【溫故知新】Java web 開發（四）JSTL 與 JDBC 的增刪改查

時間 2020-06-02

標籤溫故知新 java web 開發 jstl jdbc 增刪欄目 Java 简体版

原文原文鏈接

　　本篇開始使用 jstl 這個 jsp 的標籤庫，在同一個 Servlet 中實現處理 CRUD 請求，以及使用 jdbc 數據庫基本操做。而後你會發現 Servlet 和 jdbc 仍是有不少不方便之處，而後在篇章五將開始正式使用框架。html

　　使用 jstl 是爲了先後端分離，試想若是 jsp 中嵌入一堆的 java 代碼片斷，那樣的話前端就很難開發除非它懂 java 技術，或者得由後端開發這個。前端

　　此次咱們就作一個書籍列表的功能，記錄購買過的書籍的名稱、價格和頁數信息，能夠添加、刪除、修改單本書籍信息（同時刪除多本書籍這個須要 js 的幫助，這裏暫時不用，留在以後回顧）。

1. 簡單的邏輯梳理

　　要實現增刪改查須要設計一下流程，這裏只給出一個簡單的設計：

首頁有展現書籍列表的入口，

書籍列表頁提供一個表格展現數據庫中存入的書籍，每一個書籍（每行記錄後）都附帶編輯和刪除操做，

編輯須要跳轉到編輯頁面實現，過程當中須要查詢一次數據庫獲得數據，而不是從列表頁直接獲取傳遞，編輯書籍頁面提交給真正的編輯方法來執行入庫

刪除操做不須要專門頁面來處理，直接數據庫操做，返回結果展現到一個消息頁面，該頁面通用，

表格下方有添加數據的連接，添加須要跳轉到添加頁面實現，添加書籍頁面數據提交給真正的添加動做方法來執行入庫，

如刪除操做所述，基本頁面的操做結果由通用消息頁面展現，該頁面根據傳遞過來的字符串和URL決定跳轉到地址。

2. jstl el 表達式

　　使用 el 表達式必須關掉忽略 el 表達式的設定，以下 <%@ page isELIgnored ="false" %>，若是不關閉，則頁面上會出現不少${"sth"}，不會被解析成 jstl 標籤。

　　經常使用的 jstl 庫及其用法示例

核心庫 <%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
格式化文本庫 <%@ taglib prefix="fmt" uri="http://java.sun.com/jsp/jstl/fmt" %>
函數庫 <%@ taglib prefix="fn" uri="http://java.sun.com/jsp/jstl/functions" %>

核心庫，提供了相似語言的基礎服務，好比流程控制，輸出語句，賦值語句等。

格式化文本，顧名思義，能夠格式化時間、浮點數等。

函數庫，提供了不少函數，好比字符串的操做如子串，長度等功能，還有map、list這種集合類判空等方法，自行研究。

這裏貼幾段示例

　　第一個：用到的有核心庫裏的流程控制語句，還有循環語句，函數庫的長度方法以及處理 map 的方法。

<c:choose>
    <c:when test="${empty fileMap or fn:length(fileMap) == 0}">
        <p>您尚未上傳文件，請點擊這裏上傳：<a href=/file/add>上傳文件</a></p>
    </c:when>
    <c:otherwise>
        <c:forEach items="${fileMap}" var="entry">
            <p>文件名: ${entry.key} &nbsp;&nbsp;&nbsp; <a href="/file/download?fileName=${entry.value}" >下載</a></p>
        </c:forEach>
    </c:otherwise>
</c:choose>

　　第二個：用到的有格式化浮點數的語句java

<c:forEach items="${bookList}" var="book">
    <tr>
        <th>ids</th>
        <th><input type="checkbox" value="${book.id}" /></th>
        <td>${book.name}</td>
        <td><fmt:formatNumber value="${book.price}" pattern="0.00"/></td>
        <td>${book.pageCount}</td>
        <td><a href="/book?method=update&id=${book.id}">編輯</a> &nbsp;<a href="/book?method=delete&id=${book.id}">刪除</a></td>
    </tr>
</c:forEach>

差點忘記說了，上邊有取數的操做，好比最開始的${bookList}、${fileMap}，它是怎樣從 Servlet 傳遞到 jsp 中的呢？mysql

答案是 servlet 中用 request.setAttribute("bookList", bookList);這種方式。web

3. 數據庫操做 jdbcspring

建立數據庫鏈接的基本動做步驟sql

加載 jdbc 驅動類 DriverManager
建立鏈接（鏈接URL的格式）
建立 statement，防止 Sql 注入的 PreparedStatement
執行語句查詢 executeQuery(); 更改、刪除 executeUpdate()
處理返回結果 ResultSet
關閉鏈接

public class JDBCTest { public static void main(String[] args) { String driver = "com.mysql.jdbc.Driver"; String dbName = "spring"; String passwrod = "root"; String userName = "root"; String url = "jdbc:mysql://localhost:3308/" + dbName; String sql = "select * from users"; try { Class.forName(driver); Connection conn = DriverManager.getConnection(url, userName, passwrod); PreparedStatement ps = conn.prepareStatement(sql); ResultSet rs = ps.executeQuery(); while (rs.next()) { System.out.println("id : " + rs.getInt(1) + " name : "
                        + rs.getString(2) + " password : " + rs.getString(3)); } // 關閉記錄集
            if (rs != null) { try { rs.close(); } catch (SQLException e) { e.printStackTrace(); } } // 關閉聲明
            if (ps != null) { try { ps.close(); } catch (SQLException e) { e.printStackTrace(); } } // 關閉連接對象
            if (conn != null) { try { conn.close(); } catch (SQLException e) { e.printStackTrace(); } } } catch (Exception e) { e.printStackTrace(); } } }

4. 具體實現

由於一個 Servlet 只有一個 service 方法，因此它能處理的請求URL也只有一類，爲了實現 CRUD 操做，採用 /book?method=list這種方式以method參數區分。這裏的實現不是爲了展現技術的，因此沒有用RESTFul 的方式，也不要嫌 low，正是由於 servlet 的很差用之處，才逼着咱們去用更好用的框架好比 Spring MVC，對吧。

BookServlet.java

@WebServlet(name = "bookServlet", urlPatterns = {"/book"}) public class BookServlet extends HttpServlet { @Override public void init() { try { Class.forName("com.mysql.jdbc.Driver"); } catch (ClassNotFoundException e) { e.printStackTrace(); } } @Override public void service(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException{ request.setCharacterEncoding("UTF-8"); String method = request.getParameter("method"); Connection conn; try { conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/webpractice", "root", "Circle233???"); switch (method) { case "list": list(request, response, conn); break; case "add": add(request, response); break; case "create": create(request, response, conn); break; case "delete": delete(request, response, conn); break; case "update": update(request, response, conn); break; case "updateOp": updateOp(request, response, conn); break; default: break; } } catch (Exception e) { e.printStackTrace(); } } private void add(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { request.getRequestDispatcher("/WEB-INF/page/book_add.jsp").forward(request, response); } private void create(HttpServletRequest request, HttpServletResponse response, Connection conn) throws IOException, ServletException { String message; PreparedStatement ps = null; String insertSql = "insert into books(name, price, page_count) values(?,?,?)"; try { String name = request.getParameter("name"); String pricePara = request.getParameter("price"); double price = Double.valueOf(pricePara); String pageCountPara = request.getParameter("pageCount"); int pageCount = Integer.parseInt(pageCountPara); ps = conn.prepareStatement(insertSql); ps.setString(1, name); ps.setDouble(2, price); ps.setInt(3, pageCount); ps.executeUpdate(); message = "添加書籍成功！"; request.setAttribute("message",message); request.setAttribute("suggestURL", "/book?method=list"); request.setAttribute("declaration", "查看書籍列表"); request.getRequestDispatcher("/WEB-INF/page/message.jsp").forward(request, response); } catch (Exception e) { e.printStackTrace(); message = "添加書籍出錯！"; request.setAttribute("message",message); request.setAttribute("suggestURL", "/book?method=add"); request.setAttribute("declaration", "從新添加書籍"); request.getRequestDispatcher("/WEB-INF/page/message.jsp").forward(request, response); } finally { returnResource(null, ps, conn); } } private void delete(HttpServletRequest request, HttpServletResponse response, Connection conn) throws IOException, ServletException { String message; PreparedStatement ps = null; String deleteSql = "delete from books where id = (?)"; try { String id = request.getParameter("id"); ps = conn.prepareStatement(deleteSql); ps.setString(1, id); ps.executeUpdate(); message = "刪除書籍成功！"; request.setAttribute("message",message); request.setAttribute("suggestURL", "/book?method=list"); request.setAttribute("declaration", "查看書籍列表"); request.getRequestDispatcher("/WEB-INF/page/message.jsp").forward(request, response); } catch (Exception e) { e.printStackTrace(); message = "刪除書籍出錯！"; request.setAttribute("message",message); request.setAttribute("suggestURL", "/book?method=list"); request.setAttribute("declaration", "回到書籍列表"); request.getRequestDispatcher("/WEB-INF/page/message.jsp").forward(request, response); } finally { returnResource(null, ps, conn); } } private void update(HttpServletRequest request, HttpServletResponse response, Connection conn) throws IOException, ServletException { PreparedStatement ps = null; ResultSet rs = null; String message; try { String queryOneSql = "select * from books where id = ?"; ps = conn.prepareStatement(queryOneSql); ps.setString(1, request.getParameter("id")); rs = ps.executeQuery(); if (rs.next()) { int id = rs.getInt("id"); String name = rs.getString("name"); double price = rs.getDouble("price"); int pageCount = rs.getInt("page_count"); Book book = new Book(name, price, pageCount); book.setId(id); request.setAttribute("book", book); request.getRequestDispatcher("/WEB-INF/page/book_update.jsp").forward(request, response); } else { message = "沒有查到對應書籍，請刷新列表頁面！"; request.setAttribute("message",message); request.setAttribute("suggestURL", "/book?method=list"); request.setAttribute("declaration", "回到書籍列表"); request.getRequestDispatcher("/WEB-INF/page/message.jsp").forward(request, response); } } catch (Exception e) { e.printStackTrace(); message = "編輯書籍出錯！"; request.setAttribute("message",message); request.setAttribute("suggestURL", "/book?method=list"); request.setAttribute("declaration", "回到書籍列表"); request.getRequestDispatcher("/WEB-INF/page/message.jsp").forward(request, response); } finally { returnResource(rs, ps, conn); } } private void updateOp(HttpServletRequest request, HttpServletResponse response, Connection conn) throws IOException, ServletException { PreparedStatement ps = null; ResultSet rs = null; String message; try { String updateSql = "update books set name = ?, price = ?, page_count = ? where id = ?"; ps = conn.prepareStatement(updateSql); ps.setString(1, request.getParameter("name")); ps.setDouble(2, Double.valueOf(request.getParameter("price"))); ps.setInt(3, Integer.parseInt(request.getParameter("pageCount"))); ps.setInt(4, Integer.parseInt(request.getParameter("id"))); int result = ps.executeUpdate(); System.out.println("result: " + result); if (result == 1) { message = "修改書籍成功！"; request.setAttribute("message", message); request.setAttribute("suggestURL", "/book?method=list"); request.setAttribute("declaration", "查看書籍列表"); request.getRequestDispatcher("/WEB-INF/page/message.jsp").forward(request, response); } else { message = "修改書籍失敗！"; request.setAttribute("message", message); request.setAttribute("suggestURL", "/book?method=list"); request.setAttribute("declaration", "回到書籍列表"); request.getRequestDispatcher("/WEB-INF/page/message.jsp").forward(request, response); } } catch (Exception e) { e.printStackTrace(); message = "修改書籍失敗！"; request.setAttribute("message", message); request.setAttribute("suggestURL", "/book?method=list"); request.setAttribute("declaration", "回到書籍列表"); request.getRequestDispatcher("/WEB-INF/page/message.jsp").forward(request, response); } finally { returnResource(rs, ps, conn); } } private void list(HttpServletRequest request, HttpServletResponse response, Connection conn) throws IOException, ServletException{ PreparedStatement ps = null; ResultSet rs = null; try { List<Book> list = new ArrayList<>(); ps = conn.prepareStatement("select * from books"); rs = ps.executeQuery(); while (rs.next()) { int id = rs.getInt("id"); String name = rs.getString("name"); double price = rs.getDouble("price"); int pageCount = rs.getInt("page_count"); Book book = new Book(name, price, pageCount); book.setId(id); list.add(book); } request.setAttribute("bookList", list); request.getRequestDispatcher("/WEB-INF/page/book_list.jsp").forward(request, response); } catch (SQLException e) { e.printStackTrace(); } finally { returnResource(rs, ps, conn); } } private void returnResource(ResultSet rs, PreparedStatement ps, Connection conn) { try { if (rs != null) { rs.close(); } if (ps != null) { ps.close(); } if (conn != null) { conn.close(); } } catch (SQLException e) { e.printStackTrace(); } } }

book_add.jsp數據庫

<%-- Created by IntelliJ IDEA. User: yixin Date: 2018/7/12
  Time: 14:49
  To change this template use File | Settings | File Templates. --%>
<%@ page contentType="text/html;charset=UTF-8" pageEncoding="UTF-8" %>
<!DOCTYPE html>
<html lang="zh-cn">
<head>
    <meta charset="UTF-8">
    <title>添加新書籍</title>
</head>
<body>
    <form action="/book?method=create" method="post" > 書名：<input name="name" type="text"><br /> 價格：<input name="price" type="text"><br /> 頁數：<input name="pageCount" type="number"> <br />
        <input type="submit" value="提交"> <input type="reset" value="重置">
    </form>
    <a href="/index.html">回到首頁</a>
</body>
</html>

book_list.jsp後端

<%@ page contentType="text/html;charset=UTF-8" pageEncoding="UTF-8"%>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<%@ taglib prefix="fmt" uri="http://java.sun.com/jsp/jstl/fmt" %>
<%@ page isELIgnored ="false" %>
<!DOCTYPE html>
<html lang="zh-cn">
<head>
    <meta charset="UTF-8">
    <title>書籍展現頁</title>
</head>
<body>
<div>
    <h2><span>購買過的書籍列表</span></h2>

    <c:choose>
        <c:when test="${empty bookList}">
            <tr>沒有內容</tr>
        </c:when>
        <c:otherwise>
            <table border="1">
                <tr>
                    <th><input type="checkbox" id="chbAll"></th>
                    <th>編號</th>
                    <th>書名</th>
                    <th>價格</th>
                    <th>頁數</th>
                    <th>操做</th>
                </tr>
                <tbody>
                    <c:forEach items="${bookList}" var="book">
                        <tr>
                            <th>ids</th>
                            <th><input type="checkbox" value="${book.id}" /></th>
                            <td>${book.name}</td>
                            <td><fmt:formatNumber value="${book.price}" pattern="0.00"/></td>
                            <td>${book.pageCount}</td>
                            <td><a href="/book?method=update&id=${book.id}">編輯</a> &nbsp;<a href="/book?method=delete&id=${book.id}">刪除</a></td>
                        </tr>
                    </c:forEach>
                </tbody>
            </table>
        </c:otherwise>
    </c:choose>
    <div>
        <p>
            <a href="/book?method=add">添加書籍</a> &nbsp; <a href="/index.html">返回首頁</a>
        </p>
    </div>
</div>
</body>
</html>

book_update.jsp框架

<%-- Created by IntelliJ IDEA. User: yinjd Date: 2018/7/15
  Time: 17:33
  To change this template use File | Settings | File Templates. --%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ page isELIgnored ="false" %>
<!DOCTYPE HTML>
<html lang="zh-cn">
<head>
    <meta charset="UTF-8">
    <title>修改書籍</title>
</head>
<body>
<form action="/book?method=updateOp" method="post" > 書名：<input name="name" type="text" value="${book.name}"><br /> 價格：<input name="price" type="text" value="${book.price}"><br /> 頁數：<input name="pageCount" type="number" value="${book.pageCount}"> <br />
    <input type="hidden" name="id" value="${book.id}"><br />
    <input type="submit" value="提交">
</form>
</body>
</html>

message.jsp

<%@ page contentType="text/html;charset=UTF-8" pageEncoding="UTF-8" %>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<%@ taglib uri="http://java.sun.com/jsp/jstl/functions" prefix="fn"%>
<%@ page isELIgnored ="false" %>
<!DOCTYPE html>
<html lang="zh-cn">
<head>
    <meta charset="UTF-8">
    <title>消息頁</title>
</head>
<body>

<p>${message}</p>
<p>點擊這裏><a href="${suggestURL}">${declaration}</a></p>

</body>
</html>

index.html

<!DOCTYPE html>
<html lang="zh-cn">
<head>
    <meta charset="UTF-8">
    <title>歡迎頁</title>
</head>
<body>
    <a href="/file/list">上傳文件列表</a><br />
    <a href="/book?method=list">購買書籍列表</a>
</body>
</html>