The return value of the lambda function is not expected

[x-wk]

Describe the bug

I have a simple lambda function like this

exports.handler = async (event) => {
    return "string result";
};

when I invoke it via Java sdk the return value is: (double quotes around)

"string result"

Expected Behavior

string result

Current Behavior

As above

Reproduction Steps

As above

Possible Solution

No response

Additional Information/Context

No response

AWS Java SDK version used

2.17.194

JDK version used

jdk 11

Operating System and version

win10

[ashishdhingra]

Hi @x-wk,

Good afternoon.

Thanks for posting the issue. Could you please share the sample code to reproduce the issue?

Regards,
Ashish

[x-wk]

public static void main(String[] args) {
    LambdaClient lambdaClient = LambdaClient.create();

    //Setup an InvokeRequest
    InvokeRequest request = InvokeRequest.builder()
       .functionName("testFunction")
       .build();

    InvokeResponse res = lambdaClient.invoke(request);
    String result = res.payload().asUtf8String();
    System.out.println("string result".equals(result)); // expected true but false
    System.out.println("\"string result\"".equals(result)); // true
 }

[ashishdhingra]

@x-wk This doesn't appear to be issue with the Java SDK. I was able to replicate the same behavior using AWS SDK for .NET using the code below (used package AWSSDK.Lambda version 3.7.13.9):

using System;
using System.Text;
using Amazon.Lambda;
using Amazon.Lambda.Model;

namespace LambdaInvokeTest
{
    class Program
    {
        static void Main(string[] args)
        {
            AmazonLambdaClient lambdaClient = new AmazonLambdaClient();
            InvokeRequest request = new InvokeRequest { FunctionName = "TestFunction" };
            InvokeResponse res = lambdaClient.InvokeAsync(request).Result;
            string resultPayload = Encoding.UTF8.GetString(res.Payload.ToArray());
            Console.WriteLine(resultPayload);
            Console.WriteLine("------------");
            Console.WriteLine("string result".Equals(resultPayload)); // expected true but false
            Console.WriteLine("\"string result\"".Equals(resultPayload)); // true
        }
    }
}

This produced the following output:

"string result"
------------
False
True

Upon further analysis, it appears that the payload returned by lambdaClient.invoke() is a JSON string. Refer https://docs.aws.amazon.com/lambda/latest/dg/API_Invoke.html#API_Invoke_ResponseSyntax where it mentions that payload is The response from the function, or an error object.. Although it is not clear, but since it also returns an error object, which infers that it would be returned as a JSON.

While most languages support parsing plain strings as valid JSON (e.g. in .NET we could use code something like System.Text.Json.JsonDocument.Parse(resultPayload).RootElement.GetString()), it doesn't appear to be case with some Java libraries.

I was able to come up with a workaround using package org.json:json (example Maven dependency below).

<dependency>
  <groupId>org.json</groupId>
  <artifactId>json</artifactId>
  <version>20190722</version>
</dependency>

and code:

import software.amazon.awssdk.services.lambda.LambdaClient;
import software.amazon.awssdk.services.lambda.model.InvokeRequest;
import software.amazon.awssdk.services.lambda.model.InvokeResponse;
import org.json.JSONObject;

public class TestLambda {
    public static void main(String[] args) {
        LambdaClient lambdaClient = LambdaClient.create();

        //Setup an InvokeRequest
        InvokeRequest request = InvokeRequest.builder()
                                             .functionName("TestFunction")
                                             .build();

        InvokeResponse res = lambdaClient.invoke(request);
        String result = res.payload().asUtf8String();
        System.out.println("string result".equals(result)); // expected true but false
        System.out.println("\"string result\"".equals(result)); // true

        System.out.println("----------------\nUsing JSON parser");
        if (!result.startsWith("{")) {
            result = "{\"root\":" + result+ "}";

            JSONObject jsonObject =  new JSONObject(result);
            String jsonStringValue = jsonObject.getString("root");
            System.out.println(jsonStringValue);
            System.out.println("string result".equals(jsonStringValue)); // This should return true
            System.out.println("\"string result\"".equals(jsonStringValue)); // This should return false
        }
    }
}

Hope this helps.

Thanks,
Ashish

[x-wk]

@ashishdhingra Thank you for your reply.
Maybe I should communicate with the developer to return a JSON object instead of a string.
Forgive me, i think code like this is a little bit ugly 😅
Thanks again.