How to modify spring/jackson JSON deserialisation - json

Question

I'm trying to figure out how to adjust the way spring/jackson convert a JSON string (stored in a file) into various POJOs. For example, if I have this JSON:
{
"rates":{
"EURUSD":5.4321,
"USDHKD":1.2345
}
}
I actually want to get an instance of my 'Rates' class. Inside that I want a List containing each individual rate.
In my spring config file I created this entry:
#Bean
public ObjectMapper jsonObjectMapper() {
return new MappingJackson2HttpMessageConverter().getObjectMapper();
}
And in my service class I did this:
#Autowired
ObjectMapper jsonObjectMapper;
public Rates currentRates() {
Resource resource = this.ctx.getResource("classpath:stub/data/rates/Rates-01.json");
return this.jsonObjectMapper.readValue(resource.getURL(), Rates.class);
}
The problem is that I am trying to figure out how to take the Map containing the currencies as a single key, break those currencies in two and then create a RateEntry object containing the two currencies and the rate, before populating a list in the Rates class.
I've been looking at Spring's Conversion Service with the idea to define a converter that maps the Map to a list. i.e. this signature: Converter<Map<String, BigDecimal>, List<Rate>>. However this is based on the assumption that the JSON is first converted to standard types before the conversion service is called. An assumption I now think is incorrect.
So I'm now trying to figure out if I need to register some sort of custom ObjectMapper to handle reading directly from the JSON String data. But that sounds like over kill as I only want to adjust part of the object graph, and let the default converters handle the rest.
Any pointers appreciated. Thanks.

Answer 1

Ok, Jackson tries to stay away from structural transformations (since it's a quick-sand pit with unlimited number of general permutations). But it might be possible to use some existing features to do what you want.
First: to use Object key to indicate type, you will probably want to enable polymorphic type handling with "as object wrapper" inclusion.
So add something like:
#JsonTypeInfo(as=Include.WRAPPER_OBJECT)
for your Rates class declaration.
As to converting values into list; this might work by defining "any-setter" (see http://www.cowtowncoder.com/blog/archives/2011/07/entry_458.html), something like:
#JsonAnySetter
public void set(String key, Double value) // or "Object value")
{
list.add(new Rate(key, value));
}
I hope this helps.