I know this is old, but I ran into the same problem. And there doesn't seem to be much about it here... so I thought I would add what I learned.

I used a third-party library to aid in the endeavor: org.codehaus.jackson All of the downloads for this can be found here.

For base JSON functionality, you need to add the following jars to your project's libraries: jackson-mapper-asl and jackson-core-asl

Choose the version your project needs. (Typically you can go with the latest stable build).

Once they are imported in to your project's libraries, add the following import lines to your code:

Copy import org.codehaus.jackson.JsonGenerationException;
 import org.codehaus.jackson.map.JsonMappingException;
 import org.codehaus.jackson.map.ObjectMapper;

With the java object defined and assigned values that you wish to convert to JSON and return as part of a RESTful web service

CopyUser u = new User();
 u.firstName = "Sample";
 u.lastName = "User";
 u.email = "sampleU@example.com";

ObjectMapper mapper = new ObjectMapper();
    
try {
    // convert user object to json string and return it 
    return mapper.writeValueAsString(u);
}

  // catch various errors
  catch (JsonGenerationException e) {
    e.printStackTrace();
} 
  catch (JsonMappingException e) {
    e.printStackTrace();
}

The result should looks like this: {"firstName":"Sample","lastName":"User","email":"sampleU@example.com"}