public void ajaxValidateEmail(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException
{
    //获取邮箱
    String email = req.getParameter("email");
    //通过service得到校验结果
    boolean b = userService.ajaxValidateEmail(email);
    //发给客户端
    PrintWriter out = resp.getWriter();
    out.print(b);
}

/**
 * sql语句条件表达式
 */
public class Expression {
    // 数据库字段名
    private String name;
    // 符号（=, is null, is not null, like）
    private String operator;
    // 值
    private String value;
}

/**
 * 分页Bean，会在各层之间传递
 */
public class PageBean {
    private int pageCode; //当前页码
    private int totalRecord; //总记录数
    private int pageSize; //每页记录数
    private int totalPage; //页数
    private String url; //请求路径和参数
    private List beanList; // 结果列表
}

/**
 * 通用查询方法
 * @param expressionList sql语句的where条件
 * @param pageCode 页码
 */
private PageBean findByCriteria(List<Expression> expressionList, int pageCode) throws SQLException {
    int pageSize = 10; //每页记录数
    //通过expressionList来生成where子句
    StringBuilder whereSql = new StringBuilder("where 1=1");
    //对应sql语句中?的值
    List<Object> params = new ArrayList<Object>();
    //给where语句添加条件
    for(Expression expression : expressionList)
    {
        whereSql.append(" and ").append(expression.getName()).append(" ")
                .append(expression.getOperator()).append(" ");
        if(!expression.getOperator().equals("is null"))
        {
            whereSql.append("?");
            params.add(expression.getValue());
        }
    }

    String sql = "select count(*) from book " + whereSql.toString();
    Number number = qr.query(sql, new ScalarHandler<>(), params.toArray());
    int totalRecord = number.intValue(); //总记录数

    sql = "select * from book " + whereSql.toString() + " limit ?,?";
    params.add((pageCode-1) * pageSize); //当前页首行记录的下标
    params.add(pageSize); //查询行数
    List<Book> bookList = qr.query(sql, new BeanListHandler<Book>(Book.class), params.toArray());

    //创建pageBean，设置参数
    PageBean pageBean = new PageBean();
    //还没设置pageBean的url属性
    pageBean.setPageCode(pageCode);
    pageBean.setTotalRecord(totalRecord);
    pageBean.setPageSize(pageSize);
    pageBean.setBeanList(bookList);

    return pageBean;
}