javaweb 分页

分页在javaweb当中是一个很重要的内容，我们必须要去掌握。
在这里，具体介绍一下分页的代码实现。我们使用mysql数据库，c3p0连接池和最原始的servlet来实现分页操作。（这里没有设计接口，标准开发是需要设计相应接口的）
首先，需要引入jar包，包括mysql的jar包，c3p0的jar包，jtsl jar包，dbutils jar文件，我们用dbutils组件来对数据库进行操作。
首先配置c3p0连接池的xml文件。

<?xml version="1.0" encoding="UTF-8" ?><c3p0-config>    <default-config>         <property name="jdbcUrl">jdbc:mysql://127.0.0.1:3306/jdbc_demo</property>        <property name="driverClass">com.mysql.jdbc.Driver</property>        <property name="user">root</property>        <property name="password">root</property>        <property name="acquireIncrement">3</property>        <property name="initialPoolSize">10</property>        <property name="minPoolSize">2</property>        <property name="maxPoolSize">10</property>    </default-config></c3p0-config>

然后写一个工具类用于获取数据库的连接

public class JdbcUtils {    private static DataSource dataSource;    static {        dataSource = new ComboPooledDataSource();    }    public static QueryRunner getQueryRunner(){        return new QueryRunner(dataSource);    }}

配置工作完成，接下来创建pagebean类，用于存放分页需要用到的各种信息，在这里需要使用泛型类，这样任何类型的数据都可以用这个类来进行封装。

package entity;import java.util.List;public class PageBean<T> {    private int currentPage = 1; // 当前页 默认第一页    private int pageCount = 4; // 每页显示的行数 默认显示4行    private int totalCount; // 总记录数   totalPage/pageCount 或 totalPage/pageCount+1    private int totalPage; // 总页数    private List<T> pageData; // 分页查询到的数据    public int getCurrentPage() {        return currentPage;    }    public void setCurrentPage(int currentPage) {        this.currentPage = currentPage;    }    public int getPageCount() {        return pageCount;    }    public void setPageCount(int pageCount) {        this.pageCount = pageCount;    }    public int getTotalCount() {        return totalCount;    }    public void setTotalCount(int totalCount) {        this.totalCount = totalCount;    }    public int getTotalPage() {        if(totalCount % pageCount == 0){            totalPage = totalCount / pageCount;        }else{            totalPage = totalCount / pageCount + 1;        }        return totalPage;    }    public void setTotalPage(int totalPage) {        this.totalPage = totalPage;    }    public List<T> getPageData() {        return pageData;    }    public void setPageData(List<T> pageData) {        this.pageData = pageData;    }}

设置实体类，在这里以Employee类为例，需要注意的一点，属性名称必须和数据库一致，因为我们使用了dbutils组件。

package entity;public class Employee {    private int empId;    private String empName;    public int getEmpId() {        return empId;    }    public void setEmpId(int empId) {        this.empId = empId;    }    public String getEmpName() {        return empName;    }    public void setEmpName(String empName) {        this.empName = empName;    }}

下面进行dao的开发，创建EmployeeDao，这里主要去做的事情就是往pagebean里面封装数据

public class EmployeeDao {    public void getAll(PageBean<Employee> pb) {        //查询总记录数，设置到pb对象        int totalCount = getTotalCount();        pb.setTotalCount(totalCount);        //当前页为首页，点击上一页会出现问题，在这里判断，末页同理        if(pb.getCurrentPage() <= 0){            pb.setCurrentPage(1);        }else if(pb.getCurrentPage()>pb.getTotalPage()){            pb.setCurrentPage(pb.getTotalPage());        }        //获取当前页，计算查询起始行，返回的行数        int currentPage = pb.getCurrentPage();        int start = (currentPage - 1) * pb.getPageCount();        int end = pb.getPageCount();        //分页查询数据，把查到的数据设置到pb对象        String sql = "select * from employee limit ?,?";        QueryRunner qr = JdbcUtils.getQueryRunner();        try {            //当前页的数据            List<Employee> list = qr.query(sql, new BeanListHandler<Employee>(Employee.class), start,end);            pb.setPageData(list);        } catch (Exception e) {            throw new RuntimeException(e);        }    }    public int getTotalCount() {        String sql = "select count(*) from employee";        QueryRunner qr = JdbcUtils.getQueryRunner();        try {            Long count = qr.query(sql, new ScalarHandler<Long>());            return count.intValue();        } catch (Exception e) {            throw new RuntimeException(e);        }    }}

接下来进行service的开发，对dao里面的方法进行调用

public class EmployeeService {    private EmployeeDao employeeDao = new EmployeeDao();     public void getAll(PageBean<Employee> pb){        try {            employeeDao.getAll(pb);        } catch (Exception e) {            e.printStackTrace();        }    }}

然后进行servlet开发，需要注意，首次访问没有任何，需要判断当前页是否存在，如果不存在，则设置当前页为1

public class EmployeeServlet extends HttpServlet {    private static final long serialVersionUID = 1L;    private EmployeeService employeeService = new EmployeeService();    @Override    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {        //获取当前页        try {            String currentPage =req.getParameter("currentPage");            //判断            if(currentPage == null || "".equals(currentPage) ){                currentPage = "1";            }            int realCurrentPage = Integer.parseInt(currentPage);            //创建pageBean对象，设置当前页参数，传入service            PageBean<Employee> pageBean = new PageBean<Employee>();            pageBean.setCurrentPage(realCurrentPage);            employeeService.getAll(pageBean);            //保存pageBean到request域对象中            req.setAttribute("pageBean", pageBean);            //转发            req.getRequestDispatcher("/list.jsp").forward(req, resp);        } catch (Exception e) {            e.printStackTrace();        }    }    @Override    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {        this.doGet(req, resp);    }}

最后进行jsp页面的编写

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%><%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%><!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"><html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>分页</title></head><body>    <table align="center" border="1" cellpadding="5" cellspacing="0" width="80%">        <tr>            <td>编号</td>            <td>员工编号</td>            <td>员工姓名</td>        </tr>        <c:choose>            <c:when test="${not empty requestScope.pageBean.pageData }">                <c:forEach var="employee" items="${requestScope.pageBean.pageData }" varStatus="vs">                <tr>                        <td>${vs.count }</td>                        <td>${employee.empId }</td>                        <td>${employee.empName }</td>                </tr>                </c:forEach>            </c:when>            <c:otherwise>                <tr>                    <td colspan="3">对不起，没有你要找的数据</td>                </tr>            </c:otherwise>        </c:choose>        <tr>            <td colspan="3" align="center">                当前${requestScope.pageBean.currentPage }/${requestScope.pageBean.totalPage }页 &nbsp; &nbsp; &nbsp; &nbsp;                <a href="${pageContext.request.contextPath }/employeeServlet?currentPage=1">首页</a>                <a href="${pageContext.request.contextPath }/employeeServlet?currentPage=${requestScope.pageBean.currentPage -1} ">上一页</a>                <a href="${pageContext.request.contextPath }/employeeServlet?currentPage=${requestScope.pageBean.currentPage+1}">下一页</a>                <a href="${pageContext.request.contextPath }/employeeServlet?currentPage=${requestScope.pageBean.totalPage}">末页</a>            </td>        </tr>    </table></body></html>

ok，大功告成。