尚硅谷之JDBC

         //通用的查询方法之一：查询一行，即一个对象

         /**

          * 执行查询操作的SQL语句，SQL可以带参数(?)

          * @param clazz Class 查询的结果需要封装的实体的Class类型，例如：学生Student，商品Goods,订单Order

          * @param sql String 执行查询操作的SQL语句

          * @param args Object... 对应的每个?设置的值，顺序要与?对应

          * @return T 封装了查询结果的实体

          * @throws Exception

          */

         public static <T> T get(Class<T> clazz,String sql,Object... args)throws Exception{

                   //1、注册驱动

                   //2、获取连接

                   Connection conn = JDBCUtils.getConnection();

                   //3、对sql进行预编译

                   PreparedStatement pst = conn.prepareStatement(sql);

                   //4、设置？

                   if(args!=null && args.length>0){

                            //数组的下标是从0开始，？的编号是1开始

                            for (int i = 0; i < args.length; i++) {

                                     pst.setObject(i+1, args[i]);

                            }

                   }

                   //5、查询

                   ResultSet rs = pst.executeQuery();

                   //获取查询的结果集的元数据信息

                   ResultSetMetaData rsmd = rs.getMetaData();

                   //这是查询的结果集中，一共有几列

                   int count = rsmd.getColumnCount();

                   T t = clazz.newInstance();//要求这个Javabean类型必须有无参构造

                   while(rs.next()){

                            /*

                             * 问题？

                             * （1）sql语句中查询了几列，每一列是什么属性

                             * （2）怎么把这个值设置到Javabean的属性中

                             */

                            //循环每一行有几列

                            for (int i = 0; i < count; i++) {

                                     //第几列的名称

//                                  String columnName = rsmd.getColumnName(i+1);

                                     //如果sql中没有取别名，那么就是列名，如果有别名，返回的是别名

                                     String fieldName = rsmd.getColumnLabel(i+1);

                                     //该列的值

//                                  Object value = rs.getObject(columnName);

                                     Object value = rs.getObject(fieldName);

                                     //设置obj对象的某个属性中

                                     Field field = clazz.getDeclaredField(fieldName);//JavaBean的属性名

                                     field.setAccessible(true);

                                     field.set(t, value);

                            }

                   }

                   //5、释放资源

                   //5、释放资源

                   JDBCUtils.closeQuietly(rs,pst,conn);

                   return t;

         }