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You still don't get it; the use of shared data depends entirely on the application. Many Java web applications do not depend on shared data structures, and thus it doesn't matter if you have separate servers. Other web applications might use static or rarely changed resources, so no communication between servers is needed. Applications which do use dynamic in-memory data will need to use some sort of clustering to scale beyond multiple machines -
but this is a capability that PHP does not have at all!
Any web app using server-side sessions running on multiple servers should use session replication. Clustering is something provided by many Java servlet containers/web servers.