CROSS is a system that allows client web applications (such as JavaScript code) loaded on one domain to interact with resources located on a different domain.
Origin: An "Origin" (or source) URL consists of three parts: Protocol (scheme): Such as http or https. Domain (host): Such as a143mk.blogspot.com or localhost. Port (port number): Such as 8080 or 3000. If any of these three parts are different in a request, it is called a Cross-Origin request.
Example:
If your frontend (endpoints.com) sends a request to a backend API (api.com), it is Cross-Origin and will be intercepted by the browser. CORS is a way to relax this security restriction in a controlled manner, allowing legitimate cross-origin communication.

 
import org.springframework.web.bind.annotation.CrossOrigin; import org.springframework.web.bind.annotation.RestController; @RestController @CrossOrigin(origins = "http://localhost:3000") // Example: Allow only this Origin public class MyController { // ... methods }
Configuring CORS globally in Spring Boot is the preferred approach, as it saves you from having to apply the @CrossOrigin annotation to every controller or method and keeps the configuration centralized.
Example: This method defines global CORS settings 

package com.example.demo.config; import org.springframework.context.annotation.Configuration; import org.springframework.web.servlet.config.annotation.CorsRegistry; import org.springframework.web.servlet.config.annotation.WebMvcConfigurer; @Configuration public class CorsConfig implements WebMvcConfigurer { @Override public void addCorsMappings(CorsRegistry registry) { registry.addMapping("/**").allowedOrigins("http://localhost:3000", "https://your-client-app.com") .allowedMethods("GET", "POST", "PUT", "DELETE", "OPTIONS").allowedHeaders("*") .allowCredentials(true).maxAge(3600); } }