Also note, that Jetty 8 is EOL (End of Life) now.
You should use Jetty 9 if you can from here on out.
As for extraClasspath, that is rarely a good idea.
All you are doing is breaking the servlet spec, and servlet behavior.
And you are introducing extra work, configuration, and maintenance on yourself.
extraClasspath was never really designed for components that are scanned (like spring, cdi, guice, etc..)
Since you are obviously using spring + no web.xml it seems that you have an interest in keeping configuration and maintenance low for your webapp. So just include the spring jars in your WEB-INF/lib/ and be done with it.
If you still insist on using extraClasspath, upgrade to Jetty 9 and use the features available there to allow scanning of container jars as well.
You'll have to declare a deploy time webapp attribute called "org.eclipse.jetty.server.webapp.ContainerIncludeJarPattern"
which is a pipe-delimited list of regex expressions for jars that you want to also scan during the container annotation scanning.
eg:
".*/foo-[^/]*\.jar$|.*/bar-[^/]*\.jar$|.*/myjars/*\.jar$"
In Jetty 9.2+ you also have the quickstart features and modules available that can do the scan and configuration at build-time making deploy / startup even faster (think sub 250ms).