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.

Have you checked CloudWatch Logs for the lambda? Maybe it errors out? — Marcin
– Marcin, Commented May 11, 2020 at 23:15
@kmpt You should review the API logs. They'll show what causes the Server Error. — Marcellin Wabo
– Marcellin Wabo, Commented May 11, 2020 at 23:55
you are correct, I get 'java.lang.RuntimeException: An error occurred during JSON parsing Caused by: java.io.UncheckedIOException: com.amazonaws.lambda.thirdparty.com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of java.lang.String out of START_OBJECT token' — kmpt
– kmpt, Commented May 12, 2020 at 0:28
That sounds like it wants a String containing the JSON, not a JSONObject. — David Conrad
– David Conrad, Commented May 12, 2020 at 0:41

jellycsc · Accepted Answer · 2020-05-12 01:02:09Z

2

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();
    }
}

answered May 12, 2020 at 1:02

jellycsc

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2 Comments

kmpt Over a year ago

it seems to be executing fine now without any errors in the logs, but I still get the same message when I visit the website... I get the following in the logs every time I visit the website: 'REPORT RequestId: d28cc5cc-6af8-4045-adf7-0836620d46b3 Duration: 1.80 ms Billed Duration: 100 ms Memory Size: 128 MB Max Memory Used: 89 MB' but the message written in the body is the same. I will check it again tomorrow (maybe it takes time for the website to update?)

kmpt Over a year ago

unfortunately I was still getting the same error when I woke up... can it be something related to Strings? I really want to understand why for example using streams (see my answer) worked but not Strings...

kmpt · Accepted Answer · 2020-05-12 09:04:02Z

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();

    }

}

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