Context

5.1什么是Context

• 文本上下文(官方定义)
• 具有超长的生存周期，随服务器而存在

5.2获取Context

ServletContext con = this.getServletContext();

5.3应用

1.传递变量

因为其超长的生存周期，可以用来储存一些公共变量，还可以传递变量

context1

public class Context1 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext con = this.getServletContext();
        con.setAttribute("username","root");
        resp.setContentType("text/html");
        PrintWriter out = resp.getWriter();
        out.print("already save username");
    }

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

context2

public class Context2 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setContentType("text/html");
        ServletContext con = this.getServletContext();
        String name = (String) con.getAttribute("username");
        resp.getWriter().print("username = "+name);
    }

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

访问结果

2.请求转发

将Context1修改为如下

public class Context1 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext con = this.getServletContext();
        con.setAttribute("username","root");
        resp.setContentType("text/html");
        PrintWriter out = resp.getWriter();
        out.print("already save username");
        RequestDispatcher Rd = con.getRequestDispatcher("/Context2");
        Rd.forward(req,resp);
    }

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

则访问Context1结果如下