Convert JSON string to Map in Java

1. Overview

Converting JSON strings to Java objects (and vice versa) is a frequent need in modern Java applications, especially when dealing with RESTful web services. In this tutorial, we'll look at how to use the Jackson library to convert a JSON string representing a user into a Java Map.

Check out all the Java Jackson JSON tutorials and examples: 50+ Java Jackson JSON Tutorials with Examples

2. Development Steps

1. Set up a new Maven project.

2. Add the required Jackson dependencies.

3. Create a sample JSON string representing a user.

4. Utilize Jackson's ObjectMapper to convert this string into a Map.

5. Display the contents of the Map.

3. Create a Maven Project

There are different ways to create a simple Maven project:

Create a Simple Maven Project using the Command Line Interface

Create a Simple Maven Project using  Eclipse IDE

Create a Simple Maven Project using  IntelliJ IDEA

4. Maven Dependencies

Open the pom.xml file, and add the following Jackson data binding dependency:

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.13.0</version>
</dependency>

5. Example 1: Convert JSON string to Map in Java

import com.fasterxml.jackson.databind.ObjectMapper;
import java.util.Map;
public class MainApp {
    public static void main(String[] args) {
        // Sample JSON string
        String jsonStr = "{\"id\":1,\"name\":\"Jane Smith\",\"email\":\"jane.smith@example.com\"}";
        // Create an ObjectMapper instance
        ObjectMapper mapper = new ObjectMapper();
        try {
            // Convert JSON string to Map
            Map<String, Object> map = mapper.readValue(jsonStr, Map.class);
            // Display the map
            for (Map.Entry<String, Object> entry : map.entrySet()) {
                System.out.println(entry.getKey() + ": " + entry.getValue());
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Output:

id: 1
name: Jane Smith
email: jane.smith@example.com

Code Explanation:

1. We initiate with a sample JSON string that represents a user.

2. Next, we create an instance of the ObjectMapper class.

3. Using the readValue() method of ObjectMapper, the JSON string is converted into a Map.

4. The map's contents are displayed using a simple loop.

Note: The generic Map.class was used, implying the keys are Strings, but values are treated as Objects. This allows for flexibility in handling different data types within the JSON string.

6. Example 2: Convert Complex JSON string to Map in Java

Below is the code to convert a complex JSON string to a Java Map:

import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;
import java.util.Map;
public class JsonToMapExample {
    public static void main(String[] args) {
        try {
            // Example of complex JSON string
            String jsonString = "{"
                    + "\"name\":\"John\","
                    + "\"age\":30,"
                    + "\"address\":{"
                    + "    \"street\":\"123 Main St\","
                    + "    \"city\":\"Springfield\","
                    + "    \"zipcode\":\"12345\""
                    + "},"
                    + "\"phoneNumbers\":[\"123-456-7890\",\"987-654-3210\"]"
                    + "}";
            // Create ObjectMapper instance
            ObjectMapper objectMapper = new ObjectMapper();
            // Convert JSON string to Map
            Map<String, Object> map = objectMapper.readValue(jsonString, new TypeReference<Map<String, Object>>() {});
            // Print the Map in an organized manner
            for (Map.Entry<String, Object> entry : map.entrySet()) {
                if (entry.getValue() instanceof Map) {
                    System.out.println(entry.getKey() + " = ");
                    Map<String, String> nestedMap = (Map<String, String>) entry.getValue();
                    for (Map.Entry<String, String> nestedEntry : nestedMap.entrySet()) {
                        System.out.println("    " + nestedEntry.getKey() + " = " + nestedEntry.getValue());
                    }
                } else if (entry.getValue() instanceof List) {
                    System.out.println(entry.getKey() + " = " + entry.getValue());
                } else {
                    System.out.println(entry.getKey() + " = " + entry.getValue());
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Output:

name = John
age = 30
address =
    street = 123 Main St
    city = Springfield
    zipcode = 12345
phoneNumbers = [123-456-7890, 987-654-3210]

7. Conclusion

Jackson offers a powerful and efficient means of converting between JSON strings and Java Maps, allowing for flexible data processing within Java applications. This functionality is especially valuable when handling dynamic or unpredictable JSON structures.

Check out all the Java Jackson JSON tutorials and examples: 50+ Java Jackson JSON Tutorials with Examples