I tried to use a map based EL expression ${folder.variables["copyright"]} as Tag attribute like <jg:if exists="${folder.variables["copyright"]}"> But there was a JasperException "equal symbol expected" When I included backslashes, it worked: <jg:if exists="${folder.variables[\"copyright\"]}"> I did find nothing in the JSP 2.0 spec about having to escape quotes in Tag attributes, especially as in normal usage it is not necessary. At least the error message is not very helpful.
This would be a problem since tomcat4 using JSTL too. An alternative is to mix your quotes. But the error message does state where in the JSP file there is an error. At best this could be an enhancement.
JSP.1.6 defines rules for quoting and escape conventions. Any attributes, including ELs, must follow these rules. The error message is not perfect, but it usually includes line column numbers, which should let you locate the problems.