/**
     * 单表多条查询，将查询到的多条记录传入一个对象，然后再将这些存入一个集合中，返回这个集合
     * @param sql 传入对应的sql查询语句
     * @param parameters 传入对应的占位符的值
     * @return 返回查询到的记录转化成的对象的集合
     */
    //Object...parameters是sql语句中对应的占位符的值，是一个不定长可变参数，我们需要写一个函数来获取他
    public List<E> list(String sql,Object...parameters) {
        Connection conn = null;
        PreparedStatement st = null;
        ResultSet rs = null;
        List<E> list = new ArrayList<>();
        
        try {
            conn = JdbcUtil.getConnection();
            st = conn.prepareStatement(sql);
            setParameters(st, parameters);
            rs = st.executeQuery();
            while(rs.next()) {
        //将获取到的结果集存入一个对象中，这个我们也单独写一个函数来实现
                E obj = oneRowToObject(rs);
        //然后将对象存入一个集合中返回
                list.add(obj);
            }          
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            JdbcUtil.closeAll(rs, st, conn);
        }  
        return list;
    }

/**
 * 设置占位符
 * @param st 预处理
 * @param parameters 占位符数组
     * @return 返回存储占位符对应的对象的数组
 */
 private void setParameters(PreparedStatement st, Object[] parameters) {
        //判断是否有结果集，结果集中是否有记录
 if(parameters!=null&&parameters.length>0) {
  for(int i=0;i<parameters.length;i++) {
  try {
   st.setObject(i+1,parameters[i] );
  } catch (SQLException e) {
   e.printStackTrace();
  }
  }
 }
 }

* 把得到的一列数据存入到一个对象中
     * @param rs
     * @return
     * @throws InstantiationException
     * @throws IllegalAccessException
     * @throws SQLException
     * @throws NoSuchMethodException
     * @throws SecurityException
     * @throws IllegalArgumentException
     * @throws InvocationTargetException
     */
    @SuppressWarnings("unchecked")
    private E oneRowToObject(ResultSet rs) throws InstantiationException, IllegalAccessException, SQLException, NoSuchMethodException, SecurityException, IllegalArgumentException, InvocationTargetException {
        E obj;
        obj=(E) cls.newInstance();
        //获取结果集元数据(获取此 ResultSet 对象的列的编号、类型和属性。)
        ResultSetMetaData rd=rs.getMetaData();
        for (int i = 0; i < rd.getColumnCount(); i++) {
            //获取列名
            String columnName=rd.getColumnLabel(i+1);
            //组合方法名
            String methodName="set"+columnName.substring(0, 1).toUpperCase()+columnName.substring(1);
            //获取列类型
            int columnType=rd.getColumnType(i+1);
            Method method=null;
            switch(columnType) {
            case java.sql.Types.VARCHAR:
            case java.sql.Types.CHAR:
                method=cls.getMethod(methodName, String.class);
                if(method!=null) {
                    method.invoke(obj, rs.getString(columnName));  
                }
                break;         
            case java.sql.Types.INTEGER:
            case java.sql.Types.SMALLINT:
                method=cls.getMethod(methodName, int.class);
                if(method!=null) {
                    method.invoke(obj, rs.getInt(columnName)); 
                }
                break;
            case java.sql.Types.BIGINT:
                method=cls.getMethod(methodName, long.class);
                if(method!=null) {
                    method.invoke(obj, rs.getLong(columnName));
                }
                break;
            case java.sql.Types.DATE:
            case java.sql.Types.TIMESTAMP:
                try {
                    method=cls.getMethod(methodName, Date.class);
                    if(method!=null) {                     
                        method.invoke(obj, rs.getTimestamp(columnName));   
                    }
                } catch(Exception e) {
                    method=cls.getMethod(methodName, String.class);
                    if(method!=null) {
                        method.invoke(obj, rs.getString(columnName));  
                    }
                }              
                break;
            case java.sql.Types.DECIMAL:
                method=cls.getMethod(methodName, BigDecimal.class);
                if(method!=null) {
                    method.invoke(obj, rs.getBigDecimal(columnName));  
                }
                break;
            case java.sql.Types.DOUBLE:
            case java.sql.Types.NUMERIC:
                method=cls.getMethod(methodName, double.class);
                if(method!=null) {
                    method.invoke(obj, rs.getDouble(columnName));  
                }
                break;
            case java.sql.Types.BIT:
                method=cls.getMethod(methodName, boolean.class);
                if(method!=null) {
                    method.invoke(obj, rs.getBoolean(columnName)); 
                }
                break;
            default:
                break;
            }  
        }
        return obj;
    }  

public class test {
 public static void main(String[] args) throws InstantiationException, IllegalAccessException, NoSuchMethodException, SecurityException, IllegalArgumentException, InvocationTargetException, SQLException, IntrospectionException {
 UserDao userdao = new UserDao();
        List<User> list=userdao.list("select * from user");
 System.out.println("uid\t"+"uname\t"+"state\t"+"flag");
 for (User user : list) {
  System.out.println(user.getUid()+"\t"+user.getUname()+"\t"+user.getState()+"\t"+user.getFlag());
 }
}
}