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Module system (Java 9+)

The Java Platform Module System (JPMS) was introduced in Java 9 (Project Jigsaw). It adds modularity at a higher level than packages.

Key concepts:

  • A module is a named, self‑describing collection of code and data.
  • Each module has a module descriptor (module-info.java).
  • Modules explicitly declare which packages they export and which modules they require.

Benefits:

  • Strong encapsulation (internal packages are not accessible by default).
  • Reliable configuration (no missing dependencies at startup).
  • Scalability (can create custom runtime images with jlink).

Module declaration example (module-info.java):

module com.example.myapp {
    requires java.sql;
    requires transitive java.logging;
    exports com.example.myapp.api;
    exports com.example.myapp.util to com.example.test;
    uses com.example.spi.Service;
    provides com.example.spi.Service with com.example.impl.ServiceImpl;
}

Compiling modules:

Terminal window
javac -d mods/com.example.myapp src/com.example.myapp/module-info.java src/com.example.myapp/...

Running a module:

Terminal window
java --module-path mods -m com.example.myapp/com.example.myapp.Main

Module path vs classpath: Modules go on the module path; traditional JARs can still be on the classpath (automatic modules).

Common directives:

  • requires – module dependency.
  • requires transitive – implied readability.
  • exports – makes package accessible to other modules.
  • opens – opens package for deep reflection.
  • uses / provides – service loading.