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 an Object.class type:

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) {
    ...
}
...

This is the conversion table:

JSON Type	Java Type
object	LinkedHashMap<String,Object>
array	ArrayList<Object>
string	String
number (no fraction)	Integer, Long or BigInteger (smallest applicable)
number (fraction)	Double (configurable to use BigDecimal)
false	Boolean
null	null

It is possible to call readValue() with a more specific type. For example, if the top level JSON value is an Object, readValue() can be called upon 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

JSON to Java Code Example

JSON to Java Simple Data Binding Example

Generics Support

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

Java to JSON

The Map/List/String/Number/Boolean hierarchy can be written out as JSON by ObjectMapper.writeValue().

ByteArrayOutputStream baos = new ByteArrayOutputStream();

ObjectMapper om = new ObjectMapper();

Map root = ...

om.writeValue(baos, root);

System.out.println(new String(baos.toByteArray()));

Java to JSON Code Example

Java to JSON Simple Data Binding Example