jackson中自定义处理序列化和反序列化

http://jackyrong.iteye.com/blog/2005323

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对于一直用gson的人来说，如果单独用jackson，真是麻烦了，但还是得小结下了：
先来看下如何自定义把某个对象序列化为json:

先是对象：

Java代码

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

JACKSON一般的使用很容易,如;

Item myItem = new Item(1, "theItem", new User(2, "theUser"));
String serialized = new ObjectMapper().writeValueAsString(myItem); 

结果为：

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

如果要输出为如下的样子，比如；

{
    "id": 25,
    "itemName": "FEDUfRgS",
    "owner": 15
} 

则要自定义了，要继承JsonSerializer类，如下：

    public class ItemSerializer extends JsonSerializer<Item> {
        @Override
        public void serialize(Item value, JsonGenerator jgen, SerializerProvider provider)
          throws IOException, JsonProcessingException {
            jgen.writeStartObject();
            jgen.writeNumberField("id", value.id);
            jgen.writeStringField("itemName", value.itemName);
            jgen.writeNumberField("owner", value.owner.id);
            jgen.writeEndObject();
        }
    }  

然后

    Item myItem = new Item(1, "theItem", new User(2, "theUser"));
    ObjectMapper mapper = new ObjectMapper();  

    SimpleModule module = new SimpleModule();
    module.addSerializer(Item.class, new ItemSerializer());
    mapper.registerModule(module);  

    String serialized = mapper.writeValueAsString(myItem);  

看，相当复杂,然后看有无办法简单点，其实是有的哦；方法为：

@JsonSerialize(using = ItemSerializer.class)
public class Item {
    ...
} 

就是使用注解@JsonSerialize，然后：

    Item myItem = new Item(1, "theItem", new User(2, "theUser"));
    String serialized = new ObjectMapper().writeValueAsString(myItem);  

接下来看如何反序列化了，
同样，要反序列化的两个pojo为：

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

反序列化代码为：

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

如果要自定义反序列化，比如要反序列化的JSON为;

{
    "id": 1,
    "itemName": "theItem",
    "owner": 2
} 

则上面这样会报错：
com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException:

Unrecognized field "createdBy" (class org.baeldung.jackson.dtos.Item),

not marked as ignorable (3 known properties: "id", "owner", "itemName"])

at [Source: java.io.StringReader@53c7a917; line: 1, column: 43]

(through reference chain: org.baeldung.jackson.dtos.Item["createdBy"])

代码：

    public class ItemDeserializer extends JsonDeserializer<Item> {  

        @Override
        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("id")).numberValue();  

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

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

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

也可以用注解：

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