Jackson Simple Data Binding

Internal

• Jackson

Overview

Simple data binding extracts data from JSON and initializes an in-memory Java object hierarchy. Unlike the tree model, which uses JsonNodes, simple data binding assembles the hierarchy representing the JSON data out of Maps, Lists, Strings, Numbers, Booleans and nulls. A Map/List/String/Number/Boolean hierarchy can be written out as JSON content.

JSON to Java

JSON to Java parsing is done by ObjectMapper.readValue(), which must be called with a type corresponding to the top-level JSON data type, usually a Map.

import com.fasterxml.jackson.databind.ObjectMapper;

ObjectMapper mapper = new ObjectMapper();

Map root = mapper.readValue(inputStream, Map.class);

However, if the top level JSON value is a List, the code above will throw an exception:

Exception in thread "main" com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of java.util.LinkedHashMap out of START_ARRAY token

That is why is best if the parsing starts with an Object:

import com.fasterxml.jackson.databind.ObjectMapper;

ObjectMapper mapper = new ObjectMapper();

Object root = mapper.readValue(src, Object.class);

if (root instance of Map) {
    ...
}
else if (root instanceof List) {
    ...
}
...

It is possible to enable generic type information (like Map<String, Object>). For details, see Full Data Binding.

JSON to Java Code Example

https://github.com/NovaOrdis/playground/blob/master/json/jackson/simple-data-binding-json-to-java/src/main/java/io/novaordis/playground/json/jackson/simpledatabinding/json2java/Main.java

Java to JSON

ObjectMapper om = new ObjectMapper();

Map root = ...

om.writeValue(baos, root);