Java back-end app questions

I am just starting to look into web hosting services but I am curious how services that offer JVMs for back-end apps operate. These are pretty newbie questions. I have experience with back-end applications on a dedicated server from my previous job (where most of these questions were not an issue).

How does a shared JVM work? How does that run more than one app simultaneously and how does one app get updated w/o shutting down the whole JVM?

Java tends to use lots of memory - from people's experience how well do these dedicated JVMs with only 50M of RAM work (for small apps)?

How much control does that client usually have over the JVM options (classpath, logging, etc etc)?

How does app deployment usually work? Can the client FTP the app and then restart the JVM themselves?

thx

 

 

 

 

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