Last week I have been struggling with a small issue while developing a service using Jersey.

The goal of this service is to provide JSON object to my Web application, so called directly from the browser. This service returns in a JSON array a list of Employees, something like:

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{"employee":[
{"email":"jdoe@example.com","firstName":"John","lastName":"Doe"},
{"email":"mmajor@example.com","firstName":"Mary","lastName":"Major"}
]}

So an “employee” array, this is perfect and expected, but when my service returns a single element the returned object looks like:

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{"employee":{"email":"jdoe@example.com","firstName":"John","lastName":"Doe"}}

As you can see brackets […] are missing around the employee item. This is an issue since your client code is expecting an array.

A solution…

My application is using Jersey, the JAX-RS Reference Implementation, and JAXB for the serialization of Java Objects to JSON, as I have explained in a previous blog post. I found a solution to this by creating a new JAXB Context Resolver.

In this resolver I can control how the JSON object should be generated, here is my implementation :

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import com.grallandco.employee.service.converter.EmployeeConverter;
import javax.ws.rs.ext.ContextResolver;
import javax.ws.rs.ext.Provider;
import javax.xml.bind.JAXBContext;

import com.sun.jersey.api.json.JSONConfiguration;
import com.sun.jersey.api.json.JSONJAXBContext;

@Provider
public class JAXBContextResolver implements ContextResolver < JAXBContext > {

  private JAXBContext context;
  private Class[] types = {EmployeeConverter.class};

  public JAXBContextResolver() throws Exception {
    this.context = new JSONJAXBContext(JSONConfiguration.mapped().arrays("employee").build(),
    types);

  }

  public JAXBContext getContext(Class objectType) {
    for (Class type : types) {
      if (type == objectType) {
        return context;
      }
    }
    return null;
  }
}

First of all I declare this new class as a @Provider to say that it this class is of interest to the JAX-RS runtime.

I put in the types array the list of the Java classes that are concerned by the serialization (line#13). Then I create the ContextResolved with the different options that fulfill my requirements. You can take a look to the JAXBContextResolver Javadoc to see all the possible options available.

With this class, the service now returned the following JSON String:

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{"employee":[{"email":"jdoe@example.com","firstName":"John","lastName":"Doe"}]}

You can find a complete example (NetBeans project) here.