command line parameters.

Automatic code recompilation

GvTags supports automatic recompilation for JSP and for GTL through the TagTemplateServlet. When a page with Groovy code changes, it will automatically be recompiled. Through a special class loader the class will be replaced by the new verison. A reload of the page in the browser will show the changed version.

Also Groovy application classes in WEBINF/groovy will be automatically recompiled and reloaded when they are changed. At the moment this works for objects in request scope and Groovy Scope but not for session scope and application scope. Objects in request and grooyv scope are created again for every request and therefore they will be build from the changed class. But you should'nt keep to much in session or application scope anyway.

This makes developping GvTags applications very agile, since the edit/compile/link/go/test-cycle is minimized.

FAQ

Frequently Asked Questions see the FAQ

Getting support, bug reporting, feature request

To request support goto [Support]
To report a bug goto [Bugs]
Proposing new features [Feature Requests]

Subscribe to the [mailing list] or contact GvTags.

Hints for the bug hunter

Groovy is not very good in error reporting and GvTags is even worse. So finding errors with GvTags can easily become a nightmare.

Here some helpful hints:

Keep in mind: beans in application or session scope are not affected by an automatic recompilation.

The class definition of an already exiting bean cann't be changed in Java. A bean already instantiate will not use the changed implementation even if the groovy file has changed and is automatically compiled. To use the new implementation, the bean must be reinstatiated. For beans in application or session scope this can only be done by a restart of the application or by creating a new session.

Syntax errors in external groovy classes are not very well reported.

If an external groovy class, which is used by GvTags page, contains an syntax error, the JSP page will only show the error: class not found. Since this gives not much hints about the real error, you should use JUnit with GroovyTestCase to test external groovy classes before using them from a JSP.

Use unit tests with GroovyTestCase for external classes before calling them by GvTags

Develop in tiny steps and always test in between. In this way you can easily isolate errors.

Working with the source distribution

Other approaches

There are some other approaches for web applications in Groovy

Writing a Groovy Servlet, see [Groovlets]
Using Groovy Server Pages or GSP, see [GSP]
Have a look at [Grails]