Last updated September 20, 2023

Heroku can run Java applications across a variety of Java implementations and includes support for framework-specific workflows.

This document describes the general behavior of Heroku as it relates to the recognition and execution of Java applications. General Java support on Heroku refers to the support for all frameworks except for Play. You can read about Play framework support in the Play framework support reference.

For framework-specific tutorials visit:

• Getting Started with Java on Heroku
• Java Tutorials

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.

When a deployed application is recognized as a Java application, Heroku responds with Java app detected.

$ git push heroku master
-----> Java app detected

Build Behavior

The following command is run to build your app:

$ mvn -B -DskipTests clean dependency:list install

However, 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

The following environment variables are set in 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: With 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)

About JAVA_TOOL_OPTIONS

JAVA_TOOL_OPTIONS is directly supported by Java and 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 could pass in:

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

You can also set your own JAVA_TOOL_OPTIONS config var. Setting your own causes its value to be appended to Heroku’s defaults and take 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 a new dyno type is selected, the following settings are automatically added to JAVA_TOOL_OPTIONS:

• eco, basic, or standard-1x: -Xmx300m -Xss512k -XX:CICompilerCount=2
• standard-2x: -Xmx671m -XX:CICompilerCount=2
• performance-m: -Xmx2g
• performance-l: -Xmx12g

For Private Space dynos, the values are:

• private-s: -Xmx671m -XX:CICompilerCount=2
• private-m: -Xmx2g
• private-l: -Xmx12g

Monitoring Resource Usage

You can use additional JVM flags to monitor resource usage in a dyno. The following flags are recommended for monitoring resource usage:

-XX:+PrintGCDetails -XX:+PrintGCDateStamps -XX:+PrintTenuringDistribution -XX:+UseConcMarkSweepGC

See the troubleshooting article for more information about tuning a JVM process.

Supported Java Versions

Heroku currently uses OpenJDK 8 to run your application by default. Other OpenJDK versions are also available. Depending on the OpenJDK version you select the latest available version of that JDK uses each time you deploy your app.

Current default versions for each OpenJDK version and Heroku stack are:

The OpenJDK that your app uses is included in the slug, which affects your slug size.

Specifying a Java Version

You can specify a Java version by adding a file called system.properties to your application.

Set a property java.runtime.version in the file:

java.runtime.version=11

Refer to the supported Java versions table for accepted major version values. Because the default is 8, you don’t need this file to use Java 8.

You can also pin your JDK update version by using a value such as this:

java.runtime.version=1.8.0_202

However, we encourage you to use the more general 1.8 format, which automatically installs any security updates.

Specifying an OpenJDK Distribution

Heroku supports builds of OpenJDK from either Azul® Zulu® or Heroku. Both distributions are built from the same source and are fully compliant with the Java SE specification. Azul® Zulu® builds of OpenJDK are also TCK/JCK certified and are our recommended OpenJDK distribution.

Current default distributions of OpenJDK for each stack are:

Using a Non-default Distribution

To use a non-default distribution with your app, you must specify a Java version. Prefix the version string with either heroku- or zulu- to select the respective distribution.

To use OpenJDK 11 and explicitly select Azul® Zulu® as the OpenJDK distribution, add the following to system.properties:

java.runtime.version=zulu-11

Upgrading your Java Version

All Java apps are automatically upgraded to the latest available JDK version when and only when they’re deployed. They aren’t upgraded if the app isn’t redeployed or if a specific version is configured in the system.properties file.

Specifying a Maven Version

Heroku supports the Maven Wrapper, which is the recommended mechanism for defining a Maven version. If Heroku detects an mvnw file in the root directory of your repository, it uses this script to launch the Maven process.

You can also specify a Maven version with the system.properties file by setting a maven.version property like this:

maven.version=3.9.4

If this property is defined, the mvnw script is ignored. You are not upgraded to a newer version automatically and need to update your system.properties file to get a newer version.

The default, if you don’t specify a version and don’t use Maven Wrapper, is 3.9.4. This default will change over time and we recommend using Maven Wrapper to ensure stable builds.

Default Web Process Type

The Java buildpack automatically detects the use of the Spring Boot and Thorntail web frameworks. For Spring Boot, it creates a web process type with the following command:

java -Dserver.port=$PORT $JAVA_OPTS -jar target/*.jar

For Thorntail, the buildpack uses this command for the default web process type:

java -Dswarm.http.port=$PORT $JAVA_OPTS -jar target/*.jar

You can override these defaults or define a custom process type using a Procfile. The appropriate command depends on your app and the frameworks in use. See one of the Java tutorials for information on setting up your Procfile.

Postgres Auto-Provisioning

A mini 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

Further Reading

• Warming Up a Java Process
• Running Database Migrations for Java Apps
• Reducing the Slug Size of Java Applications