HttpServletResponse的学习

 

关于Response对象的一些方法和属性可以查看官方文档：https://javaee.github.io/javaee-spec/javadocs/

比如里面定义了许多常量：

这些都是服务器向浏览器返回的一些状态码，我们把它定义成常量，之后就不要写数字！

还有一些方法：

1、示例：向浏览器输出中文：

package test;

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

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

/**
 * Servlet implementation class ServletContextDemo2
 */
@WebServlet("/ServletContextDemo2")    //注意有了这个就不需要往web.xml文件里面添加路径映射
public class ServletContextDemo2 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub大小写要注意好了
        response.setHeader("Content-type", "text/html;charset=UTF-8"); //要告诉浏览器以什么编码打开
        String s = "中国";
        OutputStream out = response.getOutputStream();
        out.write(s.getBytes("UTF-8"));  //为了兼容性，我们设置字符编码为utf-8
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

还可以：

package test;

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

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

/**
 * Servlet implementation class ServletContextDemo2
 */
@WebServlet("/ServletContextDemo2")    //注意有了这个就不需要往web.xml文件里面添加路径映射
public class ServletContextDemo2 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        String s = "中国";
        OutputStream out = response.getOutputStream();
        //使用html的meta标签模拟一个http响应头，来控制浏览器行为
        out.write("<meta http-equiv='content-type' content='text/html;charset=UTF-8'>".getBytes());
        out.write(s.getBytes("UTF-8"));  //为了兼容性，我们设置字符编码为utf-8
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

注意：下面这种方式对于谷歌浏览器不可以，对于IE浏览器可以

package test;

import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.util.Properties;

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

/**
 * Servlet implementation class ServletContextDemo2
 */
@WebServlet("/ServletContextDemo2")    //注意有了这个就不需要往web.xml文件里面添加路径映射
public class ServletContextDemo2 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        String s = "中国";
        response.setCharacterEncoding("UTF-8");  //设置response的默认编码，控制response以什么码表像浏览器输出数据
        response.setHeader("Content-type", "text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();
        out.write(s);  //通过字符方式写数据
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

//下面两个语句一样的效果
response.setHeader("Content-type", "text/html;charset=UTF-8");
response.setContentType("text/html;charset=UTF-8");  这样也可以

2、文件下载

 

package test;

import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.util.Properties;

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

/**
 * Servlet implementation class ServletContextDemo2
 */
@WebServlet("/ServletContextDemo2")    //注意有了这个就不需要往web.xml文件里面添加路径映射
public class ServletContextDemo2 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        String path = this.getServletContext().getRealPath("/download/1.txt");
        String file_name = path.substring(path.lastIndexOf("\")+1);
        response.setHeader("content-disposition", "attachment;filename="+file_name);
        //文件写入response
        InputStream in = null;
        OutputStream out = null;
        try {
            in = new FileInputStream(path);
            int len=0;
            byte buffer[] = new byte[1024];
            out = response.getOutputStream();
            while((len = in.read(buffer))>0) {
                out.write(buffer,0,len);
            }
        }finally {  
            if(in!=null) {
                try {
                    in.close();
                }catch(Exception e){
                    e.printStackTrace();
                }
            }
        }
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

如果文件名是中文的需要经过URL编码

package test;

import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.net.URLEncoder;
import java.util.Properties;

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

/**
 * Servlet implementation class ServletContextDemo2
 */
@WebServlet("/ServletContextDemo2")    //注意有了这个就不需要往web.xml文件里面添加路径映射
public class ServletContextDemo2 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        String path = this.getServletContext().getRealPath("/download/你好.txt");
        String file_name = path.substring(path.lastIndexOf("\")+1);
        response.setHeader("content-disposition", "attachment;filename="+URLEncoder.encode(file_name,"UTF-8"));
        //文件写入response
        InputStream in = null;
        OutputStream out = null;
        try {
            in = new FileInputStream(path);
            int len=0;
            byte buffer[] = new byte[1024];
            out = response.getOutputStream();
            while((len = in.read(buffer))>0) {
                out.write(buffer,0,len);
            }
        }finally {  
            if(in!=null) {
                try {
                    in.close();
                }catch(Exception e){
                    e.printStackTrace();
                }
            }
        }
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

2、生成随机验证图片，并向浏览器输出

目录为：

ServletContextDemo2.Java内容：

package test;

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.image.BufferedImage;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.net.URLEncoder;
import java.util.Properties;
import java.util.Random;

import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class ServletContextDemo2
 */
@WebServlet("/ServletContextDemo2")    //注意有了这个就不需要往web.xml文件里面添加路径映射
public class ServletContextDemo2 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    public static final int WIDTH = 120;
    public static final int HEIGHT = 35;
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        BufferedImage image = new BufferedImage(WIDTH,HEIGHT,BufferedImage.TYPE_INT_RGB);
        Graphics g = image.getGraphics();
        //背景色
        setBackGround(g);
        //边框
        setBorder(g);
        //干扰线
        drawRandomLine(g);
        //画随机数字体旋转
        drawRandomNum((Graphics2D)g);
        
        //图形写给浏览器
        response.setContentType("image/jpeg");
        //控制浏览器不要缓存随机图片
        response.setHeader("Cache-Control", "no-cache");
        response.setHeader("Pragma", "no-cache");
        ImageIO.write(image, "jpg", response.getOutputStream());
    }

    private void drawRandomNum(Graphics2D g) {
        // TODO Auto-generated method stub
        g.setColor(Color.red);
        //设置字体为加粗宋体，高度20
        g.setFont(new Font("宋体",Font.BOLD,20));
        //在UTF-8码表中，所有汉字都在[4e00-9fa5]这个区间内
        //char c = 'u4e00'; Java内内置编码为UTF-8 ，所以u4e00就相当于一个汉字
        //常用汉字编码utf-8编码
        String data = "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";
        int x=5;
        for(int i=0;i<4;++i)
        {
            int degree = new Random().nextInt()%30; //生成-30-30的随机数
            String ch = data.charAt(new Random().nextInt(data.length()))+"";
            //degree*Math.PI/180角度变弧度,设置旋转角度
            g.rotate(degree*Math.PI/180,x,20); //基于(x,20)点旋转
            g.drawString(ch, x, 20);
            g.rotate(-degree*Math.PI/180,x,20); //调回旋转角度
            x+=30;
        }
    }

    private void drawRandomLine(Graphics g) {
        // TODO Auto-generated method stub
        g.setColor(Color.GREEN);
        for(int i=0;i<5;++i) {
            int x1 = new Random().nextInt(WIDTH); //生成[0,WIDTH)的随机数
            int y1 = new Random().nextInt(HEIGHT);
            
            int x2 = new Random().nextInt(WIDTH); //生成[0,WIDTH)的随机数
            int y2 = new Random().nextInt(HEIGHT);
            g.drawLine(x1, y1, x2, y2);
        }
    }

    private void setBorder(Graphics g) {
        // TODO Auto-generated method stub
        g.setColor(Color.BLUE);
        g.drawRect(1, 1, WIDTH-2, HEIGHT-2);
    }

    private void setBackGround(Graphics g) {
        // TODO Auto-generated method stub
        g.setColor(Color.WHITE);
        g.fillRect(0, 0, WIDTH, HEIGHT);
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

还有html内容：

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<body>
    <form action="">
        用户名：<input type="text" name="username">
        密码：<input type="password" name="password">
        认证码：<input type="text" name="checkcode"><img src="/day02/ServletContextDemo2"><br/>
        <input type="submit" value="注册">
    </form>
</body>
</html>

实现点验证图片之后换一张图片只需要改一下html内容：

<!DOCTYPE html>
<html>
<head>
    <script type="text/javascript">
        function changeimage(img){
            //img.src = img.src 后面的src是获取html里面img的src属性，赋值给img.src就代表重新访问
            //但是上面的语句不可行是因为浏览器会默认从缓存里面找图片，所以你应该把图片地址换一下
            //这样浏览器在缓存里面找不到就会向服务器发送请求
            img.src=img.src+"?"+new Date().getTime();
        }
    </script>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<body>
    <form action="">
        用户名：<input type="text" name="username">
        密码：<input type="password" name="password">
        认证码：<input type="text" name="checkcode">
        <img src="/day02/ServletContextDemo2" onclick="changeimage(this)" alt="换一张" style="cursor:hand"><br/>
        <input type="submit" value="注册">
    </form>
</body>
</html>

 

3、控制浏览器定时刷新

package test;

import java.io.IOException;
import java.util.Random;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class ServletContextDemo2
 */
@WebServlet("/ServletContextDemo2")    //注意有了这个就不需要往web.xml文件里面添加路径映射
public class ServletContextDemo2 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setHeader("refresh", "3"); //3秒刷新一次
        String data = new Random().nextInt(1000000)+"";
        response.getWriter().write(data);  //以字符形式输出数据
    }
    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

3秒之后跳转到指定页面

package test;

import java.io.IOException;
import java.util.Random;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class ServletContextDemo2
 */
@WebServlet("/ServletContextDemo2")    //注意有了这个就不需要往web.xml文件里面添加路径映射
public class ServletContextDemo2 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setHeader("Content-type", "text/html;charset=utf-8");
        //3秒之后刷新，并跳转到register.html页面
        response.setHeader("refresh", "3;url='/day02/register.html'"); 
        response.getWriter().write("本浏览器在3秒后跳转，若没有跳转，点击<a href=''>超链接</a>");  //以字符形式输出数据
    }
    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

使用expries控制浏览器缓存

package test;

import java.io.IOException;
import java.util.Random;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class ServletContextDemo2
 */
@WebServlet("/ServletContextDemo2")    //注意有了这个就不需要往web.xml文件里面添加路径映射
public class ServletContextDemo2 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //控制浏览器缓存，浏览器缓存截止于1小时后不可用
        //1000代表一秒，乘于3600就是一小时。System.currentTimeMillis()代表当前时间
        response.setDateHeader("expries", System.currentTimeMillis()+1000*3600);
        String data = "aaaaaa";
        response.getWriter().write(data);
    }
    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

重定向：

package test;

import java.io.IOException;
import java.util.Random;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class ServletContextDemo2
 */
@WebServlet("/ServletContextDemo2")    //注意有了这个就不需要往web.xml文件里面添加路径映射
public class ServletContextDemo2 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    //重定向会发送两次请求，每一次请求都是不同的servlet处理
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //重定向
        response.setStatus(302);
        response.setHeader("location", "/day02/register.html");
        
        //下面也是重定向
        response.sendRedirect("/day02/register.html");
    }
    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

getWriter和getOutputStream不能用同一个servlet处理，就比如你使用转发，这个时候是一个servlet处理两次请求，所以即使在两个文件却不能同时出现两种输出数据方式，示例：

ServletContextDemo2

package test;

import java.io.IOException;
import java.util.Random;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class ServletContextDemo2
 */
@WebServlet("/ServletContextDemo2")    //注意有了这个就不需要往web.xml文件里面添加路径映射
public class ServletContextDemo2 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    //重定向会发送两次请求，每一次请求都是不同的servlet处理
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.getOutputStream();
        this.getServletContext().getRequestDispatcher("/ServletContextDemo1").forward(request, response);
    }
    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

ServletContextDemo1

package test;

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

/**
 * Servlet implementation class ServletContextDemo1
 */
@WebServlet("/ServletContextDemo1")
public class ServletContextDemo1 extends HttpServlet {
    private static final long serialVersionUID = 1L;
    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        response.getWriter();
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

上面这样就会报错，但是换成重定向就没有问题，因为重定向的两次请求不是一个servlet处理的。