映射与Jackson有不同类型的JSON字段？

Question

我从Web服务获取JSON并且不能影响JSON格式.下面的JSON代码只是一个例子来说明问题.该字段cars可以是包含Car对象的对象,也可以是空字符串.如果我可以更改Web服务,我会将空字符串更改为空对象,"cars" : {}而不是"cars" : "".

尝试将JSON映射到此Java对象时:

public class Person {
    public int id;
    public String name;
    public Map<String, Car> cars;
}

这有效:

{
    "id" : "1234",
    "name" : "John Doe",
    "cars" : {
        "Tesla Model S" : {
            "color" : "silver",
            "buying_date" : "2012-06-01"
        },
        "Toyota Yaris" : {
            "color" : "blue",
            "buying_date" : "2005-01-01"
        }
    }
}

这失败了:

{
    "id" : "1",
    "name" : "The Dude",
    "cars" : ""
}

杰克逊处理这个案子的最佳方法是什么？如果有空字符串,我想去null场cars.我尝试过使用ACCEPT_EMPTY_STRING_AS_NULL_OBJECT,但没有用.

Answer 1

现场汽车可以包含汽车对象列表......这有效:

{
    "id" : "1234",
    "name" : "John Doe",
    "cars" : {
        "Tesla Model S" : {
            "color" : "silver",
            "buying_date" : "2012-06-01"
        },
        "Toyota Yaris" : {
            "color" : "blue",
            "buying_date" : "2005-01-01"
        }
    }
}

"cars"元素值不是列表(aka数组).它是一个JSON对象,也可以被视为一个map类型集合,但它不是一个列表.

因此,为了重新解决这个问题,我们的目标是将有时候是对象的JSON反序列化,有时将空字符串反序列化为Java Map.

为了解决这个问题,我很惊讶ACCEPT_EMPTY_STRING_AS_NULL_OBJECT没有工作.我建议在http://jira.codehaus.org/browse/JACKSON上记录一个问题.

您可以实现自定义反序列化.以下是一个示例解决方案.如果目标数据结构具有其他Map引用,则需要相应地更改此解决方案.

import java.io.File;
import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

import org.codehaus.jackson.JsonNode;
import org.codehaus.jackson.JsonParser;
import org.codehaus.jackson.JsonProcessingException;
import org.codehaus.jackson.ObjectCodec;
import org.codehaus.jackson.Version;
import org.codehaus.jackson.map.DeserializationContext;
import org.codehaus.jackson.map.JsonDeserializer;
import org.codehaus.jackson.map.ObjectMapper;
import org.codehaus.jackson.map.module.SimpleModule;
import org.codehaus.jackson.type.TypeReference;

public class Foo
{
  public static void main(String[] args) throws Exception
  {
    SimpleModule module = new SimpleModule("CarsDeserializer", Version.unknownVersion());
    module.addDeserializer(Map.class, new CarsDeserializer());

    ObjectMapper mapper = new ObjectMapper().withModule(module);

    Person person1 = mapper.readValue(new File("input1.json"), Person.class);
    System.out.println(mapper.writeValueAsString(person1));
    // {"id":1234,"name":"John Doe","cars":{"Tesla Model S":{"color":"silver","buying_date":"2012-06-01"},"Toyota Yaris":{"color":"blue","buying_date":"2005-01-01"}}}

    Person person2 = mapper.readValue(new File("input2.json"), Person.class);
    System.out.println(mapper.writeValueAsString(person2));
    // {"id":1,"name":"The Dude","cars":{}}
  }
}

class Person
{
  public int id;
  public String name;
  public Map<String, Car> cars;
}

class Car
{
  public String color;
  public String buying_date;
}

class CarsDeserializer extends JsonDeserializer<Map<String, Car>>
{
  @Override
  public Map<String, Car> deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException,
      JsonProcessingException
  {
    ObjectCodec codec = jp.getCodec();
    JsonNode node = codec.readTree(jp);
    if (!"".equals(node.getTextValue()))
    {
      ObjectMapper mapper = new ObjectMapper();
      return mapper.readValue(node, new TypeReference<Map<String, Car>>() {});
    }
    return new HashMap<String, Car>(); // or return null, if preferred
  }
}