ServletContext

什么是ServletContext?

1. Servlet环境上下文对象
2. web容器启动的时候，会为每一个web程序创建一个ServletContext对象，它代表了当前的web应用；
3. 一个web应用是由若干个JSP,JavaBean，Servlet等web组件的集合构成
4. 可以通过ServletContext对象得到以下资源信息：
   .初始化参数
   .存储在context中的对象
   .与context关联的资源
   .日志

ServletContext应用：

1.共享数据

放置数据

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

    }

读取数据

public class GetServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf-8");
        ServletContext context = this.getServletContext();
        String  name = (String) context.getAttribute("name");
        resp.getWriter().println("名字"+name);
    }

两个Servlet在同一个web应用里，所以是相同的ServletContext对象

2. 获取初始化参数

<!--初始化一些参数-->
<context-param>
    <param-name>url</param-name>
    <param-value>jdbc:mysql://localhost:3306/mybatis</param-value>
  </context-param>

//取得参数
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String  url=context.getInitParameter("url");
        resp.getWriter().println(url);
    }

3.请求转发

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        System.out.println("进入了demo4方法");
        RequestDispatcher requestDispatcher = context.getRequestDispatcher("/gp");//请求转发的路径
        requestDispatcher.forward(req,resp);//调用forward进行转发
    }

注意与请求重定向的差别：访问的路径没有改变

4.读取资源文件
Properties:

• Java路径下新建properties
• resources目录下新建properties文件
  这两个文件都被打包至同一路径下:classes(target)，俗称为classpath

    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext servletContext = this.getServletContext();
        InputStream is = servletContext.getResourceAsStream("/WEB-INF/classes/db.properties");
        Properties properties = new Properties();
        properties.load(is);
        String username = properties.getProperty("username");
        String password = properties.getProperty("password");
        resp.getWriter().println(username+":"+password);

    }