详解 Servlet 中 ServletContext、HttpServletResponse、HttpServletRequest 对象

写在前面 Github 源码地址（可以直接运行）：

• ServletContext – servlet-02
• HttpServletResponse – response
• HttpServletRequest – request

一. ServletContext

web 容器在启动的时候，它会为每个 web 程序都创建一个对应的 ServletContext 对象，它代表了当前的 web 应用；

1.1 共享数据

• HelloServlet

public class HelloServlet extends HttpServlet {
    
    
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
//        resp.getWriter().println("Hello,Serlvet"); //响应流
        //this.getInitParameter()   初始化参数
        //this.getServletConfig()   Servlet配置
        //this.getServletContext()  Servlet上下文
        ServletContext context = this.getServletContext();

        String username = "秦疆"; //数据
        context.setAttribute("username",username); //将一个数据保存在了ServletContext中，名字为：username 。值 username
        resp.getWriter().println("Hello Servlet!");
    }
}

• GetServlet

public class GetServlet extends HttpServlet {
    
    
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
        ServletContext context = this.getServletContext();
        String username = (String) context.getAttribute("username");

        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf-8");
        resp.getWriter().print("名字"+username);
    }
}

1.2 获取初始化参数

• 在 web.xml 中设置参数

    <context-param>
        <param-name>url</param-name>
        <param-value>jdbc:mysql://localhost:3306/mybatis</param-value>
    </context-param>

• ParamServlet

public class ParamServlet extends HttpServlet {
    
    

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
        ServletContext context = this.getServletContext();
        String url = context.getInitParameter("url");
        resp.getWriter().println(url);

    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
        doGet(req, resp);
    }
}

1.3 请求转发

• TransferServlet

public class TransferServlet extends HttpServlet {
    
    
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
        ServletContext context = this.getServletContext();
        System.out.println("进入了param");
        //RequestDispatcher requestDispatcher = context.getRequestDispatcher("/gp"); //转发的请求路径
        //requestDispatcher.forward(req,resp); //调用forward实现请求转发；
        context.getRequestDispatcher("/param").forward(req,resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
        doGet(req, resp);
    }
}

1.4 读取资源文件

• db.propreties

username = kobe_bryant
password = fds4f5ds4f65ds4f6ds4f

• PropServlet

public class PropServlet extends HttpServlet {
    
    
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
        InputStream is = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db.properties");

        Properties prop = new Properties();
        prop.load(is);
        String user = prop.getProperty("username");
        String pwd = prop.getProperty("password");

        resp.getWriter().print(user+":"+pwd);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
        doGet(req, resp);
    }
}

二. HttpServletResponse

web 服务器接收到客户端的 http 请求，针对这个请求，分别创建一个代表请求的 HttpServletRequest 对象，代表响应的一个 HttpServletResponse；

• 如果要获取客户端请求过来的参数：找 HttpServletRequest
• 如果要给客户端响应一些信息：找 HttpServletResponse

2.1 简单分类

• 负责向浏览器发动数据的方法

ServletOutputStream getOutputStream() throws IOException;
PrintWriter getWriter() throws IOException;

• 负责向浏览器发送响应头的常用方法

    void setCharacterEncoding(String var1);

    void setContentLength(int var1);

    void setContentType(String var1);

    void setHeader(String var1, String var2);

    void addHeader(String var1, String var2);

    void setIntHeader(String var1, int var2);

    void addIntHeader(String var1, int var2);

• 常用响应的状态码

    int SC_OK = 200;
    int SC_MULTIPLE_CHOICES = 300;
    int SC_NOT_FOUND = 404;
    int SC_INTERNAL_SERVER_ERROR = 500;

2.2 下载文件

1. 向浏览器输出消息
2. 下载文件
   1. 要获取下载文件的路径
   2. 下载的文件名是啥？
   3. 设置想办法让浏览器能够支持下载我们需要的东西
   4. 获取下载文件的输入流
   5. 创建缓冲区
   6. 获取 OutputStream 对象
   7. 将 FileOutputStream 流写入到buffer缓冲区
   8. 使用 OutputStream 将缓冲区中的数据输出到客户端！

• FileServlet

public class FileServlet extends HttpServlet {
    
    
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
        // 1. 要获取下载文件的路径
        String realPath = "D:\\Java\\IntelliJ IDEA 2020.3\\Projects\\Javaweb-study\\response\\target\\classes\\科比.jpg";
        System.out.println("下载文件的路径："+realPath);
        // 2. 下载的文件名是啥？
        String fileName = realPath.substring(realPath.lastIndexOf("\\") + 1);
        // 3. 设置想办法让浏览器能够支持(Content-Disposition)下载我们需要的东西,中文文件名URLEncoder.encode编码，否则有可能乱码
        resp.setHeader("Content-Disposition","attachment;filename="+ URLEncoder.encode(fileName,"UTF-8"));
        // 4. 获取下载文件的输入流
        FileInputStream in = new FileInputStream(realPath);
        // 5. 创建缓冲区
        int len = 0;
        byte[] buffer = new byte[1024];
        // 6. 获取OutputStream对象
        ServletOutputStream out = resp.getOutputStream();
        // 7. 将FileOutputStream流写入到buffer缓冲区,使用OutputStream将缓冲区中的数据输出到客户端！
        while ((len=in.read(buffer))>0){
    
    
            out.write(buffer,0,len);
        }

        in.close();
        out.close();
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
        doGet(req, resp);
    }
}

2.3 验证码功能

public class ImageServlet extends HttpServlet {
    
    
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
        //如何让浏览器3秒自动刷新一次;
        resp.setHeader("refresh","3");

        //在内存中创建一个图片
        BufferedImage image = new BufferedImage(80,20,BufferedImage.TYPE_INT_RGB);
        //得到图片
        Graphics2D g = (Graphics2D) image.getGraphics(); //笔
        //设置图片的背景颜色
        g.setColor(Color.white);
        g.fillRect(0,0,80,20);
        //给图片写数据
        g.setColor(Color.BLUE);
        g.setFont(new Font(null,Font.BOLD,20));
        g.drawString(makeNum(),0,20);

        //告诉浏览器，这个请求用图片的方式打开
        resp.setContentType("image/jpeg");
        //网站存在缓存，不让浏览器缓存
        resp.setDateHeader("expires",-1);
        resp.setHeader("Cache-Control","no-cache");
        resp.setHeader("Pragma","no-cache");

        //把图片写给浏览器
        ImageIO.write(image,"jpg", resp.getOutputStream());

    }

    //生成随机数
    private String makeNum(){
    
    
        Random random = new Random();
        String num = random.nextInt(9999999) + "";
        StringBuffer sb = new StringBuffer();
        for (int i = 0; i < 7-num.length() ; i++) {
    
    
            sb.append("0");
        }
        num = sb.toString() + num;
        return num;
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
        doGet(req, resp);
    }
}

2.4 实现重定向

• B一个web资源收到客户端A请求后，B他会通知A客户端去访问另外一个web资源C，这个过程叫重定向

• 方法

void sendRedirect(String var1) throws IOException;

• 测试

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    

    /*
        resp.setHeader("Location","/r/img");
        resp.setStatus(302);
         */
    resp.sendRedirect("/r/img");//重定向
}

2.5 重定向和转发的区别

相同点

• 页面都会实现跳转

不同点

• 请求转发的时候，url 不会产生变化
• 重定向时候，url 地址栏会发生变化；

三. HttpServletRequest

3.1 实现重定向（代码实例）

• index.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>登录页面</title>
</head>
<body>
<h1>登录</h1>
<div style="text-align: center">
    <%-- 以Post方式提交表单，提交到我们的login请求 --%>
    <form action="${
    
    pageContext.request.contextPath}/login" method="post">
        用户名：<input type="text" name="username"><br>
        密码：<input type="password" name="password"><br>
        爱好：
        <input type="checkbox" name="hobbys" value="女孩">女孩
        <input type="checkbox" name="hobbys" value="代码">代码
        <input type="checkbox" name="hobbys" value="唱歌">唱歌
        <input type="checkbox" name="hobbys" value="电影">电影
        <br>
        <input type="submit">
    </form>

</div>
</body>
</html>

• success.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Success</title>
</head>
<body>
    <h1>登录成功！！！</h1>
</body>
</html>

• LoginServlet.jsp

public class LoginServlet extends HttpServlet {
    
    
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
        req.setCharacterEncoding("utf-8");
        resp.setCharacterEncoding("utf-8");

        String username = req.getParameter("username");
        String password = req.getParameter("password");
        String[] hobbys = req.getParameterValues("hobbys");
        System.out.println("=============================");
        //后台接收中文乱码问题
        System.out.println(username);
        System.out.println(password);
        System.out.println(Arrays.toString(hobbys));
        System.out.println("=============================");


        System.out.println(req.getContextPath());
        //通过请求转发
        //这里的 / 代表当前的web应用
        req.getRequestDispatcher("/success.jsp").forward(req,resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
        doGet(req, resp);
    }
}

• 测试结果