Generic JSF entity converter [duplicate]

Question

I'm writing my first Java EE 6 web app as a learning exercise. I'm not using a framework, just JPA 2.0, EJB 3.1 and JSF 2.0.

I have a Custom Converter to convert a JPA Entity stored in a SelectOne component back to an Entity. I'm using an InitialContext.lookup to obtain a reference to a Session Bean to find the relevant Entity.

I'd like to create a generic Entity Converter so I don't have to create a converter per Entity. I thought I'd create an Abstract Entity and have all Entities extend it. Then create a Custom Converter for the Abstract Entity and use it as the converter for all Entities.

Does that sound sensible and/or practicable?

Would it make more sense not to have an abstract entity, just a converter that converts any entity? In that case I'm not sure how I'd obtain a reference to the appropriate Session Bean.

I've included my current converter because I'm not sure I'm obtaining a reference to my Session Bean in the most efficient manner.

package com.mycom.rentalstore.converters;

import com.mycom.rentalstore.ejbs.ClassificationEJB;
import com.mycom.rentalstore.entities.Classification;
import javax.faces.application.FacesMessage;
import javax.faces.component.UIComponent;
import javax.faces.context.FacesContext;
import javax.faces.convert.Converter;
import javax.faces.convert.ConverterException;
import javax.faces.convert.FacesConverter;
import javax.naming.InitialContext;
import javax.naming.NamingException;

@FacesConverter(forClass = Classification.class)
public class ClassificationConverter implements Converter {

    private InitialContext ic;
    private ClassificationEJB classificationEJB;

    @Override
    public Object getAsObject(FacesContext context, UIComponent component, String value) {

        try {
            ic = new InitialContext();
            classificationEJB = (ClassificationEJB) ic.lookup("java:global/com.mycom.rentalstore_RentalStore_war_1.0-SNAPSHOT/ClassificationEJB");

        } catch (NamingException e) {
            throw new ConverterException(new FacesMessage(String.format("Cannot obtain InitialContext - %s", e)), e);
        }

        try {
            return classificationEJB.getClassificationById(Long.valueOf(value));
        } catch (Exception e) {
            throw new ConverterException(new FacesMessage(String.format("Cannot convert %s to Classification - %s", value, e)), e);
        }
    }

    @Override
    public String getAsString(FacesContext context, UIComponent component, Object value) {
        return String.valueOf(((Classification) value).getId());
    }
}

Answer 1

I am using JSF 2.0 view map:

@FacesConverter("entityConverter")
public class EntityConverter implements Converter {

private static final String key = "com.example.jsf.EntityConverter";
private static final String empty = "";

private Map<String, Object> getViewMap(FacesContext context) {
    Map<String, Object> viewMap = context.getViewRoot().getViewMap();
    @SuppressWarnings({ "unchecked", "rawtypes" })
    Map<String, Object> idMap = (Map) viewMap.get(key);
    if (idMap == null) {
        idMap = new HashMap<String, Object>();
        viewMap.put(key, idMap);
    }
    return idMap;
}

@Override
public Object getAsObject(FacesContext context, UIComponent c, String value) {
    if (value.isEmpty()) {
        return null;
    }
    return getViewMap(context).get(value);
}

@Override
public String getAsString(FacesContext context, UIComponent c, Object value) {
    if (value == null) {
        return empty;
    }
    String id = ((Persistent) value).getId().toString();
    getViewMap(context).put(id, value);
    return id;
}
}

Answer 2

Well I had the same problem today, and I solved it by creating a generic ConversionHelper and using it in the converter. For this purpose I have an EntityService which is a generic SLSB that I use to perform simple CRUD operations for any entity type. Also my entities implement a PersistentEntity interface, which has a getId and setId methods and I keep them with simple primary keys. That's it.

In the end my converter looks like this:

@FacesConverter(value = "userConverter", forClass = User.class)
public class UserConverter implements Converter {

    @Override
    public Object getAsObject(FacesContext ctx, UIComponent component, java.lang.String value) {

        return ConversionHelper.getAsObject(User.class, value);
    }

    @Override
    public String getAsString(FacesContext ctx, UIComponent component, Object value) {

        return ConversionHelper.getAsString(value);
    }
}

And my conversion helper looks like this:

public final class ConversionHelper {

    private ConversionHelper() {
    }

    public static <T> T getAsObject(Class<T> returnType, String value) {

        if (returnType== null) {

            throw new NullPointerException("Trying to getAsObject with a null return type.");
        }

        if (value == null) {

            throw new NullPointerException("Trying to getAsObject with a null value.");
        }

        Long id = null;

        try {

            id = Long.parseLong(value);

        } catch (NumberFormatException e) {

            throw new ConverterException("Trying to getAsObject with a wrong id format.");
        }

        try {

            Context initialContext = new InitialContext();
            EntityService entityService = (EntityService) initialContext.lookup("java:global/myapp/EntityService");

            T result = (T) entityService.find(returnType, id);

            return result;

        } catch (NamingException e) {

            throw new ConverterException("EntityService not found.");
        }
    }

    public static String getAsString(Object value) {

        if (value instanceof PersistentEntity) {

            PersistentEntity result = (PersistentEntity) value;

            return String.valueOf(result.getId());
        }

        return null;
    }
}

Now creating converters for simple JPA entities is a matter of duplicate a converter and change 3 parameters.

This is working well for me, but I don't know if it is the best approach in terms of style and performance. Any tips would be appreciated.