This option was added when parallel capable class loading was added in JDK 7 to help users debug their parallel capable class loaders that might have bugs. Allowing general access to this functionality can cause deadlocks, so this shouldn't be an option for users. Nor is is useful for debugging parallel capable class loaders, since it may only hide their bugs.
This option is untested (and should be because it'll cause deadlocks) and should be deprecated in 17, obsoleted in 18 and removed in 19.
VM Flags for early access testing
AlwaysLockClassLoader - default false. Provided for ParallelClassLoader that has a bug in handling multiple threads in parallel. Expected to ship in product.