java - MongoDB + Apache Lucene

Question

today i doing integration mongodb with Apache lucene,using the library https://github.com/rstiller/mongo-lucene presented an example throw exception NullPointerException

MongoClient mongoClient = new MongoClient("localhost" , 27017 );
    DB db = mongoClient.getDB("test");

    DBCollection dbCollection = db.getCollection("users");

    // serializers + map-store
    DBObjectSerializer<String> keySerializer = new SimpleFieldDBObjectSerializer<String>("key");
    DBObjectSerializer<MapDirectoryEntry> valueSerializer = new MapDirectoryEntrySerializer("value");
    ConcurrentMap<String, MapDirectoryEntry> store = new MongoConcurrentMap<String, MapDirectoryEntry>(dbCollection, keySerializer, valueSerializer);

    // lucene directory
    Directory dir = new MapDirectory(store);

    // index files
    StandardAnalyzer analyzer = new StandardAnalyzer(Version.LUCENE_36);
    IndexWriterConfig config = new IndexWriterConfig(Version.LUCENE_36, analyzer);
    IndexWriter writer = new IndexWriter(dir,config); 
    Document doc = new Document();
    doc.add(new Field("title", "My file's content ...", Field.Store.YES, Field.Index.ANALYZED));
    writer.addDocument(doc);
    writer.close();

    // search index
    Query q = new QueryParser(Version.LUCENE_36, "first", analyzer).parse("Jo*");
    IndexReader reader = IndexReader.open(dir);
    IndexSearcher searcher = new IndexSearcher(reader);
    TopScoreDocCollector collector = TopScoreDocCollector.create(10, true);
    searcher.search(q, collector);
    ScoreDoc[] hits = collector.topDocs().scoreDocs;
    System.out.println(hits.length);

depencies

<dependencies>
<dependency>
  <groupId>junit</groupId>
  <artifactId>junit</artifactId>
  <version>3.8.1</version>
  <scope>test</scope>
</dependency>
<dependency>
    <groupId>org.mongodb</groupId>
    <artifactId>mongo-java-driver</artifactId>
    <version>2.10.1</version>
</dependency>
<dependency>
    <groupId>com.github.mongoutils</groupId>
    <artifactId>mongo-lucene</artifactId>
    <version>1.0</version>
</dependency>
<dependency>
    <groupId>org.apache.lucene</groupId>
    <artifactId>lucene-core</artifactId>
    <version>3.6.1</version>
</dependency>

Exception arise in string IndexWriter writer = new IndexWriter(dir,config);

Prompt in what a problem or an alternative, but not a standard search mongodb because it is not suitable.with examples

stacktrace

Exception in thread "main" java.lang.NullPointerException
at org.apache.lucene.index.SegmentInfos.getLastCommitGeneration(SegmentInfos.java:160)
at org.apache.lucene.index.SegmentInfos$FindSegmentsFile.run(SegmentInfos.java:605)
at org.apache.lucene.index.SegmentInfos$FindSegmentsFile.run(SegmentInfos.java:554)
at org.apache.lucene.index.SegmentInfos.read(SegmentInfos.java:359)
at org.apache.lucene.index.IndexReader.indexExists(IndexReader.java:1099)
at org.apache.lucene.index.IndexWriter.<init>(IndexWriter.java:1112)
at com.officeteam.Lucene.App.main(App.java:53)

Answer 1

Waitting for answers