Re: simple static question

Ben wrote:

if I don't have a reference to any object of class A, but I have a
reference to it's static queue, will the "lost" reference to class A I
used in the constructor be collected by the garbage collector? If yes
once a new object is constructed will the static queue have the same
address, or will a new address be given to it?

Unfortunately the answer is rather context-dependent. In 99% of cases the
answer is simple, but you /can/ create cases where the answer is more
complicated if you try hard enough.

In most cases, the simple answer will be correct: As long as class B is loaded
into the JVM then class A will be too, and so the queue in A will continue to
exist whether or not B keeps a reference to it. For "normal" Java programs
both A and B will be loaded for the entire lifetime of the program. So, once
the queue has been created, it will never go away again (unless you explicitly
set the static reference in A to null).

The non-simple answer applies in specialised circumstances, and /only/ applies
if someone is using custom classloaders. Unfortunately "advanced" application
frameworks (such as servlet containers, etc) /do/ make use of classloaders.
Anyway, in the more complicated case, there are again two possibilities: a
simple one which applies in nearly all cases, and a complicated one which
applies if you try hard enough.

If A and B are loaded by the same classloader, or if A is loaded by a parent of
B's classloader (which will /always/ be the case if the code in B refers to A
directly, rather than using Class.forName()), then A will live at least as long
as B. So in that case the picture is pretty-much the same as for the simple
case, except that it may be possible for both A and B to be unloaded before the
program exits (and so the queue will be discarded and cleaned up -- but that
won't usually matter because there is no longer any need for it).

The last case is most complicated, but /cannot/ apply unless you yourself are
playing games with classloaders. If the code for B does not contain any direct
references to A (no variables declared as type A, for instance), /and/ if
you've taken care to load A through a classloader which is not the one used for
B (or one of its parents), then it is possible for A to be unloaded while B is
still active. If so, then the queue will be discarded when A unloads (assuming
B has no reference to it), and if B repeats the operations it used to get a
reference to the queue the first time, then it is possible that it will see a
new queue associated with a freshly re-loaded and re-initialised version of
class A.

I hope that's not overcomplicating what you thought was a simple question. As
I say, in nearly all cases, you can treat static variables as having a lifetime
when extends to program exit.

-- chris


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