Of course there isn't a rule in the
servlet specification which says something like "Objects exceeding 12,839 bytes may not be stored in a session".
However from a practical point of view, if you store large objects in a session and you have a lot of users, and hence a lot of sessions, then you may be using too much memory. Of course "large" and "too much" have to be interpreted in terms of how much memory you have available to run your servlet container, and whether your sessions are shared among servers in a cluster, and various other things.
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