Using JSONP with JAX-RS

JSONP is a format used to work around the same origin policy. As the name implies it is a modification on the JSON format. Instead of returning a JSON object like so

{ "customerId": 123, "name": "fred", "email": "fred@bedrock.com" }

the JSON is return as a method call to a function like so

foo({ "customerId": 123, "name": "fred", "email": "fred@bedrock.com" })

JSONP is typically used in cross origin AJAX calls using the GET method. You can learn more about JSONP from this Stack Overflow discussion. The JSON-P website talks about why and when it is used.

Handling JSONP

We will examine JSONP by first looking at how we use it from the ever popular jQuery. To make an cross origin AJAX call with JSONP we use $.ajax() like so

$.ajax({
   type: "GET",
   url: "webresources/customer/1",
   dataType: 'jsonp' })
.done(function(result) {
   //Do something with the result
   console.log("----> " + JSON.stringify(result));
}).fail(function() {
   //Handle failure
   ...
});

Lets assume that our AJAX call is going to get a customer record. The HTTP request goes across to the server

GET /JSONPTest/webresources/customer/1?callback=jQuery210012143543711863458_1393227259817 HTTP/1.1
Accept: text/javascript, application/javascript, application/ecmascript, application/x-ecmascript, */*; q=0.01

and the response

HTTP/1.1 200 OK
Content-Type: application/javascript
jQuery210012143543711863458_1393227259817({"name":"barney","id":1,"email":barney@gmail.com})

Notice that the callback function name is in the query string of the request.

So to write a JAX-RS resource to handle we need to

• Handle the HTTP GET method
• Read the callback parameter in the query string. We will use the name of the function to wrap our result
• Produce the MIME type in the Accept header

The following JAX-RS resource should handle the JSONP request

@RequestScoped
@Path("/customer/{id}")
public class CustomerResource {

   @PersistenceContext EntityManager em;

   @GET
   @Produces("application/javascript")
   public String get(@PathParam("id") Integer id ,
         @QueryParam("callback") String callback) {

      Customer cust = em.find(Customer.class, id); 
      if (null == cust)
         throw WebApplicationException(Response.Status.NOT_FOUND);

      //Convert object to JSON
      JsonObjectBuilder builder = Json.createObjectBuilder();
      ...
      return (callback + "(" + builder.build().toString() + ")");
   }
}

The code is quite straightforward. We inject the callback query string into the handler which we subsequently use to wrap the result.

As you can see, its quite simple to handle JSONP.

JSONP as MessageBodyWriter

JAX-RS has a pair of SPI call MessageBodyReader and MessageBodyWriter that allows anyone to handle marshalling and unmarshalling of objects from HTTP to JAX-RS resources. I wrote a simple MessageBodyWriter , called JSONP4JAXRS, that simplifies the entire process (if you don’t mind adding 12K to to your project).

If you’re using JSONP4JAXRS, then the above CustomerResource can rewritten as follows

@RequestScoped
@Path("/customer/{id}")
public class CustomerResource {

   @PersistenceContext EntityManager em;

   @GET
   @Produces("application/javascript") 
      public String get(@PathParam("id") Integer id ) {

      Customer cust = em.find(Customer.class, id); 
      if (null == cust)
         throw WebApplicationException(Response.Status.NOT_FOUND);

      //Convert object to JSON
      JsonObjectBuilder builder = Json.createObjectBuilder();
      ...
      return (builder.build()); 
   }
}

No more fiddling around with callback parameter. The MessageBodyWriter will automatically pad the JsonObject with the callback.

But wait! There is more!

JSONP4JAXRS will automatically pad any return types according to the following rules:

Return Type	Padding (assume fn is the name of the callback function)
JsonObject	fn({ … }) where { … } is the JsonObject
JsonArray	fn([ … ]) where [ … ] is the JsonArray
Any primitives including their  wrappers	fn({“value”: the_value })
Any object	fn({“value”: toString_value_of_object})
Iterable	fn([ … ]) The Iterable is converted to a JsonArray and the elements are converted according the the above rule

Don’t forget to wrap your Iterable (eg. List<JsonObject>) in a GenericEntity if you are returning the result via a Response.

The JSONP4JAXRS contains a class call JsonWithPadding that defines the following constants for JSONP MIME types

Constant Name	MIME Type
APPLICATION_JAVASCRIPT	application/javascript
APPLICATION_JSON	application/json
APPLICATION_ECMASCRIPT	application/ecmascript
APPLICATION_X_ECMASCRIPT	application/x-ecmascript
TEXT_JAVASCRIPT	text/javascript

JSONP4JAXRS works with Java EE 7 only. JSONP4JAXRS can be downloaded here. The source and binaries are in the compile NetBeans project file. The binaries is in dist directory.

Till next time.