code: public static Object startJarVerification() {
          ProviderList currentList = getProviderList();
          ProviderList jarList = currentList.getJarList(jarVerificationProviders);
          // return the old thread-local provider list, usually null
          return beginThreadProviderList(jarList);
      }
docs: Start JAR verification. This sets a special provider list for
  the current thread. You MUST save the return value from this
  method and you MUST call stopJarVerification() with that object
  once you are done.