consider the following (simplified) eclipse project configuration.

web_base_jar (shared taglib etc.)
web_war (uses web_base_jar)

When developing our web project locally we use an exploded war layout. The compile output
of web_base_jar is directly copied (not as a jar!) to the war’s
web_war/WEB-INF/classes directory by our ide’s (eclipse with sysdeo plugin and intellij).
Our shared taglib ends up here: web_war/WEB-INF/classes/META-INF/myTaglib.tld. The taglib
is successfully picked up by the Jar Scanner configured with “scanAllDirectories”.

This leads to the following generated jsp code when myTaglib is used:

  static {
    _jspx_dependants = new java.util.HashMap<java.lang.String,java.lang.Long>(1);
    _jspx_dependants.put("file:/somepath/pc_web_war_exploded/WEB-INF/classes/META-INF/ myTaglib.tld
", Long.valueOf(-1L));
  }

The relevant code for this snippet is in org.apache.jasper.compiler.TagLibraryInfoImpl.<init>(TagLibraryInfoImpl.java:173).
Because the Taglib is a file uri (file:/C:/somepath/pc_web_war_exploded/WEB-INF/classes/META-INF/myTaglib.tld)
it is not found by ApplicationContext#getResource and the method
org.apache.jasper.JspCompilationContext.getLastModified(JspCompilationContext.java:437) returns
-1 in this case.

When the compiler checks if the current jsp is outdated it checks also all includes/dependants.
Because myTaglib is in the dependants  list of our jsps it is also checked. The check tests
the lastModified timestamp of the file against -1 and returns always true. See here org.apache.jasper.compiler.Compiler.isOutDated(Compiler.java:519)
This means that our jsps are recompiled on every request. We have some workarounds for this but it would be nice if this setup would work out of the box