javaWeb的HttpServletRequest和HttpServletResponse

HttpServletRequest

HttpServletRequest对象是封装了用户的请求信息，包括请求参数去，请求头等信息，service()f方法中的两个HttpServletRequest和HttpServletResponse的形参是由Tomcat将创建好的实例对象传进来，不需要我们关心。（如果感兴趣的话，可以看看Tomcat的底层实现，如果有时间的话，我们可以再详细讨论一下这个原理）。

HttpservletRequest的常用方法

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 Servlet01 extends HttpServlet{
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
            String url=req.getRequestURL().toString();
            System.out.println("url--->"+url);

            String uri=req.getRequestURI();
            System.out.println("uri--->"+uri);

            String qs=req.getQueryString();
            System.out.println("请求参数："+qs);

            String method=req.getMethod();
            System.out.println("请求参数"+method);

            String protocol=req.getProtocol();
            System.out.println("请求协议："+protocol);

            String context=req.getContextPath();
            System.out.println("请求站点名："+context);    

    }
}

获取请求头

import java.io.IOException;
import java.util.Enumeration;

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

public class Servlet02 extends HttpServlet{
        @Override
        protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
            String header=req.getHeader("host");
            System.out.println(header);

            System.out.println("++++++++++++++++");
            Enumeration<String> enu=req.getHeaderNames();
            while(enu.hasMoreElements()){
                System.out.println(enu.nextElement());
            }
        }
}

获取客户端请求参数

import java.io.IOException;
import java.util.Enumeration;
import java.util.Map;

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

public class Servlet03 extends HttpServlet {
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String name=req.getParameter("uname");
        System.out.println(name);

        String [] hobby=req.getParameterValues("hobby");
        for(String temp:hobby){
            System.out.println(temp);
        }

        Enumeration<String> es=req.getParameterNames();
        while(es.hasMoreElements()){
            es.nextElement();
        }

        Map<String, String[]> map =req.getParameterMap();
        for(String key:map.keySet()){
            System.out.println(key);
            for(String str:map.get(key)){
                System.out.println(str);
            }
        }
    }
}

乱码问题的解决方式

由于现在的 request 属于接收客户端的参数，所以必然有其默认的语言编码，主要是由于在解析过程中默认使用的编码方式为 ISO-8859-1(此编码不支持中文)，所以解析时一定会出现乱码。要想解决这种乱码问题，需要设置 request 中的编码方式，告诉服务器以何种方式来解析数据。或者在接收到乱码数据以后，再通过相应的编码格式还原。

方式一：req.setCharacterEncoding("UTF-8");

该方法值针对post有效，对get是无效的，但是Tomcat8 起，以后的 GET方式请求是不会出现乱码的，所以使用Tomcat8及以上的Tomcat版本，只需要设置该post的编码即可

方式二：

String str=req.getParameter("uname");
String str1=new String(str.getBytes("ISO-8859-1"),"UTF-8");

该方式借助了 String 对象的方法，该种方式对任何请求有效，是通用的，但只能针对单个参数进行编码转换来达到解决乱码的问题

请求转发

请求转发，是一种服务器的行为，当客户端请求到达后，服务器进行转发，此时会将请求对象进行保存，地址栏中的 URL 地址不会改变，得到响应后，服务器端再将响应发送给客户端，从始至终只有一个请求发出。实现方式如下，达到多个资源协同响应的效果。

        　　  servlet01.java
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 servlet01 extends HttpServlet {
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String name= req.getParameter("uname");
        req.setAttribute("uname", name);

        //转发到servlet02
        req.getRequestDispatcher("s02").forward(req,resp);
    }
}

     　　servlet02.java 
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 servlet02 extends HttpServlet {
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println(req.getAttribute("uname"));
        //转发到index.jsp页面 ，请求转发的过程中，用的是同一个request和response
        req.getRequestDispatcher("index.jsp").forward(req,resp);
    }
}

index.jsp页面
<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
</head>
<body>
欢迎${uname }来到首页
</body>
</html>

HttpServletResponse

Web 服务器收到客户端的 http 请求，会针对每一次请求，分别创建一个用于 代表请求的 request 对象和 代表响应的 response 对象。request 和 response 对象代表请求和响应：获取客户端数据，需要通过request 对象； 向客户端输出数据，需要通过 response 对象。HttpServletResponse 的主要功能用于服务器对客户端的请求进行响应，将Web 服务器处理后的结果返回给客户端。service()方法中形参接收的是HttpServletResponse 接口的实例化对象，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。

常用方法

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 Servlet04 extends HttpServlet{
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        req.setCharacterEncoding("UTF-8");
        String str=req.getParameter("uname");
    String str1=new String(str.getBytes("ISO-8859-1"),"UTF-8");
        System.out.println(str1);

        resp.addHeader("name", "zhangsan");
        resp.setHeader("name", "lisi");

        resp.setHeader("refresh", "3;url=http://www.shsxt.com");
        resp.sendError(404);

    }
}

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 Servlet04 extends HttpServlet{
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    resp.setContentType("text/html;charset=UTF-8");
        resp.getOutputStream().write("<h3> hello world</h3> ".getBytes());

将图片显示在网页上

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

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

public class Servlet05 extends HttpServlet {
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setContentType("image/jpg;charset=UTF-8");
        String realpath=req.getServletContext().getRealPath("/");
        System.out.println(realpath);

        String filepath=realpath+"1.jpg";
        File file=new File(filepath);
        if(file.exists()&& file.isFile()){
            InputStream is=new FileInputStream(file);
            byte[] bs=new byte[1024];
            int len=0;
            while((len=is.read(bs))!=-1){
                    resp.getOutputStream().write(bs,0,len);
            }
            resp.getOutputStream().close();
            is.close();
        }
        else{
            resp.setContentType("text/html;charset=UTF-8");
            resp.getWriter().write("<h1>文件不存在！</h1>");
        }

    }
}

重定向

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 Servlet06 extends HttpServlet{
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
            resp.sendRedirect("s05");

    }

}