DispatcherServlet.java

@WebServlet(urlPatterns = "/*", loadOnStartup = 1)
public class DispatcherServlet extends HttpServlet {

    private Map<String, Method> uriMappings = new HashMap<>();

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException {
        System.out.println("Getting request for " + req.getRequestURI());
        var uri = req.getRequestURI();

        if(! uriMappings.containsKey(uri)){
            resp.sendError(404, "no mapping found for request uri " + uri);
            return;
        }

        var method = getMappingForUri(uri);
        try {
            // getting new instancea
            var instance = method.getDeclaringClass().newInstance();

            // getting params
            var params = req.getParameterMap();

            // calling method with params if needed
            Object result;
            if(method.getParameterCount() > 0 ){
                result = method.invoke(instance, params);
            }
            else {
                result = method.invoke(instance);
            }

            // sending response
            resp.getWriter().print(result.toString());
        } catch (InstantiationException | IllegalAccessException e) {
            // default exception handling
            e.printStackTrace();
        }
        catch( InvocationTargetException e){
            // when getting an exception, sending it to the client
            resp.sendError(500, "exception when calling method " + method.getName() + " : " + e.getCause().getMessage());
        }
    }

    @Override
    public void init(ServletConfig config) throws ServletException {
        super.init(config);
        // on enregistre notre controller au démarrage de la servlet
        this.registerController(HelloController.class);
    }

    protected void registerController(Class controllerClass){
        System.out.println("Analysing class " + controllerClass.getName());

        // checking that the class is a controller
        if(controllerClass.getAnnotation(Controller.class) == null){
            throw new IllegalArgumentException();
        }

        for(Method method : controllerClass.getDeclaredMethods()){
            this.registerMethod(method);
        }
    }

    protected void registerMethod(Method method) {
        System.out.println("Registering method " + method.getName());

        var requestMapping = method.getAnnotation(RequestMapping.class);

        if(requestMapping == null){
            // nothing to do, no annotation found on method
            return;
        }

        if(method.getReturnType().equals(Void.TYPE)){
            // nothing to do, method returns void
            return;
        }

        var uri = requestMapping.uri();

        this.uriMappings.put(uri, method);
    }

    protected Map<String, Method> getMappings(){
        return this.uriMappings;
    }

    protected Method getMappingForUri(String uri){
        return this.uriMappings.get(uri);
    }
}