AWS Lambda Java Hello World (Java 21)

A minimal, production-ready "Hello World" AWS Lambda written in Java 21 with API Gateway, structured logging, unit tests, and an AWS SAM template for deployment.

Prerequisites

• Java 21 (JDK)
• Maven 3.9+
• AWS CLI configured (aws configure)
• AWS SAM CLI 1.100+

Project Structure

• src/main/java/com/example/hello/HelloHandler.java — Lambda handler using API Gateway proxy integration
• src/main/java/com/example/hello/Json.java — minimal JSON helper using Jackson
• src/test/java/com/example/hello/HelloHandlerTest.java — JUnit tests
• template.yaml — AWS SAM template (runtime java21)
• events/hello.json — sample API Gateway event for local testing
• pom.xml — Maven build with shading for deployable artifact

Build

mvn -q -DskipTests package

The deployable artifact will be in target/lambda-hello-1.0.0-shaded.jar.

Test

mvn -q test

Local Invoke (SAM)

Build with SAM (uses Maven under the hood):

sam build

Invoke locally with the sample event:

sam local invoke HelloFunction -e events/hello.json

Start a local API (hot-reload-like loop):

sam local start-api

Then open: http://127.0.0.1:3000/hello?name=Atul

Deploy (SAM)

First deploy (guided):

sam deploy --guided

Recommended choices during guided deploy:

• Stack Name: lambda-java-hello
• Region: your preferred region (e.g., us-east-1)
• Confirm changes before deploy: y
• Save arguments to samconfig.toml: y

Subsequent deploys become just:

sam build && sam deploy

After deploy, SAM outputs ApiUrl. Example:

https://xxxxxxxxxx.execute-api.us-east-1.amazonaws.com/Prod/hello

Try: ?name=YourName

Configuration

• Environment variable GREETING controls the greeting prefix. Default is Hello.
  • Set in template.yaml under Globals -> Function -> Environment -> Variables.

Logging

• Uses SLF4J (org.slf4j:slf4j-simple) for structured logs.
• Also writes a short line to the LambdaLogger for quick timing visibility.
• View logs via CloudWatch Logs or locally when using sam local.

Handler Signature

• Handler: com.example.hello.HelloHandler::handleRequest
• Input: APIGatewayProxyRequestEvent
• Output: APIGatewayProxyResponseEvent

Notes

• If your IDE shows package mismatch errors like main.java.com..., import the project as a Maven project. The source layout here is standard Maven (src/main/java and src/test/java).
• Java 21 runtime is specified in the SAM template (java21). Ensure your AWS region supports it.

Cleanup

To remove deployed resources:

aws cloudformation delete-stack --stack-name lambda-java-hello