Using Parameter Converters in JAX-RS

Injection Made Easy

JAX-RS provides a simple set of API components to make it easy to extract information from your HTTP requests. It takes care of all the heavy lifting – all you need to do is use the applicable annotation (from the list below) and the container will inject the data for you:

• @MatrixParam, @QueryParam, @PathParam: injects URI related info
• @HeaderParam, @CookieParam: takes care of headers and cookies sent along with an HTTP request
• @FormParam: handles form data (application/x-www-form-urlencoded content type) POSTed over HTTP

But There Are a Few Rules…

As far as automatic injection is concerned, JAX-RS applies the following constraints:

• applicable for Java primitives (or equivalent wrappers) and String (of course)
• A custom type which has a static valueOf method or public constructor (both should accept a single parameter of type String)
• Supported Collections types: List, Set (whose generic type parameters comply with above-mentioned rules)

@Path("test")
public class JAXRSInjectionTestResource{
  @HeaderParam("token")
  //Assume that Token has a public ctor with a String param
  private Token token;

  @GET
  @Path("{id}")
  @Produces("application/json")
  public CustDetails fetch(@PathParam("id") String custID){
    //this method will be invoked by a HTTP GET on http://host:port/context-root/test/42
  }
}

What About Exceptional Scenarios?

More often than not, you would need to deal with data types which do not comply with the above rules. Also, you might not have the ability to retrofit (change) the actual source. In such scenarios, you can use a ParamConverter implementation to provide custom conversion logic of your HTTP request data (String) to your desired Java type. Here is an example:

public class MyParamConverter implements ParamConverter<MyObject>{

  @Override
  public MyObject fromString(String s){
    //imagine MyObject is a third party class
    return MyObjectFactory.get(s);
  }

  @Override
  public String toString(MyObject mo){
    return mo.toString();
  }
}

We Are Not Done Yet!

The ParamConverter implementation is not the actual provider which the JAX-RS runtime accesses directly. A ParamConverterProvider implementation provides another layer of abstraction in order to select the right ParamConverter for the job.

@Provider
public class MyParamConverterProvider implements ParamConverterProvider{

    @Override
    public <T> ParamConverter<T> getConverter(Class<T> rawType, Type genericType, Annotation[] annotations) {
        if(rawType.equals(MyObject.class)){
            return (ParamConverter<T>) new MyParamConverter();
        }
        return null;
    }
}

To Be Noted…

• The JAX-RS runtime automatically passes some parameters in the getConverter method. This allows for some extra processing/decision-making logic based on the parameters passed in by the JAX-RS runtime
• The ParamConverter and ParamConverterProvider interfaces were introduced in JAX-RS 2.0 (part of Java EE 7 Platform)

Just click here for some of my previous JAX-RS posts …