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"
}

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>

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.

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"}                                                                                                                      

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\"}");
    }
}