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WTF, Java 9?! 😱

Some things break on Java 9. Here I collect short, self contained, correct (or not) examples demonstrating what fails and (hopefully) how to fix it. Note that there is no judgment involved! This is not about whether the things should break or not and whether that's good or bad - it's just about what does break.

You could be looking at this either as a beautiful static site or as a GitHub repository.


So far we have "trouble" with...


The demos are organized as Maven modules (I use Maven 3.5.0). You can run them individually (in their respective folders) or consecutively (in the root folder) with mvn clean test, maybe throwing in -fae (fail at end) to execute all even if some fail.

On Java 8, the build should be successful. To run it on Java 9:

  • define JAVA_HOME in a .mavenrc file, pointing to your Java 9 install
  • in the .mvn folders, rename the files jvm9.config to jvm.config (even the empty ones), which adds Java-9-specific command line flags required for the project

If you're on Linux, you can use the switch-to-java script. Called with either 8 or 9 as parameter, it makes sure that the build runs on the appropriate version.


Contributions are very welcome! To make sure they have high quality, make sure your demo checks as many of these boxes as possible:

  • it should be as small as possible
  • it should be a Maven module, where mvn clean test passes on Java 8 but fails on Java 9
  • it should demonstrate the behavior without much editing:
    • if compiler or Surefire need different arguments on different releases, use profiles
    • if the entire Maven run needs arguments, have a look at how .mvn/jvm.config works
  • its README should describe the problem, possibly show some code (and link to more), and also point out the solution if one is available; also, don't forget to link to it from above
  • the README should also mention the Java 8 and 9 versions on which the described behavior was observed

If you have any questions, open an issue. If want to contribute either open an issue or a PR, depending on how much you already have.

Once again, contributions are welcome! :)

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