This article is just for Googlers and my future self encountering this error. Recently I was building a Docker image from the ksqlDB code base, and whilst it built successfully the ksqlDB server process in the Docker container when instantiated failed with a UnsupportedClassVersionError:

java.lang.UnsupportedClassVersionError: javafx/beans/value/ObservableValue has been compiled by a more recent version of the Java Runtime (class file version 54.0), this version of the Java Runtime only recognizes class file versions up to 52.0

Here’s how I fixed it, courtesy of help from my colleagues Colin and Tim.

  1. Download JDK 8 ("Java SE") and install it

    • On my Mac I had an existing Java, this article showed how to list them and switch between them

      $ /usr/libexec/java_home -V

Matching Java Virtual Machines (2):
    12.0.2, x86_64:	"OpenJDK 12.0.2"	/Library/Java/JavaVirtualMachines/adoptopenjdk-12.0.2.jdk/Contents/Home
    1.8.0_241, x86_64:	"Java SE 8"	/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home

  2. Set the JAVA_HOME for your current shell to JDK 8

    export JAVA_HOME=`/usr/libexec/java_home -v 1.8`

  3. In the same shell, run

    `mvn clean`

  4. In the same shell, run the actual Maven build, which will now be done under Java 8 and should work 🤞