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https://issues.apache.org/jira/browse/SOLR-2983?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13187733#comment-13187733
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Tommaso Teofili commented on SOLR-2983:
---------------------------------------
it looks like it's a cyclic dependency problem since the SolrIndexWriter uses
the SolrIndexConfig.toIndexWriterConfig method to create an IndexWriterConfig
which is used to call the basic Lucene IndexWriter constructor while at the
same time the SolrIndexConfig.toIndexWriterConfig may need an IndexWriter to
instantiate the MergePolicy (try clause).
> Unable to load custom MergePolicy
> ---------------------------------
>
> Key: SOLR-2983
> URL: https://issues.apache.org/jira/browse/SOLR-2983
> Project: Solr
> Issue Type: Bug
> Reporter: Mathias Herberts
> Priority: Critical
> Fix For: 3.6, 4.0
>
>
> As part of a recent upgrade to Solr 3.5.0 we encountered an error related to
> our use of LinkedIn's ZoieMergePolicy.
> It seems the code that loads a custom MergePolicy was at some point moved
> into SolrIndexConfig.java from SolrIndexWriter.java, but as this code was
> copied verbatim it now contains a bug:
> try {
> policy = (MergePolicy)
> schema.getResourceLoader().newInstance(mpClassName, null, new
> Class[]{IndexWriter.class}, new Object[]{this});
> } catch (Exception e) {
> policy = (MergePolicy)
> schema.getResourceLoader().newInstance(mpClassName);
> }
> 'this' is no longer an IndexWriter but a SolrIndexConfig, therefore the call
> to newInstance will always throw an exception and the catch clause will be
> executed. If the custom MergePolicy does not have a default constructor
> (which is the case of ZoieMergePolicy), the second attempt to create the
> MergePolicy will also fail and Solr won't start.
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