The logic in rt/build.gradle takes JAVA_HOME either from the environment or from the java.home system property. However, these values differ in that ${JAVA_HOME} points to the JDK top-level directory, whereas the java.home property points to the jre directory within the JDK.

One consequence of this is that the check for $JAVA_HOME/jre/lib/ext/jfxrt.jar does not work if JAVA_HOME is not set in the environment.