A PassVerifier actually verifies a class file; it is instantiated by a Verifier. The verification should conform with a certain pass as described in The Java Virtual Machine Specification, 2nd edition. This book describes four passes. Pass one means loading the class and verifying a few static constraints. Pass two actually verifies some other constraints that could enforce loading in referenced class files. Pass three is the first pass that actually checks constraints in the code array of a method in the class file; it has two parts with the first verifying static constraints and the second part verifying structural constraints (where a data flow analysis is used for). The fourth pass, finally, performs checks that can only be done at run-time. JustIce does not have a run-time pass, but certain constraints that are usually delayed until run-time for performance reasons are also checked during the second part of pass three. PassVerifier instances perform caching. That means, if you really want a new verification run of a certain pass you must use a new instance of a given PassVerifier.