According to section 12.4.2 ("Detailed Initialization Procedure") of the
language spec:
"The implementation of the Java virtual machine is responsible for
taking care of synchronization and recursive initialization..."
So it isn't necessary to perform explicit synchronization in the static
initializer of 'Store'.
See
http://java.sun.com/docs/books/jls/second_edition/html/execution.doc.htm
l#44630 for details.
Regards,
Al.
> -----Original Message-----
> From: Rick Reumann [mailto:[EMAIL PROTECTED]
> Sent: 01 April 2003 22:20
> To: jdjlist
> Subject: [jdjlist] Singletons and is this design ok?
>
>
>
> I have a couple books on design patterns but I'm still a bit
> stumped on how to 'best' handle this situation...
>
> For the sake of explaining this I'm using an example of
> "Stores." The example of stores isn't perfect but it's easier
> to picture than my real life example.
>
> What I want to be able to is pull out a List of "items" from
> a Store object that will never change for the life of the web
> application (I know in real life this wouldn't be the case
> for Store items). Then I also have subclasses of Store -
> "Target" and "Walmart" - where I need to pull out a List of
> "specials" and these unique Lists for Walmart and Target will
> not change for those subclasses.
>
> Now each of the above classes only needs to be instantiated
> once so they can be all singletons and the Lists only need to
> be populated once. It gets a bit tricky since the creation of
> the object instances need to be thread safe and you can't
> really make the superclass (Store) a singleton if you plan to
> subclass it. So what I've done is this...
>
>
> public abstract class Store {
>
> private static Collection items = null;
> private Collection specials = null;
>
> static {
> if ( items == null ) {
> synchronized( Store.class) {
> if ( items == null ) {
> items = //get List of Items from somewhere
> }
> }
> }
> }
>
> public static Collection getItems() {
> return items;
> }
> public Collection getSpecials() {
> return specials;
> }
> }
>
>
> public class Walmart extends Store {
>
> static private Target _instance;
>
> private Walmart() {
> specials = //get specials unique for Walmart
> }
>
> public static Walmart getInstance() {
> if (_instance == null) {
> synchronized(Walmart.class) {
> if (_instance == null) {
> _instance = new Walmart();
> }
> }
> }
> return _instance;
> }
> }
>
> public class Target extends Store {
> //same thing like above but gets specials unique for Target }
>
> What I then have is a factory that figures out what Store to
> give me so in my code I can just call:
>
> Store s = SomeFactory.getStore() //and SomeFactory returns to
> me the correct type of store.
>
> The above seems to work ok, but is it a crappy design and are
> there still some multithreading issues that could screw things up?
>
> Thanks,
>
> --
> Rick Reumann
>
> ---
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