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From Trejkaz <>
Subject Re: Using IndexWriterConfig repeatedly in 3.1
Date Fri, 01 Apr 2011 22:15:48 GMT
On Sat, Apr 2, 2011 at 7:07 AM, Christopher Condit <> wrote:
> I see in the JavaDoc for IndexWriterConfig that:
> "Note that IndexWriter makes a private clone; if you need to
> subsequently change settings use IndexWriter.getConfig()."
> However when I attempt to use the same IndexWriterConfig to create
> multiple IndexWriters the following exception is thrown:
> org.apache.lucene.util.SetOnce$AlreadySetException: The object cannot be
> set twice!
>        at org.apache.lucene.util.SetOnce.set(
>        at org.apache.lucene.index.MergePolicy.setIndexWriter(
>        at org.apache.lucene.index.IndexWriter.<init>(
> Is this the intended design? Is there a way to use the configuration
> multiple times? I see that clone won't work since it's shallow...

Looking at this, it is odd indeed.  What is the benefit of having
clone() exposed if it isn't cloning any other sub-objects which
maintain state?  It's almost as if IndexWriter takes a clone to try
and prevent others modifying the object directly, but then it allows
getting its copy anyway, and on top of that, it modifies deep state
which affects the caller's copy of the object.

Solutions from the Lucene end:

  1. Remove clone() as it's misleading

  2. Make MergePolicy and other mutable things which can live in the
config Cloneable so that when the IndexWriter "takes a clone", it
actually takes a clone instead of using objects the caller passed in.

Solutions from the application end:

  1. Make an IndexWriterConfigCloner / MergePolicyCloner / ...
utility, and implement the cloning like that.

  2. Make your own builder API with cloning support, and have that
create a new IndexWriterConfig each time (but I think this was more or
less the point of IndexWriterConfig itself, since it looks like a
builder API already.)


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