Thin lambda dev server

ApiGatewayServer.java

import com.amazonaws.services.lambda.runtime.Context;
import io.javalin.Javalin;
import io.javalin.http.HandlerType;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.Map;
import java.util.regex.Pattern;
import java.util.stream.Collectors;

public class ApiGatewayServer implements AutoCloseable {
  private static final Logger LOG = LoggerFactory.getLogger(ApiGatewayServer.class);

  private final LambdaApplication app;
  private final int port;
  private Javalin javalin;

  public ApiGatewayServer(LambdaApplication app, int port) {
    this.app = app;
    this.port = port;
  }

  public void start() {
    javalin = Javalin.create().start("0.0.0.0", port);

    for (Class<? extends LambdaFunction> function : app.getFunctions()) {
      Lambda metadata = function.getAnnotation(Lambda.class);
      HandlerType method = HandlerType.valueOf(metadata.method().toUpperCase());
      String path = convertToJavalinPath(metadata.path());

      LOG.info("{} /{} ({})", method, metadata.path(), function.getCanonicalName());

      javalin.addHandler(method, path, ctx -> {
        // Build the request objects.
        Context context = new FakeContext(function);

        RequestIdentity identity = new RequestIdentity();
        identity.setSourceIp(ctx.ip());
        identity.setUserAgent(ctx.userAgent());

        RequestContext requestContext = new RequestContext();
        requestContext.setAccountId("0123456789");
        requestContext.setRequestId(context.getAwsRequestId());
        requestContext.setStage("local");
        requestContext.setIdentity(identity);

        Request request = new Request();
        request.setBase64Encoded(false);
        request.setBody(ctx.body());
        request.setHeaders(ctx.headerMap());
        request.setHttpMethod(method.name());
        request.setPath(ctx.path());
        request.setPathParameters(ctx.pathParamMap());
        request.setQueryStringParameters(ctx.queryParamMap().entrySet().stream()
          .collect(Collectors.toMap(Map.Entry::getKey, entry -> entry.getValue().get(0))));
        request.setRequestContext(requestContext);

        // Invoke the handler class.
        LambdaLauncher launcher = new LambdaLauncher(function, app.module());
        Response response = launcher.handleRequest(request, context);

        // Write the result to the Javalin context.
        for(Map.Entry<String, String> header : response.getHeaders().entrySet()) {
          ctx.header(header.getKey(), header.getValue());
        }

        ctx.status(response.getStatusCode());
        ctx.result(response.getBody());
      });
    }

    // This method does not return.
    while (!Thread.currentThread().isInterrupted());
  }

  @Override
  public void close() {
    if (javalin != null) {
      javalin.stop();
    }
  }

  private static String convertToJavalinPath(String apiGatewayPath) {
    Pattern pattern = Pattern.compile("\\{(?<VARIABLE>[^{}]+)}");
    return pattern.matcher(apiGatewayPath).replaceAll(":$1");
  }
}

Lambda.java

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE})
public @interface Lambda {
  String path();
  String method();
  String name() default "";
  String description() default "";
  int memoryMb() default 128;
  int timeoutSec() default 60;
}

LambdaFunction.java

import com.amazonaws.services.lambda.runtime.Context;
import com.amazonaws.services.lambda.runtime.RequestHandler;

public interface LambdaFunction extends RequestHandler<Request, Response> {
  
}