Iterating through nested sets of objects

[Langdon Stevenson <ls...@objectpositive.com>]

Hi

I am trying to output the contents of a set of nested Objects in a JSP 
page using the <nested> taglib.

I have three objects that are persisted using Hibernate:

1. Student - (contains a HashSet of Project called 'projects')
2. Project - (contains a HashSet of Url called 'urls')
3. Url - (contains a string)

The Hibernate persistance seems to require the use of HashSets for 
sub-objects to work.  I have tried using an ArrayList, but get Hibernate 
errors.

I set a student bean in session scope in my action and then pass control 
to the jsp.  The jsp looks something like:

<nested:root name="adamh">
   <nested:iterate id="element1" property="projects">

     <tr>
       <td>
         Name:
       </td>
       <td>
         <nested:write name="element1" property="name"/>
       </td>
     </tr>

     <tr>
       <td>
         Query:
       </td>
       <td>
         <nested:write name="element1" property="query"/>
       </td>
     </tr>

   </nested:iterate>
</nested:root>


When I run the above jsp code, it works fine and prints the name and 
query string of the project below the student details.  However when I 
add the second level of iteration like this:


<nested:root name="adamh">

   <nested:iterate id="element1" property="projects">

     <tr>
       <td>
         Name:
       </td>
       <td>
         <nested:write name="element1" property="name"/>
       </td>
     </tr>

     <tr>
       <td>
         Query:
       </td>
       <td>
         <nested:write name="element1" property="query"/>
       </td>
     </tr>

     <nested:iterate name="element2" property="urls">
       <tr>
         <td>
           <nested:write name="element2" property="url"/>
         </td>
       </tr>
     </nested:iterate>

   </nested:iterate>
</nested:root>


I get the following error:

javax.servlet.ServletException: Cannot find bean element2 in any scope

This makes sense (sort of), but leaves me with little clue as to what I 
should be doing instead.  All of the examples that I can find of using 
the <nested:iterate> tag say that I should use the form:

COMPANY: <html:text property="companyName"/>
<nested:iterate property="departments">
     Department: <nested:text property="deptName"/>
         <nested:iterate property="employees">
             Employee: <nested:text property="empName"/>
             E-mail: <nested:text property="email"/>
         </nested:iterate>
</nested:iterate>

(this from the Struttin' with Struts tutorial - 
http://www.reumann.net/struts/nested.do)

However my reading of the iterate taglib docs say that with a HashTable 
I must use the id="something" property="object property" form.  As 
mentioned, that works fine for one level of nesting, but not multiple 
levels.

Any suggestions as to how I should be modifying my JSP?  Keeping in mind 
that Hibernate appears to limit me to using a HashMap to store my nested 
objects.  I expect that there is a simple answer, but can't for the life 
of me find it.

I am using Struts 1.1 with Tomcat 4.0.6 / 4.1 on Win2k

Full listing of relevent files below.

Any assistance greatly appreciated.

Regards,
Langdon
---------------------


-- Student.java --
package adamh;

import java.io.Serializable;
import java.util.Date;
import java.util.HashSet;
import java.util.Set;

/**
  * Created by IntelliJ IDEA.
  * Student: langdons
  * Date: 12/08/2004
  * Time: 18:58:49
  * To change this template use Options | File Templates.
  */

public class Student implements Serializable {
     private String id;
     private String parentId;
     private String emailAddress;
     private String password;
     private String nickname;
     private String yearOfBirth;
     private String country;
     private String state;
     private String gender;
     private java.util.Date firstContact;
     private String referredById;
     private String agreeToRules;
     private User user;
     private Role role;
     private Set projects = new HashSet();


     public String getId() {
         return id;
     }

     public void setId(String string) {
          id = string;
     }


     public String getParentId() {
         return parentId;
     }

     public void setParentId(String string) {
          parentId = string;
     }


     public String getEmailAddress() {
         return emailAddress;
     }

     public void setEmailAddress(String string) {
          emailAddress = string;
     }


     public String getPassword() {
         return password;
     }

     public void setPassword(String string) {
          password = string;
     }


     public String getNickname() {
         return nickname;
     }

     public void setNickname(String string) {
          nickname = string;
     }


     public String getyearOfBirth() {
         return yearOfBirth;
     }

     public void setYearOfBirth(String string) {
          yearOfBirth = string;
     }


     public String getCountry() {
         return country;
     }

     public void setCountry(String string) {
          country = string;
     }


     public String getState() {
         return state;
     }

     public void setState(String string) {
          state = string;
     }


     public String getGender() {
         return gender;
     }

     public void setGender(String string) {
         gender = string;
     }


     public Date getFirstContact() {
         return firstContact;
     }

     public void setFirstContact(Date date) {
         firstContact = date;
     }


     public String getReferredById() {
         return referredById;
     }

     public void setReferredById(String string) {
         referredById = string;
     }


     public String getAgreeToRules() {
         return agreeToRules;
     }

     public void setAgreeToRules(String string) {
         agreeToRules = string;
     }


     public User getUser() {
         return user;
     }

     public void setUser(User user) {
         this.user = user;
     }


     public Role getRole() {
         return role;
     }

     public void setRole(Role role) {
         this.role = role;
     }


     public Set getProjects() {
         return this.projects;
     }

     public void setProjects(Set project) {
         this.projects = project;
     }

     public void addProject(Project project) {
         project.setStudent(this);
         projects.add(project);
     }

}

-- Project.java --

package adamh;

import java.util.Set;
import java.util.HashSet;

/**
  * Created by IntelliJ IDEA.
  * User: langdons
  * Date: 7/10/2004
  * Time: 20:39:14
  * To change this template use Options | File Templates.
  */
public class Project {

     private String id;
     private String student_id;
     private String name;
     private String query;
     private Student student;
     private Set urls = new HashSet();


     public String getId() {
         return id;
     }

     public void setId(String string) {
         id = string;
     }


     public String getStudent_id() {
         return student_id;
     }

     public void setStudent_id(String string) {
         student_id = string;
     }


     public String getName() {
         return name;
     }

     public void setName (String string) {
         name = string;
     }


     public String getQuery () {
         return query;
     }

     public void setQuery (String string) {
         query = string;
     }


     public void setStudent(Student student){
         this.student = student;
     }

     public Student getStudent() {
         return student;
     }


     public Set getUrls() {
         return urls;
     }

     public void setUrls(Set set) {
         this.urls = set;
     }

     public void addUrls(Url url){
         url.getProjects().add(url);
         urls.add(url);
     }
}


-- Url.java --

package adamh;

import java.util.Set;
import java.util.HashSet;

/**
  * Created by IntelliJ IDEA.
  * User: langdons
  * Date: 7/10/2004
  * Time: 21:13:35
  * To change this template use Options | File Templates.
  */

public class Url {

     private String id;
     private String url;
     private Set projects = new HashSet();

     public String getId() {
         return id;
     }

     public void setId(String string) {
         id = string;
     }


     public String getUrl() {
         return url;
     }

     public void setUrl (String string) {
         url = string;
     }


     public Set getProjects () {
         return this.projects;
     }

     public void setProjects (Set set) {
         this.projects = set;
     }
}

-- home.jsp --

<%@ taglib uri="/tags/struts-html" prefix="html" %>
<%@ taglib uri="/tags/struts-bean" prefix="bean" %>
<%@ taglib uri="/tags/struts-nested" prefix="nested"%>

<html>
<head>
<title><bean:message key="studentHomePage.pageTitle" /></title>

</head>
<body>
<h1><bean:message key="studentHomePage.pageHeader" /></h1>

	<table>

         <tr>
             <td colspan=2>
                 <html:messages id="message" message="true">
                     <%= message %>
                 </html:messages>
             </td>
         </tr>


         <tr>
             <td>
                 <bean:message 
key="studentRegistrationWizard.referredById" />:
             </td>
             <td>
                 <bean:write name="adamh" property="referredById" />
             </td>
         </tr>
         <tr>
             <td>
                 <bean:message 
key="studentRegistrationWizard.emailAddress" />:
             </td>
             <td>
                 <bean:write name="adamh" property="emailAddress" />
             </td>
         </tr>
         <tr>
             <td>
                 <bean:message key="studentRegistrationWizard.password" />:
             </td>
             <td>
                 <bean:write name="adamh" property="password" />
             </td>
         </tr>
         <tr>
             <td>
                 <bean:message key="studentRegistrationWizard.nickname" />:
             </td>
             <td>
                 <bean:write name="adamh" property="nickname" />
             </td>
         </tr>
		<tr>
			<td>
				<bean:message key="studentRegistrationWizard.yearOfBirth"/>
			</td>
			<td>
				<bean:write name="adamh" property="yearOfBirth" />
			</td>
		</tr>
		<tr>
			<td>
				<bean:message key="studentRegistrationWizard.country"/>
			</td>
			<td>
				<bean:write name="adamh" property="country" />
			</td>
		</tr>
		<tr>
			<td>
				<bean:message key="studentRegistrationWizard.state"/>
			</td>
			<td>
				<bean:write name="adamh" property="state" />
			</td>
		</tr>
		<tr>
			<td>
				<bean:message key="studentRegistrationWizard.gender"/>
			</td>
			<td>
				<bean:write name="adamh" property="gender" />
			</td>
		</tr>
         <tr>
             <td>
                 Projects:
             </td>
         </tr>


         <nested:root name="adamh">
             <nested:iterate id="element1" property="projects">
                     <tr>
                         <td>
                             Name:
                         </td>
                         <td>
                             <nested:write name="element1" property="name"/>
                         </td>
                     </tr>
                     <tr>
                         <td>
                             Query:
                         </td>
                         <td>
                             <nested:write name="element1" 
property="query"/>
                         </td>
                     </tr>

                     <nested:iterate name="element2" property="urls">
                         <tr>
                             <td>
                                 <nested:write name="element2" 
property="url"/>
                             </td>
                         </tr>
                     </nested:iterate>

                     <tr>
                         <td colspan="2">
                             <hr>
                         </td>
                     </tr>
             </nested:iterate>
         </nested:root>


	</table>
</body>


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