javajsonspringjackson

@RequestBody gives empty JsonObject when making a POST Request

I have the following method:

@RequestMapping(value = "/app/write", method = RequestMethod.POST, consumes = "application/json", produces = "application/json")
public
@ResponseBody
Status writeBuildData(@RequestBody JsonObject templateDataJSON){}

Basically I make an Ajax POST request sending JSON, I always get an empty JsonObject {} as a result

JsonObject templateDataJSON = "{}";

But if I use String instead of JsonObject, I get the correct value.

This app is made with Spring Mvc 4.1.4.

Dependencies:

compile 'org.codehaus.jackson:jackson-mapper-asl:1.9.13'
compile 'com.google.code.gson:gson:2.3.1'

Any idea what I am missing and why the JsonObject doesn't get injected and always gives me {}?

Solution

  • Spring no longer supports Jackson 1 as a message converter implementation.

    So your

    compile 'org.codehaus.jackson:jackson-mapper-asl:1.9.13'

    is actually meaningless to Spring.

    Your

    compile 'com.google.code.gson:gson:2.3.1'

    will cause Spring to use GsonHttpMessageConverter and, basically, do

    String json = "{\"random\":\"42\"}";
Gson gson = new Gson();
JsonObject jsonObject = gson.fromJson(json, JsonObject.class);

    JsonObject is a Gson type. Gson is aware of it and knows how to deserialize JSON object json into it. This will work correctly and will generate a JsonObject which has a value of 

    {"random":"42"}

    Since you're saying that you're getting an empty JsonObject, I can only assume that you have Jackson 2 on your classpath.

    Spring registers the Jackson HttpMessageConverter, MappingJackson2HttpMessageConverter, before the GsonHttpMessageConverter if both are present on the classpath.

    With Jackson, Spring would have deserialized your request body basically as such

    ObjectMapper mapper = new ObjectMapper();
mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
JsonObject jsonObject = mapper.readValue(json, JsonObject.class);

    which you'll note results in 

    {}

    This is because Jackson knows nothing about the type JsonObject so it has to dynamically build a deserialization strategy. That strategy depends on properties which Jackson defines as setters (for the context of deserialization) or anything annotated with @JsonProperty, which obviously JsonObject doesn't have. So it basically thinks the type JsonObject doesn't have any properties (or maybe none that appear in your custom JSON content). As such, and because it ignores any unknown properties (which would have caused it to throw exceptions), it simply returns a new, empty JsonObject object.

    One solution is to remove Jackson 2 from the classpath. Another solution is to explicitly add HttpMessageConverter instances in the order you want.