Servlet详解含实例

2023-11-16 11:45:52

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：
Servlet详解含实例

<modules>
    <module>servlet-01</module>
</modules>

子项目中：

Servlet详解含实例

<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
Servlet详解含实例

4、编写Servlet程序

  1. 编写一个普通类
  2. 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

Servlet详解含实例

7、启动测试

Servlet详解含实例

Servlet详解含实例

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：

Servlet详解含实例

  • 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>

Servlet详解含实例

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

Servlet详解含实例

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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