Coding in JSP for the first time, I need to render a variable's value to HTML. It looks like there are various ways to do this; what is the difference between these (given that I have a variable named foo)?
<%= foo %>
and
${ foo }
Asked
Active
Viewed 637 times
3
BalusC
- 1,082,665
- 372
- 3,610
- 3,555
Richard Ev
- 52,939
- 59
- 191
- 278
-
Please don't confuse JSTL with EL. I fixed the tags. To understand the difference, read http://stackoverflow.com/tags/jstl/info and http://stackoverflow.com/tags/el/info – BalusC Oct 01 '13 at 14:07
1 Answers
5
This, using an old fashioned output scriptlet which is discouraged since a decade,
<%= foo %>
does basically the same as the following in a regular scriptlet:
<% out.println(foo); %>
which in turn does basically the same as the following in a normal Java servlet class (you probably already know, JSPs ultimately get compiled and converted to a servlet class):
response.getWriter().println(foo);
where foo is thus declared as a local/instance variable. It thus prints the local/instance variable foo to the HTTP response at exactly the declared place.
This, using expression language (EL), which is the recommended approach since JSP 2.0 in 2003,
${ foo }
does basically the same as the following in a regular scriptlet, with PageContext#findAttribute():
<%
Object foo = pageContext.findAttribute("foo");
if (foo != null) out.println(foo);
%>
which is in turn equivalent to:
<%
Object foo = pageContext.getAttribute("foo");
if (foo == null) foo = request.getAttribute("foo");
if (foo == null) foo = session.getAttribute("foo");
if (foo == null) foo = application.getAttribute("foo");
if (foo != null) out.println(foo);
%>
It thus prints the first non-null occurrence of the attribute in the page/request/session/application scope to the response at exactly the declared place. If there is none, then print nothing. Please note that it thus doesn't print a literal string of "null" when it's null, on the contrary to what scriptlets do.
