Customizing the JDK

Last updated 26 June 2018

There are some cases where files need to be bundled with the JDK in order to expose functionality in the runtime JVM. For example, the inclusion of a custom certificate authority (CA) store is common. To handle such cases, Heroku will copy files designated by the app in a .jdk-overlay folder into the JDK’s directory structure.

To include additional files in the JVM, follow these instructions:

Prerequisites

• A Java app running on Heroku.

Specify a JDK version

Create a system.properties file if one does not already exist, specify the version, and commit it to git. Supported version are described in the Java Support article. The file’s contents should look something like this:

java.runtime.version=9

Then add the file to Git by running:

$ git add system.properties
$ git commit -m "JDK 7"

Create a .jdk-overlay folder

In your application’s root directory, create a .jdk-overlay folder.

$ mkdir .jdk-overlay
$ ls -la
total 24
drwxr-xr-x    9 user  staff   306 Oct 16 14:43 .
drwxr-xr-x  202 user  staff  6868 Oct 16 14:40 ..
drwxr-xr-x   13 user  staff   442 Oct 16 15:06 .git
drwxr-xr-x    3 user  staff   102 Oct 16 14:43 .jdk-overlay
-rw-r--r--    1 user  staff    45 Oct 16 14:40 Procfile
-rw-r--r--    1 user  staff  1860 Oct 16 14:40 pom.xml
drwxr-xr-x    3 user  staff   102 Oct 16 14:40 src
-rw-r--r--    1 user  staff    25 Oct 16 14:40 system.properties

Add custom files

Copy any custom files into the .jdk-overlay directory. The files will be copied to their equivalent directory in the JDK. For example, to define a custom security policy, the java.policy file could be placed in the .jdk-overlay/jre/lib/security/ directory of your app’s repository.

Adding custom certificates

You may also need to add custom certificates to the JDK’s cacerts. You may start with the keystore in your local JDK or download the base Heroku keystore. After adding the custom certificate with a command like this:

$ keytool -import -keystore cacerts -file custom.cer

You may then include the keystore in the slug by placing it in the .jdk-overlay/jre/lib/security/ directory of your app’s repository.

Now add any custom files to your Git repo like this:

$ git add .jdk-overlay
$ git commit -m "Custom JDK Files"

Then deploy your application, with the custom files, to Heroku:

$ git push heroku master
...

If you are invoking any Heroku services, you’ll need to add the Let’s Encrypt Certificate Authority to your bundle. First, download the certificates from Let’s Encrypt and save them on your local file system. You may also want to download the IdentTrust root chain as a backup. Then add both certificates to your cacerts bundle with the keytool command. Finally, add your cacerts file to your app’s .jdk-overlay directory and redeploy as before.

Verify the copy

The copies can be verified by starting a bash session on Heroku and checking the JDK directory. The JDK directory is located in $HOME/.jdk/.

For example, to verify custom certificates were copied correctly, the $HOME/.jdk/jre/lib/security/ directory can be checked.

$ heroku run bash
Running `bash` attached to terminal... up, run.1
~ $ keytool -list -keystore .jdk/jre/lib/security/cacerts
...
Your keystore contains 140 entries
...

Other examples

This method can be used for Java extensions when necessary. Though a dependency management tool, such as Maven, should be the preferred mechanism for introducing dependencies.