JRestless - Build Serverless JAX-RS Applications

3 minute read

AWS Lambda is a very interesting serverless computing service provided by Amazon. It allows you to run code without the burden of setting up a server - and even better you only pay what you use with the first 1,000,000 calls being free of charge.

The only downside is that you get locked in into that stack once you start developing applications with or rather for it.

That’s why I am working on a framework called JRestless to avoid the cloud vendor lock-in.

JRestless is designed to use AWS Lambda (and AWS API Gateway) to run your RESTful APIs and/or microservices using JAX-RS. Jersey is used to provide compliance with the JAX-RS standard and allows you to use its extensions, including integration for Spring 4.x, as well.

You can find JRestless together with example applications and documentation on GitHub: https://github.com/bbilger/jrestless

Avoiding the cloud vendor lock-in by using JAX-RS allows you to develop, test and run your applications locally and - if required - move your application from AWS Lambda to another FaaS (Function as a Service) environment or a normal application server.

JRestless abstracts away AWS Lambda (and AWS API Gateway) specifics and allows you to develop your application using JAX-RS. In case you are not familiar with JAX-RS: JAX-RS is a standard for Java to develop REST APIs.

JRestless doesn’t implement the JAX-RS APIs itself but uses Jersey - the reference implementation for JAX-RS - and provides a container for it. So you get JAX-RS compliance and you can use mostly all JAX-RS features like request/response filters, plus some of the extension Jersey provides on top which includes integration for Spring 4.x to name just one.

JRestless acts as an integration layer between FaaS environments and Jersey. Since AWS Lambda is the only Faas environment that supports Java at moment, it is the only supported environment right now. JRestless, however, is designed to support other platforms, as well, once Java is added.

You can use JRestless to develop two different types of AWS Lambda functions:

  1. Gateway functions that can be invoked through AWS API Gateway and so normal HTTP request.
  2. Service functions that can be invoked by other Lambda functions (or directly using the AWS SDK). The event object to invoke those function types is similar to a HTTP request. You can abstract the fact that you invoke a Lambda function away by using JRestless’ feign client, allowing you to transparently call those functions. You can read more about those functions on Github: jrestless-aws-service-handler

In the beginning it might be difficult to configure AWS properly and to deploy your Lambda functions. So you might be interested in the fact that you can use the serverless framework together with JRestless to get your applications deployed easily.

Since gateway functions are probably the most interesting ones, I want to end this post with a simple example.

Install serverless as described in the docs https://serverless.com/framework/docs/guide/installing-serverless/

Setup your AWS account as described in the docs https://serverless.com/framework/docs/providers/aws/setup/

Create a new function using serverless

mkdir aws-gateway-usage-example
cd aws-gateway-usage-example
serverless create --template aws-java-gradle --name aws-gateway-usage-example
rm -rf src/main/java/hello # remove the classes created by the template
mkdir -p src/main/java/com/jrestless/aws/examples # create the package structure

Replace serverless.yml with the following contents:

service: aws-gateway-usage-example-service

  name: aws
  runtime: java8
  stage: dev
  region: eu-central-1

  artifact: build/distributions/aws-gateway-usage-example.zip

    handler: com.jrestless.aws.examples.RequestHandler
      - http:
          path: sample/{proxy+}
          method: any

Replace build.gradle with the following contents:

apply plugin: 'java'

repositories {
dependencies {
task buildZip(type: Zip) {
  archiveName = "${project.name}.zip"
  from compileJava
  from processResources
  into('lib') {
    from configurations.runtime
build.dependsOn buildZip

Create a new JAX-RS resource and a JAXB DTO (src/main/java/com/jrestless/aws/examples/SampleResource.java):

package com.jrestless.aws.examples;

import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.Produces;
import javax.ws.rs.core.MediaType;
import javax.ws.rs.core.Response;
import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;

public class SampleResource {
  @Produces({ MediaType.APPLICATION_JSON, MediaType.APPLICATION_XML })
  public Response getHealth() {
    return Response.ok(new HealthStatusDto("up and running")).build();
  @XmlRootElement // for JAXB
  public static class HealthStatusDto {
    private String statusMessage;
    private HealthStatusDto() {
      // for JAXB
    HealthStatusDto(String statusMessage) {
      this.statusMessage = statusMessage;
    @XmlElement // for JAXB
    public String getStatusMessage() {
      return statusMessage;

Create the request handler (src/main/java/com/jrestless/aws/examples/RequestHandler.java):

package com.jrestless.aws.examples;

import com.jrestless.aws.gateway.GatewayResourceConfig;
import com.jrestless.aws.gateway.handler.GatewayRequestObjectHandler;

public class RequestHandler extends GatewayRequestObjectHandler {
  public RequestHandler() {
    init(new GatewayResourceConfig().packages("com.jrestless.aws.examples"));

Build your function from within the directory aws-gateway-usage-example:

gradle build

This, amongst other things, creates a deployable zip file of your function (build/distributions/aws-gateway-usage-example.zip) using the dependent task buildZip.

Now you can deploy the function using serverless:

serverless deploy

If serverless is configured correctly, it should show you an endpoint in its output.

  ANY - https://<SOMEID>.execute-api.eu-central-1.amazonaws.com/dev/sample/{proxy+}

Hit the endpoint:

curl -H 'Accept: application/json' 'https://<SOMEID>.execute-api.eu-central-1.amazonaws.com/dev/sample/health'
# {"statusMessage":"up and running"}
curl -H 'Accept: application/xml' 'https://<SOMEID>.execute-api.eu-central-1.amazonaws.com/dev/sample/health'
# <?xml version="1.0" encoding="UTF-8" standalone="yes"?><healthStatusDto><statusMessage>up and running</statusMessage></healthStatusDto>