Jake Wharton

Using Renovate to update build JDK

08 January 2025

You want to be using the latest JDK for development. Don’t use Gradle toolchains, they’ll needlessly force you to use old JDKs. You can still target and test on old JVM versions but develop with the latest and greatest. Java and the JDK are literally built for this.

Locally this hasn’t been a problem. Homebrew (or your favorite equivalent) will keep your default JDK at the latest. Keeping my GitHub actions up-to-date, however, frequently slips my mind. I find projects using 19 or 20 simply because I haven’t touched the CI build in the two years since 19 or 20 was the latest.

We’re already using Renovate to keep dependencies up to date. With a little extra programming in JSON (wince) we can have the JDK version updated to latest as well.

First, migrate the existing build JDK version in your GitHub Action to a .github/.java-version file1.

21

Next, change the setup-java action to use this file rather than a hard-coded version.

 - uses: actions/setup-java@v4
   with:
     distribution: 'zulu'
-    java-version: 21
+    java-version-file: .github/.java-version

Finally, in your renovate.json52, add a custom manager to update this file3.

ignorePresets: [
  // Ensure we get the latest version and are not pinned to old versions.
  'workarounds:javaLTSVersions',
],
customManagers: [
  // Update .java-version file with the latest JDK version.
  {
    customType: 'regex',
    fileMatch: [
      '\\.java-version$',
    ],
    matchStrings: [
      '(?<currentValue>.*)\\n',
    ],
    datasourceTemplate: 'java-version',
    depNameTemplate: 'java',
    // Only write the major version.
    extractVersionTemplate: '^(?<version>\\d+)',
  },
],

Commit, push, and wait for Renovate to send you a PR4. Now your CI build automatically tracks the latest JDK.

  1. I’m putting the .java-version file into the .github/ folder because I don’t want to force this version on people using jenv or the like. The whole point of this setup is you can build with any version of Java newer than our very, very old baseline of Java 8 (although things like Gradle have a higher minimum requirement). ↩

  2. JSON5 is just JSON with fewer quotes, more trailing commas, and actual comments. You don’t have to migrate to use this functionality (but you should!). ↩

  3. You can adapt the file-matching regex and string-matching regex to target the GitHub Action .yamls directly if you want. I choose to use the .java-version file so that I can install multiple older JDKs with a second setup-java (which won’t be updated) for testing with multiple JDKs. ↩

  4. Here’s an example sequence of PRs which did this migration: Switch to .java-version, add custom Renovate config, and Renovate’s automatic JDK bump. ↩

— Jake Wharton