refactor: make factory class constructor private

src/dao/DAOFactory.java

@@ -12,6 +12,13 @@ public class DAOFactory {
     private static final TeacherDAO teacherDAO = new TeacherDAOImpl();
     private static final LeaveRequestDAO leaveRequestDAO = new LeaveRequestDAOImpl();
 
+    /**
+     * 私有构造函数，防止实例化
+     */
+    private DAOFactory() {
+        // 私有构造函数，阻止实例化
+    }
+
     /**
      * 获取StudentDAO实例
      * @return StudentDAO实例

src/service/ServiceFactory.java

@@ -8,6 +8,13 @@ public class ServiceFactory {
     private static final TeacherService teacherService = new TeacherService();
     private static final LeaveRequestService leaveRequestService = new LeaveRequestService();
 
+    /**
+     * 私有构造函数，防止实例化
+     */
+    private ServiceFactory() {
+        // 私有构造函数，阻止实例化
+    }
+
     /**
      * 获取StudentService实例
      * @return StudentService实例