Last updated December 09, 2024

The Heroku Platform and Java buildpack has the following behavior for any type of Java application deployed.

Activation

The default build system for Java applications on Heroku is Maven. Heroku Java support for Maven is applied to applications that contain a pom.xml file.

Build Behavior

Run the command to build your app:

$ mvn -B -DskipTests clean dependency:list install

If Heroku detects an mvnw script in your application’s repository, it runs this script instead of the default Maven installation. You can override this behavior by explicitly setting a Maven version.

The Maven repo is cached between builds to improve performance.

Environment Variables

The following environment variables are set in the dyno at boot-time:

• PORT: the web process binds to this HTTP port
• JAVA_HOME: the location of the JDK install directory
• LD_LIBRARY_PATH: the location of the JDK shared libraries
• JDBC_DATABASE_URL: If a DATABASE_URL variable is present, this variable is populated with the converted form. See Connecting to Relational Databases on Heroku with Java for more information.
• JAVA_TOOL_OPTIONS: default Java options based on dyno size
• JAVA_OPTS: default Java options based on dyno size (identical to JAVA_TOOL_OPTIONS)

JAVA_TOOL_OPTIONS

Java supports JAVA_TOOL_OPTIONS and it’s intended to augment a command line in environments where the command line can’t be accessed or modified. Heroku uses this value to set default Java options based on dyno size. Since Java automatically picks it up, you don’t need to include it in your Procfile command.

You can override these settings in the Procfile command, which takes precedence over the defaults. For example, to change the default of -Xmx300m, you can pass in:

web: java -Xms150M -jar target/myapp.jar

You can also set your own JAVA_TOOL_OPTIONS config var. Setting your own appends the value to Heroku’s defaults and takes precedence. Individual options not overridden in the Procfile command or custom JAVA_TOOL_OPTIONS are still in effect.

When a Java process is started on your dyno, the following Java options are added to JAVA_TOOL_OPTIONS and automatically picked up by Java:

• -Dfile.encoding=UTF-8
• -XX:+UseContainerSupport (for Java 11 and higher)

Adjusting Environment for a Dyno Size

When you select a new dyno type some JVM flags are automatically added to JAVA_TOOL_OPTIONS.

Cedar

Cedar-generation dynos have JAVA_TOOL_OPTIONS set to the following per dyno size:

Fir

All Fir-generation dynos have JAVA_TOOL_OPTIONS set to -XX:MaxRAMPercentage=80.0.

Postgres Auto-Provisioning

A Heroku Postgres database automatically provisions on the first deploy of your Java applications. These apps must have a dependency on the Postgres JDBC driver or pgjdbc-ng driver in their pom.xml. This auto-provisioning populates the DATABASE_URL environment variable.

If you don’t want the Postgres add-on, remove it by running:

$ heroku addons:destroy DATABASE --app example-app