[#2]✨Feat: 로그인 인증 인가 코드 추가

src/main/java/univ/yesummit/global/auth/util/SecurityUtil.java

@@ -0,0 +1,27 @@
+package univ.yesummit.global.auth.util;
+
+import org.springframework.security.core.Authentication;
+import org.springframework.security.core.context.SecurityContextHolder;
+import org.springframework.security.core.userdetails.UserDetails;
+import org.springframework.security.oauth2.core.user.OAuth2User;
+
+public class SecurityUtil {
+
+    public static String getLoginUsername() {
+        Authentication authentication = SecurityContextHolder.getContext().getAuthentication();
+        if (authentication == null || !authentication.isAuthenticated()) {
+            throw new IllegalStateException("User is not authenticated");
+        }
+
+        Object principal = authentication.getPrincipal();
+        if (principal instanceof UserDetails) {
+            return ((UserDetails) principal).getUsername();
+        } else if (principal instanceof OAuth2User) {
+            return ((OAuth2User) principal).getAttribute("name"); // 필요에 따라 변경 가능
+        } else if (principal instanceof String) {
+            return (String) principal;
+        }
+
+        throw new IllegalStateException("Unknown principal type");
+    }
+}