Platform & Advanced APIs

Modules (JPMS)

The Java Platform Module System (Java 9) adds module-info.java: explicit dependencies (requires) and explicit API surface (exports). The JDK itself is modular; application adoption is optional and, outside libraries, modest.

requires declares dependencies; exports opens packages to compilers and callers
opens permits deep reflection (frameworks!) without exporting the API
Strong encapsulation: non-exported packages are inaccessible — even reflectively
Unnamed module (classpath) and automatic modules (jars on module path) ease migration
requires transitive re-exports; provides/uses wires ServiceLoader
jlink assembles a minimal runtime image from just the modules you use

module-info.java

module com.acme.orders {
    requires java.sql;
    requires transitive com.acme.common;   // callers of orders see common too

    exports com.acme.orders.api;            // public API — the ONLY visible packages
    opens com.acme.orders.model             // reflection access (e.g. for Jackson)
            to com.fasterxml.jackson.databind;

    provides com.acme.spi.Exporter
            with com.acme.orders.CsvExporter;   // ServiceLoader wiring
}

What modules fix: classpath hell (missing jars surface at startup with clear errors, not NoClassDefFoundError mid-request), accidental API (internals like sun.misc.Unsafe were used because they were reachable; exports make the contract explicit), and bloat (jlink builds a runtime with only the ~20 modules you use — a 40 MB self-contained image instead of a full JDK, relevant for containers).

Migration reality (Core Java II ch. 12): code on the classpath lives in the unnamed module and can read everything, so pre-9 apps run unchanged. A jar dropped on the module path becomes an automatic module readable by named modules. Most applications stop there; full modularization pays off mainly for libraries and platforms. The strong-encapsulation side effects, though, are universal: --add-opens flags for frameworks doing deep reflection into the JDK (Reflection).