Category: GAE

JSF 2 Custom Converter

I ran into a very strange problem in my little Google App Engine application. I’m using JSF 2 as the presentation layer. I’m using Google’s low level api for accessing the data store and have written some little utilities to convert from my objects to Google’s Entity objects.

I thought I had the mapping working OK, but had only tried setting simple String properties on my data objects. I finally tried to map a simple association, where one object would have a list of keys of other objects. In my case I have Tables. Each table can be associated with zero or more Genres. So, my Table data object has a List of Keys for the genres that it is a part of, like this:

public class Table
        extends DataObjectAbstract
        implements Serializable
    private List<Key> genres;

    // other private attributes here

    public List<Key> getGenres()
        return genres;

    public void setGenres( List<Key> genres )
        this.genres = genres;

    // other public getters and setters and other methods here

You can safely ignore DataObjectAbstract and the @Persistent annotation. They aren’t part of this blog post.

The strange behavior I was seeing was that when I would get the genre list from the data store, it would be a List<String> instead of the List<Key> I was expecting. After some debugging, I finally discovered that I was setting it as a List<String> because Strings are what I was getting from my JSF page, and thanks to Java’s type erasure with generics, my code didn’t know any better. So, the solution is a custom converter, which will allow me to pass the Keys to and from the browser.

Fortunately, in JSF 2, custom converters are even easier than they are in JSF 1.x. Here’s my simple converter class for the class. Making this even more simple is the fact that Google offers a KeyFactory that takes care of giving a URL safe representation of their Key object.

package jota.soc.ui.converter;

import javax.faces.component.UIComponent;
import javax.faces.context.FacesContext;
import javax.faces.convert.Converter;
import javax.faces.convert.FacesConverter;

 * Converter for Google Key.
 * @author Joel.Weight
@FacesConverter( value="keyConverter" )
public class KeyConverter implements Converter {

     * converts the String representation of the key back to the Object
    public Object getAsObject( FacesContext context, UIComponent component,
                               String value )
        // will throw new IllegalArgumentException if it can't parse.
        return KeyFactory.stringToKey( value );

     * converts the Key object into its String representation.
    public String getAsString( FacesContext context, UIComponent component,
                               Object value )
        if ( value instanceof Key )
            return KeyFactory.keyToString( (Key)value );
            throw new IllegalArgumentException( "Cannot convert non-key object in KeyConverter" );

With the @FacesConverter annotation, you don’t have to register the converter in faces-config or anything. You can simply reference it using the id.

Here’s the code that uses the converter.

<h:selectManyListbox id="genresBox" value="#{createTable.table.genres}">
    <f:converter converterId="keyConverter" />
    <f:selectItems value="#{createTable.genres}"
        itemLabel="#{}" />

And just like that, I’m storing Keys in the data store instead of Strings.

JSF 2.0.2 and Google App Engine

I spent a couple hours today trying to get a JSF 2 based webapp up in app-engine.  Of course, it wasn’t as easy as simply following the instructions in

configuring jsf 2.0 to run on the google app engine using netbeans

I ran into the problem detailed here  ( JNDI problem in jsf 2.0 and google app engine ) right off the bat.  I tried to use his packaged jar, but app-engine kept giving me an exception that it couldn’t load the zip file. Perhaps it was corrupted when I downloaded it or something.

If you follow his link to the file, you will see that he created a new file within the same package as the existing one and copied the implementation, commenting out the offending code.  I decided to try a different approach. I’m sure there is nothing wrong with Josh’s approach, and if it weren’t for his blog and all the ground work he had already done in finding the offending code, I probably wouldn’t have even bothered going further with JSF 2 in app engine. In any case, here’s what I did.

I downloaded the JSF 2.0.2 source from here ( jsf 2.0.2 source ) and expanded it.  I then found the file and decided to modify it.  Rather than comment out all the source, I added a new config enumeration value to the BooleanWebContextInitParameter enumerations as follows

// around line 1071 in the finished file, after the previously final enum value AllowTextChildren

Then, I check for the value of that parameter in private constructor, around line 113

        // add the isOptionEnabled call to this conditional statement
        if ( isOptionEnabled( BooleanWebContextInitParameter.EnableJNDI ) && canProcessJndiEntries()) {

When that is done, simply compile, and you have a new jsf-api/build/lib/jsf-api.jar and jsf-ri/build/lib/jsf-impl.jar that you can use in your project. For my compile, I simply copied the to and set the property to the directory in which the source was checked out.

Finally, put a new context-param in web.xml with the name com.sun.faces.enableJNDI and a value of false.

All I’ve got so far is the hello world, so we’ll see if I run into other difficulties as I use further functionality.