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。