Servlet

最新推荐文章于 2025-05-22 17:13:35 发布

wh_in_silence

最新推荐文章于 2025-05-22 17:13:35 发布

阅读量78

点赞数

文章标签： maven java servlet

本文链接：https://blog.youkuaiyun.com/wendy_gong/article/details/121156947

版权

本文详细讲解了Servlet的工作原理，包括Servlet映射路径的配置，如精确路径、通用路径和前缀/后缀匹配。还介绍了ServletContext的使用，如共享数据和获取初始化参数。此外，涵盖了响应操作，如请求转发、文件下载、验证码和重定向的示例。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

1. Servlet运行原理

在这里插入图片描述
上图来自：Servlet运行原理

2. Mapping

web.xml详解
servlet映射路径位于
使用‘/’开头，使用‘/’结尾，表示使用路径匹配，比如/foo/bar/

一个servlet可以指定一个映射路径，即精确路径匹配

<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>/hello1</url-pattern>
  </servlet-mapping>
  <servlet-mapping>
    <servlet-name>hello2</servlet-name>
    <url-pattern>/hello2</url-pattern>
  </servlet-mapping>

一个servlet可以指定通用映射路径
只使用‘/*’，表示匹配所有的请求；

<servlet-mapping>
    <servlet-name>hello</servlet-name>
    <url-pattern>/*</url-pattern>
  </servlet-mapping>

可以使用前缀或后缀等等

<servlet-mapping>
    <servlet-name>hello</servlet-name>
    <url-pattern>.do</url-pattern>
  </servlet-mapping>

在这里插入图片描述

<servlet-mapping>
    <servlet-name>hello</servlet-name>
    <url-pattern>*.do</url-pattern>
  </servlet-mapping>

连接成功

url优先级问题

 <servlet-mapping>
    <servlet-name>hello</servlet-name>
    <url-pattern>/hello</url-pattern>
  </servlet-mapping>

  <servlet-mapping>
    <servlet-name>hello2</servlet-name>
    <url-pattern>/*</url-pattern>
  </servlet-mapping>

默认走通用的，但如果指定了对应路径，会优先走指定的路径，而非通用路径。

tips:可以将web.xml的模板内容在项目下新建一个note.md保存起来，方便使用。
在这里插入图片描述

web.xml
```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"
metadata-complete="true">

</web-app>

3.ServletContext

WEB容器在启动的时候，他会为每个web程序创建一个对应的ServletContext对象，它代表了当前的web应用。

3.1 共享数据

在一个Servlet中保存的数据，可以在另一个Servlet中拿到。注意需要先提交保存数据，再访问保存的数据

package org.raylene.servlet;

import javax.servlet.ServletContext;
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.util.Date;

public class HelloServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        ServletContext servletContext = this.getServletContext();
        String username = "tom & jerry测试中文";
        servletContext.setAttribute("username",username);
    }
}

package org.raylene.servlet;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class GetContext extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setContentType("text/html;charset = utf-8");
        ServletContext servletContext = this.getServletContext();
        String username = (String)servletContext.getAttribute("username");
        resp.getWriter().println("名字："+username);
    }
}

<?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"
         metadata-complete="true">
    <servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>org.raylene.servlet.HelloServlet</servlet-class>
    </servlet>
    <servlet>
        <servlet-name>getc</servlet-name>
        <servlet-class>org.raylene.servlet.GetContext</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/hello</url-pattern>

    </servlet-mapping>
    <servlet-mapping>
        <servlet-name>getc</servlet-name>
        <url-pattern>/getc</url-pattern>
    </servlet-mapping>

</web-app>

3.2 获取初始化参数getInitParameter

<?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"
         metadata-complete="true">
    <context-param>
        <param-name>url</param-name>
        <param-value>jdbc:mysql://localhost:3006/mybatis</param-value>
    </context-param>
      
    <servlet>
        <servlet-name>getpara</servlet-name>
        <servlet-class>org.raylene.servlet.ServletDemo03</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>getpara</servlet-name>
        <url-pattern>/gp</url-pattern>
    </servlet-mapping>
   

</web-app>

package org.raylene.servlet;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class ServletDemo03 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext servletContext = this.getServletContext();

        String url = servletContext.getInitParameter("url");
        resp.setContentType("text/html;charset=utf-8");
        resp.getWriter().println("url为："+url);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doPost(req, resp);
    }
}

3.3实现请求转发

<servlet>
        <servlet-name>rd</servlet-name>
        <servlet-class>org.raylene.servlet.ServletDemo04</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>rd</servlet-name>
        <url-pattern>/rd</url-pattern>
    </servlet-mapping>

package org.raylene.servlet;

import javax.servlet.RequestDispatcher;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;


public class ServletDemo04 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setContentType("text/html;charset=utf-8");
        System.out.println("进入ServletDemo04");
        ServletContext servletContext = this.getServletContext();
        RequestDispatcher requestDispatcher = servletContext.getRequestDispatcher("/gp");//转发的请求路径
        requestDispatcher.forward(req,resp);//调用forward方法实现转发
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doPost(req, resp);
    }
}

当我们在浏览器输入rd时，==却收到了ServletDemo03对应的资源，==说明实现了转发
在这里插入图片描述

请求转发：
request.getRequestDispatcher(URL地址).forward(request, response)

处理流程：

客户端发送请求，Servlet做出业务逻辑处理。
Servlet调用forword()方法，服务器Servlet把目标资源返回给客户端浏览器。

参考链接：https://www.jianshu.com/p/29822c2c1ec0

在这里插入图片描述

3.4 读取资源文件

Properties
关于properties类的一些介绍可以参考这篇博文：Java中Properties类详解

在Java目录下新建properties
在resources目录下新建properties
发现都被打爆到同一个路径下，target下servlet-02的classes路径下，该路径称为classpath

配置pom文件

<!--在build中配置resources，来防止我们资源导出失败的问题-->
    <build>
        <resources>
            <resource>
                <directory>src/main/resources</directory>
                <includes>
                    <include>**/*.properties</include>
                    <include>**/*.xml</include>
                </includes>
                <filtering>true</filtering>
            </resource>
            <resource>
                <directory>src/main/java</directory>
                <includes>
                    <include>**/*.properties</include>
                    <include>**/*.xml</include>
                </includes>
                <filtering>true</filtering>
            </resource>
        </resources>
    </build>

注册servlet映射

<servlet>
        <servlet-name>prop</servlet-name>
        <servlet-class>org.raylene.servlet.ServletDemo06</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>prop</servlet-name>
        <url-pattern>/prop</url-pattern>
    </servlet-mapping>

package org.raylene.servlet;

import javax.servlet.ServletContext;
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.InputStream;
import java.util.Properties;


public class ServletDemo06 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext servletContext = this.getServletContext();
        InputStream is = servletContext.getResourceAsStream("/WEB-INF/classes/db.properties");
        Properties properties = new Properties();
        properties.load(is);
        String usernm = properties.getProperty("username");
        String pwd= properties.getProperty("password");
        resp.setContentType("text/html;charset=utf-8");
        resp.getWriter().print(usernm+":"+pwd);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doPost(req, resp);
    }
}

读取java目录下的文件

InputStream is = servletContext.getResourceAsStream("/WEB-INF/classes/org/raylene/servlet/aa.properties");
        Properties properties = new Properties();
        properties.load(is);

4.HttpServletResponse

Web服务器接收到客户端的请求，针对这个请求，分别创建一个代表请求的HttpServletRequest对象，和代表响应的HttpResponse对象

获取客户端请求过来的参数HttpServletRequest
给用户返回一些信息，用HttpServletResponse对象
接口HttpServletResponse继承ServletResponse

public interface HttpServletResponse extends ServletResponse

public interface ServletResponse

ServletResponse中的方法有：

String getCharacterEncoding();

    String getContentType();

    ServletOutputStream getOutputStream() throws IOException;

    PrintWriter getWriter() throws IOException;

    void setCharacterEncoding(String var1);

    void setContentLength(int var1);

    void setContentType(String var1);

    void setBufferSize(int var1);

    int getBufferSize();

    void flushBuffer() throws IOException;

    void resetBuffer();

    boolean isCommitted();

    void reset();

    void setLocale(Locale var1);

    Locale getLocale();

ServletResponse中常用的方法

ServletOutputStream getOutputStream() throws IOException;

    PrintWriter getWriter() throws IOException;//负责向浏览器发送响应数据
    void setContentType(String var1);

HttpServletResponse中有一些HTTP状态码常量及一些相应的方法
响应状态码

int SC_CONTINUE = 100;
    int SC_SWITCHING_PROTOCOLS = 101;
    int SC_OK = 200;
    int SC_CREATED = 201;
    int SC_ACCEPTED = 202;
    int SC_NON_AUTHORITATIVE_INFORMATION = 203;
    int SC_NO_CONTENT = 204;
    int SC_RESET_CONTENT = 205;
    int SC_PARTIAL_CONTENT = 206;
    int SC_MULTIPLE_CHOICES = 300;
    int SC_MOVED_PERMANENTLY = 301;
    int SC_MOVED_TEMPORARILY = 302;
    int SC_FOUND = 302;
    int SC_SEE_OTHER = 303;
    int SC_NOT_MODIFIED = 304;
    int SC_USE_PROXY = 305;
    int SC_TEMPORARY_REDIRECT = 307;
    int SC_BAD_REQUEST = 400;
    int SC_UNAUTHORIZED = 401;
    int SC_PAYMENT_REQUIRED = 402;
    int SC_FORBIDDEN = 403;
    int SC_NOT_FOUND = 404;
    int SC_METHOD_NOT_ALLOWED = 405;
    int SC_NOT_ACCEPTABLE = 406;
    int SC_PROXY_AUTHENTICATION_REQUIRED = 407;
    int SC_REQUEST_TIMEOUT = 408;
    int SC_CONFLICT = 409;
    int SC_GONE = 410;
    int SC_LENGTH_REQUIRED = 411;
    int SC_PRECONDITION_FAILED = 412;
    int SC_REQUEST_ENTITY_TOO_LARGE = 413;
    int SC_REQUEST_URI_TOO_LONG = 414;
    int SC_UNSUPPORTED_MEDIA_TYPE = 415;
    int SC_REQUESTED_RANGE_NOT_SATISFIABLE = 416;
    int SC_EXPECTATION_FAILED = 417;
    int SC_INTERNAL_SERVER_ERROR = 500;
    int SC_NOT_IMPLEMENTED = 501;
    int SC_BAD_GATEWAY = 502;
    int SC_SERVICE_UNAVAILABLE = 503;
    int SC_GATEWAY_TIMEOUT = 504;
    int SC_HTTP_VERSION_NOT_SUPPORTED = 505;

4.1 常见应用

向浏览器输出消息
下载文件
实现方法
1. 获取下载路径
2. 下载文件的名称
3. 浏览器支持下载我们需要的内容
4. 获取下载文件的输入流
5. 创建缓存区
6. 获取OutputStream对象
7. 将FileOutputStream写入buffer缓存区
8. 使用OutputStream将缓存区的对象输出到客户端

4.1 Response下载资源

IO流IO
2，3搞懂

package org.raylene.servlet;

import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.FileInputStream;
import java.io.IOException;
import java.net.URLEncoder;

public class FileServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//   1. 获取下载路径
        //获取资源路径
        //文件输入流读取资源
        String realPath = "G:\\file\\wonder.png";

        System.out.println("文件的下载路径为："+realPath);
//   2. 下载文件的名称
        //获取文件名
        //substring() 方法返回字符串的子字符串。
        //lastIndexOf() 方法可返回一个指定的字符串值最后出现的位置，
        // 在一个字符串中的指定位置从后向前搜索
        String fileName = realPath.substring(realPath.lastIndexOf("\\")+1);
//   3. 浏览器支持下载我们需要的内容
        //设置消息头，告诉浏览器，我要下载1.png这个图片
        resp.setHeader("Content-Disposition", "attachment; filename="+ URLEncoder.encode(fileName,"UTF-8"));
//   4. 获取下载文件的输入流
        FileInputStream in = new FileInputStream(realPath);

//   5. 创建缓存区
        int len = 0;
        byte[] bytes = new byte[1024];

//   6. 获取OutputStream对象
        ServletOutputStream out = resp.getOutputStream();

//   7. 将FileOutputStream写入buffer缓存区
        while((len=in.read(bytes))>0){
            out.write(bytes, 0, len);
        }
        //关闭资源
        out.close();
        in.close();


    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

4.2 验证码

resp的方法解读，缓存策略解读，图片画笔

package org.raylene.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.util.Random;

public class ImageServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doPost(req, resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //如何让浏览器5秒刷新一次
        //设置一下自动刷新功能
        //下面两个语句均可，选其一
        resp.setHeader("refresh","2");
        //resp.setIntHeader("refresh", 5);
        //在内存新建一个图片
        BufferedImage image = new BufferedImage(80,20,BufferedImage.TYPE_INT_RGB);
        //得到图片
        Graphics2D graphics = (Graphics2D) image.getGraphics();
        //设置背景颜色
        graphics.setColor(Color.pink);
        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);

        resp.setContentType("image/jpeg");
        //浏览器不缓存
        resp.setHeader("Cache-Control", "no-cache");
        resp.setHeader("Cache-Control", "no-store");
        resp.setHeader("Pragma", "no-cache");
        resp.setDateHeader("Expires", 0);

//        或
//
//        response.setHeader("Cache-Control", "no-cache");
//        response.setHeader("Pragma", "no-cache");
//        response.setDateHeader("Expires", 0);
//
        //把图片写给浏览器
        ImageIO.write(image, "jpg", resp.getOutputStream());

    }
    private String makeNum(){
        Random r = new Random();
        String snum = r.nextInt(999999) + "";
        StringBuffer sbuffer = new StringBuffer();
        //防止生成数小于6位，若小于6位，则用0填充
        for (int i = 0; i < 6-snum.length(); i++) {
            sbuffer.append("0");
        }
        String s = sbuffer.toString()+snum;
        return s;

    }
}

4.3 Response重定向

重定向：
response.sendRedirect(URL地址)
处理流程：

客户端发送请求，Servlet做出业务逻辑处理。
Servlet调用response.sendReadirect()方法，把要访问的目标资源作为response响应头信息发给客户端浏览器。
客户端浏览器重新访问服务器资源xxx.jsp，服务器再次对客户端浏览器做出响应。
在这里插入图片描述
参考链接：https://www.jianshu.com/p/29822c2c1ec0


package org.raylene.servlet;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class RedirectServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doPost(req,resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.sendRedirect("/response_war/img");
    }
}

在这里插入图片描述
注意
当进入该项目时，默认进入index.jsp，当在URL中输入red时，它会告诉浏览器，你去其他的url下找，此时，需要输入项目完整名称(除去localhost:8080这一段的后面全部)，也就是重定向可以区定向到不同的项目下。
在这里插入图片描述

在这里插入图片描述

在这里插入图片描述
重定向到另一个项目下

为什么不能重定向到另一个项目下：此时，若你在tomcat中没有配置你要跳转的项目对应的war包，则无法跳转，若已配置，则可以成功实现跳转。如图2所示

 resp.sendRedirect("/servlet_01_war/hello");

图1

图2
在这里插入图片描述
重定向与转发的区别
相同点：

页面都可以拿到对应资源

不同点

请求转发时，url不会发生变化
重定向时，url会发生变化。

5. HttpServletRequest

关于各类文件乱码：乱码参考
index.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<html>
<body>
<h2>Hello World!</h2>
<%--这里的提交路径，需要找到项目的路径--%>
<form action="${pageContext.request.contextPath}/login" method="get">
    用户名：<input type="text" name="username"><br>
    密码：<input type="password" name="password"><br>
    手机号<input type="tel" name="telp"><br>
    <input type="submit">
</form>
</body>
</html>

success.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<h1>success!</h1>
</body>
</html>

package org.raylene.servlet;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class RequestServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doPost(req,resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        System.out.println("进入了这个请求");
        String username = req.getParameter("username");
        String pwd = req.getParameter("password");

        System.out.println(username+":"+pwd);

        resp.sendRedirect("/response_war/success.jsp");
    }
}

5.1 获取参数，请求转发

index.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>登录</title>
</head>
<body>
<h1>登录</h1>
<div style="text-align: center">
<%--    以post方式提交表单，提交的路径/login--%>
    <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="bear">bear
        <input type="checkbox"name="hobby" value="football">football
        <input type="checkbox"name="hobby" value="table tennis">table tennis
        <input type="checkbox"name="hobby" value="basketball">basketball
        <input type="checkbox"name="hobby" value="swimming">swimming
       <input type="submit">
    </form>
</div>

</body>
</html>

succcess.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<h1>
    登录成功
</h1>
</body>
</html>

package org.raylene.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.util.Arrays;

public class LoginServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doPost(req,resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String username = req.getParameter("username");
        String password= req.getParameter("password");
        String[] hobbies = req.getParameterValues("hobby");
        System.out.println("---------------");
        System.out.println(username+":"+password);
        System.out.println(Arrays.toString(hobbies));
        System.out.println("==========================");
//        resp.sendRedirect("/request_war/succcess.jsp");
        //请求转发
        req.setCharacterEncoding("utf-8");
        req.getRequestDispatcher("./succcess.jsp").forward(req,resp);
    }
}