In this tutorial, we will show you few Jackson examples to convert JSON string to/from a Map.

P.S All examples are tested with Jackson 2.6.3

1. JSON string to Map

JsonMapExample.java

package com.mkyong.json;

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

import com.fasterxml.jackson.core.JsonGenerationException;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.ObjectMapper;

public class JsonMapExample {

	public static void main(String[] args) {

		try {

			ObjectMapper mapper = new ObjectMapper();
			String json = "{\"name\":\"mkyong\", \"age\":29}";

			Map<String, Object> map = new HashMap<String, Object>();

			// convert JSON string to Map
			map = mapper.readValue(json, new TypeReference<Map<String, String>>(){});

			System.out.println(map);

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

}

Output


{name=mkyong, age=29}

2. Map to JSON string

Example to convert the Map to JSON string.

MapJsonExample.java

package com.mkyong.json;

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

import com.fasterxml.jackson.core.JsonGenerationException;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.ObjectMapper;

public class MapJsonExample{

	public static void main(String[] args) {

		try {

			ObjectMapper mapper = new ObjectMapper();
			String json = "";

			Map<String, Object> map = new HashMap<String, Object>();
			map.put("name", "mkyong");
			map.put("age", 29);

			// convert map to JSON string
			json = mapper.writeValueAsString(map);

			System.out.println(json);

			json = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(map);

			// pretty print
			System.out.println(json);

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

}

Output


{"name":"mkyong","age":29}
{
  "name" : "mkyong",
  "age" : 29
}

3. Map to JSON File

JsonMapFileExample.java

package com.mkyong.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 com.fasterxml.jackson.core.JsonGenerationException;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.ObjectMapper;

public class JsonMapFileExample {

	public static void main(String[] args) {

		try {

			ObjectMapper mapper = new ObjectMapper();

			Map<String, Object> map = new HashMap<String, Object>();
			map.put("name", "mkyong");
			map.put("age", 29);

			List<Object> list = new ArrayList<>();
			list.add("msg 1");
			list.add("msg 2");
			list.add("msg 3");

			map.put("messages", list);

			// write JSON to a file
			mapper.writeValue(new File("c:\\user.json"), map);

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

}
c:\\user.json

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

4. JSON file to Map

JsonFileMapExample.java

package com.mkyong.json;

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

import com.fasterxml.jackson.core.JsonGenerationException;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.ObjectMapper;

public class Jackson2Example {

	public static void main(String[] args) {

		try {

			ObjectMapper mapper = new ObjectMapper();

			// read JSON from a file
			Map<String, Object> map = mapper.readValue(
					new File("c:\\user.json"), 
					new TypeReference<Map<String, Object>>() {
			});

			System.out.println(map.get("name"));
			System.out.println(map.get("age"));

			@SuppressWarnings("unchecked")
			ArrayList<String> list = (ArrayList<String>) map.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


mkyong
29
msg 1
msg 2
msg 3

References

  1. Wikipedia : JSON
  2. Jackson – High-performance JSON processor
  3. Jackson 2 – Object to / from JSON