How to iterate over java.util.Set in JSF

I spent quite some time trying to find a solution for the following JSF issue: it is not possible to iterate over a java.util.Set.
- ui:repeat (facelets) doesn’t work
- a4j:repeat (richfaces) doesn’t work
- c:forEach works..only in case it does not rely on a variable defined by a parent component (rich:dataTable for instance)

All above are pretty logical phenomena, as UIData relies on ordered data, and generally a Set is not ordered.

In my case I had to use a Set defined in the Hibernate (JPA) object (PersistentSet).
An important note: you should use a set in case the view order is of no matter to you.

The solution..is pretty simple. And I’ll suggest it to be a part of facelets/richfaces for the next version, unless of course there is some valid specific

reason for it not to be.

1. Define your own UI component extending an existing repeater component. I used a4j:repeat (HtmlAjaxRepeat)
2. Override the metohd getDataModel
3. Define your component in your faces-config
4. create a custom facelets tag definition
5. Define a context-variable in web.xml pointing to the facelet tag definition.

Note: for use with JSP instead of Facelets, you should define a .tld and a Tag handler, which is not an ojbect of this post.

Now let’s see the steps in detail:

1,2. Here some code:

package com.myproject.components;
import java.util.ArrayList;
import java.util.Set;

import javax.faces.model.DataModel;
import javax.faces.model.ListDataModel;

import org.ajax4jsf.component.html.HtmlAjaxRepeat;
import org.ajax4jsf.model.SequenceDataModel;

public class UIIterator extends HtmlAjaxRepeat {

   @SuppressWarnings("unchecked")
   @Override
   protected DataModel getDataModel() {
      Object current = getValue();
      if(current instanceof Set){
          return new SequenceDataModel(new ListDataModel(
                new ArrayList((Set) current)));
      }
      return super.getDataModel();
   }
}

So, as we don’t care about the order of the elements, we just create a new ArrayList out of the Set. And we can now easily return the appropirate DataModel.

3. Add this to your faces-config. (I copied it from the a4j definition)

<component>
		<description />
		<display-name>Iterator</display-name>
		<component-type>com.myproject.Iterator</component-type>
		<component-class>com.myproject.components.UIIterator</component-class>

		<component-extension>
			<component-family>javax.faces.Data</component-family>
			<renderer-type>org.ajax4jsf.components.RepeatRenderera</renderer-type>
		</component-extension>
	</component>

4. Here is the tag definition for facelets

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE facelet-taglib PUBLIC
"-//Sun Microsystems, Inc.//DTD Facelet Taglib 1.0//EN"
"http://java.sun.com/dtd/facelet-taglib_1_0.dtd">
<facelet-taglib xmlns="http://java.sun.com/JSF/Facelet">
<namespace>http://myproject.com/cust</namespace>

<tag>
<tag-name>repeat</tag-name>
<component>
<component-type>com.myproject.Iterator</component-type>
<renderer-type>org.ajax4jsf.components.RepeatRenderer</renderer-type>
</component>
</tag>

</facelet-taglib>

Save this file as /WEB-INF/facelets/custom.taglib.xml

5. Add to your web.xml

<context-param>
<param-name>facelets.LIBRARIES</param-name>
<param-value>/WEB-INF/facelets/custom.taglib.xml</param-value>
</context-param>

6. It is now ready to use
…
xmlns:cust=”http://myproject.com/cust”
…

<cust:repeat var=”myVar” value=”${aSet}”>
…
</cust:repeat>
I think it is way neater than other workarounds, like defining a custom EL Resolver.