Last updated 09 April 2020

Heroku expects a web application to bind its HTTP server to the port defined by the $PORT environment variable. Many frameworks default to port 8080, but can be configured to use an environment variable instead. In most cases, the port can be configured by adding a parameter to the java command in an application’s Procfile, but some frameworks provide a configuration file. The most common Java frameworks are listed here.

Spring Boot

Spring-Boot provides a few different mechanisms for setting the HTTP port. It can be passed an option to the executable JAR file (i.e. the options after the -jar app.jar options in your java command):

--server.port=$PORT

Or as a Java system property:

-Dserver.port=$PORT

In both cases, these options can be added to the java command in an app’s Procfile. Another option is a configuration element in the application.yml:

server:
    port: $PORT

For more info see Spring Boot documentation on Properties and Configuration

Webapp Runner (Tomcat)

Webapp Runner allows you to launch an exploded or compressed war on your filesystem into a Tomcat container with a simple java -jar command. It accepts the following option to the executable JAR file (i.e. the options after the -jar webapp-runner.jar options in your java command):

--port $PORT

When using the heroku war:deploy and Heroku Maven Plugin tools, this option is configured for you. For more information, see the Webapp Runner documentation.

Play Framework

The Play Framework, which uses Netty as its server, accepts a Java system property (i.e. an option to the java command):

-Dhttp.port=$PORT

For more info see the Play documentation for Heroku.

Dropwizard

The Dropwizard framework accepts a Java system property:

-Ddw.server.applicationConnectors[0].port=$PORT

For more information see the Dropwizard Configuration reference.

Thorntail

The Thorntail framework accepts a system property:

-Dswarm.http.port=$PORT

For more information see the WildFly Swarm documentation.

Ratpack

The Ratpack framework, which uses the Netty server, automatically detects the $PORT environment variable and configures the app to use it.

Grails

The Grails 3 framework produces a WAR file that can be run with either Tomcat or Jetty. See the Webapp Runner section for more information.

For more information see the WildFly Swarm