ServletContext对象

**1 ServletContext对象
  1)在web应用中，由服务器创建的唯一的一个对象是ServletContext
  2)ServletContext对象在每一个Servlet中取得都是相同到
  3)ServletContext对象在存取方法：
    存ServletContext.setAttribute(String,Object):void
    取ServletContext.getAttribute(String):Object

package cn.itcast.web.servlet;

import java.io.IOException;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/*
 * http://192.168.10.252:8080/day06/Demo11?username=jack回车
 */

public class Demo11 extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {
        
        //取得客户端的请求参数
        String username = request.getParameter("username");
        
        //取得唯一的一个ServletContext对象
        ServletContext context = this.getServletContext();
        System.out.println("Demo11:"+context.hashCode());
        
        //将信息存入ServletContext对象
        context.setAttribute("USERNAME",username);
    }
}
package cn.itcast.web.servlet;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/*
 * http://192.168.10.252:8080/day06/Demo12回车
 */

public class Demo12 extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {
        
        //取得唯一的一个ServletContext对象
        ServletContext context = this.getServletContext();
        System.out.println("Demo12:"+context.hashCode());
        
        //根据name取得ServletContext对象中的值
        String username = (String) context.getAttribute("USERNAME");
        
        //在浏览器中输出
        response.setContentType("text/html;charset=UTF-8");
        response.getWriter().write("用户名：" + username + "<br/>");
        
    }
}

  4)在web应用中，可以取得web应用的初始化参数

package cn.itcast.web.servlet;

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

public class Demo2 extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String email = context.getInitParameter("email");
        String tel = context.getInitParameter("tel");
        if(email!=null && tel!=null){
            response.setContentType("text/html;charset=UTF-8");
            response.getWriter().write("赵老师邮箱：" + email + "<br/>");
            response.getWriter().write("赵老师电话：" + tel + "<br/>");
        }
    }
}
<context-param>
      <param-name>email</param-name>
      <param-value>runsin0723@163.com</param-value>
  </context-param>    
  <context-param>
      <param-name>tel</param-name>
      <param-value>13632334557</param-value>
  </context-param>

  5)ServletContxt对象能够实现转发功能，即307+location

    ServletContext context = this.getServletContext();
    //定位需要转发的路径
    RequestDispatcher rd = context.getRequestDispatcher("/success.html");
    //真正转向页面
    rd.forward(request,response);

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <title>login.html</title>
    <meta http-equiv="content-type" content="text/html; charset=UTF-8">
  </head>
  <body>
      <form 
          action="/day06/LoginServlet" 
          method="post">
          <table border="1" align="center">
              <caption>用户登录[模拟MVC]</caption>
              <tr>
                  <th>用户名</th>
                  <td><input type="text" name="username"/></td>
              </tr>
              <tr>
                  <th>密码</th>
                  <td><input type="password" name="password"/></td>
              </tr>
              <tr>
                  <th>验证码</th>
                  <td>
                      <input type="text" name="checkCode"/>
                  </td>
                  <td>
                      <img src="/day06/Demo3"/>
                  </td>
              </tr>
              <tr>
                  <td colspan="2" align="center">
                      <input type="submit" value="提交"/>
                  </td>
              </tr>
          </table>
      </form>
  </body>
</html>
package cn.itcast.web.servlet;

public class LoginBean {
    public boolean validate(String username){
        boolean flag = false;
        if(username!=null && "jack".equals(username)){
            flag = true;
        }
        return flag;
    }
}
package cn.itcast.web.servlet;

import java.io.IOException;

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;

public class LoginServlet extends HttpServlet {
    public void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {
        
        //取得表单提交的参数
        String username = request.getParameter("username");
        
        //调用模型层对象
        LoginBean loginBean = new LoginBean();
        boolean flag = loginBean.validate(username);
        
        //根据返回值，转发到不同的页面
        if(flag){
            ServletContext context = this.getServletContext();
            //定位需要转发的路径
            RequestDispatcher rd = context.getRequestDispatcher("/success.html");
            //真正转向页面
            rd.forward(request,response);
        }else{
            ServletContext context = this.getServletContext();
            //定位需要转发的路径
            RequestDispatcher rd = context.getRequestDispatcher("/fail.html");
            //真正转向页面
            rd.forward(request,response);
        }
    }
}

   6)如果以传统方式读取资源文件（txt/xml/properties），是相对于web服务器的bin/目录而言

*7)如果以ServletContext方式读取资源文件（txt/xml/properties），是相对于web服务器的当前web应用目录而言
    此时/表示:当前web应用，即day06
*8)类加载器只能加载IDE工具下src目录下的资源文件，其它目录无法加载
    此时/表示:/WEB-INF/classes/目录

package cn.itcast.web.servlet;

import java.io.IOException;
import java.io.InputStream;
import java.util.Properties;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class Demo4 extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {
        
        /*传统方式
        InputStream is = new FileInputStream("src/config.properties");
        Properties props = new Properties();
        props.load(is);
        System.out.println(props.getProperty("email"));
        */
        
        ServletContext context = this.getServletContext();
        InputStream is = context.getResourceAsStream("/doc/config.properties");
        Properties props = new Properties();
        props.load(is);
        System.out.println(props.getProperty("email"));
        
    }
}
package cn.itcast.web.servlet;

import java.io.IOException;
import java.io.InputStream;
import java.util.Properties;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class Demo5 extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {
        //取得当前对象的字节码对象
        Class clazz = this.getClass();
        //取得当前对象的类加载器
        ClassLoader cl = clazz.getClassLoader();
        //通过类加载器加载资源文件
        InputStream is = cl.getResourceAsStream("/cn/itcast/web/config/config.properties");
        Properties props = new Properties();
        props.load(is);
        System.out.println(props.getProperty("email"));
    }
}

10)其实浏览器访问的静态资源，本质上是由一个缺省的Servlet来帮你处理的

package cn.itcast.web.servlet;

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class DownServlet extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {
        //NO1获取当前config文件的方式
        ServletConfig config = this.getServletConfig();
        String filePath = config.getInitParameter("filePath");
        //NO2
        File file = new File(filePath);
        response.setHeader("content-disposition","attachment;filename="+file.getName());
        //NO3
        InputStream is = new FileInputStream(file);
        OutputStream os = response.getOutputStream();
        byte[] buf = new byte[1024];
        int len = 0;
        while((len=is.read(buf))>0){
            os.write(buf,0,len);
        }
        is.close();
        os.close();
    }
}

2 关于路径的问题
  1)二种加载方式的区别？
    服务端   
    ServletContext方式：
    1)资源的文件的位置任意 
    2)加载文件时，只会以流的方式读取，不会一次性加载全部 
    3)/表示当前web应用，即day06

    类加载器方式：
    1)资源文件的位置只能放置类路径下，即IDE工具的src目录下 
    2)加载文件时，会一次性加载全部文件 
    3)/表示当前/WEB-INF/classes/ 
   2)有关/的问题？
     物理硬盘路径使用[window系统]
     网络路径使用/
     如果/表示客户端路径的话，表示tomcat/webapps/目录  
*3 缓存应用
   1)对于浏览器的刷新而言，当浏览器缓存中有资源，也会找服务端要资源，只有在回车访问情况下，才找缓存.对于不经常变化的数据，在servlet中可以为其设置合理的缓存时间值，以避免浏览器频繁向服务器发送请求，提升服务器的性能。
   2)如果要实现一种高级功能，即客户端请求动态web资源时，动态web资源发现发给客户端的数据更新了，就给客户端发送最新的数据，如果发现数据没有更新，则动态web资源就要客户端就去访问它自己缓存的数据。此种情况可以通过覆写动态web资源(即servlet)的getLastModify方法予以实现。

package cn.itcast.web.servlet;

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

/*
 * http://localhost:8080/day06/login.html回车
 * 设置静态资源[html/css/js]缓存的时间[做成一个缺省的的Servlet，路径为/]
 */

public class Demo6 extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {
        String uri = request.getRequestURI();
        //如果是一个html静态资源
        if(uri!=null && uri.endsWith("html")){
            String html = this.getServletConfig().getInitParameter("html");
            if(html!=null){
                long end = Long.parseLong(html)*1000 + System.currentTimeMillis();
                response.setDateHeader("expires",end);
            }
        }
    }
}
<servlet>
    <servlet-name>Demo6</servlet-name>
    <servlet-class>cn.itcast.web.servlet.Demo6</servlet-class>
    <init-param>
        <param-name>html</param-name>
        <param-value>86400</param-value>
    </init-param>
    <init-param>
        <param-name>css</param-name>
        <param-value>123456</param-value>
    </init-param>
    <init-param>
        <param-name>js</param-name>
        <param-value>654321</param-value>
    </init-param>
  </servlet>
package cn.itcast.web.servlet;

import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
//缓存高级应用
public class Demo7 extends HttpServlet {
    protected long getLastModified(HttpServletRequest req) {
        System.out.println("getLastModified()");
        String path = this.getServletContext().getRealPath("/doc/abc.txt");
        File file = new File(path);
        return file.lastModified();
    }
    public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {
        System.out.println("doGet()");
        ServletContext context = this.getServletContext();
        InputStream is = context.getResourceAsStream("/doc/abc.txt");
        OutputStream os = response.getOutputStream();
        byte[] buf = new byte[1024];
        int len = 0;
        while((len=is.read(buf))>0){
            os.write(buf,0,len);
        }
        is.close();
        os.close();
    }
}

*4 response的应用
  1)web容器会传入与web容器相关的HttpServletRequest和HttpServletResponse对象的实现。request和response对象即然代表请求和响应，那我们要获取客户机提交过来的数据，只需要找request对象就行了。要向客户机输出数据，只需要找response对象就行了。
  2)每次浏览器请求，web容器会创建新的请求和响应对象，这二个对象的生命周期仅限于web容器内。HttpServletResponse对象服务器的响应。这个对象中封装了向客户端发送数据、发送响应头，发送响应状态码的方法。
  3)对于字节方式来说，输出数字，respnse会编查ISO8859-1编码表，将码表的中的数字对应的字符取出来显示，即遇数字转字符
  4)如果要输出数字，提前将数字转化成字符串
  5)对于字符方式来说，可以直接输出数字
  6)将中文转成字节数组时，查UTF-8码表，如果不写的话，默认查ISO8859-1码表 
  7)以字符方式输出中文，提前设置编码方式；以字节方式输出中文，提前设置转化编码方式
    字符：   
    response.getOutputStream().write("中国".getBytes("UTF-8"));
    字节：
    response.setContentType("text/html;charset=UTF-8");
    response.getWriter().write("美国");
   8)HTML中的所有<meta/>标签，类似于response.setHeader()，目的是通知浏览器以特定的方式显示页面的信息
   9)重定向是客户端和服务端的共同行为，
     response.sendRedirect("/day06/message.html")其中/表示webapps目录

   10)response常见应用：

      •向客户端输出中文数据

      •分别以OutputStream和PrintWriter输出

      •用OutputStream输出1，客户端看到的是什么？

      •中文名文件下载

      •生成随机图片

      •发送http头，控制浏览器定时刷新网页(refresh)

      •发送http头，控制浏览器禁止缓存当前文档内容

      •多学一招：使用HTML/JSP语言里面的<meta>标签来控制浏览器行为<meta http-equiv="content-type" content="text/html; charset=UTF-8">

      •通过response实现请求重定向。

      •请求重定向指：一个web资源收到客户端请求后，通知客户端去访问另外一个web资源，这称之为请求重定向。

      •应用场景：用户登陆。

      •实现方式

      •response.sendRedirect(“/welcome.html”)

      •实现原理：

          302状态码和location头即可实现重定向

package cn.itcast.web.response;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
//向浏览器输出中文数据 
public class Demo1 extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {
        //以字节方式输出
         //response.getOutputStream().write((97+"").getBytes());
        //以字符方式输出
         //response.getWriter().write(65);
        //response.getOutputStream().write("中国".getBytes("UTF-8"));
        response.setContentType("text/html;charset=UTF-8");
        response.getWriter().write("美国");
    }
}
package cn.itcast.web.response;

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.URLEncoder;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
//中文名文件下载
public class Demo2 extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String path = context.getRealPath("/images/神奇的图片.JPG");
        File file = new File(path); 
        response.setHeader("content-disposition","attachment;filename="+URLEncoder.encode(file.getName(),"UTF-8"));
        InputStream is = new FileInputStream(file);
        OutputStream os = response.getOutputStream();
        byte[] buf = new byte[1024];
        int len = 0;
        while((len=is.read(buf))>0){
            os.write(buf,0,len);
        }
        is.close();
        os.close();
    }
}
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <title>download.html</title>
    <meta http-equiv="content-type" content="text/html; charset=UTF-8">
  </head>
  <body>    
      <a href="/day06/Demo2">下载中文文件</a>
  </body>
</html>
package cn.itcast.web.response;

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.Random;
import java.util.UUID;
import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
//生成验证码图片
public class Demo3 extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {
        response.setHeader("expires","-1");
        response.setHeader("cache-control","no-cache");
        response.setHeader("pragma","no-cache");
        //以内存中构造一副图片
        BufferedImage image = new BufferedImage(80,25,BufferedImage.TYPE_INT_RGB);
        //取得画笔
        Graphics g = image.getGraphics();
        //设置字体大小和颜色
        g.setColor(Color.YELLOW);
        g.setFont(new Font("黑体",Font.BOLD,22));
        //在图片中，画一个字符串"1A2B"
        g.drawString(getString(),20,20);
        //将内存中的图片以JPG格式输出到浏览器
        ImageIO.write(image,"JPG",response.getOutputStream());
    }
    //产生一个随机字符串
    private String getString(){
        String str = "";
        String numberAndLetter="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
        int length = numberAndLetter.length();
        for(int i=1;i<=4;i++){
            if(i==1 || i==3){//数字
                while(true){
                    Random random = new Random();
                    int index = random.nextInt(length);
                    String value = numberAndLetter.substring(index,index+1);
                    if(value.matches("[0-9]")){
                        str += value;
                        break;
                    }
                }
            }else if(i==2 || i==4){//大小字母
                while(true){
                    Random random = new Random();
                    int index = random.nextInt(length);
                    String value = numberAndLetter.substring(index,index+1);
                    if(value.matches("[A-Za-z]")){
                        str += value;
                        break;
                    }
                }
            }
        }
        return str;
    }
}
package cn.itcast.web.response;

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

public class LoginServlet extends HttpServlet {
    public void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {
        String username = request.getParameter("username");
        String password = request.getParameter("password");
        String checkCode = request.getParameter("checkCode");
        System.out.println("用户名：" + username);
        System.out.println("密码：" + password);
        System.out.println("验证码：" + checkCode);
        //重定向到message.html页面
        //这时/表示webapps目录
        response.sendRedirect("/day06/message.html");
    }
}

5 response细节
  1)以字符或字节方式输出内容的话，只能使用其中一种
  2)响应头会以叠加的方式设置，在相同的情况下，后者起决定作用 

package cn.itcast.web.response;

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

public class Demo4 extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        response.setContentType("text/html;charset=GBK");
        PrintWriter pw = response.getWriter();
        pw.write("美国");
        pw.flush();
        pw.close();
    }
}

   3)在没有关闭流的情况下，web容器帮你自动关闭所有从response中取得IO流

  4)getOutputStream和getWriter方法分别用于得到输出二进制数据、输出文本数据的ServletOuputStream、Printwriter对象。

  5)getOutputStream和getWriter这两个方法互相排斥，调用了其中的任何一个方法后，就不能再调用另一方法。

  6)Servlet程序向ServletOutputStream或PrintWriter对象中写入的数据将被Servlet引擎从response里面获取，Servlet引擎将这些数据当作响应消息的正文，然后再与响应状态行和各响应头组合后输出到客户端。

  7)Serlvet的service方法结束后，Servlet引擎将检查getWriter或getOutputStream方法返回的输出流对象是否已经调用过close方法，如果没有，Servlet引擎将调用close方法关闭该输出流对象。

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原文地址：https://www.cnblogs.com/sunhan/p/3542146.html