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Servlet详解含实例
2022-08-03 05:27:00 【耳冉鹅】
一、简介
- Servlet是sun公司开发动态web的一门技术
- Sun在这些API中提供了一个接口叫做：Servlet，若想要开发一个Servlet程序，只需要完成两个小步骤：
- 编写一个类，实现Servlet接口
- 把开发好的Java类部署到web服务器中
把实现了Servlet接口的Java程序叫做 Servlet
二、HelloServlet
Servlet接口在Sun公司有两个默认的实现类：HttpServlet、GenericServlet
1、构建项目
构建一个普通的Maven项目，删掉里面的src目录，然后在项目中建立Model
这个空的工程就是Maven主工程
2、父子工程理解
父项目中：
<modules>
<module>servlet-01</module>
</modules>
子项目中：
<parent>
<artifactId>javaweb-02-servlet</artifactId>
<groupId>com.erran</groupId>
<version>1.0-SNAPSHOT</version>
</parent>
父项目中的java子项目可以直接使用
son extends father
3、环境优化
web.xml文件头更新：
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd" version="4.0">
<display-name>Archetype Created Web Application</display-name>
</web-app>
将Maven结构搭建完整
4、编写Servlet程序
- 编写一个普通类
- 实现Servlet接口，直接继承HTTPServlet
package com.erran.servlet;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;
/** * @author erran */
public class HelloServlet extends HttpServlet {
// get、post只是请求实现的不同方式，因此可以相互调用且业务逻辑都一样
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
// 响应流
PrintWriter writer = resp.getWriter();
writer.println("HelloServlet");
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
super.doPost(req, resp);
}
}
5、编写Servlet映射
为什么需要映射
Java程序需要通过浏览器访问，而浏览器需要连接web服务器，因此需要在web服务中注册所写的Servlet，还需要给浏览器一个能够访问的路径
web.xml中添加
<servlet>
<servlet-name>hello</servlet-name>
<servlet-class>com.erran.servlet.HelloServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>hello</servlet-name>
<url-pattern>/hello</url-pattern>
</servlet-mapping>
6、配置Tomcat
注意：配置项目发布的路径
7、启动测试
执行过程：http://localhost:8080/s1/hello执行的是下面代码中的url-pattern，继而去寻找servlet-name
<servlet-mapping>
<servlet-name>hello</servlet-name>
<url-pattern>/hello</url-pattern>
</servlet-mapping>
从servlet-mapping转到servlet中，对应servlet-name，因此去寻找servlet-class，即我们所写的HelloServlet
<servlet>
<servlet-name>hello</servlet-name>
<servlet-class>com.erran.servlet.HelloServlet</servlet-class>
</servlet>
进入到doGet方法，打印“HelloServlet”，完成执行
三、Servlet原理
Servlet是由Web服务器调用，web服务器在收到浏览器请求后，会：
- Request会从service（请求）拿到请求并把请求之后的响应交给Response
- 我们编写的实现类重写这些方法
- 接收并处理请求
- 给出响应的信息
四、Mapping
一个Servlet可以指定一个映射路径
<servlet-mapping>
<servlet-name>hello</servlet-name>
<url-pattern>/hello</url-pattern>
</servlet-mapping>
一个Servlet可以指定多个映射路径
<servlet-mapping>
<servlet-name>hello</servlet-name>
<url-pattern>/hello</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>hello</servlet-name>
<url-pattern>/hello2</url-pattern>
</servlet-mapping>
一个Servlet可以指定通用映射路径
<servlet-mapping>
<servlet-name>hello</servlet-name>
<url-pattern>/hello/*</url-pattern>
</servlet-mapping>
默认请求路径
<servlet-mapping>
<servlet-name>hello</servlet-name>
<url-pattern>/*</url-pattern>
</servlet-mapping>
自定义后缀实现请求路径**(前面不能加项目映射的路径)*
<servlet-mapping>
<servlet-name>hello</servlet-name>
<url-pattern>*.html</url-pattern>
</servlet-mapping>
优先级问题
指定了固定的映射路径优先级最高，若找不到则走默认的处理请求
五、ServletContext
web容器在启动的时候，会为每个web程序都创建一个对应的ServletContext对象，代表了当前的web应用
1、共享数据
在这个Servlet中保存的数据，可以在另一个Servlet上拿到
public class Helloservlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
// this.getInitParameter(); 初始化参数
// this.getServletConfig(); Servlet配置
// this.getServletContext(); Servlet上下文
ServletContext servletContext = this.getServletContext();
String username = "名称";
// 将一个数据保存在ServletContext中
servletContext.setAttribute("username", username);
System.out.println("Hello");
}
}
public class GetServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
ServletContext servletContext = this.getServletContext();
String username = (String) servletContext.getAttribute("username");
resp.setContentType("text/html");
resp.setCharacterEncoding("utf-8");
resp.getWriter().println(username);
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
super.doPost(req, resp);
}
}
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
version="4.0">
<servlet>
<servlet-name>hello</servlet-name>
<servlet-class>com.erran.servlet.Helloservlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>hello</servlet-name>
<url-pattern>/hello</url-pattern>
</servlet-mapping>
<servlet>
<servlet-name>get</servlet-name>
<servlet-class>com.erran.servlet.GetServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>get</servlet-name>
<url-pattern>/get</url-pattern>
</servlet-mapping>
</web-app>
2、获取初始化参数
<context-param>
<param-name>url</param-name>
<param-value>jdbc:mysql://localhost:3306</param-value>
</context-param>
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
ServletContext servletContext = this.getServletContext();
String url = servletContext.getInitParameter("url");
resp.getWriter().println(url);
}
3、请求转发
<servlet>
<servlet-name>demo2</servlet-name>
<servlet-class>com.erran.servlet.Demo2</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>demo2</servlet-name>
<url-pattern>/demo2</url-pattern>
</servlet-mapping>
<servlet>
<servlet-name>demo</servlet-name>
<servlet-class>com.erran.servlet.Demo</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>demo</servlet-name>
<url-pattern>/gp</url-pattern>
</servlet-mapping>
public class Demo2 extends Helloservlet{
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
ServletContext servletContext = this.getServletContext();
// 转发的请求路径
RequestDispatcher requestDispatcher = servletContext.getRequestDispatcher("/gp");
// 调用forward实现请求转发
requestDispatcher.forward(req, resp);
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
doGet(req,resp);
}
}
六、HttpServletResponse
web服务器接收到客户端的http请求，针对这个请求，分别创建一个代表请求的HttpServletRequest对象，代表响应的一个HttpServletResponse
- 若要获取客户端请求过来的参数： HttpServletRequest
- 若要给客户端响应一些信息： HttpServletResponse
1、简单分类
负责向浏览器发送数据的方法
public ServletOutputStream getOutputStream() throws IOException;
public PrintWriter getWriter() throws IOException;
负责向浏览器发送响应头的方法
public void setDateHeader(String name, long date);
public void setIntHeader(String name, int value);
public void addHeader(String name, String value);
public void setIntHeader(String name, int value);
public void addIntHeader(String name, int value);
2、常见应用
- 向浏览器输出消息
- 下载文件
- 要获取下载文件的路径
- 下载的文件名
- 设置让浏览器能够支持下载我们需要的东西
- 获取下载文件的输入流
- 创建缓冲区
- 获取OutputStream对象
- 将FileOutputStream流写入到buffer缓冲区
- 使用OutputStream将缓冲区中的数据输出到客户端
public class FileServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
String realPath = "E:\\idea\\IntelliJ IDEA Community Edition 2019.3.1\\IdeaProjects\\javaweb-02-servlet\\response\\target\\classes\\1.jpg";
System.out.println("下载路径：" + realPath);
String fileName = realPath.substring(realPath.lastIndexOf('\\') + 1);
resp.setHeader("Content-Disposition","attachment;filename="+fileName);
FileInputStream in = new FileInputStream(realPath);
int len;
byte[] bytes = new byte[1024];
ServletOutputStream outputStream = resp.getOutputStream();
while ((len = in.read(bytes)) > 0) {
outputStream.write(bytes, 0, len);
}
in.close();
outputStream.close();
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
super.doPost(req, resp);
}
}
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd" version="4.0">
<servlet>
<servlet-name>filedown</servlet-name>
<servlet-class>com.erran.servlet.FileServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>filedown</servlet-name>
<url-pattern>/down</url-pattern>
</servlet-mapping>
</web-app>
3、验证码功能
验证：
- 前端实现
- 后端实现，需要用到Java 的图片类，生产一个图片
public class ImageServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
// 如何让浏览器5秒自动刷新一次
resp.setHeader("refresh", "3");
// 在内存中创建一个图片
BufferedImage bufferedImage = new BufferedImage(80, 20, BufferedImage.TYPE_INT_RGB);
// 得到图片 --- 笔
Graphics2D graphics = (Graphics2D) bufferedImage.getGraphics();
// 设置图片的背景颜色
graphics.setColor(Color.white);
graphics.fillRect(0, 0, 80, 20);
// 给图片写数据
graphics.setColor(Color.blue);
graphics.setFont(new Font(null, Font.BOLD, 20));
graphics.drawString(makeNum(), 0, 20);
// 告诉浏览器，这个请求用图片的方式打开
resp.setContentType("image/jpg");
// 网站存在缓存 不让浏览器缓存
resp.setDateHeader("expires", -1);
resp.setHeader("Cache-Control", "no-cache");
resp.setHeader("Pragma", "no-cache");
// 把图片写给浏览器
ImageIO.write(bufferedImage,"jpg", resp.getOutputStream());
}
// 生成随机数
private String makeNum() {
Random random = new Random();
String num = random.nextInt(9999999) + "";
StringBuffer stringBuffer = new StringBuffer();
for (int i = 0; i < 7 - num.length(); i++) {
stringBuffer.append("0");
}
String s1 = stringBuffer.toString() + num;
return num;
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
doGet(req, resp);
}
}
<servlet>
<servlet-name>ImageServlet</servlet-name>
<servlet-class>com.erran.servlet.ImageServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>ImageServlet</servlet-name>
<url-pattern>/img</url-pattern>
</servlet-mapping>
4、重定向
一个web资源收到客户端请求后，会通知客户端去访问另外一个web资源，这个过程叫做重定向
场景：
- 用户登录
public void sendRedirect(String location) throws IOException;
测试：
public class RedirectServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
// resp.setHeader("Location", "/response");
// resp.setStatus(302);
resp.sendRedirect("/img");
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
super.doPost(req, resp);
}
}
<servlet>
<servlet-name>RedirectServlet</servlet-name>
<servlet-class>com.erran.servlet.RedirectServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>RedirectServlet</servlet-name>
<url-pattern>/Redirect</url-pattern>
</servlet-mapping>
面试题：重定向与转发的区别
- 相同点
- 页面都会实现跳转
- 不同点
- 请求转发的时候，url不会产生变化 307
- 重定向的时候，url地址栏会产生变化 302
七、HttpServletRequest
HttpServletRequest代表客户端的请求，用户通过Http协议访问服务器，http请求中的所有信息会被封装到HttpServletRequest，通过这个HttpServletRequest的方法，获得客户端的所有信息
[外链图片转存失败,源站可能有防盗链机制,建议将图片保存下来直接上传(img-bx3Wodxc-1615873624563)(Servlet.assets/image-20210316132335284.png)]
获取前端传递的参数、请求转发
public class LoginServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
// 后台接收中文乱码问题
req.setCharacterEncoding("utf-8");
String username = req.getParameter("username");
String password = req.getParameter("password");
String[] hobbys = req.getParameterValues("hobby");
System.out.println("===================");
System.out.println(username);
System.out.println(password);
System.out.println(Arrays.toString(hobbys));
System.out.println("===================");
// 页面接收中文乱码问题
resp.setCharacterEncoding("utf-8");
req.getRequestDispatcher("/success.jsp").forward(req, resp);
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
doGet(req, resp);
}
}
index.jsp
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>Title</title>
</head>
<body>
<div style="text-align: center">
<form action="${pageContext.request.contextPath}/login" method="post">
用户名：<input type="text" name="username"><br>
密码: <input type="password" name="password"> <br>
爱好：
<input type="checkbox" name="hobby" value="抽烟">抽烟
<input type="checkbox" name="hobby" value="喝酒">喝酒
<input type="checkbox" name="hobby" value="烫头">烫头
<br>
<input type="submit">
</form>
</div>
</body>
</html>
web.xml
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd" version="4.0">
<servlet>
<servlet-name>login</servlet-name>
<servlet-class>com.erran.servlet.LoginServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>login</servlet-name>
<url-pattern>/login</url-pattern>
</servlet-mapping>
</web-app>
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