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## 📁 스프링 파일 업로드파일 업로드 기능은 웹 애플리케이션 개발에서 빼놓을 수 없는 중요한 요소 중 하나입니다. 이번 포스팅에서는 HTML 폼을 통한 파일 업로드 방식의 기본적인 이해부터 서블릿에서의 접근 방식, 그리고 스프링 프레임워크가 제공하는 강력한 `MultipartFile` 활용법까지 자세히 알아보겠습니다.✅ **1. 파일 업로드, 어떤 방식으로 이루어질까? (`application/x-www-form-urlencoded` vs `multipart/form-data`)**| 구분 | `application/x-www-form-urlencoded` | `multipart/form-data` || :-----------------..

1. Why Type Conversion Matters in SpringSpring often needs to convert data types between String ↔ Object, especially in:@RequestParam, @PathVariable, @ModelAttributeYAML (@Value)View rendering (Thymeleaf)Form input/output2. Converter Interfacepublic interface Converter { T convert(S source);}Generic, bidirectional type conversion (e.g. String ↔ Integer, IpPort ↔ String)Used programmatically o..

1. HTML vs API Exception HandlingHTML-based applications: Showing user-friendly error pages (4xx/5xx) is usually enough.APIs: Clients expect structured JSON with proper status codes and detailed messages—not HTML.2. Spring Boot Default Error Handling (BasicErrorController)Spring Boot uses BasicErrorController to handle all errors via the /error path.It responds with:HTML (if Accept: text/html)JS..

1. Why Error Handling is NeededTo provide user-friendly error pagesTo log error information and handle them appropriatelyAvoid exposing internal server errors (like HTTP Status 500 or 404) to users with default WAS error pages2. Servlet's Basic Error Handling Mechanism1. Throwing Exceptionsthrow new RuntimeException("Something went wrong!");If not caught, the exception propagates to the WAS and ..

1. 📌 Common Goal: Cross-Cutting Concern HandlingBoth Filters and Interceptors are used to handle common logic that should apply across multiple requests or controllers, such as:LoggingAuthentication / AuthorizationError handlingSession validation2. 🚦 Request Flow Comparison  Layer Request PathFilterHTTP Request → WAS → 🔒 Filter → Servlet (DispatcherServlet) → ControllerInterceptorHTTP Request..

Why Do We Need Cookies and Sessions?HTTP is stateless. To track logged-in users, we need a mechanism to store and transfer user identity between client and server. This is where cookies and sessions come in.1. Cookie-Based Login (Client-Side Identity)How It WorksWhen login is successful, the server stores the user's ID in a cookie:Cookie idCookie = new Cookie("memberId", String.valueOf(user.getI..

✅ OverviewBean Validation is a standard technology that allows you to handle field validation logic simply using annotations. The most commonly used implementation is Hibernate Validator. In Spring Boot, the library is automatically registered through the spring-boot-starter-validation dependency, and validation can be applied easily using @Validated or @Valid.✅ Basic Examplepublic class Item { ..

📌 1. Purpose of ValidationEnsure data integrity from client inputs.Enhance user experience by keeping user inputs and showing friendly error messages.📌 2. Validation Flow (1) Type Conversion StageDuring @ModelAttribute binding, type mismatches can occur.Without BindingResult → Spring throws a 400 Bad Request.With BindingResult → Controller is still called, and errors are stored in BindingResul..