Java Servlet 分页 - 《ZetCode 中文系列教程》

分页
Java Servlet
Bootstrap
Java Servlet 分页示例

原文： http://zetcode.com/articles/javaservletpagination/

Java servlet 分页教程显示了如何使用 Java servlet 进行分页。在示例中，Bootstrap 用于 UI。

分页

分页是将内容分为几页的过程。用户具有用于通过特定页面链接访问这些页面的导航界面。导航通常包括上一个/下一个和第一个/最后一个链接。当数据库中有大量数据或一页中显示许多项时，将使用分页。

Java Servlet

Servlet 是 Java 类，可响应特定类型的网络请求-最常见的是 HTTP 请求。 Java servlet 用于创建 Web 应用。它们在 servlet 容器（例如 Tomcat 或 Jetty）中运行。现代 Java Web 开发使用在 servlet 之上构建的框架。

Bootstrap

Bootstrap 是 Twitter 的一个 UI 库，用于创建响应式，移动优先的 Web 应用。

Java Servlet 分页示例

在以下应用中，我们从 MySQL 数据库加载数据并将其显示在表中。有一个导航系统可以遍历数据库表中的所有数据。在将数据显示在表中之前，用户可以选择表将显示多少行。

除了从数据库表中获取数据之外，我们还需要知道数据库表中所有行的数量，每页的记录数以及要在导航中显示的页面数。 SQL 语句可以计算出数据库中所有行的数量。用户以 HTML 格式选择每页的记录数。最后，从其他两个值计算分页中的页数。

countries_mysql.sql

CREATE TABLE Countries(ID BIGINT NOT NULL PRIMARY KEY AUTO_INCREMENT, 
    Name VARCHAR(100), Population INT);
INSERT INTO Countries(Name, Population) VALUES('China', 1382050000);
INSERT INTO Countries(Name, Population) VALUES('India', 1313210000);
INSERT INTO Countries(Name, Population) VALUES('USA', 324666000);
INSERT INTO Countries(Name, Population) VALUES('Indonesia', 260581000);
INSERT INTO Countries(Name, Population) VALUES('Brazil', 207221000);
INSERT INTO Countries(Name, Population) VALUES('Pakistan', 196626000);
INSERT INTO Countries(Name, Population) VALUES('Nigeria', 186988000);
INSERT INTO Countries(Name, Population) VALUES('Bangladesh', 162099000);
INSERT INTO Countries(Name, Population) VALUES('Nigeria', 186988000);
INSERT INTO Countries(Name, Population) VALUES('Russia', 146838000);
INSERT INTO Countries(Name, Population) VALUES('Japan', 126830000);
INSERT INTO Countries(Name, Population) VALUES('Mexico', 122273000);
INSERT INTO Countries(Name, Population) VALUES('Philippines', 103738000);
INSERT INTO Countries(Name, Population) VALUES('Ethiopia', 101853000);
INSERT INTO Countries(Name, Population) VALUES('Vietnam', 92700000);
INSERT INTO Countries(Name, Population) VALUES('Egypt', 92641000);
INSERT INTO Countries(Name, Population) VALUES('Germany', 82800000);
INSERT INTO Countries(Name, Population) VALUES('the Congo', 82243000);
INSERT INTO Countries(Name, Population) VALUES('Iran', 82800000);
INSERT INTO Countries(Name, Population) VALUES('Turkey', 79814000);
INSERT INTO Countries(Name, Population) VALUES('Thailand', 68147000);
INSERT INTO Countries(Name, Population) VALUES('France', 66984000);
INSERT INTO Countries(Name, Population) VALUES('United Kingdom', 60589000);
INSERT INTO Countries(Name, Population) VALUES('South Africa', 55908000);
INSERT INTO Countries(Name, Population) VALUES('Myanmar', 51446000);
INSERT INTO Countries(Name, Population) VALUES('South Korea', 68147000);
INSERT INTO Countries(Name, Population) VALUES('Colombia', 49129000);
INSERT INTO Countries(Name, Population) VALUES('Kenya', 47251000);
INSERT INTO Countries(Name, Population) VALUES('Spain', 46812000);
INSERT INTO Countries(Name, Population) VALUES('Argentina', 43850000);
INSERT INTO Countries(Name, Population) VALUES('Ukraine', 42603000);
INSERT INTO Countries(Name, Population) VALUES('Sudan', 41176000);
INSERT INTO Countries(Name, Population) VALUES('Algeria', 40400000);
INSERT INTO Countries(Name, Population) VALUES('Poland', 38439000);

该 SQL 脚本在 MySQL 中创建Countries表。

$ tree
.
├── nb-configuration.xml
├── pom.xml
└── src
    ├── main
    │   ├── java
    │   │   └── com
    │   │       └── zetcode
    │   │           ├── bean
    │   │           │   └── Country.java
    │   │           ├── service
    │   │           │   ├── CountryService.java
    │   │           │   └── ICountryService.java
    │   │           └── web
    │   │               └── ReadCountries.java
    │   ├── resources
    │   └── webapp
    │       ├── index.html
    │       ├── listCountries.jsp
    │       ├── META-INF
    │       │   └── context.xml
    │       └── WEB-INF
    └── test
        └── java

这是项目结构。

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" 
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 
http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>com.zetcode</groupId>
    <artifactId>JavaServletPagination</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>war</packaging>
    <name>JavaServletPagination</name>
    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <maven.compiler.source>1.8</maven.compiler.source>
        <maven.compiler.target>1.8</maven.compiler.target>
    </properties>
    <dependencies>
        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>javax.servlet-api</artifactId>
            <version>3.1.0</version>
            <scope>provided</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-jdbc</artifactId>
            <version>5.0.2.RELEASE</version>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.45</version>
        </dependency>        
        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>jstl</artifactId>
            <version>1.2</version>
        </dependency>
    </dependencies>
    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-war-plugin</artifactId>
                <version>2.3</version>
                <configuration>
                    <failOnMissingWebXml>false</failOnMissingWebXml>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

这是 Maven POM 文件。 javax.servlet-api工件用于 servlet。 spring-jdbc依赖项用于JdbcTemplate库，该库简化了 Java 中的数据库编程。 mysql-connector-java是 Java 语言的 MySQL 驱动程序。 jstl依赖项为 JSP 页面提供了一些附加功能。 maven-war-plugin负责收集 Web 应用的所有工件依赖项，类和资源，并将它们打包到 Web 应用存档（WAR）中。

context.xml

<?xml version="1.0" encoding="UTF-8"?>
<Context path="/JavaServletPagination"/>

在 Tomcat context.xml文件中，我们定义了上下文路径。它是 Web 应用的名称。

Country.java

package com.zetcode.bean;
public class Country {
    private String name;
    private int population;
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getPopulation() {
        return population;
    }
    public void setPopulation(int population) {
        this.population = population;
    }
}

Country bean 从Countries数据库表中保留一行。

ReadCountries.java

package com.zetcode.web;
import com.zetcode.bean.Country;
import com.zetcode.service.CountryService;
import java.io.IOException;
import java.util.List;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@WebServlet(name = "ReadCountries", urlPatterns = {"/ReadCountries"})
public class ReadCountries extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        int currentPage = Integer.valueOf(request.getParameter("currentPage"));
        int recordsPerPage = Integer.valueOf(request.getParameter("recordsPerPage"));
        CountryService countryService = new CountryService();
        List<Country> countries = countryService.findCountries(currentPage, 
                recordsPerPage);
        request.setAttribute("countries", countries);
        int rows = countryService.getNumberOfRows();
        int nOfPages = rows / recordsPerPage;
        if (nOfPages % recordsPerPage > 0) {
            nOfPages++;
        }
        request.setAttribute("noOfPages", nOfPages);
        request.setAttribute("currentPage", currentPage);
        request.setAttribute("recordsPerPage", recordsPerPage);
        RequestDispatcher dispatcher = request.getRequestDispatcher("listCountries.jsp");
        dispatcher.forward(request, response);
    }
}

ReadCountries Servlet 确定将从请求属性中检索多少数据，并从数据库表中读取指定的行数。

@WebServlet(name = "ReadCountries", urlPatterns = {"/ReadCountries"})

Java 类用@WebServlet注解修饰。它映射到ReadCountries URL 模式。

response.setContentType("text/html;charset=UTF-8");

Servlet 将以 HTML 输出数据，并且数据的编码设置为 UTF-8。

int currentPage = Integer.valueOf(request.getParameter("currentPage"));
int recordsPerPage = Integer.valueOf(request.getParameter("recordsPerPage"));

从请求中我们得到两个重要的值：当前页和每页的记录数。

CountryService countryService = new CountryService();
List<Country> countries = countryService.findCountries(currentPage, 
        recordsPerPage);
request.setAttribute("countries", countries);

CountryService是用于连接到数据库并读取数据的服务类。检索国家列表并将其设置为请求的属性。稍后将由目标 JSP 页面使用。

int rows = countryService.getNumberOfRows();
int nOfPages = rows / recordsPerPage;
if (nOfPages % recordsPerPage > 0) {
    nOfPages++;
}

我们使用getNumberOfRows()服务方法从数据库表中获取所有行的数目。我们计算导航中的页面数。

request.setAttribute("noOfPages", nOfPages);
request.setAttribute("currentPage", currentPage);
request.setAttribute("recordsPerPage", recordsPerPage);

页数，当前页和每页的记录数是我们建立分页所需的值。

RequestDispatcher dispatcher = request.getRequestDispatcher("listCountries.jsp");
dispatcher.forward(request, response);

处理被转发到listCountries.jsp页面。

ICountryService.java

package com.zetcode.service;
import com.zetcode.bean.Country;
import java.util.List;
public interface ICountryService  {
    public List<Country> findCountries(int currentPage, int numOfRecords);
    public int getNumberOfRows();
}

ICountryService包含两种签约方法：findCountries()和getNumberOfRows()。

CountryService.java

package com.zetcode.service;
import com.zetcode.bean.Country;
import java.sql.SQLException;
import java.util.List;
import java.util.logging.Level;
import java.util.logging.Logger;
import org.springframework.jdbc.core.BeanPropertyRowMapper;
import org.springframework.jdbc.core.JdbcTemplate;
import org.springframework.jdbc.datasource.SimpleDriverDataSource;
public class CountryService implements ICountryService {
    @Override
    public List<Country> findCountries(int currentPage, int recordsPerPage)  {
        List<Country> countries = null;
        int start = currentPage * recordsPerPage - recordsPerPage;
        try {
            String sql = "SELECT * FROM Countries LIMIT ?, ?";
            SimpleDriverDataSource ds = new SimpleDriverDataSource();
            ds.setDriver(new com.mysql.jdbc.Driver());
            ds.setUrl("jdbc:mysql://localhost:3306/testdb");
            ds.setUsername("testuser");
            ds.setPassword("test623");
            JdbcTemplate jtm = new JdbcTemplate(ds);
            countries = jtm.query(sql, new Object[] {start, recordsPerPage}, 
                    new BeanPropertyRowMapper(Country.class));
        } catch (SQLException ex) {
            Logger.getLogger(CountryService.class.getName()).log(Level.SEVERE, 
                null, ex);
        }
        return countries;
    }    
    @Override
    public int getNumberOfRows() {
        int numOfRows = 0;
        try {
            String sql = "SELECT COUNT(Id) FROM Countries";
            SimpleDriverDataSource ds = new SimpleDriverDataSource();
            ds.setDriver(new com.mysql.jdbc.Driver());
            ds.setUrl("jdbc:mysql://localhost:3306/testdb");
            ds.setUsername("testuser");
            ds.setPassword("test623");
            JdbcTemplate jtm = new JdbcTemplate(ds);
            numOfRows = jtm.queryForObject(sql, Integer.class);
        } catch (SQLException ex) {
            Logger.getLogger(CountryService.class.getName()).log(Level.SEVERE, 
                null, ex);
        }
        return numOfRows;
    }
}

CountryService包含两种合同方法的实现。

String sql = "SELECT * FROM Countries LIMIT ?, ?";

SQL LIMIT子句用于获取当前页面的行数。

JdbcTemplate jtm = new JdbcTemplate(ds);
countries = jtm.query(sql, new Object[] {start, recordsPerPage}, 
        new BeanPropertyRowMapper(Country.class));

JdbcTemplate用于执行 SQL 语句。在BeanPropertyRowMapper的帮助下，行自动映射到Country bean。

String sql = "SELECT COUNT(Id) FROM Countries";

通过此 SQL 语句，我们从数据库表中获取行数。

index.html

<!DOCTYPE html>
<html>
<head>
    <title>Home page</title>
    <meta charset="UTF-8">
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.6/css/bootstrap.min.css">        
</head>
<body class="m-3">
<h1>Show countries</h1>
<form action="ReadCountries">
    <input type="hidden" name="currentPage" value="1">
    <div class="form-group col-md-4">
        <label for="records">Select records per page:</label>
        <select class="form-control" id="records" name="recordsPerPage"> 
            <option value="5">5</option> 
            <option value="10" selected>10</option>
            <option value="15">15</option>
        </select>
    </div>
    <button type="submit" class="btn btn-primary">Submit</button>
</form>
<script src="https://code.jquery.com/jquery-3.1.1.slim.min.js" ></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/tether/1.4.0/js/tether.min.js" ></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.6/js/bootstrap.min.js" ></script>
</body>
</html>

这是主页。它包含一个 HTML 表单，用于通过select标签选择每页的记录数。该表单使用 Bootstrap 库中的样式类。提交表单后，处理将发送到ReadCountries Servlet。

<input type="hidden" name="currentPage" value="1">

该表单包含一个隐藏的input标记，该标记将currentPage参数设置为 1。

<select class="form-control" id="records" name="recordsPerPage"> 
    <option value="5">5</option> 
    <option value="10" selected>10</option>
    <option value="15">15</option>
</select>

select标签允许每页选择 5、10 或 15 条记录。

<button type="submit" class="btn btn-primary">Submit</button>

提交按钮执行表单。

listCountries.jsp

<%@page contentType="text/html" pageEncoding="UTF-8"%>
<%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>Countries</title>
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.6/css/bootstrap.min.css">
</head>
<body class="m-3">
<div class="row col-md-6">
    <table class="table table-striped table-bordered table-sm">
        <tr>
            <th>Name</th>
            <th>Population</th>
        </tr>
        <c:forEach items="${countries}" var="country">
            <tr>
                <td>${country.getName()}</td>
                <td>${country.getPopulation()}</td>    
            </tr>
        </c:forEach>
    </table>
</div>
<nav aria-label="Navigation for countries">
    <ul class="pagination">
        <c:if test="${currentPage != 1}">
            <li class="page-item"><a class="page-link" 
                href="ReadCountries?recordsPerPage=${recordsPerPage}&currentPage=${currentPage-1}">Previous</a>
            </li>
        </c:if>
        <c:forEach begin="1" end="${noOfPages}" var="i">
            <c:choose>
                <c:when test="${currentPage eq i}">
                    <li class="page-item active"><a class="page-link">
                            ${i} <span class="sr-only">(current)</span></a>
                    </li>
                </c:when>
                <c:otherwise>
                    <li class="page-item"><a class="page-link" 
                        href="ReadCountries?recordsPerPage=${recordsPerPage}&currentPage=${i}">${i}</a>
                    </li>
                </c:otherwise>
            </c:choose>
        </c:forEach>
        <c:if test="${currentPage lt noOfPages}">
            <li class="page-item"><a class="page-link" 
                href="ReadCountries?recordsPerPage=${recordsPerPage}&currentPage=${currentPage+1}">Next</a>
            </li>
        </c:if>              
    </ul>
<script src="https://code.jquery.com/jquery-3.1.1.slim.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/tether/1.4.0/js/tether.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.6/js/bootstrap.min.js"></script>
</body>
</html>

listCountries.jsp在表格和分页系统中显示数据。 Bootstrap 用于使 UI 响应并看起来不错。

<table class="table table-striped table-bordered table-sm">

table，table-striped，table-bordered和table-sm都是 Bootstrap 类。

<c:forEach items="${countries}" var="country">
    <tr>
        <td>${country.getName()}</td>
        <td>${country.getPopulation()}</td>    
    </tr>
</c:forEach>

使用 JSTL 的forEach标签，我们可以显示当前页面的所有数据。

<c:if test="${currentPage != 1}">
    <li class="page-item"><a class="page-link" 
        href="ReadCountries?recordsPerPage=${recordsPerPage}&currentPage=${currentPage-1}">Previous</a>
    </li>
</c:if>

使用c:if标签，我们仅在存在前一个链接时显示它。在链接中，我们将recordsPerPage和currentPage值传递给请求对象。

<c:forEach begin="1" end="${noOfPages}" var="i">
    <c:choose>
        <c:when test="${currentPage eq i}">
            <li class="page-item active"><a class="page-link">
                    ${i} <span class="sr-only">(current)</span></a>
            </li>
        </c:when>
        <c:otherwise>
            <li class="page-item"><a class="page-link" 
                href="ReadCountries?recordsPerPage=${recordsPerPage}&currentPage=${i}">${i}</a>
            </li>
        </c:otherwise>
    </c:choose>
</c:forEach>

使用forEach标签，我们显示所有页面链接。

Java Servlet 分页 - 图1

图：Java Servlet 分页

该示例显示了一个装有数据和分页系统的表。当前选择的页面突出显示。

在本教程中，我们展示了如何使用 Java Servlet 在 Web 应用中创建分页系统。

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