Json to Java Mapping | 易学教程

问题

I want to map below Json data to java object of List<Map<String, String>> type.

Sample Json:

{ 
  {
    a:b,
    c:d
  },
  {
    e:f,
    g:h,
    i:j
  },
  { 
   h:k
  }
}

Here a:b represents key-value pair. So a:b and c:d will be mapped to first map of the list and so on. one way to do this is by building JSON tree and access each node and store the pair into the map.

Is there a better way to do this (cleaner approach)?

回答1:

Here is the code to read a List<Map<String, String>> using the Jackson library, with your example data as input:

public class JsonTest {
public static void main(String[] args) throws Exception {
    final String json
        = "[\n"
        + "    {\n"
        + "        \"a\":\"b\",\n"
        + "        \"c\":\"d\"\n"
        + "    },\n"
        + "    {\n"
        + "        \"e\":\"f\",\n"
        + "        \"g\":\"h\",\n"
        + "        \"i\":\"j\"\n"
        + "    },\n"
        + "    {\n"
        + "        \"h\":\"k\"\n"
        + "    }\n"
        + "]"; // [{a:b,c:d},{e:f,g:h,i:j},{h:k}]   
    ObjectMapper mapper = new ObjectMapper();
    TypeFactory factory = TypeFactory.defaultInstance();
    List<Map<String, String>> list = mapper.readValue(json,factory
        .constructCollectionType(List.class, factory
                .constructMapType(Map.class, String.class, String.class)));
    System.out.println(list.toString());
}
}

Note: I had to fix your outermost braces from {} to [], which is the correct JSON list syntax.

回答2:

You can use Jackson to achieve this task through below example code

public class JacksonExample {
public static void main(String[] args) {

    ObjectMapper mapper = new ObjectMapper();

    try {

        // Convert JSON string from file to Object
        User user = mapper.readValue(new File("C:\\user.json"), User.class);
        System.out.println(user);

        // Convert JSON string to Object
        String jsonInString = "{\"age\":33,\"messages\":[\"msg 1\",\"msg 2\"],\"name\":\"xyz\"}";
        User user1 = mapper.readValue(jsonInString, User.class);
        System.out.println(user1);

    } catch (JsonGenerationException e) {
        e.printStackTrace();
    } catch (JsonMappingException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }

}

}

回答3:

First I made few changes to your json to make it valid

{
"key": 
    [{
        "a": "b",
        "c": "d"
    },
    {
        "e": "f",
        "g": "h",
        "i": "j"
    }, 
    {
        "h": "k"
    }]

}

Please find the below code that I have tried out :

ObjectMapper mapper = new ObjectMapper();
ObjectNode objectNode1 = mapper.createObjectNode();
Map<String, Object> i = mapper.readValue(new File("J:/temp/sample.json"), Map.class);
System.out.println(i.get("key"));
System.out.println(i.values());

Output :

//For System.out.println(i.get("key"));

[{a=b, c=d}, {e=f, g=h, i=j}, {h=k}]

//For System.out.println(i.values());

[[{a=b, c=d}, {e=f, g=h, i=j}, {h=k}]]

If the above approach helps you, Make a right decision based on your needs either i.get("key") or i.values()

回答4:

Just use gson library to Json to object and object to Json

import java.lang.reflect.Type;
import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;

Object to Json

Gson gson = new Gson();
Student obj=new Student();
String jsonInString = gson.toJson(obj);

Json To Object

Student obj = gson.fromJson(jsonInString, Student.class);

来源：https://stackoverflow.com/questions/40628324/json-to-java-mapping

标签

java

jackson