Proper JSON output with JAX-RS

2013-03-10

I had to develop a REST webservice endpoint recently in our JavaEE application. After we figured out what data the client needs we came to an agreement that we will produce JSON output. I knew JAX-RS can do that, so I simply wrote a method like this:

 1@GET
 2@Produces(MediaType.APPLICATION_JSON)
 3@Path("/get/{param1}/{param2}")
 4public List<ApprovedEntrant> getEntrants(
 5            @PathParam("param1") final String param1,
 6            @PathParam("param2") final String param2) {
 7
 8     final List<ApprovedEntrant> entrants = new LinkedList<ApprovedEntrant>();
 9
10     // put elements in the list
11
12     return entrants;
13}

It seems to work as expected but sometimes we noticed weird results with the default JSON serialization of JAX-RS in Glassfish 3 (maybe Jersey?):

in case of an empty list (!), it produces "null" string output
if the list contains only one element, the result is the element in JSON object, instead of a JSON array with one element

Of course, I could import the official JSON Java library and do some toString() or object transform tricks, but I don’t want to introduce another dependency in my widely used domain project. I want to solve it in the webservice layer, without manual object / list JSON transformations. After some research I found the Jackson project which has a smarter JAX-RS JSON provider.

First, I added the new dependency to my webservice project:

1<dependency>
2    <groupId>com.fasterxml.jackson.jaxrs</groupId>
3    <artifactId>jackson-jaxrs-json-provider</artifactId>
4    <version>2.1.4</version>
5</dependency>

Then I had to modify my code a bit, but I think it’s quite normal and doesn’t contain ugly JSON objects and arrays.

 1private static ObjectMapper mapper = new ObjectMapper();
 2
 3@GET
 4@Produces(MediaType.APPLICATION_JSON)
 5@Path("/get/{param1}/{param2}")
 6public String getEntrants(
 7            @PathParam("param1") final String param1,
 8            @PathParam("param2") final String param2) {
 9
10     final List<ApprovedEntrant> entrants = new LinkedList<ApprovedEntrant>();
11
12     // put elements in the list
13
14     String out = "";
15     try {
16         out = mapper.writeValueAsString(entrants);
17     } catch (JsonProcessingException ex) {
18         //error handling
19     }
20
21     return out;
22}

#coding #java #json #rest