Servlet详解

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Servlet详解

一、什么是Servlet

1、Servlet 是 JavaEE 规范之一。规范就是接口
2、Servlet 就 JavaWeb 三大组件之一。三大组件分别是：Servlet 程序、Filter 过滤器、Listener 监听器。
3、Servlet 是运行在服务器上的一个 java 小程序，它可以接收客户端发送过来的请求，并响应数据给客户端.
4、把实现了Servlet接口的Java程序叫做，Servlet。

二、手动实现HelloServlet程序

1、导入依赖

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<!--添加Servlet和JSP依赖-->
<dependency>
    <groupId>jakarta.servlet.jsp</groupId>
    <artifactId>jakarta.servlet.jsp-api</artifactId>
    <version>3.0.0</version>
    <scope>provided</scope>
</dependency>
<dependency>
    <groupId>jakarta.servlet</groupId>
    <artifactId>jakarta.servlet-api</artifactId>
    <version>5.0.0</version>
    <scope>provided</scope>
</dependency>

2、编写servlet类继承HttpServlet

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public class HelloServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//        ServletOutputStream outputStream = resp.getOutputStream();
        System.out.println("进入了doGet");
        //设置编码，防止中文乱码，默认是ISO-8859-1
        resp.setContentType("text/html"); //ajax异步请求为application/json格式
        resp.setCharacterEncoding("utf-8");
        PrintWriter out = resp.getWriter(); //响应流
        out.print("HelloServlet");
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

3、到 web.xml 中去配置映射

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<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="https://jakarta.ee/xml/ns/jakartaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="https://jakarta.ee/xml/ns/jakartaee
https://jakarta.ee/xml/ns/jakartaee/web-app_5_0.xsd"
version="5.0" metadata-complete="true">
    <!-- servlet 标签给 Tomcat 配置 Servlet 程序 --> 
    <servlet> 
        <!--servlet-name 标签 Servlet 程序起一个别名（一般是类名） -->
        <servlet-name>HelloServlet</servlet-name>
        <!--servlet-class 是 Servlet 程序的全类名--> 
        <servlet-class>com.nichu.servlet.HelloServlet</servlet-class> 
    </servlet> 
    <!--servlet-mapping 标签给2 servlet 程序配置访问地址-->
    <servlet-mapping> 
        <!--servlet-name 标签的作用是告诉服务器，我当前配置的地址给哪个 Servlet 程序使用--> <servlet-name>HelloServlet</servlet-name>
        <!--url-pattern 标签配置访问地址 <br/>
 / 斜杠在服务器解析的时候，表示地址为：http://ip:port/工程路径 <br/>
 /hello 表示地址为：http://ip:port/工程路径/hello <br/> -->
        <url-pattern>/hello</url-pattern> 
    </servlet-mapping>
</web-app>

4、配置Tomcat

5、启动项目，访问路径

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之后，会：

四、Mapping问题

1. 一个Servlet可以指定一个映射路径

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

2. 一个Servlet可以指定多个映射路径

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<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>
<servlet-mapping>
    <servlet-name>hello</servlet-name>
    <url-pattern>/hello3</url-pattern>
</servlet-mapping>
<servlet-mapping>
    <servlet-name>hello</servlet-name>
    <url-pattern>/hello4</url-pattern>
</servlet-mapping>
<servlet-mapping>
    <servlet-name>hello</servlet-name>
    <url-pattern>/hello5</url-pattern>
</servlet-mapping>

3. 一个Servlet可以指定通用映射路径

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

4. 默认请求路径

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<!--默认请求路径-->
<servlet-mapping>
    <servlet-name>hello</servlet-name>
    <url-pattern>/*</url-pattern>
</servlet-mapping>

5. 指定一些后缀或者前缀等等….

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<!--可以自定义后缀实现请求映射
    注意点，*前面不能加项目映射的路径
    hello/sajdlkajda.nichu
    -->
<servlet-mapping>
    <servlet-name>hello</servlet-name>
    <url-pattern>*.nichu</url-pattern>
</servlet-mapping>

6. 优先级问题

指定了固有的映射路径优先级最高，如果找不到就会走默认的处理请求（404）；

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<!--404-->
<servlet>
    <servlet-name>error</servlet-name>
    <servlet-class>com.kuang.servlet.ErrorServlet</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>error</servlet-name>
    <url-pattern>/*</url-pattern>
</servlet-mapping>

五、ServletContext

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，它代表了当前的web应用

应用：

1. 共享数据

  • 在一个Servlet中写入的数据，在另一个Servlet中可以访问。
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public class Servlet01 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        PrintWriter writer = resp.getWriter();
        writer.print("hello,Servlet");
        ServletContext contest = this.getServletContext();
        String name="倪矗";
        contest.setAttribute("name",name);
    }

}
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public class Servlet02 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf-8");
        String name = (String) context.getAttribute("name");
        resp.getWriter().print("name:"+name);
    }
}

2. 获得初始化参数

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<!--  配置一下web应用初始化参数-->
  <context-param>
    <param-name>url</param-name>
    <param-value>jdbc:mysql://localhost:3306/mybatis</param-value>
  </context-param>
<!-- 添加映射 -->
  <servlet>
    <servlet-name>url</servlet-name>
    <servlet-class>com.nichu.servlet.Servlet03</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>url</servlet-name>
    <url-pattern>/DB</url-pattern>
  </servlet-mapping>
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public class Servlet03 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //创建context对象
        ServletContext context = this.getServletContext();
        //获得初始化参数
        String url = context.getInitParameter("url");
        resp.getWriter().println(url);
    }
}

3. 请求转发(getRequestDispatcher)

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public class Servlet04 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        //获得请求转发的路径
        RequestDispatcher requestDispatcher = context.getRequestDispatcher("/DB");
        //调用forward方法实现请求转发
        requestDispatcher.forward(req,resp);
    }

}

4. 读取资源文件

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//创建properties文件
username=root
password=12354654
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protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    ServletContext context = this.getServletContext();
        //以流的方式获取资源文件
    InputStream is = context.getResourceAsStream("/WEB-INF/classes/db.properties");
        //新建Properties对象
    Properties properties = new Properties();
        //加载得到的流
    properties.load(is);
        //从流中获取指定参数
    String usr = properties.getProperty("username");
    String pwd = properties.getProperty("password");
    resp.getWriter().println(usr+":"+pwd);
}

访问测试即可；

五、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；

  • 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

1. 简单分类

负责向浏览器发送数据的方法

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ServletOutputStream getOutputStream() throws IOException;
PrintWriter getWriter() throws IOException;

负责向浏览器发送响应头的方法

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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);

响应的状态码

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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;

2. 下载文件

  1. 要获取下载文件的路径
  2. 下载的文件名是啥？
  3. 设置想办法让浏览器能够支持下载我们需要的东西
  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OutputStream将缓冲区中的数据输出到客户端！
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protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    //1.获取下载文件的路径
    String realPath ="/Users/haikez/Desktop/javaweb/javaweb-01-servlet/javaweb-02-servlet/src/main/resources/1.png";
    //2.获取要下载的文件名
    String filename = realPath.substring(realPath.lastIndexOf("/")+1);
    //3.设置想办法让浏览器能够支持(Content-Disposition)下载我们需要的东西,中文文件名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(new File(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OutputStream将缓冲区中的数据输出到客户端！
    while ((len=in.read(buffer)) != -1) {
        out.write(buffer,0,len);
    }
    in.close();
    out.close();
}

4. 验证码功能

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public class ImageServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //如何让浏览器3秒自动刷新一次;
        resp.setHeader("refresh","3");
        //在内存中创建一个图片
        BufferedImage image = new BufferedImage(80,20,BufferedImage.TYPE_INT_RGB);
        //得到图片
        Graphics2D g = (Graphics2D) image.getGraphics(); //笔
        //设置图片的背景颜色
        g.setColor(Color.white);
        g.fillRect(0,0,80,20);
        //给图片写数据
        g.setColor(Color.BLUE);
        g.setFont(new Font(null,Font.BOLD,20));
        g.drawString(makeNum(),0,20);
        //告诉浏览器，这个请求用图片的方式打开
        resp.setContentType("image/jpeg");
        //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(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);
    }
}

5. 实现重定向(resp.sendRedirect(“/r/success.jsp”))


B一个web资源收到客户端A请求后，B会通知A去访问另外一个web资源C，这个过程叫重定向。常见场景：

  • 用户登录

重定向和转发的区别？

  • 请求转发url不会产生变化，重定向会发生变化
  • 请求转发可以携带参数，重定向不能携带参数
  • 请求转发只能发送一个请求，重定向至少发送两次请求

简单实现登录重定向:

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<%--这里提交的路径，需要寻找到项目的路径--%>
<%--${pageContext.request.contextPath}代表当前的项目--%>
<form action="${pageContext.request.contextPath}/login" method="get">
    用户名：<input type="text" name="username"> <br>
    密码：<input type="password" name="password"> <br>
    <input type="submit">
</form>
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@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);
    //重定向时候一定要注意，路径问题，否则404；
    resp.sendRedirect("/r/success.jsp");
}
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<servlet>
  <servlet-name>requset</servlet-name>
  <servlet-class>com.kuang.servlet.RequestTest</servlet-class>
</servlet>
<servlet-mapping>
  <servlet-name>requset</servlet-name>
  <url-pattern>/login</url-pattern>
</servlet-mapping>
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<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<h1>Success</h1>
</body>
</html>

六、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；

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@WebServlet("/req")
public class ReqServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("url路径："+req.getRequestURL());
        System.out.println("uri路径："+req.getRequestURI());
        System.out.println("context路径："+req.getContextPath());
        req.getRequestDispatcher("/file").forward(req,resp);
    }
}

狂笔记

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