Read a Environment Variable in Java with Websphere

I've a little problem with Websphere application server 7.0 (WAS7) and the reading of Environment Varaibles.

With TomCat, I've defined a variable as

<Environment name="myVar" type="java.lang.String" value="myVarOnServeur"

and I read it with a lookup on the initialContext :

Context ctx = new InitialContext();
String myVar = (String) ctx.lookup( "java:comp/env/myVar" );

and it works!

But with Websphere, I define a environment variable on the GUI but I can't read it in my java code. I've a NamingException.

_{(source: fullahead.org)}

How can I do to fix my problem?

I don't see anything there that says that those entries can be read via ctx.lookup( "java:comp/env/..." );