It looks like You have more than one org.json:json dependency on your classpath.

Looking at it:
org.springframework.boot:spring-boot-starter-test depends on
com.jayway.jsonpath:json-pathwhich in turn brings
org.json:json which is much older than your version (v. 20140107)

You could try excluding this transitive dependency from spring-boot-starter-test:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-test</artifactId>
    <scope>test</scope>
    <exclusions>
        <exclusion>
            <groupId>org.json</groupId>
            <artifactId>json</artifactId>
        </exclusion>
    </exclusions>
</dependency>

... but it's possible that there is something within json-path which depends on something from the older json library, which is no longer in the newer version, so proceed with caution and test everything thoroughly.

There is also a chance that something else brings org.json:json.

To verify, please run mvn dependency:tree and search in the produced output for org.json:json.

As per Spring Boot docs 4.3 Customize the Jackson ObjectMapper section:

Any beans of type com.fasterxml.jackson.databind.Module are automatically registered with the auto-configured Jackson2ObjectMapperBuilder and are applied to any ObjectMapper instances that it creates. This provides a global mechanism for contributing custom modules when you add new features to your application.

Therefore, if you provide a @Bean of type JsonOrgModule it will be automatically applied to the default ObjectMapper created at startup.

For example:

@SpringBootApplication
public class Application {

    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }

    @Bean
    public JsonOrgModule jsonOrgModule() {
        return new JsonOrgModule();
    }

}

@SpringBootTest
class ObjectMapperTests {

    @Autowired
    ObjectMapper defaultObjectMapper;

    @Test
    void defaultObjectMapperShouldWriteJsonObject() throws JSONException, JsonProcessingException {
        // Given
        var jsonObject = new JSONObject().put("username", "eHayik");

        // When
       var json = defaultObjectMapper.writeValueAsString(jsonObject);

       // Then
       assertThat(json).isEqualTo("{\"username\":\"eHayik\"}");
    }
}

Add under

 <artifactId>spring-boot-starter-test</artifactId>
    <scope>test</scope>

The following exclusion:

 <scope>test</scope>
    <exclusions>
        <exclusion>
            <groupId>com.vaadin.external.google</groupId>
            <artifactId>android-json</artifactId>
        </exclusion>
    </exclusions>

Similarly, for Gradle projects:

testCompile("org.springframework.boot:spring-boot-starter-test") {
    exclude group: "com.vaadin.external.google", module:"android-json"
}

I tried with your code with a sample spring boot project and I get the error,

No converter for [class org.json.JSONObject]

The reason for this error is explained clearly here. To reiterate the answer, JSONObject classes don't have getters and hence the error. By default spring-boot starter web dependency has Jackson web support which can convert any POJO class to JSON object. So as the answer by @süleyman-can using a POJO is the right way to handle this.

In case, you can't use a POJO class because the fields in the response will be different for each request. For example, you have to send

{"a": "b"}

for one response and

{"c": "d"}

for another response, you can always use Map<String, String> like this,

@RequestMapping(value = "/test", method = RequestMethod.GET)
@ResponseBody
public Map<String, String> test() {
  Map<String, String> test = new HashMap<>();
  test.put("name","caroline");
  return test;
}

and the response would come like this,

{"name":"caroline"}                                                                                                                      

Add json jar to your classpath

or use java -classpath json.jar ClassName

Or add this to your maven pom.xml depedencies:

<dependency>
    <groupId>org.json</groupId>
    <artifactId>json</artifactId>
    <version>20090211</version>
</dependency>

EDIT

Although spring boot has a dependency on json-simple it's defined as an optional dependency. So you need to add the dependency in your pom as it will not be downloaded automatically with spring boot.

Steps

1. Add this to your pom.xml

   <dependency>
       <groupId>com.googlecode.json-simple</groupId>
       <artifactId>json-simple</artifactId>
       <version>1.1.1</version>
   </dependency>
   

2. Then Refresh dependencies running the command

   mvn package
   

3. Refresh/re-import project if you are using IDE(such as JetBrains Intellij Idea).

4. Import the Package in your class

   import org.json.simple.JSONObject;
   

5. Use and Enjoy

   import org.json.simple.JSONObject;
   public class Application {
   
       public static void main(String[] args)
   
           JSONObject.escape("");
           System.out.println("Done");
       }
   }