Question

I know i can convert JSON to an object using HJackson. Below is the code snippet to do it.

    JsonFactory jsonFactory = new JsonFactory();
    ObjectMapper mapper = new ObjectMapper(jsonFactory);
    DTO1 user = mapper.readValue(new File("c:\\oneFile.json"),
                DTO1.class);

But my requirment is to convert JSON to multiple objects. In my case to multiple objects like DTO1, DTO2…. and also to String variables if possible.

Is there an API in Jackson to perform this operation.
Or is there a way where i can manually pick and store them in corresponding DTO’s(Objects).

Regards
Karan

Answer 1

From http://wiki.fasterxml.com/JacksonInFiveMinutes (but also look at TypeFactory in Jackson 1.3+) :

Data Binding with Generics

In addition to binding to POJOs and “simple” types, there is one additional variant: that of binding to generic (typed) containers. This case requires special handling due to so-called Type Erasure (used by Java to implement generics in somewhat backwards compatible way), which prevents you from using something like Collection.class (which does not compile).

So if you want to bind data into a Map you will need to use:

  Map<String,User> result = mapper.readValue(src, new TypeReference<Map<String,User>>() { });

where TypeReference is only needed to pass generic type definition (via anynomous inner class in this case): the important part is > which defines type to bind to.

If you don’t do this (and just pass Map.class), call is equivalent to binding to Map (i.e. “untyped” Map), as explained above.

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If you mean polymorphic deserialization, check this link http://programmerbruce.blogspot.com/2011/05/deserialize-json-with-jackson-into.html which discusses using the jackson annotations to assist the mapper in picking the right object for the job.

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I wrote this today (I’m sure it can be improved) for simply taking a JSON string and always converting it into a List of DTOs (or whatever it can be converted into). This could be called with any type, and could be modified to return a single instance, or List, or whatever you wished. OBJECT_MAPPER is a static ObjectMapper variable.

  public static <T> List<T> getFromJSONCollection(String jsonString, final Class<T> type) throws IOException {
    try {
      return OBJECT_MAPPER.readValue(jsonString, TypeFactory.defaultInstance().constructCollectionType(ArrayList.class, type));
    } catch (JsonMappingException e) {
      return getFromJSON(jsonString, type);
    }
  }

  public static <T> List<T> getFromJSON(final String jsonString, final Class<T> type) throws IOException {
    return new ArrayList<T>() {{ add(OBJECT_MAPPER.readValue(jsonString, type));}};
  }