12 November 2019

How to Convert Java Map to JSON

development

There are a number of ways to convert a Java Map into JSON. It is quite common to convert Java Arrays and Maps into JSON and vice versa.

In this post, we look at 3 different examples to convert Java Map to JSON. We will be using Jackson, Gson and org.json libraries.

Java Map to JSON using Jackson

The following example uses Jackson Core and Jackson Binding to convert Java Map to JSON.

In order to use the Jackson libraries, we first need to add them to our pom.xml file:

<dependencies>
    <dependency>
        <groupId>com.fasterxml.jackson.core</groupId>
         <artifactId>jackson-core</artifactId>
        <version>2.9.8</version>
    </dependency>
    <dependency>
        <groupId>com.fasterxml.jackson.core</groupId>
        <artifactId>jackson-databind</artifactId>
        <version>2.9.8</version>
    </dependency>
</dependencies>

Then:

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import org.junit.jupiter.api.Test;

import java.util.HashMap;
import java.util.Map;

public class ConvertJavaMapToJson {

    @Test
    public void convertMapToJson() {
        Map<String, String> elements = new HashMap();
        elements.put("Key1", "Value1");
        elements.put("Key2", "Value2");
        elements.put("Key3", "Value3");

        ObjectMapper objectMapper = new ObjectMapper();

        try {
            String json = objectMapper.writeValueAsString(elements);
            System.out.println(json);
        } catch (JsonProcessingException e) {
            e.printStackTrace();
        }
    }
}

Output:

{"Key2":"Value2","Key1":"Value1","Key3":"Value3"}

As can be seen from the output, the order of the elements in the JSON are not the same as the order we added them to the map.

To retain the order, we need to use SortedMap instead.

e.g.

SortedMap<String, String> elements = new TreeMap();

Output:

{"Key1":"Value1","Key2":"Value2","Key3":"Value3"}

Java Map to JSON using Gson

The following example uses Gson library to convert Java Map to JSON, but first, we need to add Gson as a dependency to pom.xml file.

<dependencies>
    <dependency>
        <groupId>com.google.code.gson</groupId>
        <artifactId>gson</artifactId>
        <version>2.8.5</version>
    </dependency>
</dependencies>

Then:

import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;
import org.junit.jupiter.api.Test;

import java.lang.reflect.Type;
import java.util.HashMap;
import java.util.SortedMap;
import java.util.TreeMap;

public class ConvertJavaMapToJson {

    @Test
    public void convertMapToJson() {
        SortedMap<String, String> elements = new TreeMap();
        elements.put("Key1", "Value1");
        elements.put("Key2", "Value2");
        elements.put("Key3", "Value3");

        Gson gson = new Gson();
        Type gsonType = new TypeToken<HashMap>(){}.getType();
        String gsonString = gson.toJson(elements,gsonType);
        System.out.println(gsonString);
    }
}

Output:

{"Key1":"Value1","Key2":"Value2","Key3":"Value3"}

Java Map to JSON using org.json

The following example uses org.json library to convert Java Map to JSON, but first, we need to add org.json as a dependency to pom.xml file.

<dependencies>
    <dependency>
        <groupId>org.json</groupId>
        <artifactId>json</artifactId>
        <version>20180813</version>
    </dependency>
</dependencies>

Then:

import org.json.JSONObject;
import org.junit.jupiter.api.Test;

import java.util.HashMap;
import java.util.Map;

public class ConvertJavaMapToJson {

    @Test
    public void convertMapToJson() {
        Map<String, String> elements = new HashMap<>();
        elements.put("Key1", "Value1");
        elements.put("Key2", "Value2");
        elements.put("Key3", "Value3");

        JSONObject json = new JSONObject(elements);

        System.out.println(json);
    }
}

Output:

{"Key2":"Value2","Key1":"Value1","Key3":"Value3"}

12 November 2019