AWS Lambda with Java and SnapStart

Running Java on AWS Lambda is straightforward with the managed runtime, and SnapStart dramatically improves cold starts for certain workloads.

Handler Interface

Implement RequestHandler for API Gateway proxy events:

public class HelloHandler implements RequestHandler<APIGatewayProxyRequestEvent, APIGatewayProxyResponseEvent> {
  @Override
  public APIGatewayProxyResponseEvent handle(APIGatewayProxyRequestEvent input, Context context) {
    var response = new APIGatewayProxyResponseEvent();
    response.setStatusCode(200);
    response.setBody("{\"message\":\"Hello from Lambda\"}");
    return response;
  }
}

SAM Template with SnapStart

Enable SnapStart in your CloudFormation or SAM template:

Resources:
  HelloFunction:
    Type: AWS::Serverless::Function
    Properties:
      Runtime: java21
      Handler: com.example.HelloHandler::handleRequest
      SnapStart:
        ApplyOn: PublishedVersions
      MemorySize: 512
      Timeout: 30

When SnapStart Helps

SnapStart snapshots initialized JVM state after startup. It works best for frameworks with heavy classpath initialization—measure p99 latency before and after enabling it in your region.