Are global variables in the included file (using <jsp:include/>) available in the file that includes it?

No: using <jsp:include /> you make a dynamic include, that will result in 2 different servlet to be generated: the outside one will call the included one and return its context in place of the <jsp:include/> tag.

Using <%@ include file="include.jsp" %> will make outside variables to be seen in the included page, as this will result in a compilation-time static include generating 1 single Servlet to accomplish JSP requests.

If you want to pass some variables from an outside page to an included one with a dynamic include you have to pass some parameters as shown following:

<%
  ...
  String myvar = "hello";
%>
  <jsp:include page="included.jsp" flush="true" >
    <jsp:param name="myvar" value="<%= myvar %>" />
  </jsp:include>