why does this AWS Lambda code in Java return "internal server error" when invoked via an API gateway?

Question

From my understanding, if you return the following json string: {"headers":{"Content-Type":"application/json"},"body":"hello world","statusCode":200}

then when visiting the page https://....eu-west-2.amazonaws.com/prod/ , you should see the phrase "hello world" written. Instead, I get {"message": "Internal server error"}

Below is the JAVA code I am using to return the above json string:

import com.amazonaws.services.lambda.runtime.Context;
import com.amazonaws.services.lambda.runtime.RequestHandler;
import org.json.simple.JSONObject;
import java.util.Map;

public class Handler implements RequestHandler<Map<String, String>, JSONObject>{
    @Override
    public JSONObject handleRequest(Map<String, String> stringStringMap, Context context) {

        JSONObject obj = new JSONObject();
        JSONObject obj2 = new JSONObject();
        obj2.put("Content-Type", "application/json");
        obj.put("statusCode", 200);
        obj.put("headers", obj2);
        obj.put("body", "hello world");
        return obj;
    }
}

What am I doing wrong? It works fine on the console when I test the function, but when I go to the page of the API it gives me an internal server error.

Answer 1

The error message you posted in the comment has already explained what the problem is. Basically, lambda handler is not able to convert the input object to String . You can solve this by changing Map<String, String> to Map<String, Object> or general Object .

Also, as @david pointed out in the comment, you should return a JSON String instead of a JSONObject . So the fully working code could be something like this:

import com.amazonaws.services.lambda.runtime.Context;
import com.amazonaws.services.lambda.runtime.RequestHandler;
import org.json.simple.JSONObject;
import java.util.Map;

public class Handler implements RequestHandler<Map<String, Object>, String>{
    @Override
    public String handleRequest(Map<String, Object> stringStringMap, Context context) {

        JSONObject obj = new JSONObject();
        JSONObject obj2 = new JSONObject();
        obj2.put("Content-Type", "application/json");
        obj.put("statusCode", 200);
        obj.put("headers", obj2);
        obj.put("body", "hello world");
        return obj.toString();
    }
}

Answer 2

For anyone interested, the following code worked fine for me:

import com.amazonaws.services.lambda.runtime.Context;
import com.amazonaws.services.lambda.runtime.RequestStreamHandler;
import org.json.simple.JSONObject;

import java.io.*;

public class Handler implements RequestStreamHandler{
    @Override
    public void handleRequest(InputStream inputStream, OutputStream outputStream, 
                                      Context context) throws IOException {

        JSONObject obj = new JSONObject();
        JSONObject obj2 = new JSONObject();
        obj2.put("Content-Type", "application/json");
        obj.put("statusCode", 200);
        obj.put("headers", obj2);
        obj.put("body", "hello world");

        OutputStreamWriter writer = new OutputStreamWriter(outputStream, "UTF-8");
        writer.write(obj.toString());
        writer.close();

    }

}