When you have an @Context field in a JAXRS class (maybe JAXWS too?) there is a select list of possible proxy classes. This method checks for the proxy class using an if/else/if/else... structure, but there is no final else clause with a good exception. Instead the method attempts to instantiate a null class name (proxyClassName), which throws a NullPointerException. This leaves the method wrapped inside a RuntimeException. A common forgetful change someone might make is to add
Which leaves a massive stack trace leaving just an NPE and Spring also tends to obscure the stack-trace of this issue. A better error would be to suggest that the given class ...HttpSession is not a valid Context type. No proxy classes can be created for this type.