JavaWeb-6-Servlet

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跟着狂神学JavaWeb -> 6. Servlet

6.1 Servlet简介

• Servlet就是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Servlet
• 如果想要开发一个Servlet程序，只需要完成两个步骤：
  • 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

6.2 HelloServlet

1. 构建一个普通的Maven项目，删除src目录，以后的就在这个项目里面建立Module；这个空的工程就是Maven的主工程

2. 关于Maven父子工程的理解：

   • 父工程中会有：

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

   • 子工程中会有：

   <parent>
       <artifactId>javaweb-02-servlet</artifactId>
       <groupId>org.example</groupId>
       <version>1.0-SNAPSHOT</version>
   </parent>
   

   • 父工程中的Jar包子工程可以使用

3. Maven环境优化

   • 修改web.xml为最新的配置

   <?xml version="1.0" encoding="UTF-8"?>
   <web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            xmlns="http://xmlns.jcp.org/xml/ns/javaee"
            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>
   

   • 将Maven结构搭建完整

   

4. 编写一个Servlet程序

   • ①编写一个普通的类
   • ②实现Servlet接口，直接继承HttpServlet

   public class HelloServlet extends HttpServlet {
       //由于get/post只是请求实现的不同方式，可以互相调用，业务逻辑都一样
       @Override
       protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
           PrintWriter writer = resp.getWriter(); /*响应流*/
           writer.print("Hello, Servlet");
       }
   
       @Override
       protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
           super.doPost(req, resp);
       }
   }
   

5. 编写Servlet的映射

   为什么需要映射？

   • 因为这里写的是Java程序，但是需要通过浏览器来访问，而浏览器需要连接web服务器，所以需要在web服务器中注册这里写的Servlet，还需要给一个浏览器能够访问的路径：

   <!--注册Servlet-->
       <servlet>
           <servlet-name>hello</servlet-name>
           <servlet-class>com.ano.servlet.HelloServlet</servlet-class>
       </servlet>
       <!--Servlet的请求路径-->
       <servlet-mapping>
           <servlet-name>hello</servlet-name>
           <url-pattern>/hello</url-pattern>
       </servlet-mapping>
   

6. 配置Tomcat

7. 启动测试

   • 浏览器输入：localhost:8080/s1/hello

   遇到的问题：利用Tomcat 10.0.4 构建类servlet报错：类HelloServlet不是Servlet？

   • 解决办法：参考

   • 用到的两个依赖：

   <!-- https://mvnrepository.com/artifact/org.apache.tomcat/tomcat-servlet-api -->
   <dependency>
       <groupId>org.apache.tomcat</groupId>
       <artifactId>tomcat-servlet-api</artifactId>
       <version>10.0.4</version>
   </dependency>
   <!-- https://mvnrepository.com/artifact/org.apache.tomcat/tomcat-jsp-api -->
   <dependency>
       <groupId>org.apache.tomcat</groupId>
       <artifactId>tomcat-jsp-api</artifactId>
       <version>10.0.4</version>
   </dependency>
   

6.3 Servlet原理

6.4 Mapping问题

• 一个Servlet可以指定一个映射路径
• 一个Servlet可以指定多个映射路径
• 一个Servlet可以指定通用映射路径
• 默认请求路径
• 可以自定义后缀实现请求路径
• 优先级问题

6.5 ServletContext 应用

Web容器在启动的时候，它会为每个Web程序都创建一个对应的ServletContext对象，就代表的当前的web应用。

6.5.1. 共享数据

/**
 * @author ano
 * 放置数据的类
 */
public class HelloServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        //data
        String username = "南柱赫";
        //将data保存在了ServletContext中
        context.setAttribute("username",username);


        System.out.println("hello");
    }
}

/**
 * @author ano
 * 获取数据的类
 */
public class GetServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        ServletContext context = this.getServletContext();
        String username = (String) context.getAttribute("username");
        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf-8");
        PrintWriter writer = resp.getWriter();
        writer.println(username);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doPost(req, resp);
    }
}

• web.xml配置

<?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.ano.servlet.HelloServlet</servlet-class>

    </servlet>

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

    <servlet>
        <servlet-name>getc</servlet-name>
        <servlet-class>com.ano.servlet.GetServlet</servlet-class>
    </servlet>

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

</web-app>

测试访问结果：

6.5.2. 获取初始化参数

• web.xml中的配置

<!--配置一些web应用的初始化参数-->
<context-param>
    <param-name>url</param-name>
    <param-value>jdbc:mysql://localhost:3306/mybatis</param-value>
</context-param>

<servlet>
    <servlet-name>getp</servlet-name>
    <servlet-class>com.ano.servlet.GetParam</servlet-class>
</servlet>

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

• 获取初始化参数的类

/**
 * @author wangjiao
 */
public class GetParam extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String url = context.getInitParameter("url");
        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf-8");
        resp.getWriter().println(url);
    }

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

• 测试结果

6.5.3. 请求转发

/**
 * @author ano
 * ServletContext-->请求转发
 */
public class ReqDispatch extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        context.getRequestDispatcher("/getp").forward(req,resp);
    }
}

• web.xml中的配置

<servlet>
    <servlet-name>reqD</servlet-name>
    <servlet-class>com.ano.servlet.ReqDispatch</servlet-class>
</servlet>

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

• 测试结果：

6.5.4 读取资源文件

在java目录下新建db.properties文件;

在resources目录下新建db.properties文件

发现：都被打包在同一个路径下：class，俗称这个路径为classpath

• db.properties

username=root
password=123456

• 读取资源类

/**
 * @author ano
 */
public class PropertiesServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        InputStream inputStream = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db.properties");

        Properties properties = new Properties();
        properties.load(inputStream);

        String username = properties.getProperty("username");
        String password = properties.getProperty("password");

        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf-8");

        resp.getWriter().print(username + ":" + password);
    }

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

• web.xml配置

<servlet>
    <servlet-name>propS</servlet-name>
    <servlet-class>com.ano.servlet.PropertiesServlet</servlet-class>
</servlet>

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

• 测试结果

6.6 HttpServletResponse

• web服务器接收到客户端的http请求，针对这个请求，分别会创建一个代表请求的HttpServletRequest对象，代表响应的一个HttpServletResponse对象；
• 如果要获取客户端请求过来的参数，找HttpServletRequest;
• 如果要给客户端响应一些信息，找HttpServletResponse.

6.6.1 简单分类

• 负责向浏览器发送数据的方法

ServletOutputStream getOutputStream() throws IOException;

PrintWriter getWriter() throws IOException;

• 负责向浏览器发送响应头的方法

void setCharacterEncoding(String var1);

void setContentLength(int var1);

void setContentLengthLong(long var1);

void setContentType(String var1);

void setDateHeader(String var1, long var2);

void addDateHeader(String var1, long var2);

void setHeader(String var1, String var2);

void addHeader(String var1, String var2);

void setIntHeader(String var1, int var2);

void addIntHeader(String var1, int var2);

• 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;

6.6.2 下载文件

常见应用包括：向浏览器输出消息；下载文件等。

• 下载文件步骤：
  1. 获取下载文件的路径
  2. 下载的文件名
  3. 设置想办法让浏览器能够支持需要下载的文件
  4. 获取下载文件的输入流
  5. 创建缓冲区
  6. 获取OutputStream对象
  7. 将FileOutputStream流写入到buffer缓冲区
  8. 使用OutputStrean将缓冲区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 {
        //1. 获取下载文件的路径
        //String realPath = this.getServletContext().getRealPath("/babe.jpg");
        String realPath = "E:\\workspace\\javaweb-02-servlet\\response\\src\\main\\resources\\babe.jpg";
        System.out.println("下载文件的路径：" + realPath);
        //2. 下载的文件名
        String fileName = realPath.substring(realPath.lastIndexOf("\\") + 1);
        //3. 设置想办法让浏览器能够支持需要下载的文件,如果是中文文件名，用URLEncoder.encode编码，否则有可能乱码
        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. 创建缓冲区
        int len = 0;
        byte[] buffer = new byte[1024];
        //6. 获取OutputStream对象
        ServletOutputStream out = resp.getOutputStream();
        //7. 将FileOutputStream流写入到buffer缓冲区,使用OutputStrean将缓冲区的数据输出到客户端
        while((len = in.read(buffer)) > 0) {
            out.write(buffer,0,len);
        }
        in.close();
        out.close();
    }

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

6.6.3 验证码功能

验证码如何实现：

• 前端实现；
• 后端实现：需要用到Java的图片类，生成一个图片

public class ImageServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //1.让浏览器5s刷新一次
        resp.setHeader("refresh","3");
        //2.在内存种创建一个图片
        BufferedImage image = new BufferedImage(80, 20, BufferedImage.TYPE_INT_RGB);
        //3.得到图片
        //笔
        Graphics2D g = (Graphics2D)image.getGraphics();
        //设置图片的背景颜色
        g.setColor(Color.WHITE);
        g.fillRect(0,0,80,20);
        //4.给图片写数据
        g.setColor(Color.BLUE);
        g.setFont(new Font(null,Font.BOLD,20));
        g.drawString(makeNum(),0,20);

        //5.告诉浏览器，这个请求用图片的方式打开
        resp.setContentType("image/jpeg");
        //不让浏览器缓存
        resp.setDateHeader("expires",-1);
        resp.setHeader("Cache-Control","no-cache");
        resp.setHeader("Pragma","no-cache");

        //6.把图片写给浏览器
        ImageIO.write(image,"jpg",resp.getOutputStream());

    }

    /**
     * 生成随机数
     */
    private String makeNum() {
        Random random = new Random();
        String num = random.nextInt(9999999) + "";
        StringBuffer sb = new StringBuffer();
        for (int i = 0; i < 7-num.length(); i++) {
            sb.append("0");
        }
        num = sb.toString() + num;
        return num;
    }

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

6.6.4 实现重定向

• 一个Web资源（B）收到客户端（A）请求后，会通知客户端（A）去访问另一个Web资源（C），这个过程叫做重定向。

void sendRedirect(String var1) throws IOException;

• 常见场景：用户登录等

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

        /*sendRedirect()原理：*/
        //resp.setHeader("Location","/r/image");
        //resp.setStatus(HttpServletResponse.SC_MOVED_TEMPORARILY);
        resp.sendRedirect("/r/image");
    }
}

面试题：重定向和转发的区别：

• 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
  • 重定向时，url地址栏会发生变化。302

• 示例：

/**
 * @author wangjiao
 */
public class RequestTest extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //处理请求
        String username = req.getParameter("username");
        String password = req.getParameter("password");

        System.out.println(username + ":" + password);
        //重定向时注意路径问题，否则会404
        resp.sendRedirect("/r/success.jsp");
    }

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

web.xml

<servlet>
    <servlet-name>request</servlet-name>
    <servlet-class>com.ano.servlet.RequestTest</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>request</servlet-name>
    <url-pattern>/login</url-pattern>
</servlet-mapping>

index.jsp

<html>
<body>
<h2>Hello World!</h2>
<%--这里提交的路径，需要寻找到项目的路径--%>
<form action="login" method="get">
    用户名：<input type="text" name="username">
    密码：<input type="password" name="password">
    <input type="submit">
</form>
</body>
</html>

success.jsp

<html>
<head>
    <title>Title</title>
</head>
<body>
<h1>Success</h1>
</body>
</html>

6.7 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中，通过这个HttpServletRequest的方法，获得客户端的所有请求信息。

6.7.1 获取前端传递的参数，请求转发

• 获取前端参数的主要方法有以下4个，但常用的是前两个。

String getParameter(String var1);

String[] getParameterValues(String var1);

Enumeration<String> getParameterNames();

Map<String, String[]> getParameterMap();

public class LoginServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        String[] hobbies = req.getParameterValues("hobbies");
        System.out.println(username);
        System.out.println(password);
        System.out.println(Arrays.toString(hobbies));

        resp.setCharacterEncoding("utf-8");
        //通过请求转发
        req.getRequestDispatcher(req.getContextPath() + "/success.jsp").forward(req,resp);
    }

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

web.xml

<servlet>
    <servlet-name>LoginServlet</servlet-name>
    <servlet-class>com.ano.servlet.LoginServlet</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>LoginServlet</servlet-name>
    <url-pattern>/login</url-pattern>
</servlet-mapping>

index.jsp

<%--
  Created by IntelliJ IDEA.
  User: wangjiao
  Date: 2021/8/5
  Time: 10:57
  To change this template use File | Settings | File Templates.
--%>
<%@ 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: left">
    <%--以post方式提交表单，提交到login请求--%>
    <form action="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="hobbies" value="女明星">女明星
        <input type="checkbox" name="hobbies" value="男明星">男明星
        <input type="checkbox" name="hobbies" value="唱歌">唱歌
        <input type="checkbox" name="hobbies" value="电影">电影
        <input type="submit">
    </form>
</div>

</body>
</html>

success.jsp

<%--
  Created by IntelliJ IDEA.
  User: wangjiao
  Date: 2021/8/5
  Time: 11:12
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<h1>登录成功！</h1>
</body>
</html>

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