code: public void putSessionObject(@Nonnull final String key, @Nullable final Object obj) {
      this.lock();
      try {
        if (obj == null) {
          this.sessionObjects.remove(key);
        } else {
          this.sessionObjects.put(key, obj);
        }
      } finally {
        this.unlock();
      }
    }
docs: Put session object for key.
  
  @param key key of he object, must not be null
  @param obj object to be placed, if null then object will be removed
  @since 1.4.2