brettdargan.com » hateoas http://brettdargan.com/blog λ Thoughts and rants Fri, 28 May 2010 01:35:46 +0000 en hourly 1 http://wordpress.org/?v=3.2.1 HATEOAS for legacy object models http://brettdargan.com/blog/2009/01/14/hateoas-for-legacy-object-models/ http://brettdargan.com/blog/2009/01/14/hateoas-for-legacy-object-models/#comments Tue, 13 Jan 2009 15:44:59 +0000 admin http://brettdargan.com/blog/?p=196

One of the best aspects of RESTful orientated services is getting your resources connected, possibly even achieving HATEOAS. Making that jump with with legacy object models or data can be tricky, depending on how intrusive you want to be.

There is such a large amount of legacy data in various forms, usually with minimal links/relations that could be leveraged.

So it is a worthy challenge to try and do it in the least intrusive manner possible.

This is one approach that is reasonably decoupled from an existing legacy model.

There are three main parts to this technique:

  1. Enhance your model objects so they have a placeholder/collection so you can accumulate links
  2. Annotate your model at interesting points with link templates, around attributes or methods or classes
  3. Use a custom ContainerResponseFilter class of jersey to scan for annotations of interest, then evaluate your link templates (at run time), using the data of your models and any request based information you may require.

I chose to enhance my model classes using AspectJ at compile time.

Download the source

mvn glassfish:run
./curl_test #will hit two urls

and you should see some results, with links like this:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<country xmlns:ns2="http://www.w3.org/1999/xlink">
  <links>
    <link rel="" ns2:type="locator"
        ns2:href="http://localhost:8080/hateoas4legacy/rs/cities/Brisbane"/>
    <link rel=""
        ns2:type="locator" ns2:href="http://localhost:8080/hateoas4legacy/rs/cities/Sydney"/>
    <link rel=""
        ns2:type="locator" ns2:href="http://localhost:8080/hateoas4legacy/rs/cities/Melbourne"/>
    <link rel=""
        ns2:type="locator" ns2:href="http://localhost:8080/hateoas4legacy/rs/countries/AU"/>
  </links>
  <cities>
    <city>
      <links/>
      <name>Brisbane</name>
    </city>
    <city>
        <links/>
        <name>Sydney</name>
    </city>
    <city>
      <links/>
      <name>Melbourne</name>
    </city>
  </cities>
  <isoCode>AU</isoCode>
  <name>Australia</name>
</country>

All countries Results

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<countries>
<country xmlns:ns2="http://www.w3.org/1999/xlink">
  <links>
    <link rel=""
        ns2:type="locator" ns2:href="http://localhost:8080/hateoas4legacy/rs/countries/AU"/>
  </links>
  <cities/>
  <isoCode>AU</isoCode>
  <name>Australia</name>
</country>
<country xmlns:ns2="http://www.w3.org/1999/xlink">
  <links>
    <link rel=""
        ns2:type="locator" ns2:href="http://localhost:8080/hateoas4legacy/rs/countries/USA"/>
  </links>
  <cities/>
  <isoCode>USA</isoCode>
  <name>United States of America</name>
</country>
</countries>

It is very crufty, but does work in the simple cases. I haven't had a chance to demonstrate all of the features yet.

I'm likely to further flesh out this example as it is interesting in other RESTful related ways.

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