JSON response from string in Javalin

Question

I am trying to create the response in JSON format from a string in Javalin. JSON object must be from a string, not from class.

public static void main(String[] args) {

    Javalin app = Javalin.create().start(9000);

    String jsonString = "{'test1':'value1','test2':{'id':0,'name':'testName'}}";
    JsonObject jsonObject= JsonParser.parseString(jsonString).getAsJsonObject();
            
    app.error(404, ctx -> ctx.json(jsonObject));

    ...

}

With the code above, I am getting a 500 server error.

Answer 1

If you are using Gson, then switch to using Jackson, when using Javalin methods such as ctx.json().

If you used Maven to import Javalin, and if you used the javalin-bundle, then you will already have Jackson, for example:

<dependency>
    <groupId>io.javalin</groupId>
    <artifactId>javalin-bundle</artifactId>
    <version>3.13.3</version>
</dependency>

If you only used javalin and not javalin-bundle then you will need to add Jackson yourself - for example:

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.12.2</version>
</dependency>

The following is the code from the question, but re-worked to use Jackson (and to use valid JSON):

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;
import io.javalin.Javalin;

public class App {

    public static void main(String[] args) throws JsonProcessingException {
        Javalin app = Javalin.create().start(9000);
        String jsonString = "{\"test1\":\"value1\",\"test2\":{\"id\":0,\"name\":\"testName\"}}";
        ObjectMapper objectMapper = new ObjectMapper();
        JsonNode jsonNode = objectMapper.readTree(jsonString);
        app.error(404, ctx -> ctx.json(jsonNode));
    }
}