Recursion in JSF (c:forEach vs. ui:repeat)

I am trying to build a navigation tree via recursion in JSF. I have defined a navigationNode component as:

<composite:interface>
    <composite:attribute name="node" />
</composite:interface>

<composite:implementation>
<ul>
    <ui:repeat value="#{navigationTreeBean.getChildrenForNode(cc.attrs.node)}" var="child">
        <li><navigation:navigationNode node="#{child}" /></li>
    </ui:repeat>
</ul>
</composite:implementation>

My tree is declared as:

rootNode = new DefaultMutableTreeNode(new NodeData("Dashboard", "dashboard.xhtml"), true);
DefaultMutableTreeNode configurationsNode = new DefaultMutableTreeNode(new NodeData("Configurations", "configurations.xhtml"), true);
rootNode.add(configurationsNode);

I call component by:

<nav:navigationNode node="#{rootNode}" />

The problem is, this results in StackOverflowError.

There are a few references to building recursion in JSF (for example, c:forEach vs ui:repeat in Facelets). The common problem seems to be mixing the build-time and render-time components/tags. In my case:

My composite component is actually a tag, which is executed when the tree is built
ui:repeat is an actual JSF component, which is evaluated when the tree is rendered

Is the child component navigation:navigationNode actually processed before the ui:repeat component? If so, what object is it using for #{child}? Is it null (doesn't seem so)? Is the problem here that the child component is actually created without even caring about the ui:repeat and so each time a new child component is created even though it is not necessarily wanted?

The c:forEach vs ui:repeat in Facelets article has a separate section for this (recursion). The suggestion is to to use c:forEach instead. I tried this, however it is still giving me the same StackOverflowError, with different trace that I cannot make sense of.

I know that we can also build components by extending UIComponent, but that approach (writing html in Java code) seems ugly. I would rather use MVC style / templates. However, if there are no other ways, do I have to implement this sort of recursion as UIComponent instead?

Solution

JSF's built-in declarative tags are ill-suited for handling this sort of recursion. JSF builds a stateful component tree that is persisted between requests. If the view is restored in a subsequent request, the view state may not reflect changes in the model.

I would favour an imperative approach. You have two options as I see it:

Use the binding attribute to bind a control (e.g. some form of panel) to a backing bean that provides the UIComponent instance and its children - you write code to instantiate the UIComponent and add whatever children you want. See the spec for the binding attribute contract.
Write a custom control, implementing some of: a UIComponent; a Renderer; a tag handler; meta-data files (delete as appropriate - you do some or all of these depending on what you are doing and how and in which version of JSF).

Perhaps another option is to pick up a 3rd party control that already does this.

UPDATE: If one is using the very useful OmniFaces library (you should if you don't already), there is the <o:tree> which has no html generation whatsoever but was specifically designed to support usecases like this.

<o:tree value="#{bean.treeModel}" var="item" varNode="node">
    <o:treeNode>
        <ul>
            <o:treeNodeItem>
                <li>
                    #{node.index} #{item.someProperty}
                    <o:treeInsertChildren />
                </li>
            </o:treeNodeItem>
        </ul>
    </o:treeNode>
</o:tree>

EDIT:

Here's a model-driven approach that doesn't involve writing custom components or backing-bean-generated component trees. It's kind of ugly.

The Facelets view:

<?xml version='1.0' encoding='UTF-8' ?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
   "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml"
      xmlns:h="http://java.sun.com/jsf/html"
      xmlns:ui="http://java.sun.com/jsf/facelets">
  <h:head><title>Facelet Tree</title></h:head>
  <h:body>
    <ul>
      <ui:repeat value="#{tree.treeNodes}" var="node">
        <h:outputText rendered="#{node.firstChild}"
                value="&lt;ul&gt;" escape="false" />
        <li>
          <h:outputText value="#{node.value}" />
        </li>
        <ui:repeat rendered="#{node.lastChild and empty node.kids}"
            value="#{node.lastChildLineage}" var="ignore">
          <h:outputText
              value="&lt;/ul&gt;" escape="false" />
        </ui:repeat>
      </ui:repeat>
    </ul>
  </h:body>
</html>

The managed bean:

@javax.faces.bean.ManagedBean(name = "tree")
@javax.faces.bean.RequestScoped
public class Tree {
  private Node<String> root = new Node(null, "JSF Stuff");

  @PostConstruct
  public void initData() {
    root.getKids().add(new Node(root, "Chapter One"));
    root.getKids().add(new Node(root, "Chapter Two"));
    root.getKids().add(new Node(root, "Chapter Three"));
    Node<String> chapter2 = root.getKids().get(1);
    chapter2.getKids().add(new Node(chapter2, "Section A"));
    chapter2.getKids().add(new Node(chapter2, "Section B"));
  }

  public List<Node<String>> getTreeNodes() {
    return walk(new ArrayList<Node<String>>(), root);
  }

  private List<Node<String>> walk(List<Node<String>> list, Node<String> node) {
    list.add(node);
    for(Node<String> kid : node.getKids()) {
      walk(list, kid);
    }
    return list;
  }
}

A tree node:

public class Node<T> {
  private T value;
  private Node<T> parent;
  private LinkedList<Node<T>> kids = new LinkedList<>();

  public Node(Node<T> parent, T value) {
    this.parent = parent;
    this.value = value;
  }

  public List<Node<T>> getKids() {return kids;}
  public T getValue() { return value; }

  public boolean getHasParent() { return parent != null; }

  public boolean isFirstChild() {
    return parent != null && parent.kids.peekFirst() == this;
  }

  public boolean isLastChild() {
    return parent != null && parent.kids.peekLast() == this;
  }

  public List<Node> getLastChildLineage() {
    Node node = this;
    List<Node> lineage = new ArrayList<>();
    while(node.isLastChild()) {
        lineage.add(node);
        node = node.parent;
    }
    return lineage;
  }
}

Output:

*  JSF Stuff
      o Chapter One
      o Chapter Two
            + Section A
            + Section B 
      o Chapter Three

I would still bite the bullet and write a custom tree control.