MODULES & MIGRATION

Module Migration Strategies: Bottom-Up vs Top-Down

Bottom-Up: Start with leaf dependencies (no dependencies), work up to main app.
Top-Down: Start with main application, dependencies become automatic modules.

// BOTTOM-UP: Convert dependencies first
// Step 1: UtilLib (leaf) -> DatabaseLib -> ServiceLayer -> MainApp
module com.company.util {
    exports com.company.util.string;
    // No requires - leaf module
}

// TOP-DOWN: Convert main app first, deps are automatic modules
module com.company.myapp {
    requires service.layer;      // automatic module from service-layer.jar
    requires commons.lang3;      // automatic module from commons-lang3.jar
}

// Automatic module naming: "jackson-core-2.13.jar" -> "jackson.core"

✅ Bottom-Up: Guaranteed to work, lower risk, clear dependencies
❌ Bottom-Up: Slower benefits, need to wait for third-party libs

✅ Top-Down: Quick wins, immediate benefits, independent of third parties
❌ Top-Down: Automatic module names can change, less predictable

💡 Learning Tip: Bottom-up = “Foundation first” (solid but slow), Top-down = “Roof first” (fast but requires careful reinforcement later). Most projects should use top-down for practicality.