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Date: Tue, 7 Jul 2009 12:34:14 -0700 (PDT)
From: "Oliver Heger (JIRA)" <jira@apache.org>
To: issues@commons.apache.org
Subject: [jira] Commented: (LANG-496) A generic implementation of the Lazy
 initialization pattern
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    [ https://issues.apache.org/jira/browse/LANG-496?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12728318#action_12728318 ] 

Oliver Heger commented on LANG-496:
-----------------------------------

Using a LazyInitializer should be straightforward. The only complication is that for this use case two objects - the list and the set - are involved, while the initializer only creates a single one. So we would have to create a class that combines these two objects (as a private nested class in {{LocaleUtils}}):

{code}
private static class LocaleData {
    final List<Locale> cAvailableLocaleList;

    final Set<Locale> cAvailableLocaleSet;

    public LocaleData() {
        List<Locale> list = Arrays.asList(Locale.getAvailableLocales());
        cAvailableLocaleList = Collections.unmodifiableList(list);
        cAvailableLocaleSet = Collections.unmodifiableSet(cAvailableLocaleList);
    }
}
{code}

Now we can create an initializer class:

{code}
private static class LocaleInitializer extends LazyInitializer<LocaleData> {
    protected LocaleData initialize() {
        return new LocaleData();
    }
}
{code}

An object of this class is declared as static field in {{LocaleUtils}} instead of the list and the set fields, e.g.

{code}
private static final LocaleInitializer localeInitializer = new LocaleInitializer();
{code}

Then the methods for accessing the list or the set simply query this initializer object, for instance:

{code}
public static Set<Locale> availableLocaleSet() {
    return localeInitializer.get().cAvailableLocaleSet;
}
{code}

Note that no synchronization is required as this is handled by the {{LazyInitializer}} base class.

As a side note: I think in the current implementation of {{LocaleUtils}} the list and the set have to be declared as *volatile*. Otherwise changes to these fields are not guaranteed to be visible for other threads immediately.

> A generic implementation of the Lazy initialization pattern
> -----------------------------------------------------------
>
>                 Key: LANG-496
>                 URL: https://issues.apache.org/jira/browse/LANG-496
>             Project: Commons Lang
>          Issue Type: New Feature
>            Reporter: Oliver Heger
>             Fix For: 3.0
>
>         Attachments: LazyInitializer.patch
>
>
> This is a fully functional implementation of the double-check idiom for lazy initialization of an instance field as discussed in Joshua Bloch's "Effective Java".
> If there is interest, this could be the first element of a set of helper classes related to concurrent programming.

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