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>[email protected]</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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