Java Modules (JPMS)

Introduced in Java 9, the Java Platform Module System (JPMS) allows you to group related packages and resources into a module.

A module is a named, self-describing collection of code and data.

Why Modules?

1. Strong Encapsulation: You can hide internal packages from other modules.
2. Reliable Configuration: Explicit dependencies (requires) ensure all needed modules are present at startup.
3. Smaller Runtime: You can create custom JREs containing only the modules your app needs (using jlink).

Defining a Module

A module is defined in a file named module-info.java at the root of the source directory.

Structure:

module com.mycompany.mymodule {
    // Directives
}

Key Directives

1. requires

Specifies that this module depends on another module.

module com.mycompany.app {
    requires java.sql;
    requires com.mycompany.utils;
}

2. exports

Specifies that the packages in this module are accessible to other modules.

module com.mycompany.utils {
    exports com.mycompany.utils.math; // Only this package is visible
    // com.mycompany.utils.internal is HIDDEN
}

3. requires transitive

If module A requires transitive module B, then any module reading A also implicitly reads B.

module com.mycompany.api {
    requires transitive java.logging;
}

Example Project Structure

src/
  com.hello/
    module-info.java
    com/
      hello/
        HelloWorld.java

module-info.java:

module com.hello {
    // No dependencies (implicitly requires java.base)
}

Key Takeaways

• Encapsulation: Modules control which packages are public.
• Dependencies: module-info.java clearly defines what your code needs.
• Java 9+: Modules are a core part of modern Java.
• Migration: You can still use the classpath (unnamed module) for legacy apps.