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From "Alexander Apanasovich (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LANG-609) Support lazy initialization using atomic variables
Date Mon, 27 Sep 2010 15:12:33 GMT

    [ https://issues.apache.org/jira/browse/LANG-609?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12915328#action_12915328
] 

Alexander Apanasovich commented on LANG-609:
--------------------------------------------

There is one more variant of the implementation, which should not have that drawback described
in class javadocs:

??The drawback is that the {@link #initialize()} method can be called multiple times which
may be problematic if the creation of the managed object is expensive. As a rule of thumb
this initializer implementation is preferable if there are not too many threads involved and
the probability that multiple threads access an uninitialized object is small. If there is
high parallelism, {@link LazyInitializer} is more appropriate.??

{code}
/**
 * Concurrent lazy initializer which guarantees concurrent access and initialization during
the first access.
 * This implementation also guarantees that the factory method (initialize) will be only invoked
once.
 */
public abstract class AtomicInitializer<T> {
    private AtomicReference<AtomicInitializer<T>> factory = new AtomicReference<AtomicInitializer<T>>();
    private AtomicReference<T> instance = new AtomicReference<T>();

    /**
     * Get (and initialize, if not initialized yet) the required object
     *
     * @return lazily initialized object
     */
    public final T get() throws ConcurrentException {
        T result;

        while ((result = instance.get()) == null) {
            if (factory.compareAndSet(null, this)) {
                instance.compareAndSet(null, initialize());
            }
        }

        return result;
    }

    /**
     * Factory method, which is invoked once during first call to {@link #get()}
     *
     * @return new object
     */
    protected abstract T initialize() throws ConcurrentException;
}
{code}

Sometimes the initialize method is expensive or dangerous to be called more than once, so
this implementation will guarantee single initialization and concurrent access while having
possibly a bit higher speed than LazyInitializer, because there is no synchronization. 


> Support lazy initialization using atomic variables
> --------------------------------------------------
>
>                 Key: LANG-609
>                 URL: https://issues.apache.org/jira/browse/LANG-609
>             Project: Commons Lang
>          Issue Type: New Feature
>          Components: lang.concurrent.*
>            Reporter: Oliver Heger
>            Priority: Minor
>             Fix For: 3.0
>
>         Attachments: AtomicInitializer.patch
>
>
> In the new concurrent package there is already the {{LazyInitializer}} class implementing
the double-check idiom for lazy initialization of instance fields. Another way to perform
initialization lazily would be the usage of atomic variables. This solution would not need
synchronization at all. If there is low contention (not many threads in parallel), it is more
efficient than synchronized access. Its drawback is that the initialization may be done multiple
times if multiple threads access the initializer almost at the same time (however, it can
be guaranteed that always the same object created by the initializer is returned).
> This is a proposal to add an {{AtomicInitializer}} class as an alternative to {{LazyInitializer}}
that operates on {{AtomicReference}} variables.
> In order to make the initializers smoothly exchangable, it would make sense for them
to share a common interface. This interface could also be implemented by the {{BackgroundInitializer}}
class.

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