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Choosing a Java runtime to install has become… interesting. Simon Ritter of Azul and Matt Raible of Okta have done great write-ups on how to select the right JDK, and in this post you’ll learn how to use a few of those options on Heroku.

We’ll cover:

Azul Zulu JDK
Amazon Corretto JDK
AdoptOpenJDK
GraalVM

If you don’t care which of these you use, then you’re probably best off using the default OpenJDK build provided by Heroku. You can do this by following the Heroku documentation for Java and picking your desired major, minor, or update version. But if you think you want an alternative JDK, you’ll probably want to start with Zulu.

Using Zulu

Zulu is a 100% free and open source distribution of OpenJDK from Azul Systems. It’s a great choice as an alternative JVM on Heroku because it’s officially supported. You can use Zulu by creating a system.properties file in the root of your app with the following contents:

java.runtime.version=zulu-1.8.0_202

Add this file to Git, and run git push heroku master as described in the Heroku guide to getting started with Java. Then run the following command to confirm that it worked:

$ heroku run java -version
Running java -version on ⬢ mighty-brook-32719... up, run.4864 (Free)
openjdk version "1.8.0_202"
OpenJDK Runtime Environment (Zulu 8.36.0.1-CA-linux64) (build 1.8.0_202-b05)
OpenJDK 64-Bit Server VM (Zulu 8.36.0.1-CA-linux64) (build 25.202-b05, mixed mode)

In addition to JDK 8, you can use Zulu JDK 11 and 12 by defining the appropriate version strings in your system.properties file. For more information, see the Heroku Dev Center.

Using AdoptOpenJDK

AdoptOpenJDK is a community of Java User Group (JUG) members, Java developers, and vendors who provide free and open source builds of OpenJDK. You can use AdoptOpenJDK binaries by adding a third-party buildpack to your Heroku app in conjunction with an official buildpack. To do so, run these commands:

$ heroku buildpacks:set jdk/adopt
$ heroku buildpacks:add heroku/java

Then deploy as before, and you’ll be using an AdoptOpenJDK binary. Run the following command to confirm that it worked:

$ heroku run java -version
Running java -version on ⬢ mighty-brook-32719... up, run.8770 (Free)
openjdk version "1.8.0_202"
OpenJDK Runtime Environment (AdoptOpenJDK)(build 1.8.0_202-201903270428-b08)
OpenJDK 64-Bit Server VM (AdoptOpenJDK)(build 25.202-b08, mixed mode)

You can customize the AdoptOpenJDK version with environment variables or a system.properties file.

Using Corretto

Corretto is a no-cost, multiplatform, production-ready distribution of the OpenJDK from Amazon. You can use it on Heroku by adding a third-party buildpack to your Heroku app in conjunction with an official buildpack. To do so, run these commands:

$ heroku buildpacks:set jdk/corretto
$ heroku buildpacks:add heroku/java

Then execute git push heroku master as before, and you’ll be running with a Corretto binary. Run the following command to confirm that it worked:

$ heroku run java -version
Running java -version on ⬢ mighty-brook-32719... up, run.3636 (Free)
openjdk version "1.8.0_202"
OpenJDK Runtime Environment Corretto-8.202.08.2 (build 1.8.0_202-b08)
OpenJDK 64-Bit Server VM Corretto-8.202.08.2 (build 25.202-b08, mixed mode)

The Corretto buildpack has a few configuration options, which you can learn about in the buildpack’s README.

Using GraalVM

The last JDK option is a little different than the others. GraalVM is a universal virtual machine for running applications written in JavaScript, Python, Ruby, and R as well as JVM-based languages like Java, Scala, Kotlin, Clojure, and more. You can use it on Heroku by adding a third-party buildpack to your Heroku app in conjunction with an official buildpack. To do so, run these commands:

$ heroku buildpacks:set jdk/graal
$ heroku buildpacks:add heroku/java

Then deploy as before, and you’ll be using GraalVM. Run the following command to confirm that it worked:

$ heroku run java -version
Running java -version on ⬢ mighty-brook-32719... up, run.9591 (Free)
openjdk version "1.8.0_202"
OpenJDK Runtime Environment (build 1.8.0_202-20190206132807.buildslave.jdk8u-src-tar--b08)
OpenJDK GraalVM CE 1.0.0-rc14 (build 25.202-b08-jvmci-0.56, mixed mode)

The Graal buildpack has a few configuration options, which you can learn about in the repo’s README file.

So Many Choices

As mentioned before, it’s probably best to stick with the default JDK provided by Heroku. But these are all great choices. For more information, see the Heroku documentation on Java.