域对象、请求对象、响应对象、请求重定向的应用

最新推荐文章于 2022-11-13 04:40:12 发布

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最新推荐文章于 2022-11-13 04:40:12 发布

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分类专栏： Java JavaWeb

本文链接：https://blog.youkuaiyun.com/M_ount/article/details/79719785

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这篇博客详细介绍了Web应用中的一些关键概念，包括从web.xml配置信息的解析到Config对象获取Servlet配置，再到域对象如何获取全局配置和服务器文件。文中还探讨了请求转发的操作，通过实例展示了其工作原理。同时，重点讨论了HttpServletResponse响应对象在文件下载中的应用，以及请求重定向的机制，区分了它与请求转发的区别，并列举了三种设置请求重定向的方法。最后，文章也提及了HttpServletRequest请求对象在处理请求中的作用。

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web.xml配置信息：

<!-- 设置全局配置信息 -->
<context-param>
    <param-name>key</param-name>
    <param-value>value</param-value>
</context-param>

<servtel>
    <servtel-name>demo01</servtel-name>
    <servtel-class>com.lanou3g.Demo01</servtel-class>
</servtel>
<init-param>
    <param-name>username</param-name>
    <param-value>zhangsan</param-value>
</init-param>
<servtel-mapping>
    <servtel-name>demo01</servtel-name>
    <url-pattern>/demo01</url-pattern>
</servtel-mapping>

<servtel>
    <servtel-name>demo02</servtel-name>
    <servtel-class>com.lanou3g.Demo02</servtel-class>
</servtel>
<servtel-mapping>
    <servtel-name>demo02</servtel-name>
    <url-pattern>/demo02</url-pattern>
</servtel-mapping>

Config对象获取Servlet配置信息：

public class Demo01 extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletConfig config = this.getServletConfig();
        String value = config.getInitParameter("username");
        System.out.println(value);
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

小结：先获取config对象，再通过getInitParameter（username相当于键值对中的键）方法获取对应值。

域对象获取全局配置信息：

public class Demo01 extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext application = this.getServletContext();
        String value = application.getInitParameter("key");
        System.out.println(value);
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

域对象获取服务器文件并读取：

public class Demo01 extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext application = this.getServletContext();
        String path = application.getRealPath("/WEB-INF/a.properties");
        Properties properties = new Properties();
        properties.load(new FileInputStream(path));
        System.out.println(properties.getProperty("key"));
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

请求转发：

概论：通过浏览器对服务器请求一个Servlet，但该Servlet无法完成，需跳转到另一个Servlet的操作。
注意：此过程中浏览器只发起一次请求。

这里写图片描述

举例：

public class Demo01 extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext application = this.getServletContext();
        RequestDispatcher dispatcher = application.getRequestDispatcher("/demo02");
        dispatcher.forward(request, response);
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

public class Demo02 extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

小结：先获取域对象application，再通过域对象的方法获取请求转发器dispatcher，最后通过dispatcher转发器的forward方法进行转发.

HttpServletResponse（响应对象）:

组成：响应行：http/1.1-->协议/状态码
      响应头：告诉浏览器我要做什么。
      例如响应给你的文件需要下载，以什么编码格式解析数据
      响应体：响应回浏览器的数据

注意：在同一个Servlet不能同时使用response中获取的字符流和字节流。

public class Demo01 extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //  设置服务器的编码格式为utf-8
        response.setCharacterEncoding("UTF-8");
        //  添加响应头
        response.setHeader("Content-type", "text/html;charset=UTF-8");
        //  第二种写法
        //  response.setContentType("text/html;charset=UTF-8");
        //  从响应对象HttpServletRequest中获取流对象
        //  注意：该流对象要从响应中获取
        PrintWriter out = response.getWriter();
        out.write("好开心");
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

文件下载：

public class Demo01 extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //  获取图片路径
        String path = this.getServletContext().getRealPath("/WEB-INF/classes/团子大家族.png");
        //  字符串切割 获取图片的名字
        int index = path.lastIndexOf("/");
        String filename = path.substring(index + 1);
        //  修改文件名字的字符集
        filename = new String(filename.getBytes(), "iso-8859-1");
        //  图片名字格式响应头
        response.setHeader("content-disposition", "attachment;filename=" + filename);
        //  浏览器文件下载的响应头
        response.setHeader("content-type", "image/png");
        //  读取图片
        FileInputStream fis = new FileInputStream(path);
        //  注意：需要获取response当中字节流
        ServletOutputStream sos = response.getOutputStream();
        //  边读边写
        int len = 0;
        byte[] b = new byte[1024];
        while ((len = fis.read(b)) != -1) {
            //  响应回浏览器
            sos.write(b, 0, len);
        }
        //  注意：关闭创建的流
        fis.close();
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

请求重定向：

概论：浏览器请求了一个Servlet，该Servlet给浏览器响应了一个带有重定向的访问地址的响应头，浏览器接到响应后，重新发起对访问地址的请求的操作。

这里写图片描述

请求重定向和请求转发的区别：

请求重定向：发起两次请求，请求地址发生了变化。
请求转发：只是一次请求。

设置请求重定向的三种方法：

一、添加请求头的方法：

public class Demo01 extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("我要借钱");
        System.out.println("我没有，去找demo08");
        response.setHeader("location", "/sh-web-servlet02/demo08");
        response.setStatus(302);
        System.out.println("我去了");
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

二、添加刷新头的方法：

public class Demo01 extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setIntHeader("refresh", 1);
        response.getWriter().write(Math.random() + "");
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

三、类似刷新的方法：

public class Demo01 extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        response.setHeader("refresh", "3;url=/sh-web-servlet02/demo08");
        response.getWriter().write("3秒后跳转");
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

HttpServletRequest（请求对象）：

组成：请求行、请求头、请求体。

public class Demo08 extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //  获取请求网址
        //  http://localhost:8080/sh-web-servlet02/demo08
        System.out.println(request.getRequestURL());
        //  /sh-web-servlet02/demo08
        System.out.println(request.getRequestURI());
        //  获取请求类型（用浏览器直接请求都是get请求）
        System.out.println(request.getMethod());
        //  获取请求路径（相对路径）
        System.out.println(request.getContextPath());

        //  获取请求中携带的参数
        //  参数是你提交表单时，表单的name属性
        String username = request.getParameter("username");
        String password = request.getParameter("password");
        System.out.println(username + "  " + password);

        //  判断浏览器
        //  可以通过请求中的信息获取到用户使用浏览器
        String header = request.getHeader("User-Agent");
        //  Firefox Chrome
        System.out.println(header);
        if (header.toLowerCase().contains("firefox")) {
            System.out.println("用的是火狐");
        } else if (header.toLowerCase().contains("chrome")) {
            System.out.println("用的是谷歌");
        } else {
            System.out.println("其他浏览器");
        }
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }

}