Servlet如何创建计数器？
在这里，我们有一个使用servlet创建计数器的简单示例。每次访问页面时，该servlet都会更新点击计数器，并以图像的形式显示点击数。该图像是在运行时使用JavaGraphic2D和ImageIO类生成的。
要存储点击数据，请创建一个表，hits其中包含一个名为的字段counter，并将计数器的初始值设置为零。下面是HitCounterServletservlet代码。
package org.nhooo.example.servlet;
import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.io.OutputStream;
import java.sql.*;
public class HitCounterServlet extends HttpServlet {
public HitCounterServlet() {
super();
}
protected void doGet(HttpServletRequest req, HttpServletResponse res)
throws ServletException, IOException {
updateHitCounter();
getHitCounterImage(req, res);
}
protected void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
updateHitCounter();
getHitCounterImage(request, response);
}
private void updateHitCounter() {
Connection connection = getConnection();
try {
//通过增加点击数来更新点击计数器表
//用户每次访问我们的页面时都进行计数。
String sql = "UPDATE hits SET counter = counter + 1";
PreparedStatement stmt = connection.prepareStatement(sql);
stmt.executeUpdate();
} catch (SQLException e) {
e.printStackTrace();
} finally {
closeConnection(connection);
}
}
private void getHitCounterImage(HttpServletRequest req,
HttpServletResponse res)
throws IOException {
Connection connection = getConnection();
String hits = "";
try {
//从数据库获取当前命中计数器。
String sql = "SELECT counter FROM hits";
PreparedStatement stmt = connection.prepareStatement(sql);
ResultSet rs = stmt.executeQuery();
while (rs.next()) {
hits = rs.getString("counter");
}
} catch (SQLException e) {
e.printStackTrace();
} finally {
closeConnection(connection);
}
//创建要发送到浏览器的柜台图片。
BufferedImage buffer = new BufferedImage(50, 20, BufferedImage.TYPE_INT_RGB);
Graphics2D g = buffer.createGraphics();
g.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON);
g.setFont(new Font("Monospaced", Font.PLAIN, 14));
g.setColor(Color.WHITE);
g.fillRect(0, 0, 50, 20);
g.setColor(Color.BLACK);
g.drawString(hits, 0, 20);
res.setContentType("image/png");
OutputStream os = res.getOutputStream();
ImageIO.write(buffer, "png", os);
os.close();
}
private Connection getConnection() {
Connection connection = null;
try {
Class.forName("com.mysql.jdbc.Driver");
connection = DriverManager.getConnection(
"jdbc:mysql://localhost/nhooo", "nhooo", "nhooo123");
} catch (Exception e) {
e.printStackTrace();
}
return connection;
}
private void closeConnection(Connection connection) {
try {
if (connection != null && !connection.isClosed()) {
connection.close();
}
} catch (SQLException e) {
e.printStackTrace();
}
}
}要配置servlet，您需要web.xml按照以下步骤更新文件：
<?xml version="1.0" encoding="UTF-8"?>
<web-app id="WebApp_ID" version="2.4"
xmlns="http://java.sun.com/xml/ns/j2ee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
<display-name>nhooo-example</display-name>
<servlet>
<display-name>HitCounter</display-name>
<servlet-name>HitCounter</servlet-name>
<servlet-class>
org.nhooo.example.servlet.HitCounterServlet
</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>HitCounter</servlet-name>
<url-pattern>/HitCounter</url-pattern>
</servlet-mapping>
</web-app>要在JSP页面上显示点击数计数器图片，请创建带有源点指向我们HitCoutnerServletservlet的图片标签。
Visited for: <img src="http://localhost:8080/app-name/HitCounter" alt="Hit Counter"/> times.