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https://issues.apache.org/jira/browse/LUCENE-4440?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13464778#comment-13464778
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Uwe Schindler commented on LUCENE-4440:
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bq. Isn't there a way to ensure that NamedSPILoader would finish before
forName() is accessed?
Unfortunately not, this is a chicken-and-egg problem: If you access Codec.class
in any way (e.g. already at the time when you subclass it!), it is classloaded.
After classloaded, it is initialized, so its static <clinit> method is called.
This method runs new NamedSPILoader(Codec.class) [the first line in the class
source code], and this one loads all codecs which are listed in
META-INF/services. As e.g. AppendingCodec is listed there, it is loaded by
NamedSPILoader and its ctor is called. As we are still in the stacktrace inside
NamedSPILoader.<init>, this call did not yet return to caller (Codec.<clinit>),
so the assignment of the static final loader field is not yet finished.
Unfortunately, AppendingCodec.<init> calls Codec.forName() and hits the NPE,
because the field is not yet initialized. You have to say: You cannot call
*any* method of Codec at that time!
> FilterCodec should take a delegate Codec in its ctor
> ----------------------------------------------------
>
> Key: LUCENE-4440
> URL: https://issues.apache.org/jira/browse/LUCENE-4440
> Project: Lucene - Core
> Issue Type: Improvement
> Components: core/codecs
> Reporter: Shai Erera
> Assignee: Shai Erera
> Priority: Minor
> Attachments: LUCENE-4440.patch
>
>
> FilterCodec has a delegate() method through which an extension can return its
> delegate Codec. This method is called on every Codec method. Adrien, on
> LUCENE-4391, failed to pass a Codec in the ctor, since he couldn't called
> Codec.forName().
> Instead, we should just pass e.g. new Lucene40Codec(). I'll post a patch
> shortly.
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