The java objects instantiated for JSP Custom Tags can now be pooled and reused. This significantly boosts the performance of JSP pages which use custom tags.
That page also says that web.xml can contain an "enablePooling" option for that, and that its default value is true.
After the doEndTag invocation, the tag handler is available for further invocations (and it is expected to have retained its properties).
So what I do is reset all local variables to their default value just before doEndTag() returns
%A Author
%B Secondary Title (of a Book or Conference Name)
%C Place Published
%D Year
%E Editor /Secondary Author
%F Label
%G Language
%H Translated Author
%I Publisher
%J Journal Name
%K Keywords
%L Call Number
%M Accession Number
%N Number (Issue)
%O Alternate Title
%P Pages
%Q Translated Title
%R DOI
%S Tertiary Title
%T Title
%U URL
%V Volume
%W Database Provider
%X Abstract
%Y Tertiary Author / Translator
%Z Notes
%0 Reference Type
%1 Custom 1
%2 Custom 2
%3 Custom 3
%4 Custom 4
%6 Number of Volumes
%7 Edition
%8 Date
%9 Type of Work
%? Subsidiary Author
%@ ISBN/ISSN
%! Short Title
%# Custom 5
%$ Custom 6
%] Custom 7
%& Section
%( Original Publication
%) Reprint Edition
%* Reviewed Item
%+ Author Address
%^ Caption
%> File Attachments
%< Research Notes
%[ Access Date
%= Custom 8
%~ Name of Database
J. Illig, A. Hotho, R. Jäschke, and G. Stumme. Knowledge Processing and Data Analysis, volume 6581 of Lecture Notes in Computer Science, Springer Berlin / Heidelberg, 10.1007/978-3-642-22140-8_9.(2011)