Gradle and Java Version Compatibility: A Beginner's Guide

No matter how long you've been working with a technology or a platform, such as Android, you always encounter weird errors. The more time you spend, the more edge cases and new things you will learn.

I feel relatively comfortable with Gradle due to my experience with Android over the years. But I encountered a weird issue last week in a perfectly working Android project.

I could run the project just fine with Android Studio, but when running ./gradlew assembleDebug command to generate a build it was failing with:

Execution failed for task ':app:kspDebugKotlin'. 
  > Error while evaluating property 'compilerOptions.jvmTarget' 
    of task ':app:kspDebugKotlin'.    
    > Failed to calculate the value of property 'jvmTarget'.       
      > Unknown Kotlin JVM target: 20 

It was complaining that my Java version was incompatible with Gradle. You see each Gradle version is compatible with specific Java versions - see this compatibility table here (source).

But if my system's Java JDK version was incompatible with Gradle, the run should have been failing from Android Studio as well, right?

What I had missed, is that Android Studio comes with an "embedded" Java JDK and it's not using your system-installed JDK. You can see and select which JDK Android Studio is using by going to Settings -> Build, Execution & Deployment -> Build tools -> Gradle. There you can change the version, see what is the current version, and even easily download and install new JDKs with one click!

To solve my issue of running ./gradlew assembleDebug in the command line, I needed to install the same JDK version as the embedded JDK that Android Studio was using. To change the Java version that is used in a command line session (not system-wide), just update the JAVA_HOME environment variable for the OS you are using.

In case you are on MacOS, you can list the available JDKs in your system by running:

/usr/libexec/java_home -V

Then update the JAVA_HOME env variable with the selected version:

export JAVA_HOME=`/usr/libexec/java_home -v 17`

And run this to confirm:

java -version

That's it! For the current terminal session, this JDK will be used, and you can run ./gradlew assembleDebug or any other command that requires a specific Java version you want.

Happy coding!