JavaWeb--Servlet

Servlet

Servlet简介

Servlet是sun公司开发动态web的一门技术
Sun公司在这些API中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。

HelloServlet

HelloServlet.java

package edu.cqupt.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("<h1>Hello Servlet!</h1>");
        System.out.println("----------------------------------");
        System.out.println("The First Servlet is running.");
        System.out.println("-----------------------------------");
    }

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

配置web.xml

<!DOCTYPE web-app PUBLIC
        "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
        "http://java.sun.com/dtd/web-app_2_3.dtd" >

<web-app>
    <servlet>
        <servlet-name>home</servlet-name>
        <servlet-class>edu.cqupt.servlet.HelloServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>home</servlet-name>
        <url-pattern>/home</url-pattern>
    </servlet-mapping>
</web-app>

Servlet原理

Mapping问题

一个Servlet可以指定一个映射路径

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

一个Servlet可以指定多个映射路径

<servlet-mapping>
     <servlet-name>home</servlet-name>
     <url-pattern>/home</url-pattern>
</servlet-mapping>
<servlet-mapping>
     <servlet-name>home</servlet-name>
     <url-pattern>/home2</url-pattern>
</servlet-mapping>
<servlet-mapping>
     <servlet-name>home</servlet-name>
     <url-pattern>/home3</url-pattern>
</servlet-mapping>

一个Servlet可以指定通用映射路径

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

一个Servlet可以指定默认映射路径（一般将默认的映射路径设置为404页面，找不到就走到默认路径）

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

一个Servlet可以指定一些后缀或者前缀等...

<!-- *前面不能加任何项目映射-->
<servlet-mapping>
     <servlet-name>home</servlet-name>
     <url-pattern>*.qingjiang</url-pattern>
</servlet-mapping>

优先级
- 指定固有的映射路径优先级最高，找不到，就会走默认的处理请求

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；

共享数据

在一个Servlet中，可以在另一个Servlet中拿到，实现了Servlet之间的通信

public class HelloServlet  extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("Hello");
        //this.getInitParameter()   初始化参数
        //this.getServletConfig()   Servlet配置
        //this.getServletContext()  Servlet上下文
        ServletContext context1 = this.getServletContext();
        //将一个数据保存到 ServletContext中，名字为：username,值为username
        resp.setContentType("text/html");
        resp.setCharacterEncoding("UTF-8");
        String username = "shilin.z";
        context1.setAttribute("username",username);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

public class GetServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context2 = this.getServletContext();
        String username = (String) context2.getAttribute("username");

        resp.setContentType("text/html");
        resp.setCharacterEncoding("UTF-8");
        resp.getWriter().print("s1传输的内容: " + username);
    }

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

		<servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>edu.cqupt.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>edu.cqupt.servlet.GetServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>getc</servlet-name>
        <url-pattern>/getc</url-pattern>
    </servlet-mapping>

测试结果：
先 http://localhost:8080/s2/hello，不然输出为null

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://local:3306/mybatis</param-value>
    </context-param>	

		<servlet>
        <servlet-name>getp</servlet-name>
        <servlet-class>edu.cqupt.servlet.Servlet03</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>getp</servlet-name>
        <url-pattern>/getp</url-pattern>
    </servlet-mapping>

  @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String url = context.getInitParameter("url");
        resp.getWriter().print(url);
    }

测试结果:

请求转发(不是重定向）

@Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("进入了Servlet04");
        ServletContext context = this.getServletContext();
        //RequestDispatcher requestDispatcher = context.getRequestDispatcher("/getp");    //转发的请求路径
        //requestDispatcher.forward(req,resp);    //调用forward实现请求转发
        context.getRequestDispatcher("/getp").forward(req,resp);
    }

	<servlet>
        <servlet-name>sd4</servlet-name>
        <servlet-class>edu.cqupt.servlet.Servlet04</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>sd4</servlet-name>
        <url-pattern>/sd4</url-pattern>
    </servlet-mapping>

测试结果:

读取资源文件

Properties类
- 在java目录下新建properties
- 在resources目录下新建properties
发现：都被打包到了同一路径下：classes，我们俗称路径为classpath

需要在本项目的pom.xml中配置resources

    <!-- 在bulid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>

需要一个文件流：
Servlet05.java

public class Servlet05 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
       	//获得文件流
        InputStream is = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db.properties");
        Properties prop = new Properties();
        prop.load(is);
        String user  = prop.getProperty("username");
        String pwd = prop.getProperty("password");
        resp.getWriter().print(user + ":" + pwd);
    }

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

db.properties

username=root
password=123456

web.xml

<servlet>
        <servlet-name>sd5</servlet-name>
        <servlet-class>edu.cqupt.servlet.Servlet05</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>sd5</servlet-name>
        <url-pattern>/sd5</url-pattern>
    </servlet-mapping>

测试结果:

**

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，一个代表响应的HttpServletResponse对象；

如果要获取客户端请求过来的参数：找HttpServletRequset对象
如果要给客户端响应一些信息：找HttpServletResponse对象

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);

响应的状态码

int SC_CONTINUE = 100;
int SC_OK = 200;
int SC_MULTIPLE_CHOICES = 300;
int SC_NOT_FOUND = 404;
int SC_INTERNAL_SERVER_ERROR = 500;
.....

常见应用

向浏览器输出消息

（resp.getwriter().print()）

下载文件

获取下载文件的路径
下载的文件名
设置想办法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 {
        // 1.获取下载文件的路径
        //String realPath = this.getServletContext().getRealPath("/dali.jpg");
        String realPath = "E:\CodePlace\Java\idea\狂神说Java\Maven\javawebMaven\javaweb\servlet-03-response\target\classes\dali.jpg";
        System.out.println("下载文件的路径为：" + 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下载我们需要的东西
        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();
    }

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

验证码功能

验证怎么来的？

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. 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_3BYTE_BGR);
        //得到图片
        Graphics2D g = (Graphics2D)image.getGraphics();
        // 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("宋体",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
        boolean write = ImageIO.write(image, "jpg",resp.getOutputStream());
    }
    //生产随机数
    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++) {   //保证生成的随机数只有7位
            sb.append("0");  //不足7位用0填充
        }
        num = sb.toString() + num;
        return num;
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

    <servlet>
        <servlet-name>img</servlet-name>
        <servlet-class>edu.cqupt.servlet.ImageServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>img</servlet-name>
        <url-pattern>/img</url-pattern>
    </servlet-mapping>

实现重定向

B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。
** 常见场景：**

用户登录：登录成功，跳转到另外的页面。

public class RedirectServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        
//        resp.setHeader("Location", "/s3/img");
//        resp.setStatus(302);
        resp.sendRedirect("/s3/img");
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

面试题：请求和重定向的区别？

相同点
- 页面都会跳转
不同点
- 请求转发，url地址不会发生变化 307
- 重定向，url地址会发生变化 302

public class RequestTestServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("进入这个请求了----");
        // 处理请求
        String username = req.getParameter("username");
        String pwd = req.getParameter("password");
        System.out.println(username + ":" + pwd);
        // 重定向一定要注意，路径问题，否则就会404
        resp.sendRedirect("/s3/home.jsp");
    }

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

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

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方法，获得客户端的所有信息。

获取前端传递的参数和请求转发

public class LoginServlet  extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setCharacterEncoding("utf-8");
        req.setCharacterEncoding("utf-8");
        //获取前端传递参数
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        String[] hobbies = req.getParameterValues("hobbies");
        System.out.println("---------------");
        System.out.println(username);
        System.out.println(password);
        System.out.println(Arrays.toString(hobbies));
        System.out.println("---------------");

        //重定向
        // resp.sendRedirect("/s4/success.jsp");
        //通过请求转发
        System.out.println(req.getContextPath());
        //这里的/代表当前的web应用
        req.getRequestDispatcher("/success.jsp").forward(req,resp);
    }

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

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原文地址：https://www.cnblogs.com/sinlearn/p/13558545.html