<%
    response.setHeader("Cache-Control", "no-cache"); //Forces caches to obtain a new copy of the page from the origin server
    response.setHeader("Cache-Control", "no-store"); //Directs caches not to store the page under any circumstance
    response.setDateHeader("Expires", 0); //Causes the proxy cache to see the page as "stale"
    response.setHeader("Pragma", "no-cache"); //HTTP 1.0 backward compatibility

    System.out.println("Validando...");
    if (session.getAttribute("usuario") == null) {
        response.sendRedirect("main.jsp");
    }
%>