Question

I have an application which makes use of an external library (Jackson), and the method I need requires a class literal as an argument. So if I wish to parse my JSON string into a User object:

ObjectMapper mapper = new ObjectMapper();
User user = mapper.readValue(new File("user.json"), User.class);

Now, I wish to use this method dynamically (i.e. parse different JSON strings using the same line of code). For example:

String json1 = "{"type":"jacket",...}";
String json2 = "{"type":"sweater",...}";

Object object = mapper.readValue(json1/json2, ???); 
//returns a Jacket object OR Sweater object based on the "type" key
//i.e. use Jacket.class as the 2nd argument if "type" is "jacket"
//OR Sweater.class if "type" is "sweater"

//After getting the deserialized object,
//if object is Jacket, cast as a Jacket
//if object is Sweater, cast as a Sweater

Of course, the JSON string in question can be for any class, so I can’t simply hard-code an if-else loop. I’ve looked at custom serializers, but frankly am quite lost at what it’s talking about, and would like some help in how I can go about this.

In summary, I need some way to first define a class literal from a String, and then cast the resulting Object into the specific class (but my focus is on getting readValue to work dynamically).

Answer 1

Looks like you need a mapping somewhere between JSON type variable and Java class type.

Generally result should be something like this map:

Map<String, Class<? extends YourSupertype>> map = new HashMap<>();
map.put("sweater", Sweater.class);
map.put("jacket", Jacket.class);

Just store possible clothing types somewhere in a file, then do something like:

String clothingType = nextEntryFromFile();
String className = constructClassNameFromClothingType(clothingType);
map.put(clothingType, Class.forName(className));