Added custom annoation to confirm the user's ownership of the shortened link.

src/main/java/com/linkurlshorter/urlshortener/link/UrlShortValidator.java

@@ -12,7 +12,7 @@
  */
 @Documented
 @Retention(RetentionPolicy.RUNTIME)
-@Target({ElementType.FIELD, ElementType.METHOD, ElementType.ANNOTATION_TYPE})
+@Target({ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER, ElementType.ANNOTATION_TYPE})
 @Constraint(validatedBy = UrlShortValidatorImpl.class)
 public @interface UrlShortValidator {
 

src/main/java/com/linkurlshorter/urlshortener/user/ShortenedLinkOwnerValidation.java

@@ -0,0 +1,41 @@
+package com.linkurlshorter.urlshortener.user;
+
+import jakarta.validation.Constraint;
+import jakarta.validation.Payload;
+
+import java.lang.annotation.*;
+
+/**
+ * The annotation used to validate user ownership of the shortened link.
+ * The validation is performed using {@link ShortenedLinkOwnerValidationImpl}.
+ *
+ * @author Vlas Potoskyi
+ */
+@Documented
+@Retention(RetentionPolicy.RUNTIME)
+@Target({ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER, ElementType.ANNOTATION_TYPE})
+@Constraint(validatedBy = ShortenedLinkOwnerValidationImpl.class)
+public @interface ShortenedLinkOwnerValidation {
+    /**
+     * The message to be used to inform about the failed validation.
+     * Default: "".
+     *
+     * @return Error message.
+     */
+    String message() default "";
+
+    /**
+     * Groups to which this constraint belongs. Default: empty array.
+     *
+     * @return Constraint groups.
+     */
+    Class<?>[] groups() default {};
+
+    /**
+     * Parameters that can be used to configure the constraint.
+     * Default: an empty array.
+     *
+     * @return The parameters of the constraint.
+     */
+    Class<? extends Payload>[] payload() default {};
+}

src/main/java/com/linkurlshorter/urlshortener/user/ShortenedLinkOwnerValidationImpl.java

@@ -0,0 +1,54 @@
+package com.linkurlshorter.urlshortener.user;
+
+import com.linkurlshorter.urlshortener.link.LinkService;
+import jakarta.validation.ConstraintValidator;
+import jakarta.validation.ConstraintValidatorContext;
+import lombok.RequiredArgsConstructor;
+import org.springframework.security.core.Authentication;
+import org.springframework.security.core.context.SecurityContextHolder;
+
+import java.util.Objects;
+import java.util.UUID;
+
+/**
+ * Implementation of user ownership verification for a shortened link.
+ * The check is performed using the link service {@link LinkService} and the user service {@link UserService}.
+ */
+@RequiredArgsConstructor
+public class ShortenedLinkOwnerValidationImpl implements ConstraintValidator<ShortenedLinkOwnerValidation, String> {
+    private final LinkService linkService;
+    private final UserService userService;
+
+    /**
+     * Checks user ownership of the shortened link.
+     *
+     * @param shortLink The shortened link to check ownership for.
+     * @param context   The context of the constraint check.
+     * @return true if the current user owns the link; false otherwise.
+     */
+    @Override
+    public boolean isValid(String shortLink, ConstraintValidatorContext context) {
+        if (shortLink == null || shortLink.isEmpty()) {
+            context.buildConstraintViolationWithTemplate("Invalid short link!");
+            return false;
+        }
+        UUID currentUserId = getCurrentUserId();
+        UUID linkUserId = linkService.findByShortLink(shortLink).getId();
+        if (!Objects.equals(currentUserId, linkUserId)) {
+            context.buildConstraintViolationWithTemplate("You cannot do this!");
+            return false;
+        }
+        return true;
+    }
+
+    /**
+     * Gets the ID of the current user.
+     *
+     * @return The ID of the current user.
+     */
+    private UUID getCurrentUserId() {
+        Authentication authentication = SecurityContextHolder.getContext().getAuthentication();
+        String userEmail = authentication.getName();
+        return userService.findByEmail(userEmail).getId();
+    }
+}