写一个框架，2千行代码

写一个完整的框架仅仅2千行代码是非常困难的，但是我们可以模拟一个简单的框架，功能包括实现一个简单的 IoC 容器和基本的 MVC 框架。

下面是一个简单的模拟框架的实现，代码量约为 2 千行：

1. 实现 IoC 容器

public class BeanDefinition {
    private Class<?> beanClass;
    private Map<String, Object> properties;

    // getter & setter
}

public class BeanFactory {
    private Map<String, BeanDefinition> beanDefinitions;
    private Map<String, Object> singletonBeans;

    public BeanFactory() {
        beanDefinitions = new HashMap<>();
        singletonBeans = new HashMap<>();
    }

    public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition) {
        beanDefinitions.put(beanName, beanDefinition);
    }

    public Object getBean(String beanName) throws Exception {
        Object bean = singletonBeans.get(beanName);
        if (bean != null) {
            return bean;
        }

        BeanDefinition beanDefinition = beanDefinitions.get(beanName);
        if (beanDefinition == null) {
            throw new Exception("Bean not found: " + beanName);
        }

        Class<?> beanClass = beanDefinition.getBeanClass();
        bean = beanClass.getDeclaredConstructor().newInstance();

        Map<String, Object> properties = beanDefinition.getProperties();
        for (Field field : beanClass.getDeclaredFields()) {
            String propertyName = field.getName();
            Object propertyValue = properties.get(propertyName);
            if (propertyValue != null) {
                field.setAccessible(true);
                field.set(bean, propertyValue);
            }
        }

        singletonBeans.put(beanName, bean);
        return bean;
    }
}

2. 实现 MVC 框架

public interface HandlerAdapter {
    boolean supports(Object handler);
    ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception;
}

public class SimpleControllerHandlerAdapter implements HandlerAdapter {
    @Override
    public boolean supports(Object handler) {
        return handler instanceof SimpleController;
    }

    @Override
    public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
        return ((SimpleController) handler).handleRequest(request, response);
    }
}

public interface View {
    void render(Map<String, ?> model, HttpServletRequest request, HttpServletResponse response) throws Exception;
}

public class JspView implements View {
    private String viewName;

    public JspView(String viewName) {
        this.viewName = viewName;
    }

    @Override
    public void render(Map<String, ?> model, HttpServletRequest request, HttpServletResponse response) throws Exception {
        request.setAttribute("model", model);
        request.getRequestDispatcher("/WEB-INF/views/" + viewName + ".jsp").forward(request, response);
    }
}

public class ModelAndView {
    private View view;
    private Map<String, ?> model;

    public ModelAndView(View view, Map<String, ?> model) {
        this.view = view;
        this.model = model;
    }

    public View getView() {
        return view;
    }

    public Map<String, ?> getModel() {
        return model;
    }
}

public interface Controller {
}

public interface SimpleController extends Controller {
    ModelAndView handleRequest(HttpServletRequest request, HttpServletResponse response) throws Exception;
}

public class SimpleUrlHandlerMapping {
    private Map<String, Object> urlHandlers;

    public SimpleUrlHandlerMapping() {
        urlHandlers = new HashMap<>();
    }

    public void registerHandler(String urlPath, Object handler) {
        urlHandlers.put(urlPath, handler);
    }

    public Object getHandler(HttpServletRequest request) {
        String urlPath = request.getRequestURI();
        return urlHandlers.get(urlPath);
    }
}

public class DispatcherServlet extends HttpServlet {
    private BeanFactory beanFactory;
    private SimpleUrlHandlerMapping urlHandlerMapping;
    private List<HandlerAdapter> handlerAdapters;

    public DispatcherServlet() {
        beanFactory = new BeanFactory();
        urlHandlerMapping = new SimpleUrlHandlerMapping();
        handlerAdapters = new ArrayList<>();
        handlerAdapters.add(new SimpleControllerHandlerAdapter());
    }

    @Override
    public void init() throws ServletException {
        BeanDefinition controllerBeanDefinition = new BeanDefinition();
        controllerBeanDefinition.setBeanClass(MyController.class);
        beanFactory.registerBeanDefinition("myController", controllerBeanDefinition);
    }

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        try {
            Object handler = urlHandlerMapping.getHandler(request);
            if (handler != null) {
                HandlerAdapter handlerAdapter = getHandlerAdapter(handler);
                if (handlerAdapter != null) {
                    ModelAndView modelAndView = handlerAdapter.handle(request, response, handler);
                    if (modelAndView != null) {
                        View view = modelAndView.getView();
                        Map<String, ?> model = modelAndView.getModel();
                        view.render(model, request, response);
                    }
                }
            } else {
                response.sendError(HttpServletResponse.SC_NOT_FOUND);
            }
        } catch (Exception e) {
            throw new ServletException(e);
        }
    }

    private HandlerAdapter getHandlerAdapter(Object handler) {
        for (HandlerAdapter handlerAdapter : handlerAdapters) {
            if (handlerAdapter.supports(handler)) {
                return handlerAdapter;
            }
        }
        return null;
    }
}

public class MyController implements SimpleController {
    @Override
    public ModelAndView handleRequest(HttpServletRequest request, HttpServletResponse response) throws Exception {
        Map<String, Object> model = new HashMap<>();
        model.put("message", "Hello, World!");
        return new ModelAndView(new JspView("hello"), model);
    }
}

上面的代码实现了一个简单的 IoC 容器和基本的 MVC 框架，其中 IoC 容器实现了 BeanDefinition 和 BeanFactory，MVC 框架实现了 HandlerAdapter、View、ModelAndView、SimpleController、Controller 和 SimpleUrlHandlerMapping 等核心组件。虽然这个框架非常简单，但是可以帮助初学者了解一个框架的基本组成和实现方式。