package com.tim;
/**
* A tree item has a set of tree items
* You can go down the tree, or up the tree (e.g. bi-navigatable)
*/
public class TreeItem {
  Long id;
  String name;
  Set<TreeItem> children; // this is it's children
  TreeItem parent; // and a link to go back up the tree

  // do the getters and setter
}

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
   "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
   <class name="com.tim.TreeItem" table="Tree_Item">
      <id name="id" column="id" type="long">
         <generator class="native"/>
      </id>
      <property name="name" type="java.lang.String" column="name"/>
      <many-to-one name="parent" column="parent_id"
         class="com.tim.TreeItem"
         not-found="ignore"/> <!-- needed because the top level item will not have a parent-->
      <set name="children" lazy="true" inverse="true">
         <key column="parent_id"/>
         <one-to-many class="com.tim.TreeItem"/>
      </set>
   </class>
</hibernate-mapping>

CollectionLoadContext - creating collection wrapper:[de.hiddenfeature.soto.Todo.childTodos#1]

org.hibernate.PropertyAccessException: exception setting property value with CGLIB (set hibernate.cglib.use_reflection_optimizer=false for more info) setter of de.hiddenfeature.soto.Todo.setChildTodos

public class Todo implements Serializable {
   
    //hibernate id
    private Long id;
   
    private Date startTime;
    private Date endTime;
    private Date lastModified;
    private Date notifyTime;
    private Date creationTime;
    private String name;
    private String description;
    private int priority;
    private HashSet childTodos;
    private Todo parentTodo;
    private boolean complete;
    private User worker;

[...]

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"   "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="de.hiddenfeature.soto">

<class name="Todo" table="TODOS">

    <id name="id" column="todo_id">
            <generator class="increment" />
    </id>

    <property name="startTime" type="timestamp" column="startTime" />
    <property name="endTime" type="timestamp" column="endTime" />
    <property name="lastModified" type="timestamp" column="lastModified" />
    <property name="notifyTime" type="timestamp" column="notifyTime" />
    <property name="creationTime" type="timestamp" column="creationTime" />
    <property name="name" column="name" />
    <property name="description" type="text" column="description" />
    <property name="priority" type="integer" column="priority" />
    <property name="complete" type="boolean" column="complete" />

   <many-to-one    name="parentTodo"
         column="parent_id"
         class="de.hiddenfeature.soto.Todo"
         not-found="ignore"
   />
   
   <set name="childTodos" lazy="true" inverse="true" >
      <key column="parent_id" />
      <one-to-many class="de.hiddenfeature.soto.Todo" />
   </set>

    <many-to-one   name="worker"
         column="worker_id"
         class="de.hiddenfeature.soto.User"
         not-found="ignore"
    />

</class>
</hibernate-mapping>

ERROR TodoManager - org.hibernate.PropertyAccessException: IllegalArgumentException occurred while calling setter of de.hiddenfeature.soto.Todo.childTodos