Thanks for your answer. I already thought about testing with the URLEncoder.class :
<% String url = URLEncoder.encode("test.jsp?code=" + value, "UTF-8"); %>
The returned url seems to be correct (test.jsp%3Fcode%3Dtest) but it doesn't works even if there is no specific character. The following error message is thrown: "The requested resource (/test.jsp%3Fcode%3Dtest) is not available"
Could you tell me what am I doing wrong? Or how can I use JSTL in this example (I hadn't ever use it).