1.
发表留言：
首先，创建一个HTML页面，包含一个表单用于输入
留言内容，提交表单时向Servlet发送POST请求。在Servlet中，获取表单
数据，将
留言内容存入
数据库，
并重定向到
留言显示页面。
HTML页面：
```
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>
发表留言</title>
</head>
<body>
<h1>
发表留言</h1>
<form action="AddMessageServlet" method="post">
<label for="username">用户名：</label>
<input type="text" id="username" name="username"><br>
<label for="content">
留言内容：</label>
<textarea id="content" name="content"></textarea><br>
<input type="submit" value="提交">
</form>
</body>
</html>
```
AddMessageServlet：
```java
@WebServlet("/AddMessageServlet")
public class AddMessageServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
request.setCharacterEncoding("UTF-8");
String username = request.getParameter("username");
String content = request.getParameter("content");
MessageDao dao = new MessageDao();
dao.addMessage(username, content);
response.sendRedirect("ShowMessageServlet");
}
}
```
MessageDao中addMessage方法：
```java
public void addMessage(String username, String content) {
Connection conn = null;
PreparedStatement stmt = null;
try {
conn = DBUtil.getConnection();
String sql = "INSERT INTO message(username, content) VALUES (?, ?)";
stmt = conn.prepareStatement(sql);
stmt.setString(1, username);
stmt.setString(2, content);
stmt.executeUpdate();
} catch (SQLException e) {
e.printStackTrace();
} finally {
DBUtil.close(stmt, conn);
}
}
```
2.
留言显示：
创建一个Servlet用于显示所有
留言，从
数据库中获取所有
留言并将其显示在HTML页面中。
ShowMessageServlet：
```java
@WebServlet("/ShowMessageServlet")
public class ShowMessageServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
List<Message> messages = new MessageDao().getAllMessages();
request.setAttribute("messages", messages);
request.getRequestDispatcher("showMessage.jsp").forward(request, response);
}
}
```
showMessage.jsp：
```jsp
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>
留言板</title>
</head>
<body>
<h1>
留言板</h1>
<a href="addMessage.html">
发表留言</a>
<hr>
<c:forEach items="${messages}" var="message">
<p>${message.username}说：${message.content}</p>
<form action="DeleteMessageServlet" method="post">
<input type="hidden" name="id" value="${message.id}">
<input type="submit" value="删除">
</form>
<hr>
</c:forEach>
</body>
</html>
```
MessageDao中getAllMessages方法：
```java
public List<Message> getAllMessages() {
Connection conn = null;
PreparedStatement stmt = null;
ResultSet rs = null;
List<Message> messages = new ArrayList<>();
try {
conn = DBUtil.getConnection();
String sql = "SELECT id, username, content FROM message";
stmt = conn.prepareStatement(sql);
rs = stmt.executeQuery();
while (rs.next()) {
int id = rs.getInt(1);
String username = rs.getString(2);
String content = rs.getString(3);
Message message = new Message(id, username, content);
messages.add(message);
}
} catch (SQLException e) {
e.printStackTrace();
} finally {
DBUtil.close(rs, stmt, conn);
}
return messages;
}
```
3. 删除
留言：
添加一个表单用于删除
留言，向Servlet发送POST请求，从
数据库中删除指定
留言，
并重定向到
留言显示页面。
DeleteMessageServlet：
```java
@WebServlet("/DeleteMessageServlet")
public class DeleteMessageServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
int id = Integer.parseInt(request.getParameter("id"));
MessageDao dao = new MessageDao();
dao.deleteMessage(id);
response.sendRedirect("ShowMessageServlet");
}
}
```
MessageDao中deleteMessage方法：
```java
public void deleteMessage(int id) {
Connection conn = null;
PreparedStatement stmt = null;
try {
conn = DBUtil.getConnection();
String sql = "DELETE FROM message WHERE id = ?";
stmt = conn.prepareStatement(sql);
stmt.setInt(1, id);
stmt.executeUpdate();
} catch (SQLException e) {
e.printStackTrace();
} finally {
DBUtil.close(stmt, conn);
}
}
```
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