How to return a partial JSON response using Java?

Question

I'm building a RESTful API and want to provide developers with the option to choose which fields to return in the JSON response. This blog post shows examples of how several API's (Google, Facebook, LinkedIn) allow developers to customize the response. This is referred to as partial response.

An example might look like this:

/users/123?fields=userId,fullname,title

In the example above the API should return the userId, fullName and title fields for User "123".

I'm looking for ideas of how to implement this in my RESTful web service. I'm currently using CXF (edit: and Jackson) but willing to try another JAX-RS implementation.

Here's what I currently have. It returns a full User object. How can I return only the fields the API caller wants at runtime based on the "fields" paramaeter? I don't want to make the other fields Null. I simply don't want to return them.

@GET
@Path("/{userId}")
@Produces("application/json")
public User getUser(@PathParam("userId") Long userId, 
    @DefaultValue("userId,fullname,title") @QueryParam("fields") String fields) {

User user = userService.findOne(userId);

StringTokenizer st = new StringTokenizer(fields, ",");
while (st.hasMoreTokens()) {

    // here's where i would like to select only the fields i want to return

}
return user;
}

UPDATE:

I followed unludo's link which then linked to this: http://wiki.fasterxml.com/JacksonFeatureJsonFilter

With that info I added @JsonFilter("myFilter") to my domain class. Then I modified my RESTful service method to return String instead of User as follows:

@GET
@Path("/{userId}")
@Produces("application/json")
public String getUser(@PathParam("userId") Long userId,
                    @DefaultValue("userId,fullname,title") @QueryParam("fields") String fields) {

    User user = userService.findOne(userId);

    StringTokenizer st = new StringTokenizer(fields, ",");
    Set<String> filterProperties = new HashSet<String>();
    while (st.hasMoreTokens()) {
        filterProperties.add(st.nextToken());
    }

    ObjectMapper mapper = new ObjectMapper();
    FilterProvider filters = new SimpleFilterProvider().addFilter("myFilter",
                SimpleBeanPropertyFilter.filterOutAllExcept(filterProperties));

    try {
        String json = mapper.filteredWriter(filters).writeValueAsString(user);
        return json;
    } catch (IOException e) {
        e.printStackTrace();
    return e.getMessage();
    }
}

I need to do more testing but so far so good.

Answer 1

If you use Jackson (a great JSON lib - kind of the standard for Java I believe), you may use the @View annotation to filter what you want in the resulting object.

I understand that you want something dynamic so it's a bit more complicated. You will find what you are looking for here: http://www.cowtowncoder.com/blog/archives/2011/02/entry_443.html (look at 6. Fully dynamic filtering: @JsonFilter).

I would be interested in the solution you will find.