To invoke Groovy scripts the built-in object $groovy has to be used. For example:
#set( $a = $groovy.invoke("test.groovy","aaa") )
This code will invoke the script test.groovy which is usually in the same directory where the source files are. The file test.groovy should contain a Groovy script. The argument to the script in the example above is the string "aaa". You can specify arbitrary number of arguments. The script can access these values through the variable args.
The return value of the script in the example above will be stored into the Velocity variable $a. This can be a string and can be inserted into the html output or it can be a reference to any object that you can invoke later using the Velocity notation $a.method()
The groovy scripts have access to a context object named context>>. This objects is of type <<<GroovyContext. This object is created before the first pass starts and is maintained during the execution of the scripts. Using this object a script may know the name of the template file, the output file, the source directory, the target directory, the actual pass.