There are various options for compiling Groovy code and then either running it or using the Java objects it creates in Java code.
Compling Groovy code to bytecode using a script
There is an Ant task called _groovyc_ which works pretty similarly to the _javac_ Ant task which takes a bunch of groovy source files and compiles them into Java bytecode. Each groovy class then just becomes a normal Java class you can use inside your Java code if you wish.
Indeed the generated Java class is indistinguishable from a normal Java class, other than it implements the GroovyObject interface.
Compiling Groovy code to bytecode using Ant and Maven
The groovyc Ant task is implemented by the Groovyc class.
You can see an example of this in action inside Groovy's maven.xml file (just search for 'groovyc')
There is also an excellent article on DeveloperWorks which will show you how to compile Groovy code from within Maven, similarly to what is done with Ant.
Dynamically using Groovy inside Java applications
If you don't want to explicitly compile groovy code to bytecode you can just
embed groovy directly into your Java application.
As well as Java 1.4 and the Groovy jar we also depend at runtime on the ASM library (asm and asm-tree mainly), as well as Antlr. You can also use the groovy-all-xxx.jar from your GROOVY_HOME/embeddable directory, which embeds ASM and Antlr in its own namespace, to avoid Jar version hell.