AWS Lambda errors with Java Lambdas and Serverless framework (502 Malformed Lambda proxy response)

Using a basic Servlerless framework event config like this:

functions:
  hello:
    handler: kh.javalambda.HelloLambda::myHandler
    events:
      - http:
        path: hello
        method: get 

…will create an API Gateway config with the Lambda Proxy feature enabled.

This sends the request as a JSON object to the Lambda. This includes HTTP headers, queryStringParameters, pathParameters and the request body. The Lambda is expected to have an appropriate parameter type for the incoming JSON payload otherwise you’ll get a Jackson parsing error like this:

An error occurred during JSON parsing: java.lang.RuntimeException
 java.lang.RuntimeException: An error occurred during JSON parsing
 Caused by: java.io.UncheckedIOException: com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of java.lang.String out of START_OBJECT token
  at [Source: lambdainternal.util.NativeMemoryAsInputStream@1bce4f0a; line: 1, column: 1]
 Caused by: com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of java.lang.String out of START_OBJECT token
  at [Source: lambdainternal.util.NativeMemoryAsInputStream@1bce4f0a; line: 1, column: 1

Use a Map<String, Object> as shown in the docs and the incoming request JSON will get passed in as this map.

Even if you are calling the Lambda with a GET request and not intending to POST or PUT a request body, you still need to have the Map<String, Object> parameter to receive the JSON event containing all the headers and params passed from API gateway.

Return Type

If you return a regular String or anything other than the expected response payload, from your Lambda will cause API Gateway to fail parsing the expected JSON response:

Wed Jan 13 07:34:32 UTC 2021 : Endpoint response body before transformations: "HelloLambda: hello null" Wed Jan 13 07:34:32 UTC 2021 : Execution failed due to configuration error: Malformed Lambda proxy response Wed Jan 13 07:34:32 UTC 2021 : Method completed with status: 502

With the API Gateway Lambda Proxy feature enabled, API Gateway expects response payloads from your Lambda to match the format shown here in the docs. Serverless framework generates a class called APIGatewayResponse that matches the expected format. Either create your own POJO that matches the expected return format or use the generated APIGatewayResponse class.

The generated sample Handler class shows this in use:

return ApiGatewayResponse.builder()
  .setStatusCode(200)
  .setObjectBody(responseBody)
  .setHeaders(Collections.singletonMap("X-Powered-By", "AWS Lambda & serverless"))
  .build();