在Web应用程序Java中存储静态实例变量

Question

Suppose i have a static instance variable which stores Lists<?> . As it is marked static the contents stored in this variable stay till the class is unloaded.

Now take the case of a web app. suppose i am using a framework like play and jpa as persistence.

In case i use this variable to persist using jpa. but as its static instance variable everything will be in memory till the app reloads. As its a web app everything being persisted from that variable will be also stored in memory till the app reloads. so won't the jvm run out of memory if i don't unload the class?

Also what is the best practice of using instance variable for persistence

Answer 1

I don't think persisting those objects with JPA (or any similar technology) will change anything. When you persist something, you are just saving it somewhere (in this case a database). Those objects will still exist in memory after persisted.

If those are referenced by something (in your case those lists in the static var(s)) they can't be garbage collected, and so if you keep adding elements to those lists and don't remove them you will eventually run out of memory.

Also note that if you store those objects in a static variable, they may be shared by a lot of objects (ex: instances of that class trying to add, remove or count the number of elements) and you may run into concurrency problems.