使用 Jackson 反序列化成 Map

Question

I have a JSON object with two attributes: "key" which is a string, and "value" which can be deserialized into a Java bean.

{ "key": "foo", "value": "bar" }

The question is, given a list of such objects, can I deserialize it into a Map?

[{"key": "foo1", "value": "bar1"}, {"key": "foo2", "value": "bar2"}] -> Map<String, String>

Currently using Jackson-databind 2.1

Answer 1

You can easily convert above JSON to List<Map<String, String>> :

import java.util.List;
import java.util.Map;

import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.type.CollectionType;

public class JacksonProgram {

    public static void main(String[] args) throws Exception {
        ObjectMapper mapper = new ObjectMapper();
        CollectionType mapCollectionType = mapper.getTypeFactory().constructCollectionType(List.class, Map.class);
        List<Map<String, String>> result = mapper.readValue(json, mapCollectionType);
        System.out.println(result);
    }
}

Above program prints:

[{key=foo1, value=bar1}, {key=foo2, value=bar2}]

Answer 2

Since your structure does not match, you have two basic options:

1. Use two-phase handling: use Jackson to bind into intermediate representation that does match (which could be JsonNode or List<Map<String,Object>> ), and then do conversion to desired type with simple code
2. Write custom serializer and/or deserializer

Jackson does not support extensive structural transformations (there are some simple ones, like @JsonUnwrapped ), so this kind of functionality is unlikely to be added to databind module. Although it could be added as an extension module, if these "smart Map" types of structures are commonly used (they seem to be, unfortunately).

Answer 3

Had the same problem and was surprised Jackson wasn't able to natively handle it. Solution I went with was to create a custom setter on the object I was trying to marshal into:

public class somePojo {
  private Map<String, String> mapStuff;

  ...

  public void SetMapStuff(List<Map<String, String> fromJackson){
    mapStuff= new HashMap<>();
    for (Map<String, String> pair : fromJackson) {
      put(pair.get("key"), pair.get("value"));
    }
  }
}

Jackson is smart enough to find that setter to and can happily pass it the List.