weird behavior of IndexReader.indexExists()

[wenjie zheng]

I created an index with more than 30,000 text files.
I used indexExists() to determine either to create a new index or to add
docs to the existing index.

But when the num of docs in the index was over 3,000 (sometimes 3,400,
sometimes 3,200), the indexExists function returns false, so I ended up
recreating a new index.

Here is my code:
if index exists, we will add files to it, otherwise, create a new index. In
either case, an IndexingThread will be spawn to do that.
        if(IndexReader.indexExists(indexDir)){
               logger.info("Working on existing index ...");
               IndexingThread.startIndexingThread(Username, new
File(propsFile), new File(indexDir), docs,
                             new StandardAnalyzer(), false);
         }else{
               logger.info("Create a new index ...");
               IndexingThread.startIndexingThread(Username, new
File(propsFile), new File(indexDir), docs,
                              new StandardAnalyzer(), true);
          }


inside the startIndexingThread function, I am calling the following function
to add files to the index:
   /**
    * Add an array of Files to an index
    *
    * @param propsFile the properties file
    * @param indexDir  the folder where index files will be created in
    * @param docs      an array of Files to be add to the index
    * @param analyzer  any Analyzer object
    */
   public void addFiles(File propsFile, File indexDir, File[] docs,
Analyzer analyzer, boolean overwrite) throws Exception {
       Properties props = new Properties(new FileInputStream(propsFile));
       if(overwrite || IndexReader.indexExists(indexDir)){ //either
overwrite or working on an existing index
           Directory index = FSDirectory.getDirectory(indexDir, overwrite);
           IndexWriter writer = new IndexWriter(index, analyzer,
overwrite);

           FileIndexer indexer = new FileIndexer(props);

           long start = new Date().getTime();
           indexer.index(writer, docs);
           writer.optimize();
           writer.close();     // close the writer
           index.close();      // close the index Directory
           long end = new Date().getTime();

           logger.info("Total time: " + (end - start) + " ms");

       }else{
           logger.error("Index files are not found: " +
indexDir.getAbsolutePath() + ", overwrite = false");
       }
   }

Thanks,
Wenjie