It's seems one of the many posts you can find in internet that helps you to generate a tree-menu using javascript/java/... What I want to illustrate is a method that use recursion in back-end (java code that generate the Tree structure) and front-end (a gsp that shown the tree calling itself!!).

In fact, the start point, is that our page is not a "simple" page but is written as grails templates.
Template is a grails way to get your front-end code structured and provide an highly re-usable mechanism that you can call simply using a defined taglib.

class TreeMenu {

    def addNode = {nodeElement, machine, tagList ->
        def nodes = [:]
        nodes[machine.hostName] =  machine
        def newList = tagList - nodeElement
        newList?.each {currentTag ->
            nodes[currentTag.name] = addNode(currentTag, machine, newList)
        }

        nodes
    }
}

Is an extraction of my program codes... in the original version the data structure is not a simple Map but I've a complex object, so I can do, for example, a check if there is a node with current provided name and so on.
What this code try to do, is to add a Machine to each Tree-Tag I'm sending to function as a List.
"nodeName" is the current node where I want my machine
"machine" is the object name I want in my tree
"tagList" is the list of all tree-node where my machine will be put

For example, I could have this situation

machine: "TryMachine"
tagList: ["A", "B"]

TreeMenu.addNode("Root", machine, tagList)

The result of this method invokation will be:

 Root 
 |-> A
 |   |->B
 |   |  |TryMachine
 |   |TryMachine
 |-> B
 |   |TryMachine
 |   |-> A
 |   |   |TryMachine
 | TryMachine

Display the tree
The extraordinary feature offers by grails is, as I said, the usage of recursion on the front-end, that make you able to create a page without insertion of some java codes: all just with default grails taglibs.
Here an example gets from my code:

<g:each in="${nodes}" var="element">
    <g:if test="${element.value instanceof Machine}">
            ${element.name}
    </g:if>
    <g:else>
    <g:machineList template="/templates/machineTree" data="${element}"/>
    </g:else>
</g:each>

And in your page, where you want to put your tree, you can just simply call the template:

<g:machineList template="/templates/machineTree" data="${treeData}"/>

Is a just a simple example (and, in fact, I'm not sure that with mods I've done to create this post, all work well :P), If you want you can make some improvements to this code, attaching, for example, javascript functions to get your tree-node opened and closed, or some other kinds of object type.u