Servlet之请求与响应

HTTP请求的结构

HTTP请求包含三部分：请求行、请求头、请求体
请求行：说明请求方法，请求的Url，HTTP的版本信息

请求头：包含了很多辅助性的信息，可以为程序处理提供额外的数据支持

请求头中的内容	作用
Accept-Language:zh-CN	当前浏览器所设置的语言
User-Agent:Mozilla/4.0(compatible;MSIE 8.0;Windows NT 6.1)	说明浏览器和操作系统的环境
Content-Length:112	代表内容的长度
Cache-Control:no-cache	缓存的设置
Cookie:…………	Cookie的设置

请求体：参数：get没有请求体，需要注意的是get请求，把参数放在URL中，其实是没有请求体的；只有Post请求才有请求体

HTTP的Get请求

在Tomcat中编写get请求的简单案例

//Servlet文件
package com.java.servlet;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
import java.io.IOException;

@WebServlet(name = "RequestServlet", value = "/RequestServlet")
public class RequestServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.getWriter().println("This is get method");//得到Get请求后，响应后，在页面输出
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.getWriter().println("This is post method");//得到Post请求后，响应后，在页面输出
    }
}

<!--html文件-->
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<form action="RequestServlet" method="get"><!--链接RequestServlet文件，发送get请求-->
    <input name="username">
    <input name="password" type="password">
    <input type="submit">
</form>
</body>
</html>

Query String是Get传参的标志

HTTP的Post请求

//Servlet文件
package com.java.servlet;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
import java.io.IOException;

@WebServlet(name = "RequestServlet", value = "/RequestServlet")
public class RequestServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.getWriter().println("This is get method");//得到Get请求后，响应后，在页面输出
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.getWriter().println("This is post method");//得到Post请求后，响应后，在页面输出
    }
}

<!--html文件-->
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<form action="RequestServlet" method="post"><!--链接RequestServlet文件，发送get请求-->
    <input name="username">
    <input name="password" type="password">
    <input type="submit">
</form>
</body>
</html>

Form Data是Post请求的标志

利用请求头开发多端应用

情况展示：

平时，同一个网站，在电脑上和手机上显示效果完全不同，这背后的本质就是程序通过读取请求头（User-Agent）来判断是手机还是电脑

发现不同版本上的显示效果不同，然后请求头中的User-Agent也不同

获取当前请求头中的User-Agent信息

package com.java.servlet;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
import java.io.IOException;

@WebServlet(name = "UserAgeServlet", value = "/UserAgeServlet")
public class UserAgeServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String userAgent =request.getHeader("User-Agent");//获取请求头中的“User-Agent”的值
        response.setContentType("text/html;charset=utf-8");//设置响应，让其支持中文
        response.getWriter().println(userAgent);//响应输出页面的值
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    }
}

根据User-Agent信息，判断是PC端还是移动端

判断出是PC端还是移动端，方便后续区别处理

package com.java.servlet;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
import java.io.IOException;

@WebServlet(name = "UserAgeServlet", value = "/UserAgeServlet")
public class UserAgeServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String userAgent =request.getHeader("User-Agent");//获取请求头中的“User-Agent”的值
        response.setContentType("text/html;charset=utf-8");//设置响应，让其支持中文
        response.getWriter().println(userAgent);//响应输出页面的值，这里的值为userAgent的值
        String output="";
        if (userAgent.indexOf("Windows NT")!=-1){//indexof查找字符串，找到字符串返回的是大于0的数字，所以当不等于-1的时候一定是找到了这个值
            output="<h1>这是PC端首页</h1>";
        }else if (userAgent.indexOf("iPhone")!=-1 || userAgent.indexOf("Android")!=-1){
            output="<h1>这是移动端首页</h1>";
        }
        response.getWriter().println(output);
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    }
}

服务器的响应的结构

请求是浏览器向服务器发送的请求，而响应则是服务器返回给浏览器的显示结果

HTTP响应包含三部分：响应行、响应头、响应体
Content-Type：text/html表示，把响应体中内容当成text/html去解释；浏览器会按照text/html的方式去解释响应体中的内容
HTTP常见状态码
如url网址输错的时候，会报404；如果Java的代码出了错误，会报500等；浏览器可以根据响应返回状态码，来知道服务器是处理成功还是出现了问题
网页中的响应内容与响应体
常见状态码演示
404：例如这里我少输出了文件名字

500：服务器的内部错误，去寻找java源代码中的错误

ContentType的作用

ContentType决定浏览器采用何种方式对响应体进行处理
浏览器可以理解成是一个解释器（解释器响应体中的内容，并体现在浏览器页面上）；Content-Type属性值，就是浏览器对响应体内容采取何种方式；其默认是text/html

ContentType的示例

Content-Type设置为”text/html”

package com.java.servlet;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
import java.io.IOException;

@WebServlet(name = "ContentTypeServlet", value = "/ContentTypeServlet")
public class ContentTypeServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String output="<h1><a href='http://www.baidu.com'><span>百度</span></a></h1>";
        response.setContentType("text/html;charset=utf-8");//设置ContentType的值为text/html
        response.getWriter().println(output);
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    }
}

Content-Type设置为”text/plain”

package com.java.servlet;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
import java.io.IOException;

@WebServlet(name = "ContentTypeServlet", value = "/ContentTypeServlet")
public class ContentTypeServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String output="<h1><a href='http://www.baidu.com'><span>百度</span></a></h1>";
        response.setContentType("text/plain;charset=utf-8");//设置ContentType的值为text/plain
        response.getWriter().println(output);
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    }
}

Content-Type设置为”text/xml”

package com.java.servlet;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
import java.io.IOException;

@WebServlet(name = "ContentTypeServlet", value = "/ContentTypeServlet")
public class ContentTypeServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String output="<h1><a href='http://www.baidu.com'><span>百度</span></a></h1>";
        response.setContentType("text/xml;charset=utf-8");//设置ContentType的值为text/xml
        response.getWriter().println(output);
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    }
}

Content-Type设置为”application/x-msdownload”

package com.java.servlet;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
import java.io.IOException;

@WebServlet(name = "ContentTypeServlet", value = "/ContentTypeServlet")
public class ContentTypeServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String output="<h1><a href='http://www.baidu.com'><span>百度</span></a></h1>";
        response.setContentType("text/xml;charset=utf-8");//设置ContentType的值为application/x-msdownload
        response.getWriter().println(output);
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    }
}

请求转发与重定向

以前接触的都是一个Servlet内处理；而实际情况中，是需要多个Servlet之间跳转的，而Servlet之间的跳转方式为：请求转发和响应重定向

多个Servlet(JSP)之间跳转有两种方式
request.getRequestDispatcher().forward() - 请求转发
response.sendRedirect() - 响应重定向

Servlet请求转发的原理

请求转发是服务器内部跳转，只会产生一次请求
请求转发的语句是：request.getRequestDispatcher().forward

Servlet请求转发案例

案例分析：例如在登录这个功能上，在登录界面，输入用户名和密码后并点击登录后；其会跳转到登录成功后的那个界面。这个案例主要是为了说明在实际开发中，Servlet是需要在不同页面上跳转显示的

请求转发的设置

请求转发的效果

可以看到地址栏的url没有改变

分析：输入http://localhost:8080/servlet_advanced/direct/check后，会先进入CheckLoginServlet这个Servlet，并打印"用户登录成功“

然后请求转发到IndexServlet的这个Servlet上，并触发了IndexServlet中的输出语句：This is index page！

Servlet响应重定向的原理

重定向则是浏览器端跳转，会产生两次请求
响应重定向语句是：response.sendRedirect()

设置请求自定义属性

请求允许创建自定义属性
设置请求属性：request.setAttribute(属性名，属性值)
获取请求属性：Object attr = request.getAttribute(属性名)

请求转发的情况下，获取自定义属性示例

package com.java.servlet.direction;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
import java.io.IOException;

@WebServlet(name = "CheckLoginServlet", value = "/direct/check")//文件名CheckLoginServlet，路径/direct/check
public class CheckLoginServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("用户登录成功");
        request.setAttribute("username","admin");//请求转发的情况下设置属性名和属性值
        //request.getRequestDispatcher()方法相当于是一个请求分发器
        //实现了请求转发的功能,/direct/check会跳转到/direct/index下
        request.getRequestDispatcher("/direct/index").forward(request,response);

    }

package com.java.servlet.direction;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
import java.io.IOException;

@WebServlet(name = "IndexServlet", value = "/direct/index")//文件名IndexServlet，路径/direct/index
public class IndexServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.getWriter().println("This is index page!"+name);
        String username= (String) request.getAttribute("username");
    }

效果：

响应重定向的情况下，获取自定义属性示例

package com.java.servlet.direction;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
import java.io.IOException;

@WebServlet(name = "CheckLoginServlet", value = "/direct/check")//文件名CheckLoginServlet，路径/direct/check
public class CheckLoginServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("用户登录成功");
        request.setAttribute("username","admin");//请求转发的情况下设置属性名和属性值
	    response.sendRedirect("/servlet/direct/index");//响应重定向需要增加contextPath(即上下文路径)

    }

package com.java.servlet.direction;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
import java.io.IOException;

@WebServlet(name = "IndexServlet", value = "/direct/index")//文件名IndexServlet，路径/direct/index
public class IndexServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.getWriter().println("This is index page!"+name);
        String username= (String) request.getAttribute("username");
    }

效果：