Heroku buildpack for Java

This is the official Heroku buildpack for Java apps. It uses Maven 3.9.4 to build your application and OpenJDK 8 to run it. However, the JDK version can be configured as described below.

The buildpack will detect your app as Java if it has a pom.xml file, or one of the other POM formats supports by the Maven Polyglot plugin, in its root directory. It will use Maven to execute the build defined by your pom.xml and download your dependencies. The .m2 folder (local maven repository) will be cached between builds for faster dependency resolution. However neither the mvn executable nor the .m2 folder will be available in your slug at runtime.

For more information about using Java and buildpacks on Heroku, see these Dev Center articles:

• Heroku Java Support
• Introduction to Heroku for Java Developers
• Deploying Tomcat-based Java Web Applications with Webapp Runner
• Deploy a Java Web Application that launches with Jetty Runner
• Using a Custom Maven Settings File
• Using Grunt with Java and Maven to Automate JavaScript Tasks

• Tomcat Webapp-Runner Example
• Spring Boot Example

Create a system.properties file in the root of your project directory and set java.runtime.version=1.8.

Example:

$ ls
Procfile pom.xml src

$ echo "java.runtime.version=1.8" > system.properties

$ git add system.properties && git commit -m "Java 8"

$ git push heroku main
...
-----> Java app detected
-----> Installing OpenJDK 1.8... done
-----> Installing Maven 3.3.3... done
...

You can define a specific version of Maven for Heroku to use by adding the Maven Wrapper to your project. When this buildpack detects the presence of a mvnw script and a .mvn directory, it will run the Maven Wrapper instead of the default mvn command.

If you need to override this, the system.properties file also allows for a maven.version entry (regardless of whether you specify a java.runtime.version entry). For example:

java.runtime.version=1.8
maven.version=3.3.9

There are three config variables that can be used to customize the Maven execution:

• MAVEN_CUSTOM_GOALS: set to clean dependency:list install by default
• MAVEN_CUSTOM_OPTS: set to -DskipTests by default
• MAVEN_JAVA_OPTS: set to -Xmx1024m by default

These variables can be set like this:

$ heroku config:set MAVEN_CUSTOM_GOALS="clean package"
$ heroku config:set MAVEN_CUSTOM_OPTS="--update-snapshots -DskipTests=true"
$ heroku config:set MAVEN_JAVA_OPTS="-Xss2g"

Other options are available for defining a custom settings.xml file.

To make changes to this buildpack, fork it on Github. Push up changes to your fork, then create a new Heroku app to test it, or configure an existing app to use your buildpack:

# Create a new Heroku app that uses your buildpack
heroku create --buildpack <your-github-url>

# Configure an existing Heroku app to use your buildpack
heroku buildpacks:set <your-github-url>

# You can also use a git branch!
heroku buildpacks:set <your-github-url>#your-branch

For example if you want to have Maven available to use at runtime in your application, you can copy it from the cache directory to the build directory by adding the following lines to the compile script:

for DIR in ".m2" ".maven" ; do
  cp -r $CACHE_DIR/$DIR $BUILD_DIR/$DIR
done

This will copy the local Maven repo and Maven binaries into your slug.

Commit and push the changes to your buildpack to your GitHub fork, then push your sample app to Heroku to test. Once the push succeeds you should be able to run:

$ heroku run bash

and then:

$ ls -al

and you'll see the .m2 and .maven directories are now present in your slug.

Licensed under the MIT License. See LICENSE file.