Question

So I have a fairly complex applet structure from a couple years back, and I have a question about a global class that is in it. Basically, I have three applets on the same page and they interact with each other in different ways. The part I’m confused about is the fact that they all share global (static) variables from the same class. How exactly is this happening? I was under the impression that each applet would have its own little environment and would thus be unable to access the same static variables as the others? For example if I have this in applet A:

Globals.globalVar = 5;

And this in applet B:

int x = Globals.globalVar;

And I ensure that B starts AFTER A then x will become 5. I would like to know how exactly this is handled in the JVM, as it doesn’t quite make any sense to me. Does this behaviour imply that if you have two separate java applications running at the same time they can access each others static variables? Thanks beforehand.

Answer 1

Because that’s how it’s defined, for backward-compatibility reasons. See http://download.oracle.com/javase/6/docs/technotes/guides/jweb/applet/applet_execution.html#cache:

Normally, if two applets have the same codebase and archive
parameters, they will be loaded by the same class loader instance.
This behavior is required for backward compatibility, and is relied on
by several real-world applications. The result is that multiple
applets on the same web page may access each others’ static variables
at the Java language level, effectively allowing the multiple applets
to be written as though they comprised a single application.

While this feature enables certain kinds of applications to be
conveniently written, it has certain drawbacks. It interferes with
termination of applets, in particular when multiple instances of the
same applet are active. It makes the programming model for applets
more complex, since it is under specified exactly when the static
fields of an applet will be re-initialized, and when they will be
maintained from run to run of the same applet. It causes imprecise
behavior of certain user interface operations within the Java Plug-in
due to the inability to identify exactly which applet initiated a
particular request.

For this reason, the new Java Plug-in provides a way to opt out of the
use of the classloader cache on an applet by applet basis.