JavaWeb（三）Servlet

本文主要是介绍JavaWeb（三）Servlet，对大家解决编程问题具有一定的参考价值，需要的程序猿们随着小编来一起学习吧！

1、servlet简介

• Servlet是sun公司开发动态web的一门技术
• sun公司提供了一个接口叫做Servlet，如果你想开发一个Servlet程序，只需要完成两个步骤：

1. 编写一个类，继承HttpServlet类
2. 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

2、HelloServlet

   2.1  编写servlet

package com.zjb.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;

public class HelloServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        PrintWriter writer = resp.getWriter();
        writer.print("HelloServlet");
    }

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

   2.2 编写servlet映射

    我们写的是java程序，但是要通过浏览器访问，而浏览器需要连接web服务器，所以我们需要在web服务中注册我们写的servlet，还需要给他一个浏览器能够访问的路径。

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

3.servlet原理

Servlet是由web服务器调用，web在收到浏览器请求之后，

4. Mapping问题

   4.1、 一个Servlet可以指定一个映射路径

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

   4.2、一个Servlet可以指定多个映射路径

    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/helloServlet</url-pattern>
    </servlet-mapping>
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/helloServlet1</url-pattern>
    </servlet-mapping>
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/helloServlet2</url-pattern>
    </servlet-mapping>

   4.3、一个Servlet可以指定通用映射路径

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

  4.4、 默认请求路径

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

  4.5、指定一些后缀或者前缀等等

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

 4.6.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；

  做个验证：

<?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_3_1.xsd"
         version="3.1"
         metadata-complete="true">
    <!--注册servlet-->
    <servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>com.zjb.servlet.HelloServlet</servlet-class>
    </servlet>
    <servlet>
        <servlet-name>error</servlet-name>
        <servlet-class>com.zjb.servlet.ErrorServlet</servlet-class>
    </servlet>
    <!--servlet的请求路径-->
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/helloServlet</url-pattern>
    </servlet-mapping>
    <!--404 ： 映射路径给默认的映射路径-->
    <servlet-mapping>
        <servlet-name>error</servlet-name>
        <url-pattern>/*</url-pattern>
    </servlet-mapping>
</web-app>

package com.zjb.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;

public class ErrorServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf-8");
        PrintWriter writer = resp.getWriter();
        writer.print("<h1>404</h1>");
    }

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

启动访问，发现当访问路径是/helloServlet时，依然可以正常访问。

5.ServletContext对象（获取上下文）

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。

5.1.共享数据

在一个servlet中保存的数据，可以在另外一个servlet中拿到。

package com.zjb.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 ServletContextServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //获取Servlet上下文
        ServletContext servletContext = this.getServletContext();

        String username = "上下文";
        servletContext.setAttribute("username",username);


        System.out.println("向ServletContext对象中存入用户名:"+username);


    }
}

package com.zjb.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.PrintWriter;

public class GetServletContextServlet 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");
        PrintWriter writer = resp.getWriter();
        writer.print("<h1>用户名:"+username+"</h1>");

    }
}

<?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_3_1.xsd"
         version="3.1"
         metadata-complete="true">
    <!--注册servlet-->
    <servlet>
        <servlet-name>setContext</servlet-name>
        <servlet-class>com.zjb.servlet.ServletContextServlet</servlet-class>
    </servlet>
    <servlet>
        <servlet-name>getContext</servlet-name>
        <servlet-class>com.zjb.servlet.GetServletContextServlet</servlet-class>
    </servlet>
    <!--servlet的请求路径-->
    <servlet-mapping>
        <servlet-name>setContext</servlet-name>
        <url-pattern>/setC</url-pattern>
    </servlet-mapping>
    <servlet-mapping>
        <servlet-name>getContext</servlet-name>
        <url-pattern>/getC</url-pattern>
    </servlet-mapping>
</web-app>

启动并访问

5.2. 获取初始化参数

 <!--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/demo</param-value>
    </context-param>

package com.zjb.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.PrintWriter;

/**
 * 获取初始化参数
 **/
public class GetInitParamServlet 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");
        resp.setCharacterEncoding("utf-8");
        PrintWriter writer = resp.getWriter();
        writer.print("<h1>数据库连接路径:"+url+"</h1>");
    }

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

5.3.请求转发

package com.zjb.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;

/**
 * ServletContext 实现请求转发
 */
public class GetRequestDispatcherServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext servletContext = this.getServletContext();
        //转发的路径
        RequestDispatcher requestDispatcher = servletContext.getRequestDispatcher("/getIp");
        //调用forward方法实现请求转发
        requestDispatcher.forward(req,resp);
    }

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

转发的时候路径是不会发生改变的。

5.4.读取资源文件

Properties

• Servle在java目录下新建properties文件
• 在resources目录下新建properties文件

发现：都被打包到了同一个路径下：classes，我们俗称这个路径为类路径classpath; 

package com.zjb.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.io.PrintWriter;
import java.util.Properties;

/***
 * ServletContext 实现读取资源文件
 */
public class GetResourceServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext servletContext = this.getServletContext();
        InputStream resourceAsStream = servletContext.getResourceAsStream("/WEB-INF/classes/db.properties");

        Properties properties = new Properties();
        properties.load(resourceAsStream);
        String username = properties.getProperty("username");
        String password = properties.getProperty("password");

        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf-8");
        PrintWriter writer = resp.getWriter();
        writer.print("<h1>用户名:"+username+",密码:"+password+"</h1>");
    }

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



}

6.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。

6.1 获取前端传递的参数

    String getParameter(String var1);

    String[] getParameterValues(String var1);

6.2  请求转发

package com.zjb.request;

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;

/**
 * HttpServletRequest 请求转发
 */
public class GetParamServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        req.setCharacterEncoding("utf-8");
        resp.setCharacterEncoding("utf-8");
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        System.out.println(username+":"+password);

        String[] hobbies = req.getParameterValues("hobbies");
        System.out.println("hobbies:"+Arrays.toString(hobbies));

        //请求转发
        req.getRequestDispatcher("/success.jsp").forward(req,resp);

    }

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

7.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，代表响应一个HttpServletResponse；

• 如果要获取客户端请求过来的参数：找HttpServletRequest
• 如果要获取客户端响应的一些信息：找HttpServletResponse

7.1. 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);

负责向浏览器发送响应状态码的方法

 void setStatus(int var1);

响应状态码的常量 

    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;

7.2 常见应用

   1. 向浏览器输出信息

   2.下载文件 

package com.zjb.response;

import javax.servlet.ServletContext;
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;

/**
 * HttpServletResponse 实现下载文件
 * 实现步骤：
 * 1.要获取下载文件的路径
 * 2.下载的文件名是什么
 * 3.想办法设置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,使用OutPutStream将缓冲区中的数据输出到客户端
 */
public class DownloadResponseServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //获取要下载的文件路径
        String realPath = "C:\\lesson\\learn-javaweb\\servlet-01\\target\\classes\\1.jpg";
        System.out.println("下载文件的路径:"+realPath);
        //下载文件的文件名
        String fileName = realPath.substring(realPath.lastIndexOf("\\") + 1);
        System.out.println("文件名:"+fileName);
        //设置让浏览器能够支持下载我们需要的东西,使用URLEncoder.encode来解决文件名是中文乱码
        resp.setHeader("Content-Disposition","attachment;filename="+ URLEncoder.encode(fileName,"UTF-8"));
        //获取下载文件的输入流
        FileInputStream fileInputStream = new FileInputStream(realPath);
        //创建缓冲区
        int len = 0;
        byte[] buffer = new byte[1024];
        //获取OutPutStream对象
        ServletOutputStream outputStream = resp.getOutputStream();
        //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 = fileInputStream.read(buffer))>0){
            outputStream.write(buffer,0,len);
        }
        fileInputStream.close();
        outputStream.close();
    }

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

 3. 验证码功能

package com.zjb.response;

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;

/**
 * HttpServletResponse 实现验证码功能
 */
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 bufferedImage = new BufferedImage(80,20,BufferedImage.TYPE_INT_RGB);
        //得到一个2D画笔
        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(makeRandomNum(),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("Prama","no-cache");

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

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

    public String makeRandomNum(){
        Random random = new Random();
        String num = String.valueOf(random.nextInt(99999999));
        StringBuffer sb = new StringBuffer();
        for (int i = 0; i < 7-num.length(); i++) {
            sb.append("0");
        }
        return sb.toString() + num;
    }
}

4.response重定向（重点）

 一个web资源收到客户端请求后，他会通知客户端去访问另外一个web资源，这个过程叫重定向。

 void sendRedirect(String var1) throws IOException;

package com.zjb.response;

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

/**
 * HttpServletResponse 实现重定向
 */
public class RedirectServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("进入重定向方法");
        /**
         * 重定向的原理:
         * resp.setHeader("location","/servlet/image");
         * resp.setStatus(HttpServletResponse.SC_MOVED_TEMPORARILY);
         * 就等价于 resp.sendRedirect("/servlet/image")
         */
        //重定向的时候一定要注意路径问题
       resp.sendRedirect(req.getContextPath()+"/image");

    }

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

我们发现路径发生了改变，并且状态码是302 。

面试题：请你聊聊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
• 重定向的时候，url地址栏会发生变化

 常见场景：

• 用户登录

package com.zjb.response;

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 LoginDemoServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("进入到该登录方法");
        String username = req.getParameter("username");
        String password = req.getParameter("password");

        System.out.println(username+":"+password);
        //重定向
        resp.sendRedirect("/servlet/success.jsp");
    }

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

  index.jsp 

<html>
<body>
<h2>Hello World!</h2>
<%-- ${pageContext.request.contextPath} 代表当前的项目--%>
<form action="${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>
</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>

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