Leon Rosenberg wrote:
> Hello,
>
> On Sun, Jun 14, 2009 at 7:01 PM, Jonathan
> Mast<jhmast.develo...@gmail.com> wrote:
>> I've not done anything with EJBs and I'm not sure what exactly you mean by
>> static "properties". I have however dealt with reducing instantiations in
>> servlets. I simply created a BeanBag class with static methods to each one
>> of my beans; these are not "proper" beans, but where simply objects that
>> were formerly used in JSP via the jsp:useBean directive.
>>
>> Here is the general pattern:
>>
>> class BeanBag {
>> private static SomeBean someBean = null;
>>
>> public static synchronized getSomeBean() {
>> if (someBean == null) someBean = new SomeBean();
>> return someBean;
>> }
>> }
>
> This is clearly the worst possible variant of doing it. Since
> getSomeBean is called ALL the time and each call is synchronized you
> are virtually giving up your concurrency and lining up all requests.
> This will cause you serious headache as soon as the site becomes
> decent traffic.
>
> There are at least two better ways to do it, static initizialization
> and double checked locking (later only on post 1.5 vms).
> 1.
> class BeanBag {
> private static final SomeBean someBean = new SomeBean();
>
> public static SomeBean getSomeBean() {
> return someBean;
> }
> }
>
> The difference is that the initialization happens at the loading time
> of the class which is guaranteed be the jvm to be synchronized
> (executed only by one thread) so there will be only one instance of
> someBean and it will be available at first call to getSomeBean.
This variation is useful, if the goal is the Singleton pattern.
http://en.wikipedia.org/wiki/Initialization_on_demand_holder_idiom
p
> 2. DLC - double checked locking, this is a very powerful but
> hot-discussed pattern, useful if the instantiation of someBean is
> really costly and you want to postpone it until it really needed. In
> this case:
> class BeanBag {
> private static volatile SomeBean someBean = null;
>
> public static SomeBean getSomeBean() {
> if (someBean==null){
> synchronized(BeanBag.class){
> if (someBean==null){
> someBean = new SomeBean();
> }
> }
> }
> return someBean;
> }
> }
>
>
>> I have now numerous Servlets, JSPs and POJOs that use BeanBag to obtain
>> singleton instances of my beans. Its worked great for me.
>
> yet :-)
>
> regards
> Leon
>
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