Getting Started with Custom Deserialization in Jackson – 在Jackson中开始使用自定义反序列化

最后修改: 2014年 1月 13日


1. Overview


This quick tutorial will illustrate how to use Jackson 2 to deserialize JSON using a custom Deserializer.

这个快速教程将说明如何使用Jackson 2使用一个定制的解串器来解串JSON。

To dig deeper into other cool things we can do with Jackson 2, head on over to the main Jackson tutorial.


2. Standard Deserialization


Let’s start by defining two entities and see how Jackson will deserialize a JSON representation to these entities without any customization:


public class User {
    public int id;
    public String name;
public class Item {
    public int id;
    public String itemName;
    public User owner;

Now let’s define the JSON representation we want to deserialize:


    "id": 1,
    "itemName": "theItem",
    "owner": {
        "id": 2,
        "name": "theUser"

And finally, let’s unmarshal this JSON to Java Entities:


Item itemWithOwner = new ObjectMapper().readValue(json, Item.class);

3. Custom Deserializer on ObjectMapper


In the previous example, the JSON representation matched the Java entities perfectly.


Next, we will simplify the JSON:


    "id": 1,
    "itemName": "theItem",
    "createdBy": 2

When unmarshalling this to the exact same entities, by default, this will of course fail:


Unrecognized field "createdBy" (class org.baeldung.jackson.dtos.Item), 
not marked as ignorable (3 known properties: "id", "owner", "itemName"])
 at [Source:; line: 1, column: 43] 
 (through reference chain: org.baeldung.jackson.dtos.Item["createdBy"])

We’ll solve this by doing our own deserialization with a custom Deserializer:


public class ItemDeserializer extends StdDeserializer<Item> { 

    public ItemDeserializer() { 

    public ItemDeserializer(Class<?> vc) { 

    public Item deserialize(JsonParser jp, DeserializationContext ctxt) 
      throws IOException, JsonProcessingException {
        JsonNode node = jp.getCodec().readTree(jp);
        int id = (Integer) ((IntNode) node.get("id")).numberValue();
        String itemName = node.get("itemName").asText();
        int userId = (Integer) ((IntNode) node.get("createdBy")).numberValue();

        return new Item(id, itemName, new User(userId, null));

As we can see, the deserializer is working with the standard Jackson representation of JSON — the JsonNode. Once the input JSON is represented as a JsonNode, we can now extract the relevant information from it and construct our own Item entity.

正如我们所看到的,反序列化器正在使用Jackson的标准JSON表示法 – JsonNode。一旦输入的JSON被表示为JsonNode,我们现在就可以从中提取相关信息并构建我们自己的Item实体。

Simply put, we need to register this custom deserializer and deserialize the JSON normally:


ObjectMapper mapper = new ObjectMapper();
SimpleModule module = new SimpleModule();
module.addDeserializer(Item.class, new ItemDeserializer());

Item readValue = mapper.readValue(json, Item.class);

4. Custom Deserializer on the Class


Alternatively, we can also register the deserializer directly on the class:


@JsonDeserialize(using = ItemDeserializer.class)
public class Item {

With the deserializer defined at the class level, there is no need to register it on the ObjectMapper — a default mapper will work fine:


Item itemWithOwner = new ObjectMapper().readValue(json, Item.class);

This type of per-class configuration is very useful in situations in which we may not have direct access to the raw ObjectMapper to configure.


5. Conclusion


This article showed how to leverage Jackson 2 to read nonstandard JSON input as well as how to map that input to any Java entity graph with full control over the mapping.

这篇文章展示了如何利用Jackson 2来读取非标准的JSON输入,以及如何将该输入映射到任何Java实体图,并对映射进行完全控制。

The implementation of all these examples and code snippets can be found over on GitHub. It’s a Maven-based project, so it should be easy to import and run as it is.