Java Packages and Modules: Organizing Code & JPMS Architecture

As software grows in size, organizing hundreds of source files into logical namespaces is critical. In Java, Packages provide directory-based namespace encapsulation, while the Java Platform Module System (JPMS) provides modular boundaries across packages.

1. Creating and Declaring a Package

To assign a class to a package, place the package statement at the very top of the .java file:

// Must be the first non-comment line in the source file:
package com.veewom.banking.services;

public class TransactionService {
    public void processTransfer() {
        System.out.println("Processing bank transfer...");
    }
}

2. Importing Classes from Other Packages

To use a class residing in another package, import it explicitly:

// Explicit Single Class Import (Recommended for clarity):
import com.veewom.banking.services.TransactionService;

// Wildcard Package Import (Imports all classes in package):
import java.util.*;

public class AppLauncher {
    public static void main(String[] args) {
        TransactionService service = new TransactionService();
        service.processTransfer();
    }
}

3. Introduction to Java 9+ Modules (JPMS / Project Jigsaw)

JPMS enforces Strong Encapsulation. Even if a class is declared public, it is completely hidden from external modules unless its package is explicitly exported in module-info.java:

// module-info.java (Root of module source directory)
module com.veewom.banking {
    // Requires dependent modules
    requires java.sql;
    requires org.apache.commons.lang3;

    // Exports specific packages to consumers
    exports com.veewom.banking.services;
    exports com.veewom.banking.models;
}