Ninja is a full-stack web framework for Java, built on Guice dependency injection, FreeMarker templating, and Jackson for JSON/XML serialization. It provides a complete MVC architecture with routing, controllers, filters, sessions, flash scope, i18n, validation, caching, file uploads, WebSockets, email sending, database migrations (via Flyway), and metrics collection. The framework is modular, with separate packages for core, servlet integration, standalone server (Jetty), database, metrics, email, WebSockets, and testing utilities. In production, Ninja applications are deployed either as standalone servers (embedded Jetty) or in servlet containers (Tomcat, Wildfly). The framework handles request routing through a central Routes.java file or annotation-based routing (JaxyRoutes), invokes controller methods with automatic parameter injection and validation, renders templates (FreeMarker) or JSON/XML responses, and manages client-side sessions with signed/encrypted cookies. It includes built-in support for CSRF protection, basic authentication, and secure filters. Key entry points include the Bootstrap class (application startup), NinjaDefault (core request handling), RouterImpl (route matching), ControllerMethodInvoker (parameter injection), and TemplateEngineManager (template rendering). External dependencies include Guice, FreeMarker, Jackson, EhCache/Memcached, Flyway, Apache Commons, and Codahale Metrics. The framework demonstrates strong engineering practices with clean abstractions (filter chain, argument extractors, template engine interface, cache provider), extensive test coverage, and a modular design that allows selective use of features. However, some areas show technical drift with deprecated APIs, aging library versions, and duplicated test code across modules.