module-info.java
Declare a module.
The Module System
The Java Platform Module System (JPMS), introduced in Java 9, lets you group packages into named modules with explicit dependencies and boundaries.
The declaration lives in a special file: module-info.java.
Why Modules
Before modules, the classpath was a flat, unstructured bag of JARs. Anyone could access any public class, and missing dependencies surfaced only at runtime.
Modules add strong encapsulation and reliable configuration verified at compile and launch time.