There is a bug in the Eclipse Equinox implementation of HttpContextManager (version 1.0.0.v20070608). It throws an array out-of-bounds exception on line 91:
for (int j = 0; j < resourceMappingElements.length; j++) {
IConfigurationElement resourceMappingElement = resourceMappingElements[i];i is the wrong index variable. It should be j.
This code will only work when there is only one context with no more than one resource mapping.