Java: any problems/negative sides of keeping SoftReference to ArrayList in HttpSession?

Question

My code is doing the following (just as an example, and the reason that I specify package path to java.lang.ref.SoftReference is to note that it's not my own implementaiton :-):

...
List<String> someData = new ArrayList<String>();
someData.add("Value1");
someData.add("Value2");
...
java.lang.ref.SoftReference softRef = new SoftReference(someData);
...
HttpSession session = request.getSession(true);
session.setAttribute("mySoftRefData", softRef);
...

and later:

...
java.lang.ref.SoftReference softRef = session.getAttribute("mySoftRefData");
if (softRef != null && softRef.get() != null) {
   List<String> someData = (List<String>)softRef.get();
   // do something with it.
}
...

Any disadvantages? Which I do not see? Thank you!

Answer 1

The obvious disadvantage is that the list might disappear unpredictably. As the session is garbage collected after it expires anyway, I don't really see a use case for SoftReference. If the list gets considerably big (at least considerably enough to justify using a SoftReference) I'd rather suggest different storage (DB, temporary files).

Answer 2

It's a very good idea ti put data that are not 100% needed for the application into a disposable cache. During peak hours they will be discarded, saving more resources for more pressing needs.

Way to go, in short.

Answer 3

If you don't reference it anywhere else in the code yourself and the JVM has run the Garbage Collector, then you may risk that the reference won't be in the session anymore. The chance is however little, less than when using a weak reference, but still, it is there.

I wouldn't do that in a webapplication. If it is pure session scoped data (e.g. logged-in user, shopping cart, etc), then just put it in the session scope the normal way. If the session expires or invalidates, then anything which is not referenced anywhere else will be garbage collected at any way. The session scope is not intented to act as a "soft" cache. Or if it is actually request scoped data, then rather store it in the request scope. Else use another kind of data store.