servlet servlet请求与响应

request

客户端浏览器发出的请求被封装成一个HttpServletRequest对象。所有的信息包括请求的地址，请求的参数，提交的数据，上传的文件，客户端的Ip地址甚至客户端操作系统都包含在HttpServletRequest对象中。

response

servlet不仅能输出文本与图片，还能输出其他格式数据，例如word，excel，pdf，mps等

只要正确设定输出类型及输出流。不同的输出类型需要声明不同的Context-Type属性，例如JPG图片是“image/jpeg",而word则是”application/msword"。

实例，response生成图片验证码,response.setContentType("image/jpeg");

package com.servlet;
import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics2D;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.Random;

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

import com.sun.image.codec.jpeg.JPEGCodec;
import com.sun.image.codec.jpeg.JPEGImageEncoder;


public class IdentityServlet extends HttpServlet{
    
    private static final long serialVersionUID = 1L;
    
    public static final char[] CHARS = {'2','3','4','5','6','7','8',
        '9','A','B','C','D','E','F','G','H','Z','K','L','M'
        ,'N','P','Q','R','S','T','U','V','W','X','Y','Z'};
                     //don't iclude 0,O,1,I,which difficult to identify
    public static Random random = new Random();
    
    public static String getRandomString(){
        StringBuffer buffer = new StringBuffer();
        for(int i = 0; i <6; i++){
            buffer.append(CHARS[random.nextInt(CHARS.length)]);
        }
        return buffer.toString();
    }
    
    public static Color getRandomColor(){
        return new Color(random.nextInt(255),random.nextInt(255),
                 random.nextInt(255));
    }
    
    public static Color getReverseColor(Color c){
        return new Color(255 - c.getRed(), 255 - c.getGreen(),
                255 - c.getBlue());
    }
    
    public void doGet(HttpServletRequest request, 
            HttpServletResponse response)
                    throws ServletException,IOException{
        //have to set the output type
        response.setContentType("image/jpeg");
        String randomString = getRandomString();
        //put it in the session
        request.getSession(true).setAttribute("randomString", randomString);
        
        //width of pic
        int width = 100;
        //heighth of pic
        int height = 30;
        
        //color of pic content
        Color color = getRandomColor();
        //color of background
        Color reverse = getReverseColor(color);
        
        //create a colorful pic
        BufferedImage bi = new BufferedImage(width,height,
                BufferedImage.TYPE_INT_RGB);
        Graphics2D g = bi.createGraphics();
        //text style
        g.setFont(new Font(Font.SANS_SERIF,Font.BOLD,16));
        g.setColor(color);
        //draw background
        g.fillRect(0, 0, width, height);
        g.setColor(reverse);
        //draw random text
        g.drawString(randomString, 18, 20);
        //draw the points
        for(int i = 0,n = random.nextInt(100); i < n; i++){
            g.drawRect(random.nextInt(width), random.nextInt(height), 1, 1);
        }
        
        ServletOutputStream out = response.getOutputStream();
        JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(out);
        encoder.encode(bi);
        out.flush();
    } 
}

web.xml:

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5" 
    xmlns="http://java.sun.com/xml/ns/javaee" 
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee 
    http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
  <display-name></display-name>    
  <servlet>
       <servlet-name>IdentyServlet</servlet-name>
       <servlet-class>com.servlet.IdentityServlet</servlet-class>
  </servlet>
  <servlet-mapping>
       <servlet-name>IdentyServlet</servlet-name>
       <url-pattern>/servlet/IdentityServlet</url-pattern></servlet-mapping>
</web-app>

效果：