In JDK 9, 8 modules are there which are based on directories named as repos.
Code must be written as:
$ROOT/jdk/src/java.base ... $ROOT/langtools/src/java.compiler ...
After Java 10, JDK forests are arranged into a single repository to streamline development. And now the code must be written as:
$ROOT/src/java.base $ROOT/src/java.compiler ...