對數據庫進行查詢操作
1.查詢指定記錄
以設計一個簡單的登錄界面爲例
步驟：
（1）啓動數據庫，選擇相應的表（此項操作可詳見上篇博客，此處我選擇的仍然是test數據庫下的t_user表）
當前表中所有記錄：
（2）打開eclipse,在當前項目中的WebContent目錄下新建一個jsp文件（我這裏命名爲signin.jsp），在源文件目錄下選擇合適的包新建一個Servlet文件（我這裏命名爲SigninServlet.java），以用作連接數據庫，處理登錄界面發送的請求參數等。
（3）選擇用戶進行登錄需要驗證的字段，相應的在jsp文件中編輯源代碼（此處我選擇的是使用username登陸，並進行密碼驗證；爲提高查詢效率，可在該列添加索引）。
signin.jsp
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>用戶登陸</title>
</head>
<body>
<form action="/blog/SigninServlet" method="post">
登錄名稱：<input name="username" type="text" /><br /><p></p>
密 碼：<input name="password" type="password" /><br />
<p><input type="submit" /></p>
</form>
</body>
</html>
顯示效果：
（4）編輯SigninServlet.java源代碼
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
//定義變量接收參數
String username = request.getParameter("username");
String password = request.getParameter("password");
//爲登陸時參數的輸入設置條件
boolean error = false;
String errorMsg = " ";
if (StringUtils.isEmpty(username)) {
error = true;
errorMsg = "用戶名不允許爲空";
} else if (StringUtils.isEmpty(password)) {
error = true;
errorMsg = "密碼不允許爲空";
}
//如果上述條件不滿足，則跳轉到error.jsp(源代碼可見上篇博客)，並給出相應提示
if (error) {
request.setAttribute("errorMsg", errorMsg);
RequestDispatcher dispatcher = request.getRequestDispatcher("error.jsp");
dispatcher.forward(request, response);
return;
}
Connection connection = null;
Statement statement = null;
try {
//連接數據庫，創建Statment接口
connection = (Connection) DBUtil.getConnection();
statement = (Statement) connection.createStatement();
//準備sql語句
String sql = "select * from t_user where username = '"+username+"' and password = '"+password+"'";
System.out.println(sql);
// 執行sql語句
ResultSet resultSet = statement.executeQuery(sql);
resultSet.last();
//判斷登陸是否成功，如果成功則轉到success.jsp頁面,如失敗則跳轉到error.jsp頁面，給出提示
int rowCount = resultSet.getRow();
if (rowCount <= 0) {
request.setAttribute("errorMsg", "用戶名或密碼錯誤");
RequestDispatcher dispatcher = request.getRequestDispatcher("error.jsp");
dispatcher.forward(request, response);
return;
}
request.setAttribute("msg", "登陸成功");
RequestDispatcher dispatcher = request.getRequestDispatcher("success.jsp");
dispatcher.forward(request, response);
return;
//捕獲執行try操作中的異常
} catch (Exception e) {
// TODO: handle exception
e.printStackTrace();
//關閉數據庫連接和Statement接口
} finally {
if (connection != null) {
try {
connection.close();
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
if (statement != null) {
try {
statement.close();
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
request.setAttribute("errorMsg", "系統異常");
RequestDispatcher dispatcher = request.getRequestDispatcher("error.jsp");
dispatcher.forward(request, response);
}
}
success.jsp
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>status</title>
</head>
<body>
<h1 style="color: red; text-align: center;">
${msg}<!-- EL表達式 -->
</h1>
<a href="/blog/showUsers">查看所有註冊用戶</a><!-- 設置了一個鏈接，用於直接跳轉到查看所有用戶信息頁面 -->
</body>
</html>
如果登錄成功則頁面顯示：
2.查詢表中所有記錄
鑑於很多步驟都與前面的相同，所以這裏就不做詳細說明，只簡單補充
（1）定義usreList.jsp文件用於顯示所有用戶信息界面
需導入jstl-1.2.jar包和standard-1.1.2.jar包（網上下載後直接copy到lib目錄下）
usreList.jsp
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<%@ taglib uri="http://java.sun.com/jsp/jstl/fmt" prefix="fmt"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>所有用戶信息</title>
</head>
<body>
<table>
<tr>
<th>id</th>
<th>郵箱</th>
<th>電話號碼</th>
<th>用戶名</th>
<th>顯示暱稱</th>
<th>密碼</th>
<th>創建時間</th>
<th>更新時間</th>
</tr>
<!-- -jstl表達式 -->
<c:forEach items="${userList}" var="u" varStatus="status">
<tr >
<td>${u.id}
</td>
<td>${u.email}
</td>
<td>${u.phoneNumber}
</td>
<td>${u.username}
</td>
<td>${u.nickname}
</td>
<td>${u.password}
</td>
<td> <fmt:formatDate value="${u.createTime}" pattern="yyyy-MM-dd HH:mm:ss" />
</td>
<td><fmt:formatDate value="${u.updateTime}" pattern="yyyy-MM-dd HH:mm:ss" />
</td>
</tr>
</c:forEach>
</table>
</body>
</html>
（2）定義ShowUsersServlet處理請求
ShowUsersServlet
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
Connection connection = null;
Statement statement = null;
try {
connection = (Connection) DBUtil.getConnection();
statement = (Statement) connection.createStatement();
// 準備sql語句
String sql = "SELECT * FROM t_user";//sql語句，不區分大小寫
System.out.println(sql);
ResultSet resultSet = statement.executeQuery(sql);
List<User> userList = new ArrayList<User>();//封裝不限長度的數組
//遍歷結果
while (resultSet.next()) {
User user = new User();
user.setId(resultSet.getLong("id"));
user.setEmail(resultSet.getString("email"));
user.setPhoneNumber(resultSet.getString("phone_number"));
user.setUsername(resultSet.getString("username"));
user.setNickname(resultSet.getString("nickname"));
user.setPassword(resultSet.getString("password"));
user.setCreateTime(resultSet.getTimestamp("create_time"));
user.setUpdateTime(resultSet.getTimestamp("update_time"));
userList.add(user);
}
request.setAttribute("userList", userList);
RequestDispatcher dispatcher = request.getRequestDispatcher("userList.jsp");
dispatcher.forward(request, response);
return;
} catch (Exception e) {
// TODO: handle exception
e.printStackTrace();
//
} finally {
if (connection != null) {
try {
connection.close();
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
if (statement != null) {
try {
statement.close();
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
request.setAttribute("errorMsg", "系統異常");
RequestDispatcher dispatcher = request.getRequestDispatcher("error.jsp");
dispatcher.forward(request, response);
}
完成登錄後點擊鏈接顯示：
拓展：
JDBC resultSet關於時間的顯示
（1）resultSet.getDate(); 顯示日期部分——> yyyy -mm-dd
（2）resultSet.getTime(); 顯示時間部分——> hh:mm:ss
（3）resultSet.getTime(); 顯示日期和時間——> yyyy -mm-dd hh:mm:ss