AWS Lambda errors with Java Lambdas and Serverless framework

Using a basic Servlerless framework event config like this:

    handler: kh.javalambda.HelloLambda::myHandler
      - 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: 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()
  .setHeaders(Collections.singletonMap("X-Powered-By", "AWS Lambda & serverless"))

Updating my React Sudoku Solver app: Replacing Flux with Redux (part 2)

In the first part of converting my Sudoku solver React app from using Flux to Redux I looked at creating the Redux Store, Action Creators, reducers, and connecting Components to the Store. In this part 2 I’m taking a look at using redux-thunk for making my api calls.

Async API calls are integrated into a Redux app by using a ‘thunk’, support is provided by adding react-thunk to your app:

npm install --save redux-thunk

react-thunk is a middleware for your Redux Store. It looks at each Action dispatched to your Store and if it’s a function instead of an Action object it executes it.

Where you create your store with createStore(), add an import for applyMiddleware and thunk:

import { applyMiddleware, createStore } from 'redux';
import thunk from 'redux-thunk';
const middlewares = [thunk];

Pass a call to applyMiddleware as a param to createStore() passing this middlewares array. In my app I’ve already passed the devToolsEnhancer() to createStore():

let store = createStore(puzzleDataReducer, devToolsEnhancer());

Naively, I thought I’d be able to add thunk and devToolsEnhancer() in the array and pass like this:

const middlewares = [thunk, devToolsMiddleware()];
let store = createStore(puzzleDataReducer, applyMiddleware(...middlewares));

… but this leads to some rather cryptic errors and I’ve honestly no idea what this means or how to resolve it:

VM1047:1 Uncaught TypeError: t.apply is not a function
     at :1:47480
     at :1:33759
     at :1:26192
     at eval (redux.js:602)
     at eval (redux.js:646)
     at createStore (redux.js:79)

Luckily a quick search found this article, suggesting the way to combine these enhancers is to use redux’s compose() function. Import compose from redux:

import { createStore, applyMiddleware, compose } from 'redux';

Use compose() to combine thunk and the devToolsEnhancer() call like this:

   applyMiddleware(thunk, otherMiddleware()),
   window.devToolsExtension ? window.devToolsExtension() : f => f

In my case I don’t have other middleware to include, so my createStore() was originally this:

let store = createStore(puzzleDataReducer, devToolsEnhancer());

and now including thunk and using compose() it looks like this:

let store = createStore(puzzleDataReducer, compose(
    window.devToolsExtension ? window.devToolsExtension() : f => f

Next I need to move my Flux Action that makes my api call to a function that is returned by an Action Creator. Since react-thunk looks for functions returned by Action Creators instead of Action objects and executes them. this was a relatively minor change moving the function from my original Action to my Action Creator.

At this point I’ve got a working app, migrated from Flux to Redux. You can check the final result on GitHub here. I’ve still got some cleanup to do – in particular I don’t have a clean separation between my reducer code and my Action Creator code. I’ll code back and do some cleanup later, and also deploy a parallel copy of the same app to AWS. Right now the original Flux based app is deployed here. I’ll deploy the final Redux based app shortly.

Creating a new AWS Lambda project with Serverless

Assuming the Serverless cli is already installed (here), init a new project with ‘serverless’ and answer the following questions:

% serverless       
 Serverless: No project detected. Do you want to create a new one? Yes
 Serverless: What do you want to make? AWS Node.js
 Serverless: What do you want to call this project? lambda-example
 Project successfully created in 'lambda-example' folder.

To deploy, run ‘serverless deploy’

AWS CloudFormation basics – part1

Collection of notes, templates and tips for building AWS CloudFormation templates.

The basic structure of CloudFormation files (in JSON):

  "Resources" : {
    "ExampleResourceName" : {
      "Type" : "AWS::?::?",
      "Properties" : {
        "Example" : "propertyvalue"
  • Resources: the AWS services to be provisioned. There can be multiple repeating Resource elements in this section, to provision/configure multiple services in a stack
  • ExampleResourceName: a name for each resource being provisioned
  • Type: the AWS type for the resource, e.g. AWS::S3::Bucket
  • Properties: properties for the service being provisioned/configured

AWS CLI commands:

aws cloudformation create-stack --stack-name STACK-NAME
  --template-body file://template-file.json
  --parameters ParameterKey=example1,ParameterValue=value1    
aws cloudformation list-stacks
aws cloudformation delete-stack --stack-name STACK-NAME

If your stack creates IAM resources, you’ll also need to pass:


Otherwise you’ll see this error:

An error occurred (InsufficientCapabilitiesException) when calling the CreateStack operation: Requires capabilities : [CAPABILITY_NAMED_IAM]