feat: improve registration validation

src/main/java/com/openisle/controller/GlobalExceptionHandler.java

@@ -3,12 +3,19 @@ package com.openisle.controller;
 import org.springframework.http.ResponseEntity;
 import org.springframework.web.bind.annotation.ExceptionHandler;
 import org.springframework.web.bind.annotation.RestControllerAdvice;
+import com.openisle.exception.FieldException;
 
 import java.util.Map;
 
 @RestControllerAdvice
 public class GlobalExceptionHandler {
 
+    @ExceptionHandler(FieldException.class)
+    public ResponseEntity<?> handleFieldException(FieldException ex) {
+        return ResponseEntity.badRequest()
+                .body(Map.of("error", ex.getMessage(), "field", ex.getField()));
+    }
+
     @ExceptionHandler(Exception.class)
     public ResponseEntity<?> handleException(Exception ex) {
         return ResponseEntity.badRequest().body(Map.of("error", ex.getMessage()));

src/main/java/com/openisle/exception/FieldException.java

@@ -0,0 +1,19 @@
+package com.openisle.exception;
+
+import lombok.Getter;
+
+/**
+ * Exception carrying a target field name. Useful for reporting
+ * validation errors to clients so they can display feedback near
+ * the appropriate input element.
+ */
+@Getter
+public class FieldException extends RuntimeException {
+    private final String field;
+
+    public FieldException(String field, String message) {
+        super(message);
+        this.field = field;
+    }
+}
+

src/main/java/com/openisle/service/PasswordValidator.java

@@ -1,6 +1,7 @@
 package com.openisle.service;
 
 import com.openisle.model.PasswordStrength;
+import com.openisle.exception.FieldException;
 import org.springframework.beans.factory.annotation.Value;
 import org.springframework.stereotype.Service;
 
@@ -14,7 +15,7 @@ public class PasswordValidator {
 
     public void validate(String password) {
         if (password == null || password.isEmpty()) {
-            throw new IllegalArgumentException("Password cannot be empty");
+            throw new FieldException("password", "Password cannot be empty");
         }
         switch (strength) {
             case MEDIUM:
@@ -31,34 +32,34 @@ public class PasswordValidator {
 
     private void checkLow(String password) {
         if (password.length() < 6) {
-            throw new IllegalArgumentException("Password must be at least 6 characters long");
+            throw new FieldException("password", "Password must be at least 6 characters long");
         }
     }
 
     private void checkMedium(String password) {
         if (password.length() < 8) {
-            throw new IllegalArgumentException("Password must be at least 8 characters long");
+            throw new FieldException("password", "Password must be at least 8 characters long");
         }
         if (!password.matches(".*[A-Za-z].*") || !password.matches(".*\\d.*")) {
-            throw new IllegalArgumentException("Password must contain letters and numbers");
+            throw new FieldException("password", "Password must contain letters and numbers");
         }
     }
 
     private void checkHigh(String password) {
         if (password.length() < 12) {
-            throw new IllegalArgumentException("Password must be at least 12 characters long");
+            throw new FieldException("password", "Password must be at least 12 characters long");
         }
         if (!password.matches(".*[A-Z].*")) {
-            throw new IllegalArgumentException("Password must contain uppercase letters");
+            throw new FieldException("password", "Password must contain uppercase letters");
         }
         if (!password.matches(".*[a-z].*")) {
-            throw new IllegalArgumentException("Password must contain lowercase letters");
+            throw new FieldException("password", "Password must contain lowercase letters");
         }
         if (!password.matches(".*\\d.*")) {
-            throw new IllegalArgumentException("Password must contain numbers");
+            throw new FieldException("password", "Password must contain numbers");
         }
         if (!password.matches(".*[^A-Za-z0-9].*")) {
-            throw new IllegalArgumentException("Password must contain special characters");
+            throw new FieldException("password", "Password must contain special characters");
         }
     }
 }

src/main/java/com/openisle/service/UserService.java

@@ -3,6 +3,7 @@ package com.openisle.service;
 import com.openisle.model.User;
 import com.openisle.model.Role;
 import com.openisle.service.PasswordValidator;
+import com.openisle.exception.FieldException;
 import com.openisle.repository.UserRepository;
 import lombok.RequiredArgsConstructor;
 import org.springframework.security.crypto.bcrypt.BCryptPasswordEncoder;
@@ -26,7 +27,7 @@ public class UserService {
         if (byUsername.isPresent()) {
             User u = byUsername.get();
             if (u.isVerified()) {                           // 已验证 → 直接拒绝
-                throw new IllegalStateException("User name already exists");
+                throw new FieldException("username", "User name already exists");
             }
             // 未验证 → 允许“重注册”：覆盖必要字段并重新发验证码
             u.setEmail(email);                              // 若不允许改邮箱可去掉
@@ -40,7 +41,7 @@ public class UserService {
         if (byEmail.isPresent()) {
             User u = byEmail.get();
             if (u.isVerified()) {                           // 已验证 → 直接拒绝
-                throw new IllegalStateException("User email already exists");
+                throw new FieldException("email", "User email already exists");
             }
             // 未验证 → 允许“重注册”
             u.setUsername(username);                        // 若不允许改用户名可去掉