In this short article, you'll learn how to convert a JSON string to a Map in Java and vice versa by using the Jackson library.

Dependencies

To add Jackson to your Gradle project, add the following dependency to build.gradle file:

implementation 'com.fasterxml.jackson.core:jackson-databind:2.10.0'

For Maven, include the below dependency to your pom.xml file:

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.10.0</version>
</dependency>

Convert JSON String to Map

To convert a JSON string to a Java Map with the same properties and keys, you can use the readValue() method from ObjectMapper:

try {
    // JSON string
    String json = "{\"name\":\"John Doe\",\"email\":\"john.doe@example.com\"," +
            "\"roles\":[\"Member\",\"Admin\"],\"admin\":true}";

    // convert JSON string to Java Map
    Map<String, Object> map = new ObjectMapper().readValue(json, Map.class);

    // print map keys and values
    for (Map.Entry<String, Object> entry : map.entrySet()) {
        System.out.println(entry.getKey() + "=" + entry.getValue());
    }

} catch (Exception ex) {
    ex.printStackTrace();
}

You should see the following output printed on the console:

name=John Doe
email=john.doe@example.com
roles=[Member, Admin]
admin=true

To convert a JSON string stored in a file into a Java map, you can pass an instance of File to readValue(), as shown below:

Map<String, Object> map = new ObjectMapper().readValue(
        Paths.get("user.json").toFile(), 
        Map.class
);

Convert Map to JSON String

To convert a Java Map to a JSON string, you can use the writeValueAsString() method from the ObjectMapper class:

try {
    // create a map
    Map<String, Object> map = new HashMap<>();
    map.put("name", "John Deo");
    map.put("email", "john.doe@example.com");
    map.put("roles", new String[]{"Member", "Admin"});
    map.put("admin", true);

    // convert map to JSON string
    String json = new ObjectMapper().writeValueAsString(map);

    // print JSON string
    System.out.println(json);

} catch (Exception ex) {
    ex.printStackTrace();
}

You should see the following JSON printed on the console:

{"roles":["Member","Admin"],"name":"John Deo","admin":true,"email":"john.doe@example.com"}

If you want to write the converted Map directly to a JSON file, use the writeValue() method instead:

new ObjectMapper().writeValue(Paths.get("user.json").toFile(), map);

For more Jackson examples, check out the How to read and write JSON using Jackson in Java tutorial.

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