Day-94,95 Understanding Servlets, JSP, Annotations and CORS

Servlets: Backend Logic and Request Handling

A Servlet is a Java program that runs on a server and handles HTTP requests and responses. Servlets are part of the Java EE (Jakarta EE) ecosystem and are primarily responsible for backend logic.

• Definition: A Servlet is a Java class that processes requests and generates responses.
• Role: Manages business logic and request handling.

JSP (Java Server Pages): Presentation Layer

JSP is used for the presentation layer of an application.

• Definition: JSP combines HTML and Java code.
• Role: Used to display dynamic content (UI part).
• Difference from Servlets: While Servlets usually handle business logic, JSP focuses on presentation.

Annotations

• Java Annotations provide metadata (example: @override).
• Spring Boot Annotations reduce boilerplate code, handle Dependency Injection (DI), configure REST APIs, ORM, security, and more.

CORS (Cross-Origin Resource Sharing)

CORS is a security feature in browsers that controls cross-origin requests.

• Same-Origin Policy: By default, browsers block requests from different origins.
• Cross-Origin Requests: If your React app runs on port 3000 and Spring Boot backend runs on 8080, the browser blocks the request without CORS configuration.
• Solution: Add @CrossOrigin annotation in your Spring Boot REST controller.
• Prevents malicious cross-site data access.
• Allows safe communication between frontend (React) and backend (Spring Boot).

Example Workflow: React + Spring Boot

1. React app runs at localhost:3000.
2. Spring Boot backend runs at localhost:8080.
3. Without CORS, the browser blocks the request.
4. Adding @CrossOrigin allows cross-origin communication.