servletcontext的作用详细介绍

1、什么是servletcontext
web容器在启动的时候，他会为每个web程序创建一个对应的servletcontext对象，他代表的是当前的web应用：
共享数据
我在这个Servlet中保存的数据，可以在宁一个servlet中拿到：
例子如下

设置数据

public class Servlettest extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request,response);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String userName="小米";
        context.setAttribute("userName",userName);
    }
		 		

获取数据

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

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String userName =(String)context.getAttribute("userName");
        System.out.println(userName);
    }

web.xml的配置

 <servlet>
        <servlet-name>gethello</servlet-name>
        <servlet-class>com.xiaoming.servlet.GetServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>gethello</servlet-name>
        <url-pattern>/gethello</url-pattern>
    </servlet-mapping>
    <servlet>
        <servlet-name>sethello</servlet-name>
        <servlet-class>com.xiaoming.servlet.Servlettest</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>sethello</servlet-name>
        <url-pattern>/sethello</url-pattern>
    </servlet-mapping>

测试结果：

二、servletcontext的方法介绍
servlet获取初始化参数

 <context-param>
        <param-name>url</param-name>
        <param-value>jdbc:mysql://localhost:8080/test</param-value>
    </context-param>

protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf-8");
        ServletContext servletContext = this.getServletContext();
        String url = servletContext.getInitParameter("url");
        resp.getWriter().print(url);
    }

请求转发

 protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setCharacterEncoding("utf-8");
        resp.setContentType("text/html");
        ServletContext context = this.getServletContext();
        context.getRequestDispatcher("/hello").forward(req,resp);
    }

读取properties资源文件
在Java和resource的目录下创建一个文件都会生成到classes文件下
classes被称为类路径。
下载文件的案例

protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//        String path = this.getServletContext().getRealPath("/1.png");
        String path =   "E:\\learn\\javaweb\\java_servlet\\ResponseServlet\\src\\main\\resources\\1.png";
        System.out.println("文件的绝对路径"+path);
        String filename = path.substring(path.lastIndexOf("\\") + 1);
        resp.setHeader("Content-Disposition","attachment;filename="+ URLEncoder.encode(filename,"utf-8"));
        FileInputStream stream = new FileInputStream(path);
        int  len=0;
        byte[] buffer = new byte[1024];
        ServletOutputStream outputStream = resp.getOutputStream();
        while((len=stream.read(buffer))>0){
            outputStream.write(buffer,0,len);
        }
        outputStream.close();
        stream.close();
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }

在Java中生成验证码图片

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setHeader("refresh","3");
//        在内存中创建一个图片

        BufferedImage Image = new BufferedImage(80, 20, BufferedImage.TYPE_3BYTE_BGR);
        Graphics2D g = (Graphics2D)Image.getGraphics();//得到一个笔
        g.setColor(Color.white);//设置背景颜色
        g.fillRect(0,0,80,20);//用笔把画布进行填充
        //生成随机数
        g.setColor(Color.blue);
        g.setFont(new Font(null,Font.BOLD,20));
        g.drawString(makeNum(),0,20);
        //告诉浏览器，这个文件用图片方式打开
       response.setContentType("image/jpg");
       response.setDateHeader("expires",-1);
       response.setHeader("Cache-Control","no-cache");
       response.setHeader("pragma","no-cache");
       //把图片写给浏览器
        ImageIO.write(Image,"jpg", response.getOutputStream());

    }
    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");
        }
        String s = sb.toString() + num;
        return s;
    }
}

重定向

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.sendRedirect("/test3/image");
    }

重定向和转发都会跳转页面，但是转发的url地址栏不会发生变化而重定向地址栏会发生变化。