Automatic Let’s Encrypt Certificates with Apache Tomcat / Spring Boot


I implemented a solution for fetching and renewing TLS certs without restart via Let’s Encrypt that works with standalone and embedded Tomcat as well as Spring Boot. It’s packaged into a Docker image, allowing for easy reuse. The certs are fetched and renewed using dehydrated, a bash script with few dependencies. This makes the TLS requesting agnostic to the application or framework in use,  as long as the application is shipped as a Docker image. The downside is, that it’s not a plain Java solution, i.e. it requires an additional process (within the container) and a Linux environment.

Having used cloud-native products like Caddy, Traefik, CertManager or Micronaut that naturally obtain and renew Let’s Encrypt certificates for HTTPS communication, I wondered if there is an easy way to use a feature such as this in a web server as long-established as Apache Tomcat. You might say: Tomcat usually  is used behind reverse proxies that do all the TLS offloading. But then there’s something like the Apache Portable Runtime (APR) based Native library for Tomcat that brings the heart of the Apache Web Server to Tomcat as a system library and with it it’s high TLS performance, that might even outperform NGINX. So obviously, this should be possible.

Surprisingly, I couldn’t find a solution with batteries included. So, let’s quickly implement our own. How hard can it be?

Dehydrated tomcat

Presenting: letsencrypt-tomcat – it packs everything needed for automatic TLS with tomcat into a single docker image:

  • dehydrated to manage certs via Let’s Encrypt,
  • tomcat-reloading-connector for hot reloading certs at runtime after renewal,
  • an init system (dumb-init) for properly handling tomcat and dehydrated processes,
  • an entrypoint script that starts up tomcat and dehydrated as well as
  • a pre-compiled version of Apache Portable Runtime (APR) and JNI wrappers for APR used by Tomcat (libtcnative), so tomcat delivers the best TLS performance possible.

This sounds like a lot of complicated stuff, but as the examples bellow will show, it’s rather easy to use. If you care about the details, check the “Challenges” bellow.

From the image everything can be copied as needed in the actual application’s Dockerfile. It’s not intended to as a base image (“inheritance”), but as a mere file archive to copy everything from a single source.


The way to use depends on the tomcat “kind” in use: standalone, embedded Tomcat orSpring Boot (which also uses embedded tomcat under the hood).

There’s one thing the setup for all kinds have in common: you pass the FQDN as environment variable DOMAIN to the container at startup, e.g. with docker run -e <image>

If necessary more parameters (optional) are available.

The remaining setup depends on the kind of tomcat used.

Sprint Boot

  • pom.xml or build.gradle: Add tomcat-reloading-connector to your dependencies (example)
  • Dockefile:
    • Copy /letsencrypt and /lib folder to the root (/) of your application’s docker image (example),
    • Set ENTRYPOINT or CMD to execute / and pass the usual command for starting your spring boot app as parameter, for example:
      ENTRYPOINT [ "/", "java", "-jar", "/app/app.jar" ]
  • or .yaml (example):
    • Configure Tomcat to serve Let’s Encrypt challenges: spring.resources.staticLocations=file:/static
    • Configure paths for certificate files: server.ssl.certificateKeyFile, etc.
    • Serve both HTTP and HTTPS ports
  • In the code that creates your Spring Boot app: Setup tomcat-reloading-connector: @Import(ReloadingTomcatServletWebServerFactory.class) (complete example)

Embedded tomcat

  • pom.xml or build.gradle: Add tomcat-reloading-connector to your dependencies (example)
  • Dockefile:
    • Copy /letsencrypt and /lib folder to the root (/) of your application’s docker image (example),
    • Set ENTRYPOINT or CMD to execute / and pass the usual command for starting your app as parameter, for example:
      ENTRYPOINT [ "/", "java", "-jar", "-Dcatalina.home=/tmp", "/app/app.jar" ]
  • In the code that creates embedded tomcat:
    • Configure Tomcat to serve Let’s Encrypt challenges (example)
    • Use tomcat-reloading-connector and configure paths for certificate files: ReloadingTomcatConnectorFactory.addHttpsConnector(tomcat, HTTPS_PORT, PK, CRT, CA); (complete example)

Standalone tomcat

  • Dockefile:
    • copy the following to the root (/) of your application’s docker image (example):
      • /letsencryptfolder,
      • tomcat-reloading-connector and
      • (if necessary) /lib folder (already included in bitnami-tomcat used in our example) ,
    • Make standalone tomcat serve Let’s Encrypt challenges (example)
  • server.xml:
    • Use tomcat-reloading-connector (example)
    • Configure paths for certificate files (example)

Real-world example

My use case for using this whole thing is my git-based wiki smeagol-galore, which relies on letsencrypt-tomcat. It already had a quite complex startup process, so I decided here to not use the meta-entrypoint but to copy the parts needed into the existing entrypoint.


Now that we have seen working examples, it’s a good time to sum up technical challenges and decisions encountered within the process. Turns out the whole indeavor had more challenges than expected.

Choosing an ACME implementation

As of 09/2020 there doesn’t seem to be a drop-in web app for tomcat or a spring boot annotation that just handles the ACME protocol used by Let’s encrypt for us.

There are Java libraries that promise to help achieving this, most prominently acme4j. But looking at the docs or this tutorial the whole process seems rather complicated. There’s the spring-boot-starter-acme which seems be able to get a cert. To use the cert, we have to restart the server, though. It also does not renew, is still a SNAPSHOT and hasn’t been updates for some years.

A completely different alternative would be to use the certbot client. But then it brings it’s own server, which would require our application to stop and start again, once certbot has finished renewing the cert.

This is where dehydrated comes in handy: it implements the whole ACME protocol used by Let’s encrypt, in a single bash script with no other dependencies than bash, curl and OpenSSL. It’s also easy to use and does not bring it’s own webserver. That’s good, because we already have a webserver: Tomcat.

How to best integrate a bash script and a tomcat process into a single artifact? Why not use an OCI (a.k.a. Docker) image? Two processes in one container? But doesn’t this violate the “one process per container” mantra?
I’m convinced that if there are good reasons for it, why not use two coherent processes in one container? We might even start chaning the mantra to “one thing per container“. The fact that there a multiple popular init systems for Docker images makes me think that I’m not the only one holding this opinion.

So let’s combine tomcat with dehydrated within a Docker container.

Integrating Tomcat and Dehydrated

The basic idea is:

  • Tomcat starts,
  • Dehydrated starts the certificate signing process via Let’s Encrypt,
  • serves the challenge via Tomcat, then
  • downloads the certificate, so that finally
  • Tomcat can server the certificate.
  • Once a day, Dehydrates checks again if the certs must be renewed.

This results in two “interfaces” between Tomcat and Dehydrated:

  • the folder where Dehydrated places the challenges must be served by Tomcat at http://$DOMAIN/.well-known/acme-challenge/and
  • the cert files, written by Dehydrated and read by Tomcat.

Depending on the kind of Tomcat, configuring the cert files and making Tomcat serve the the challenge folder on the HTTP port requires a different setup (see “Usage” above).

Self-signed cert

Once this is done, let’s start Tomcat. Only it does not start without certificate. But we need it started, in order to answer Let’s Encrypt’s challenge.
Who was first, the chicken or the egg?

One solution: create a self-signed cert at startup (see With a validity of 30 days or less, dehydrated will try to renew the cert right away.

Synchronize startup of Tomcat and Dehydrated

Tomcat might get its HTTP ports ready to serve traffic in a matter of seconds but, depending on the application, it could als take minutes. The latter resulting in Dehydrated failing to get the the certificate from Let’s Encrypt. Solution: Poll the HTTP port, waiting for a succesfull HTTP status code.

PEM vs KeyStore

With Tomcat’s default Java implementation of HTTPS (Http11NioProtocol) we would also have issues with the certificate file formats, because it requires a Java Keystore, whereas Let’s Encrypt provides private key, certificate and CA as PEM. However, we are using APR (Http11AprProtocol), which also requires PEM. So it just works™️.

Reloading the certificate

Believe it or now: At this point we have a valid certificate. Only that Tomcat still serves the old self-signed one.

Next stop, how to make Tomcat reload the cert? Tomcat 9.x (might even have been backported to 8.5) provides an AbstractHttp11Protocol.reloadSslHostConfigs() method. But how to call it?
Dehydrated offers a hook for calling “something” after success.  But how to call this method inside Tomcat from a shell script? JMX? Or via an URL request on Tomcat Manager web app? These solution would all require configuration, increase complexity and might even impair security.

The other option is to call the method from within Tomcat, by implementing our own protocol class. It could regularly check and reload the certificate on change. But then polling is just a waste of resources and it will always take longer than something that is called on change.
Other idea: Set a file watch. For better reuse, maintainance and separation of concerns, I implemented this in a separate project, tomcat-reloading-connector (too late I figured out that “tomcat-reloading-protocol” would be a better fit 🙈 ) and published it on maven central. While implementing, I came to the realization that reloading “the certificate” should be an atomic operation, containing all certificate-related files. After all, what I refer to as “the certificate” above, actually consists of a number of files, like private key, cert and CA. Reloading only one file might result in an inconsistent state. In order to avoid this, I added a reload delay of a few seconds. A changing file within the folder of the certificate results in all SSLHostConfigs being reloaded a couple of seconds later. Still not perfect, as the delay is a compromise between a fast reload and an inconsistent SSLHostConfig. For starters, let’s stick with it. For Dehydrated 3s seemed fine from my empirical studies. If it does not work for you, just define what ever suits your needs via the TOMCAT_DELAY_RELOAD_CERTIFICATES_MILLIS environment variable.


Creating a dehydrated Tomcat wasn’t straightforward. The most laborious task being the reload of the certificate, resulting in a separate project: tomcat-reloading-connector.

With letsencrypt-tomcat most of the work has been done, allowing for rather easy reuse. However, for each project, there still is some bootstrapping to be done. A 3rd party solution such as this can never compete with built-in support for Let’s Encrypt as offered by other products. On the other hand there now is a reliable solution that can run on all kinds of Tomcats be it standalone or embedded, even with Spring Boot.

Future work

That’s all for now. As always, there some work is still to be done for the curios reader.

Getting letsencrypt-tomcat to work wit Let’s Encrypt’s TLS-ALPN-01 challenge would be useful to get rid of the HTTP port. It generally is supported by Dehydrated but might be challengin to realize using Tomcat.

One issue with relying on a custom Dockerfile for creating the artifact is that it does not work intuitively with Cloud Native Build Packs. Spring Boot 2.3 comes with built-in support for building OCI images without Dockerfile. How can we integrate this with letsencrypt-tomcat? It should be possible to just add another layer, and adapt the entrypoint.

Continuous Delivery to Maven Central with Travis CI

Having used continuous delivery in my professional life for years, I would never want to miss it, even with my private projects. Recently, I had the challenge of deploying an artifact to maven central. I implemented this before using Jenkins. However, there’s no way (I know of) to get a Jenkins As A Service for free for OSS projects. So it was convenient to use TravisCI.

Surprisingly, I didn’t find a turnkey solution online. So I had to come up with my own solution. Here’s how it goes:

language: java
- openjdk8

  - |
    cat << EOF > settings.xml

- TMP_KEY="$(mktemp)"
- echo "${PK_BASE64}" | base64 -d > "${TMP_KEY}"
- export PGP_SECRETKEY="keyfile:${TMP_KEY}"

- ./mvnw clean package

skip_cleanup: true
provider: script
script: bash ./mvnw -Prelease -s settings.xml deploy -DskipTests
branch: master

- '$HOME/.m2/repository'
- '$HOME/.sonar/cache'

This requires four environment variables defined in your travis-ci project and a profile in your pom.xml:

  • OSSRH_USER – Username to Nexus
  • OSSRH_TOKEN – Access Token to Nexus
  • PGP_PASSPHRASE in the format literal:PASSPHRASE
  • PK_BASE64 – Content of your PK, base64 encoded, create with e.g. base64 -w0 pk.asc

Details will be described in the following paragraphs.


If you haven read a guide on how to deploy to maven central, do it now. The requirements, keys, signing, etc. are not within the scope of this article.

Repository credentials / settings.xml

  • The OSSRH_USER and OSSRH_TOKEN are needed for authenticating against the nexus repository where the artifacts are deployed. While you could use your username and password, I recommend to create an access token, though. It could be revoked more easily.
  • Credentials are passed to maven via settings.xml. In order to make this process secure, we create a settings.xml in the travis.yaml and tell it to read the actual credentials from the environment. We need to escape the $ because it is not supposed to be interpreted by the shell that writes the settings.xml, but by maven when reading the settings.xml
  • When calling maven, we tell it to use the local settings.xml via the -s parameter.

Signing artifacts

For signing the artifacts, kohsuke’s pgp-maven-plugin (from 2014 😲) still seems the easiest way for headless use such as in CI environments.
First, we have to specify the plugin in the pom.xml. My example pom.xml shows how.

We then can easily pass the passphrase for the private key to the plugins using the env var: PGP_PASSPHRASE. Note that we can’t just copy the plain passphrase, we have to tell the plugin, that’s its a literal, by using the format literal:PASSPHRASE.

For the private key itself, there are two challenges:

  1. How do we get the key into travis?
  2. How do we get the key into maven?

How do we get the key into travis?

Unfortunately, other than Jenkins, travis does not seem to provide a way of uploading secret files. If I get the docs right, I could encrypt the private key and push it into my repo. Has anything ever felt so wrong as pushing a private key into a public repository? So I tried loading it into an env var. Here the docs tell me to “escape any Bash special characters”, which is quite a challenge for a private key. Workaround: Base64 encode the whole thing before pasting it into travis.

How do we get the key into maven?

Now that the private key is ready in an env var, it turns out that the plugin’s sole open issue (opened 2018) is that it can’t read a private key from an env var.
So, while base64 decoding the private key, it is pasted into a temporary file, which is then passed to the plugin via an env var.


Only 20 lines of yaml to continuously deliver artifacts to maven central, including signing and all. Not so bad, is it?

Please keep in mind that the version within your pom.xml decides where the artifact is deployed to:

  • If it ends in -SNAPSHOT it is deployed to the SNAPSHOT repository
  • Otherwise, it’s deployed to the releases repository and promoted to maven central.

GraalVM (a bit) beyond Hello World


GraalVM native images are a real innovation for Java apps, providing much smaller Images and faster startup, especially useful for apps that are horizontally autoscaled. However, this causes a higher development effort, especially for existing apps. If included in new apps from the start, this effort should be smaller, but on the other hand we’re pinned to JDK8 with native images.

After working as developer with Java and Docker for many years (and more recently also as trainer) it’s obvious to me that Docker and Java are not a perfect match. As for all interpreted languages, the interpreter (the JVM, in case of Java) imposes a burden in both size and startup time on our Docker image/container.

Java shares this fate with other platforms such as .net, node.js and ruby. To make a point, the follwoing table shows a brief overview of the current “alpine” variants (i.e. the smallest) of those platform’s base images:

Image Size(MB)
 adoptopenjdk/openjdk8:jre8u212-b04-alpine 48 36
node:12.0.0-alpine 26
ruby:2.6.3-alpine3.9 26

(The sizes are the compresses size of all layers of the images within DockerHub. The image sizes is calculated as described here).

In contrast, nativly compiled apps written in Go or C/C++ can be built from scratch (if statically compiled), starting of at 0 MB.

GraalVM to the rescue

About a year ago, Oracle announced the first release candiadate of GraalVM offering better interoperability for Java Programms with non-JVM languages and precompiled native images with instant start up and low memory footprint (using ahead-of-time (AOT) compilation). In the announcement they stated that Twitter already uses GraalVM in production to efficiently run their Scala workloads.

From the start, this sounded like a innovation to me. As I’m always a bit low on free time to play around with technologies, I started by following the topic by reading articles that crossed my way, such as

Great introductions, though they all have one thing in common: They compile a single “Hello World” Java file into a native image.

This made me wonder if it’s also that easy for more real-life projects and what pitfalls there are. Especially regarding the known limitations for native image generation, like reflection.

Getting the hands dirty

My first approach was to naively adopt the Dockerfiles for two existing projects to produce native Images. Containing the compilation in Dockerfiles has the advantage of not requiring anything installed locally and is also a kind of “infrastructure as code”, providing deterministic results and allowing for version control.

The projects I chose were

BTW – As they both failed in the beginning (with GraalVM 1.0.0-rc14), I had a look on Java frameworks that offically support GraalVM native images. See my article: Short comparison: Building Graal Native Images with Quarkus, Micronaut and Helidon.

When I started, the latest version was GraalVM 1.0.0-rc14 (the 14th release candidate for version 19.0.0 🤔). For both projects I stumbled upon a number of confusing (to me) errors (like NullPointerExceptions) that all magically vanished once I updated to the “ready for production use” version GraalVM 19.0.0 (as soon as it was available).
Nice work by the GraalVM team 👍  (
if anyone is interested in the errors, I carved them into the Dockerfile or Git Commit messages, see fernflower and colander).

Here are my most interesting findings (I will elaborate on them bellow):

  • the fernflower CLI app works with a Docker image of only 5.3 MB in size! This is really revolutionary for a Java app 🎉
    Note that it only compiles statically (which is what I wanted anyway, to be able to use a scratch Docker image)
  • the colander native image also compiles (it’s also only 5 MB). However, the app does not work, because of it’s dependencies. These were the kind of real-world problem findinds I was looking for.

Findings in detail

Happy days

As said, the GraalVM native image build for fernflower works like charm. In fact, I was so charmed I created an automated build at DockerHub, that regularly builds Docker images for the latest version of fernflower, using different base images: schnatterer/fernflower-docker.

So if you’d ever need a Java decompiler just do a docker run --rm -v $(pwd):/src schnatterer/fernflower and 5MB later you’ll have you’re decompiler ready.
These images also allow for a nice comparison of image sizes for different Java base images.

Image Size(MB)
native/scratch image

5 vs 45MB comparing native image to regular JRE. Stunning, isn’t it?

BTW – this is the final Dockerfile to build the native image.

Facing real world challenges

Of course, I love it when a plan comes together. But on the other hand I suspected this wouldn’t always be the case for a such complex a thing as GraalVM native images. Here are the issues I encountered for the colander app (as said before, these issues are well documented known limitations of GraalVM):

  • When starting the native image, there’s not much output to the console.
    Reason: Not surprisingly, unlike the jar, the native image does not contain a logback.xml. In order to fix this, we would have to copy the logback.xml manually into the final docker image during the build.
  • The command line option --help does not show any options.
    Reason: These options are defined in annotations, and are read at runtime using reflection via the JCommander framework. How to fix?
    Configure the reflection for native image generation. Fortunately, some Java CLI frameworks like picocli support generating the config files out of the box. So migrating would also be an option.
  • Colander can’t show its own version name.
    Reason: The version name is read from the file using the cloudogu/versionName library. The file is (again, not surprisingly) is not contained in the native image. This made me wonder if it wouldn’t be much simpler to read the version name from a Java constant. A hard-coded value just feels more read-only than a text file such as the I added this feature to the library using annotation processors.


The issues encountered definitely proof my hypothesis, that GraalVM native images, while a technological innovation that provides a lot of potential, causes extra efforts during development. So we have to decide if these extra efforts are justified by the advantages.

I presume that, if planned from the beginning, the efforts are a lot less, because we can take GraalVM support into account when making our technical decisions. There are frameworks like Quarkus, Micronaut and Helidon that have native support for GraalVM, minimizing the extra effort. For large existing apps, though this does not help. The colander example is really small (with only about 1k of net LOC) and already causes a lot of effort.

So would I use GraalVM native images in production?

I probably wouldn’t migrate existing applications, except they run at a massive scale and the potential savings (faster horizontal scaling, smaller memory footprint) justify the effort for building the native image in the first place. For new projects, I would assess building GraalVM native images from the start, if sticking with JDK8 is OK.  Native images only support JDK8, as of GraalVM 19.0.0.
In the long run most popular Java frameworks, not only Quarkus, Micronaut and Helidon are likely to support native image generation. For now, Spring is still a WIP, also mentioned in the GraalVM 19.0 release announcement. If I had a teams with profound Spring experience, I would only switch to another framework for a good reason.
On the other hand, if developing using the microservices architecture pattern, first experiences with GraalVM native images could be gained by implementing small services using GraalVM.

Short comparison: Building Graal Native Images with Quarkus, Micronaut and Helidon

The technological innovations of the last years such as the adoption of containers, cloud-native technologies, the microservice architectural style, the inception of GraalVM and the end of JavaEE (as we know it) has energized the Java framework market.

As of May 2019, there are at least three frameworks supporting GraalVM native images out of the box, targeting cloud-native microservices:

  • Quarkus,
  • Micronaut and
  • Helidon.

As building GraalVM native images is a bit challenging, I was curious to find out how these three frameworks keep up with their promises. I worked through the respective getting started guides and wrote down some similarities and differences resulting in this short (and surely incomplete) comparison of the three frameworks. See the following table for an overview.

General comparison

Quarkus Micronaut Helidon
Core Project Source quarkusio/quarkus micronaut-projects/micronaut-core oracle/helidon
Started/Backed By RedHat objectcomputing Oracle
First Commit 2018-06-22 2017-03-06 2018-08-28
GitHub Stars (05/2019) 1693 2283 1371
GitHub Contributers (05/2019) 88 120 26
# Commits (05/2019) 3970 5907 617
Supported languages Java, Kotlin Java, Groovy, Kotlin Java
Supported build tools mvn, Gradle mvn, Gradle mvn
Supported APIs for graal native Microprofile, vert.x,


Micronaut, ReactiveX/RxJava Helidon SE (Microprofile, without native image)
Programming paradigms for graal native Imperative, reactive Reactive, imperative? Reactive (imperative, without native image)
Code generation via mvn plugin CLI (mn) mvn archetype
Getting Started (Graal native) Guide Docs Blog
Resulting src of Getting Started schnatterer/quarkus-getting-started schnatterer/micronaut-getting-started schnatterer/helidon-getting-started
Size of getting started docker image
Getting started base docker image fedora-minimal alpine-glibc scratch
Getting started uses native image? N N Y
Size of getting started in scratch docker image

All three projects are rather young (grandpa Micronaut is about 2 years old as of 05/2019) but have what looks like extensive documentation at first glance. The only thing that made my stumble a bit was that Helidon’s docs don’t return a result for “graal”. I later found a brand new getting started with graal on oracle’s developers blog. Hopefully, this will be added to the docs soon.

There are a couple of notable differences between the three frameworks:

    • Programming style (reactive vs. imperative)
      • Quarkus explicitly supports both (reactive as an extension),
      • Helidon claims to support both, but only reactive in conjunction with native images right now
      • Micronaut is reactive only From the docs it seems that micronaut focuses on reactive, but blocking approaches are supported (see Graeme Rocher’s comment).
    • Language
      • Micronaut and Quarkus both support Java and Kotlin.
        Micronaut also supports Groovy 🎉 (having Graeme Rocher, the creator of Grails, on board it’s probably a must)
      • Helidon only supports Java
    • Build tool / code generation
      • Micronaut and Quarkus support Maven and Gradle.
        • Quarkus uses a Maven plugin for code generation (bad luck for Gradle users) whereas
        • Micronaut brings its own CLI tool that thankfully can easily be installed using sdkman.
      • Helidon supports only Maven and has only initial code generation support via a Maven archetype.
    • Kubernetes
    • Community
      Hard to tell. The amount of discussions on my tweet about Quarkus makes me think they’re the ones that are most interested in feedback and people getting involved.

GraalVM native Image / Docker Image

  • The Dockerfiles provided by the getting started of Quarkus and Micronaut each require an external Maven build.
    The images base on fedora-minimal (resulting in a 44MB compressed image) or alpine-glibc (resulting in a 32MB compressed image) respectively.
    A base image containing a libc is required because the native image is linked dynamically.
  • Helidon provides a proper self-contained Dockerfile that can be built by simply calling docker build, not requiring anything locally (except Docker, of course).
    Here, the native image is linked statically. Therefore the binary can run in an empty scratch image (resulting in an 8MB compressed image).

Bearing in mind that a Java 8 JRE Image requires about 100MB (debian) or 50 MB (alpine), 44MB or even 32MB for a small webapp is not so bad. OTOH the 8 MB for the statically linked image are a real revelation, leaving me stunned.

The fact that Helidon plays well with GraalVM shouldn’t be too surprising, as they both are official Oracle products.

Beyond getting started

As Quarkus was the first framework I tried, I wondered why they rely on fedora and not just compile a static binary (later, I learned about some of their reasons on twitter). So I tried a couple of other images, eventually setting the switch for creating a static binary and using a scratch image. Voilà: It results in a 7MB image, even a wee bit smaller than the Helidon one. See the table bellow for an overview of images and their features and sizes (taken from the README of my getting started repo).

Base Image Size Shell Package Manager libc Basic Linux Folders Static Binary Dockerfile
fedora 📄
debian 📄
alpine-glibc 📄
distroless-base 📄
busybox 📄
distroless-static 📄
scratch 📄

I applied more or less the same on Micronaut. Here, the scratch image is only 5 MB smaller than the alpine one – 27 MB. This is not too surprising, because the plain alpine-glibc image is only about 6MB. It also felt like the native image generation took longer and needed more memory (observed with docker stats).

As for Helidon’s self-contained, scratch image containing only a static binary, there was not much to be done. I only extend the Dockerfile by a maven cache stage for faster Docker builds.

There’s one last thing I changed in all Dockerfiles: Don’t run as root. I used the USER statement in the Dockerfile. docker run -u ... would also be fine. This way, it’s much more unlikely that possible vulnerabilities (such as CVE-2019-5736 in runc) are exploited.

So summing up: Quarkus and Helidon can be used to create really small docker images, Micronauts are “only” small 😉. It’s worth mentioning that I didn’t look what features are included in those images, so maybe it’s a bit naive to just compare the minimal sizes resulting from the individual getting started guides.

Going even further

If I were to continue my comparison at this point (which I won’t because it’s only a short comparison) I would look into the following features of each framework:

  • integration and unit testing,
  • extensions (e.g. Cloud Native features, Tracing, Monitoring, etc.)


So, for a new green field project, which one of e frameworks would I use?

As far as I can tell after completing the getting started, all three look promising. As for all architectural decisions, I’d definitely try to build a walking skeleton (technical roundtrip) before finally deciding, in order to gain more field experience and find out what’s beyond getting started.

I’d base this decision on the experience or preferences of the team

  • reactive vs. imperative
  • Maven vs. Gradle
  • Java vs. Kotlin (or even groovy)
  • APIs – Microprofile, vert.x, RxJava

Personally, I like the fact that Quarkus builds on existing APIs such as Microprofile, so existing experience can be reused for faster results. It also seems to me the most flexible of the three, supporting Java, Kotlin, Maven, Gradle, reactive and imperative.

As for native images, I’d definitely either try it from the beginning or stick to a regular JRE. I suppose switching from plain JRE-based to native could be complicated for an existing app, due to the native image limitations. If the app under development does not have the requirement to be scaled horizontally, this could be an argument for skipping the native image part. But this is beyond the scope of this article.

As for the docker image – it’s obviously not only the size that matters. An image without shell and package manager is always more secure but harder to debug.



  • 2019/05/17: John Clingan pointed out that Quarkus supports Kubernetes resource generation an multiple reactive extensions
  • 2019/05/19: As commentef by Graeme Rocher’s, Micronaut also supports blocking workloads

The pragmatic migration to JUnit 5

This article shows how get from JUnit 3.x / 4.x to JUnit 5.x as fast as possible.

Just a short clarification of the term “JUnit 5” (from the user guide) before we take off:

JUnit 5 = JUnit Platform + JUnit Jupiter + JUnit Vintage


  • Platform provides the Maven and Gradle Plugins and is the extension point for IDE integration,
  • Vintage contains legacy JUnit 4 API and engine,
  • Jupiter contains the new JUnit 5 API and engine.

Step 1 – Run existing tests with JUnit 5 vintage

The first thing we do is to replace the existing junit:junit depedency with the following


For Gradle see this article.

For a real world example see this commit.

Note: After upgrading fromjunit:junit:4.12 to org.junit.vintage:junit-vintage-engine:5.1.0 the execution order of @Rule seems to have changed: They seem to be now executed sequentially (from top to bottom, as defined in the test class).

Step 2 – Getting started with JUnit Jupiter and the Platform

Now lets go from vintage to the fancy new stuff. Just add the Jupiter dependency and empower surefire to use the JUnit platform:


	<!-- ... -->

Make sure to juse either surefire 2.19.1 or 2.21.0+, as there seem to be a bug in the versions in between.

As above, for Gradle see this article.

For a real world example see this commit.

As of now, we’re ready to write new tests with JUnit Jupiter.

Here’s a pragmatic aproach how to introduce JUnit 5 from here:

  • Use the new API and all the new features for new test classes.
  • Don’t try to migrate all existing tests. It causes a lot of effort with no direct business value.
  • Instead, apply the boyscout rule by gradually migrating existing tests before they need to be changed.

When getting started wiht JUnit Jupiter you will recognize that some familiar features of JUnit now have a new API or can be achieved using different concepts. After that, there are a some new features to explore.

One way to get accustomed to the new API and concrepts is to migrate some (not all) existing tests, preferably the most complex ones. This way, you will find out how to use the new concepts and which limitations there still might be about JUnit Jupiter (e.g JUnit 4 rules that have not been ported to extensions).

Step 3 – Get accustomted to the API changes in JUnit Jupiter

There are some simple API changes but also two major concept changes: Rules and Runners are gone.

Simple API changes

  • public modifier can be removed (class and methods)
  • org.junit.Test➡️ org.junit.jupiter.api.Test
  • org.junit.Assert.assertX ➡️ org.junit.jupiter.api.Assertions.assertX
    (except assertThat)
  • Order of parameters changed in assert methods. The message parameter is now after expected and actual parameters! This can be a pitfall when migrating, because the message (strings) might silently turn to expected, if you just change the import.
  • assertThat is no longer part of the JUnit API. Instead, just use your favorite assertion library as AssertJ, Google truth or even hamcrest.
  • @Before ➡️ @BeforeEach
  • @After ➡️ @AfterEach
  • @BeforeClass ➡️ @BeforeAll
  • @AfterClass ➡️ @AfterAll
  • @Ignore ➡️ @Disabled
  • @Category ➡️ @Tag

For a real world example see this commit.

Make sure to not mix the APIs, because the tests are either run by the Jupiter or the vintage Engine, which will ignore unknown annotations.

Note that IntellI has a quick fix for migrating JUnit 4 to JUnit Jupiter. However, as of version 2018.1 this seems to only affect @Test, no asserts, exceptions, rules or runners.

Advanced API changes

Basically, Runners and Rules are replaced by Extensions, where one test class can have more than one extension.
However, some Runners have not been ported to Extensions, yet. For those you can try to use@EnableRuleMigrationSupport (see Temporary Folders). If this does not work, you will have to stick with the JUnit 4 API and vintage Engine for now.

Exceptions & Timeouts

Exceptions no longer need a Rule or the expected param in @Test. Instead, the API provides an assert mechanism now.

ExpectedException and @Test(expected = Exception.class) ➡️ assertThrows(Exception.class,() -> method());

For a real world example see this commit.

The same applies to timeouts:

@Test(timeout = 1) ➡️ assertTimeout(Duration.ofMillis(1), () ->method());


Instead of the mockito runner, we use the new extension, which comes in a separate module.


@RunWith(MockitoJUnitRunner.class) ➡️ @ExtendWith(MockitoExtension.class)

For a real world example see this commit.

Temporary Folders

Until there is an Extension, we can use @EnableRuleMigrationSupport from this module:


With this we can use the new API (org.junit.jupiter.api.Test). Howerver, rules and classes must stay public. ClassRules seem not to work.

For a real world example see this commit.

Other Rules

Here are some more rules and their equivalent in JUnit Jupiter.

  • @RunWith(SpringJUnit4ClassRunner.class) ➡️ @ExtendWith(SpringExtension.class)
  • stefanbirkner/system-rules, such as ExpectedSystemExit
    Work in progress! That is, these tests will have remain on the JUnit 4 APIs for now.
  • TestLoggerFactoryResetRule from slf4j-test
    No progress to be seen.
    Could be replaced by logback-spike. For a real world example see this commit.
  • Of course this list is non-exhaustive, there are a lot more runners I have not stumbled upon, yet.

Step 4 – Make use of new features in JUnit Jupiter

Just using the same features with different API is boring, right?
JUnit Jupiter offers some long-awaited features that we should make use of!
Here are some examples:

Optional: Further Reading

Automatic checks for vulnerabilities in Java project dependencies

 Java aktuell published an article I wrote on a topic at work for TRIOLOGY GmbH.

You can find an English version on the TRIOLOGY Blog: Automatic checks for vulnerabilities in Java project dependencies. The article shows an approach to keeping your Java project dependencies free of known vulnarabilities (e.g. CVEs) using the OWASP Dependency check with Jenkins and Maven. There also is an example project on GitHub.

The original article PDF (in German) is available for download here: Automatisierte Überprüfung von Sicherheitslücken in Abhängigkeiten  von Java-Projekten.

TRIOLOGY also published a short Q&A on the article, which can be found here.

Maven: Create a more sophisticated build number

Earlier this year, while working on a project for TRIOLOGY GmbH, I once again used maven to write a version name into an application, using the mechanism described in my post Maven: Create a simple build number. As a more sophisticated version name was required for this project, we expanded it by a time stamp, SCM information (branch and commit), build number and a also created a special name for releases. You can find a how-to here – Version names with Maven: Creating the version name – which is the first part of a small series of blog posts on this topic.

The second part shows how the version name can be read from within the application. While writing the examples for the post, I wondered how many times I must have implemeted reading a version name from a file in Java. Way too often! So I decided that this would be the very last time I had to do it, and extracted the logic into a small library: versionName, availble on GitHub. What it does and how to use it is described in the second part of the post: Version names with Maven: Reading the version name.

Hopefully, this will be useful for someone else. Funny enough, in the new project I’m on, I’m about to reuse it once again. I’m glad I don’t have to write it again. Here’s to reusability 🍺

Building GitHub projects with Jenkins, Maven and SonarQube 5.2 on OpenShift

Time for an update of the post Building GitHub projects with Jenkins, Maven and SonarQube 4.1.1 on OpenShift, because SonarQube 5.2 is out: It’s the first version since 4.1.1 that can be run on OpenShift. That is, it’s the first version of SonarQube 5 and the first one that contains Elasticsearch and many other features that are now available on OpenShift!
Interested? Then let’s see how to set up SonarQube on OpenShift.

  • If you’re starting from scratch, just skip to this section.
  • If you got a running instance of SonarQube
    • make sure to back up you instance before you continue:
      rhc snapshot save -a --filepath
      ssh 'snapshot' > sonar.tar.gz
    • Then pull the git repository just like in step 2,
    • wait until the app has started and visit

      SonarQube will update it’s database during the process.

    • If you followed this post to set up your SonarQube instance and therefore use an SSH tunnel to access the SonarQube database, note that you can now get rid of this workaround. From SonarQube 5.2 the analyses can be run without direct contact to the database.
      That is, you can also remove the database connection from your the configuration of the SonarQube plugin in jenkins.

Install new SonarQube instance

To install SonarQube 5.2, execute the following steps on your machine:

  1. rhc app create sonar diy-0.1 postgresql-9.2

    Make sure to remember the login and passwords!

  2. git rm -r diy .openshift misc
    git remote add upstream -m master
    git pull -s recursive -X theirs upstream master
    git push
  3. Login to your SonarQube instance at

    Note that the initial setup may take some minutes. So be patient.
    The default login and passwords are admin / admin.
    You might want to change the password right away!

Basic installation Jenkins

Basically, the following is an updated (and a lot simpler) version of my post about SonarQube 4.1.1.

  1. Create Jenkins app
    rhc app create jenkins jenkins-1
  2. Install Plugins
    Browse to Update Center

    and hit Check Now (as described here).
    Then go to the Available tab and install

    1. Sonar Plugin,
    2. GitHub plugin,
    3. embeddable-build-status (if you’d like to include those nifty build badges in you

    Then hit Install without restart or Download and install after restart. If necessary, you can restart your app anytime like so

    rhc app restart -a jenkins
  3. Set up maven settings.xml to a writable location.
    • SSH to Jenkins
      mkdir $OPENSHIFT_DATA_DIR/.m2
      echo -e "<settings><localRepository>$OPENSHIFT_DATA_DIR/.m2</localRepository></settings>" > $OPENSHIFT_DATA_DIR/.m2/settings.xml
    • Browse to Configure System

      Default settings provider: Settings file in file system
      File path=$OPENSHIFT_DATA_DIR/.m2/settings.xml

  4. Either see my post on how to introduce a dedicated slave node to this setup or
    set up the Jenkins master to run its own builds as follows (not recommended on small gears, as you might run out of memory pretty fast during builds):
    Go to Configure System

    and set
    # of executors: 1

  5. Setup sonar plugin (the following bases on SonarQube Plugin 2.3 for Jenkins)
    On the Jenkins frontend, go to Configure System
    • Global properties,
      tick Environment variables
      Click Add
      See here for more information.
    • Setup the Runner:
      Navigate to SonarQube Runner
      Click Add SonarQube Runner
    • Then set up the plugin itself
      Navigate to SonarQube
      tick Enable injection of SonarQube server configuration as build environment variables
      and set the following
      Server URL:

      Sonar account login: admin
      Sonar account password: (default: admin)

    • Hit Save

Configure build for a repository

Now lets set up our first build.

  1. Go to

    Item name:
    (Unfortunately, Maven projects do not work due to OpenShift’s restrictions.)
    Hit OK

  2. On the next Screen
    GitHub project:

    Source Code Management:

    Branch Specifier (blank for 'any'): origin/master
    Build Triggers: Tick Build when a change is pushed to GitHub
    Build Environment: Tick Prepare SonarQube Scanner environment
    Build | Execute Shell

    # Start the actual build
    mvn clean package  $SONAR_MAVEN_GOAL --settings $OPENSHIFT_DATA_DIR/.m2/settings.xml$SONAR_HOST_URL
  3. I’d also recommend the following actions
    Post-build Actions| Add post-build action| Publish JUnit test result report
    Test report XMLs=target/surefire-reports/TEST-.xml*
    Post-build Actions| Add post-build action| E-mail Notification
  4. Hit Apply.
  5. Finally, press Save and start you first build. Check Jenkins console output for errors. If everything succeeds you should see the result of the project’s analysis on SonarQube’s dashboard.

Using Custom Maven / JDK version when building with Jenkins on OpenShift

[EDIT (2016-06-02): OpenShift now provides different JDK “alternatives”, e.g.


So you might want to skip the steps bellow, regarding a custom JDK. The steps described for using a custom maven still apply, however.


In previous posts I pointed out how to build GitHub projects with Jenkins, Maven and SonarQube and how to run these builds on dedicated Jenkins slaves. The following shows how to replace the “stock” versions of maven and JDK that are provided by OpenShift.

At the time of writing OpenShift features Maven 3.0.4 and OpenJDK Server 1.7.0_85. Why would you want to change those? Best example is a Java8 project to be build on Jenkins. Can we just advise Jenkins to download the newest Oracle JDK and we’re good to go? Nope, it’s not that simple on OpenShift! Jenkins does download the new JDK, sets the JAVA_HOME variable and the correct PATH, but maven is always going to use the stock JDK. Why? Running this command provides the answer

$ cat `which mvn`
prog=$(basename $0)
export JAVA_HOME=/usr/lib/jvm/java
export JAVACMD=$JAVA_HOME/bin/java
export M2_HOME=/usr/share/java/apache-maven-3.0.4
exec $M2_HOME/bin/$prog "$@"

The stock maven is setting its own environment variables that cannot be overridden by Jenkins!

So, in order to exchange the JDK, we need to exchange maven first.

  • SSH to the machines where your builds are executed (e.g. your slave node). The following example show what to do for maven 3.3.3:
    mkdir maven
    cd maven
    tar -xvf apache-maven-3.3.3-bin.tar.gz
    rm apache-maven-3.3.3-bin.tar.gz
  • Edit maven config
    vi $OPENSHIFT_DATA_DIR/maven/apache-maven-3.3.3/conf/settings.xml

    Add the following to the tag (replace by your OpenShift UID first)


    (press i button for edit mode, insert, then press esc button, enter :wq, finally press return button)

  • Browse to

    Set Environment variables

  • And that’s it, your builds are now running on the custom maven!
    This allows for using a specific JDK in Jenkins. You could just choose a specific JDK via Jenkins console. This is comfortable, but has one disadvantage: It takes a lot of memory (approx. 600MB per JDK), because the JDK is stored twice – compressed in cache to be sent to slave and again uncompressed to be used on the master. If you got enough memory, you’re done here.

    However, In case you’re running a small gear with only 1GB of memory, you might want to save a bit of your precious memory. The following example shows how to do so for JDK 8 update 51 build 16.
    On SSH:

    mkdir jdk
    cd jdk
    wget --no-check-certificate --no-cookies --header "Cookie: oraclelicense=accept-securebackup-cookie"
    tar -xvf jdk-8u51-linux-x64.tar.gz
    rm jdk-8u51-linux-x64.tar.gz
  • Then go to Jenkins

    JDK installations | JDK

Building GitHub projects on Jenkins slaves on OpenShift

This post showed how to build GitHub projects with Jenkins, Maven and SonarQube 4 on OpenShift. For starters, it used the Jenkins master node for running build jobs. However, when running on a small gear, the master node might run out of memory pretty fast, resulting in a reboot of the node during builds.

In order to resolve this issue, there are two options:

  • limitting the memory of the build or
  • running the build on a slave node.

As spawning additional nodes is easy in a PaaS context such as OpenShift and provides a better performance than running builds with small memory, the slave solution seems to be the better approach.

This post shows how.

  1. Create new DYI app as a slave node (a how-to can be found here), name the node e.g. slave
  2. Create node in Jenkins
    1. Go to Jenkins web UI and create new node:
    2. Set the following values:
      Remote FS root:/app-root/data folder on slave. Typically this is /var/lib/openshift//app-root/data/jenkins, you can find out by SSHing to the slave node and calling

      echo $OPENSHIFT_DATA_DIR/app-root/data/jenkins

      Labels: Some label to use within builds to refer the node, e.g. OS Slave #1
      Host: the slave’s hostname, e.g.

    3. Add Credentials
      Private Key File:Path to a private key file that is authorized for your OpenShift account. In the <a href="”>first post this path was used: /var/lib/openshift//app-root/data/git-ssh/id_rsa. Note: $OPENSHIFT_DATA_DIR seems not to work here.
      BTW: You can change the credentials any time later via this URL
  3. Prepare slave node: Create same environment as on master in the <a href="”>first post
    1. Create folder structure
      mkdir $OPENSHIFT_DATA_DIR/jenkins
      mkdir $OPENSHIFT_DATA_DIR/.m2
      echo -e "<settings><localRepository>$OPENSHIFT_DATA_DIR/.m2</localRepository></settings>" > $OPENSHIFT_DATA_DIR/.m2/settings.xml
    2. Copy SSH directory from master to same directory on slave, e.g.
      scp -rp -i $OPENSHIFT_DATA_DIR/.ssh $OPENSHIFT_DATA_DIR/.ssh <slave's UID>@slave-<your account>
    3. As the different cartridges (jenkins and DIY) have different environment variables for their local IP addresses ($OPENSHIFT_JENKINS_IP vs $OPENSHIFT_DIY_IP) we’ll have to improvise at this point. There are two options: Either
      1. Replace all occurrences of $OPENSHIFT_JENKINS_IP
        In all builds and in

        Sonar | Sonar installations
        Database URL: jdbc:postgresql://$OPENSHIFT_DIY_IP:15555/sonarqube

      2. Create an $OPENSHIFT_JENKINS_IP environment variable on your slave machine
        rhc env set OPENSHIFT_JENKINS_IP=<value of  $OPENSHIFT_DIY_IP> -a slave

        You can find out the value of $OPENSHIFT_DIY_IP by SSHing to the slave and execute

        echo $OPENSHIFT_DIY_IP 
      3. I’d love to hear suggesstions that do better 😉
  4. Adapt Build
    Easiest way is to not use the master at all.
    To do so, go to

    and set # of executors to 0.
    Hit Apply

  5. Limit memory usage.
    Make sure the slave does not run out of memory (which leads to a restart of the node):
    Global properties | Environment variables
    name: MAVEN_OPTS
    value: -Xmx512m
    Hit Save.
  6. Now run a build. It should run on the slave and hopefully succeed 🙂

See also Libor Krzyzanek’s Blog: Jenkins on Openshift wi… | JBoss Developer