OK, I added a ‘private Long id’ to Country, and ran your sample.

I added some debug so the line numbers are different, but the error is occurring on line :

‘Class clazz = entity.getClass();’ in the ‘getResourceLinkFields’ method.

Stacktrace below:

Hit ENTER for redeploy
Entity is : [com.example.rest.model.jaxb.Country@7f6a92, com.example.rest.model.jaxb.Country@2dbcf0]
clazz is : class com.example.rest.model.jaxb.Country
declared fields of : com.example.rest.model.jaxb.Country@7f6a92 are : [Ljava.lang.reflect.Field;@cf64f8
Apr 4, 2009 3:01:27 PM org.apache.catalina.core.StandardWrapperValve log
SEVERE: StandardWrapperValve[Jersey Web Application]: PWC1406: Servlet.service() for servlet Jersey Web Application threw exception
java.lang.NullPointerException
at com.example.rest.connectedness.filter.ResourceLinkResponseFilterDumb.getResourceLinkFields(ResourceLinkResponseFilterDumb.java:101)
at com.example.rest.connectedness.filter.ResourceLinkResponseFilterDumb.addResourceLinks(ResourceLinkResponseFilterDumb.java:65)
at com.example.rest.connectedness.filter.ResourceLinkResponseFilterDumb.getResourceLinkFields(ResourceLinkResponseFilterDumb.java:109)
at com.example.rest.connectedness.filter.ResourceLinkResponseFilterDumb.addResourceLinks(ResourceLinkResponseFilterDumb.java:65)
at com.example.rest.connectedness.filter.ResourceLinkResponseFilterDumb.filter(ResourceLinkResponseFilterDumb.java:46)
at com.sun.jersey.server.impl.application.WebApplicationImpl._handleRequest(WebApplicationImpl.java:581)
at com.sun.jersey.server.impl.application.WebApplicationImpl.handleRequest(WebApplicationImpl.java:514)
at com.sun.jersey.server.impl.application.WebApplicationImpl.handleRequest(WebApplicationImpl.java:505)
at com.sun.jersey.spi.container.servlet.ServletContainer.service(ServletContainer.java:359)

Thanks for your help,

Ross

This is great stuff, it is exactly what I need to do. However, my domain objects have an ID of type ‘Long’, and the ‘getResourceLinkFields’ method in the filter is falling over with an NPE. I am having trouble modifying the code to work with Longs rather than strings like in your example.

Can you suggest how to update the code to work in my case ?

This is all about getting a degree of decoupling from your model objects through to your representations aka views. We all know this separation has worked well for us in the past.

You could avoid this separation and implement it in a simpler fashion, but it you would only solve it for a specific problem or set of classes.

There are three main parts of this technique:
1. Enhance your model objects so they have a placeholder/collection so you can accumulate links
2. Annotate your model at interesting members where you want links
3. In a ContentResponseFilter, scanning for annotations of interest and inserting them into the collection

Step 1, I used aspectj, to keep things decoupled, but you could take a simpler and more intrusive approach and add in a Links collection to your hibernate objects that isn’t persisted. To use aspectj on this over your hibernate objects, would likely lead to complications.

In Step 2, If you are doing really simple links like parent to child etc. you probably could get away with this.

In Step 3, You can scan any parts of your class, scanning annotations is convenient, but you could just as easily scan for particular classes or fields, but once again that is more coupling than the annotation scanning.

In my sample code I had link generation being done by a LinkTemplateProcessor, but maybe you could have a HibernateAssociationProcessor.

Largely it will depend on the links you need to generate and whether or not you can get enough contextual information from your hibernate annotation.
Some links you will want to drill down, others you may want to jump back up to a parent, or even just a particular page for a particular user, like user preferences.

More specifically when you have links that aren’t in hibernate mapping, how are you going to specify and generate them?

Sure you can insert them directly, probably in your ContainerResponseFilterDumb, it’s just a matter of choice.

Overall, for really simple links, I guess you could probably use Hibernate.
But for some of the other links, it may just be easier to use the LinkTemplate annotations I’m suggesting.

The ContainerResponseFilterDumb that I’ve implemented, is really shabby and inefficient. Time permitting, I should improve on it, probably using something like scannotations.sf.net, it may be a good fit.

On a side note, I personally would not advocate using Hibernate for this approach in general as it takes away from the designing of your resources free (kind of) of the constraints of database implementation.

But yes I’m pragmatic enough to understand there are times when you just need to pump stuff out.