private void registerInstantiators() {
    GemfireHttpSession.registerInstantiator();
    DeltaQueuedSessionAttributes.registerInstantiator();
    DeltaSessionAttributes.registerInstantiator();
  }
This is where all the initialization happens.
extensions/geode-modules-session/src/main/java/org/apache/geode/modules/session/filter/SessionCachingFilter.java