How To Convert Java Map To / From JSON (Jackson)

This time, we are doing the same, but replace user define object with Map.

Map is more suitable in a simple and short JSON processing, for complex JSON structure, you should always create an user defined object for easy maintainability.

In this tutorial, we show you how to use Map to construct the JSON data, write it to file. And also how to read JSON from file, and display each value via Map.

1. Write JSON to file

Jackson example to use Map to construct the entire JSON data structure, then write it to file named “user.json“.

import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import org.codehaus.jackson.JsonGenerationException;
import org.codehaus.jackson.map.JsonMappingException;
import org.codehaus.jackson.map.ObjectMapper;
 
public class JacksonExample {
     public static void main(String[] args) {
 
 ObjectMapper mapper = new ObjectMapper();
 
 Map<String, Object> userInMap = new HashMap<String, Object>();
 userInMap.put("name", "harit");
 userInMap.put("age", 29);
 
 List<Object> list = new ArrayList<Object>();
 list.add("msg 1");
 list.add("msg 2");
 list.add("msg 3");
 
 userInMap.put("messages", list);
 
 try {
 
   // write JSON to a file
   mapper.writeValue(new File("c:\\user.json"), userInMap);
 
 } catch (JsonGenerationException e) {
   e.printStackTrace();
 } catch (JsonMappingException e) {
   e.printStackTrace();
 } catch (IOException e) {
   e.printStackTrace();
 }
     }
}

Output – user.json”

{
 "age":29,
 "name":"harit",
 "messages":["msg 1","msg 2","msg 3"]
}

2. Read JSON from file

Read JSON data from file, convert it to Map and display the values.

import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Map;
import org.codehaus.jackson.JsonGenerationException;
import org.codehaus.jackson.map.JsonMappingException;
import org.codehaus.jackson.map.ObjectMapper;
import org.codehaus.jackson.type.TypeReference;
 
public class JacksonExample {
     public static void main(String[] args) {
 
 ObjectMapper mapper = new ObjectMapper();
 
 try {
 
    // read JSON from a file
    Map<String, Object> userInMap = mapper.readValue(
  new File("c:\\user.json"), 
  new TypeReference<Map<String, Object>>() {});
 
    System.out.println(userInMap.get("name"));
    System.out.println(userInMap.get("age"));
 
    ArrayList<String> list = 
  (ArrayList<String>) userInMap.get("messages");
 
    for (String msg : list) {
  System.out.println(msg);
    }
 
 } catch (JsonGenerationException e) {
        e.printStackTrace();
 } catch (JsonMappingException e) {
    e.printStackTrace();
 } catch (IOException e) {
    e.printStackTrace();
 }
     }
}

Output

harit
29
msg 1
msg 2
msg 3