优化目录结构

backend/src/main/java/com/openisle/service/UsernameValidator.java

@@ -0,0 +1,22 @@
+package com.openisle.service;
+
+import com.openisle.exception.FieldException;
+import org.springframework.stereotype.Service;
+
+/**
+ * Simple validator for usernames.
+ */
+@Service
+public class UsernameValidator {
+    /**
+     * Validate the username string.
+     *
+     * @param username the username to validate
+     */
+    public void validate(String username) {
+        if (username == null || username.isEmpty()) {
+            throw new FieldException("username", "Username cannot be empty");
+        }
+    }
+}
+