Reinstate testing of Tomcat JSP samples using Java 9

[snicoll]

The JSP samples themselves work just fine but the integration tests that are exercising the sample fail.

2017-09-29 11:52:10.395 ERROR 62623 --- [o-auto-1-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [The absolute uri: [http://www.springframework.org/tags] cannot be resolved in either web.xml or the jar files deployed with this application] with root cause

org.apache.jasper.JasperException: The absolute uri: [http://www.springframework.org/tags] cannot be resolved in either web.xml or the jar files deployed with this application
	at org.apache.jasper.compiler.DefaultErrorHandler.jspError(DefaultErrorHandler.java:55) ~[tomcat-embed-jasper-8.5.21.jar:8.5.21]
	at org.apache.jasper.compiler.ErrorDispatcher.dispatch(ErrorDispatcher.java:293) ~[tomcat-embed-jasper-8.5.21.jar:8.5.21]
	at org.apache.jasper.compiler.ErrorDispatcher.jspError(ErrorDispatcher.java:80) ~[tomcat-embed-jasper-8.5.21.jar:8.5.21]
	at org.apache.jasper.compiler.TagLibraryInfoImpl.generateTldResourcePath(TagLibraryInfoImpl.java:251) ~[tomcat-embed-jasper-8.5.21.jar:8.5.21]
	at org.apache.jasper.compiler.TagLibraryInfoImpl.<init>(TagLibraryInfoImpl.java:122) ~[tomcat-embed-jasper-8.5.21.jar:8.5.21]
	at org.apache.jasper.compiler.Parser.parseTaglibDirective(Parser.java:434) ~[tomcat-embed-jasper-8.5.21.jar:8.5.21]

[wilkinsona]

An app run via its main method (in your IDE for example) also won't work

[wilkinsona]

This is a limitation of Jasper's StandardJarScanner. When running via the main method (rather than as a packaged war), TLDs are found as a result of StandardJarScanner scanning the class path. It does so by walking up the ClassLoader hierarchy and examine the URLs of any URLClassLoaders that it finds. When running via a main method using Java 9, the ClassLoader that has the TLD-containing jars on its class path is not a URLClassLoader so the TLDs are not found. It works fine when packaging the application as a war because the TLD-containing jars are found by virtue of being packaged in WEB-INF/lib.

[wilkinsona]

Short of copying and modifying StandardJarScanner, I can't see a good way to fix this. While public and non-final, StandardJarScanner isn't particularly amenable to being extended. In particular, there's no way to provide an alternative way to get the URLs that are considered to be on the class path and the method that does most of the heavy lifting (process) is private.

[wilkinsona]

@markt-asf Could some changes to StandardJarScanner be considered in Tomcat 8.5.x? If process was protected and there was a scanClassPath method that we could override then I think we could get this working by falling back to looking at the class path used to launch the JVM (a reasonable approximation in a Boot app for what the scanner currently does in searching up the class loader hierarchy).

[markt-asf]

This looks like something that Tomcat will need to fix generally for the embedded use case for Java 9.