How to read Lucene 3.2 index by Lucene 4.10?

Question

Getting Lucene 4.10 read 3.2 version indexes Upgraded to 4.10 still need to read 3.2 indexes. Deployed jre 7 as required. Made all changes within a existing code base which became erroneous. Still need to read 3.2 indexes before going to take on re-indexing. How to read existing 3.2 indexes by Lucene 4.10 ( what changes to make if any in a code )

Answer 1

You can use IndexUpgrader, something like:

IndexUpgrader upgrader = new IndexUpgrader(myIndexDirectory, Version.LUCENE_4_10_0);
upgrader.upgrade();

or run it from the command line:

java -cp lucene-core.jar org.apache.lucene.index.IndexUpgrader myIndexDirectory

Answer 2

You can set the codec used to decode the indexes in the IndexWriterConfig. Lucene3xCodec would be the codec to use here:

IndexWriterConfig config = new IndexWriterConfig(Version.LATEST, analyzer);
config.setCodec(new Lucene3xCodec());
IndexWriter writer = new IndexWriter(directory, config);
IndexSearcher searcher = new IndexSearcher(new DirectoryReader.open(writer));

Bear in mind, this codec is strictly read-only. Any attempt to add, delete, or update a document will result in an UnsupportedOperationException being thrown. If you wish to support writing to the index, you must upgrade your index (see my original answer).