A. If you use maven, an useful way to debug clashing jars is:
mvn dependency:tree
For example, for an exception:
java.lang.SecurityException: class "javax.servlet.HttpConstraintElement"'s signer information does not match signer information of other classes in the same package
we do:
mvn dependency:tree|grep servlet
Its output:
[INFO] +- javax.servlet:servlet-api:jar:2.5:compile
[INFO] +- javax.servlet:jstl:jar:1.2:compile
[INFO] | +- org.eclipse.jetty.orbit:javax.servlet.jsp:jar:2.2.0.v201112011158:compile
[INFO] | +- org.eclipse.jetty.orbit:javax.servlet.jsp.jstl:jar:1.2.0.v201105211821:compile
[INFO] | +- org.eclipse.jetty.orbit:javax.servlet:jar:3.0.0.v201112011016:compile
[INFO] +- org.eclipse.jetty:jetty-servlet:jar:9.0.0.RC2:compile
shows clashing servlet-api 2.5 and javax.servlet 3.0.0.x.
B. Other useful hints (how to debug the security exception and how to exclude maven deps) are at the question at Signer information does not match.