null pointer exception in indexwriter.close (using ramdirectory in google app engine)

I am working on getting lucene indexing working on Google App Engine. I am using a ramdirectory to make the index and then serializing it (the ramdirectory object) to memcache and blobstore for persistent storage. http://code.google.com/appengine/docs/java/blobstore/overview.html#Writing_Files_to_the_Blobstore For search I just deserialize it and use in my searches.

I am facing a null pointer exception when I close the indexwriter.

I think that might have something to do with the fact that only the following libraries are supported in google app engine. http://code.google.com/appengine/docs/java/jrewhitelist.html

I am using lucene 3.5.0 and app engine java version 1.6.1

The following is the stack trace which i get

java.lang.NullPointerException
at org.apache.lucene.store.DataOutput.writeString(DataOutput.java:103)
at org.apache.lucene.store.DataOutput.writeStringStringMap(DataOutput.java:189)
at org.apache.lucene.index.SegmentInfo.write(SegmentInfo.java:623)
at org.apache.lucene.index.SegmentInfos.write(SegmentInfos.java:394)
at org.apache.lucene.index.SegmentInfos.prepareCommit(SegmentInfos.java:872)
at org.apache.lucene.index.IndexWriter.startCommit(IndexWriter.java:4601)
at org.apache.lucene.index.IndexWriter.prepareCommit(IndexWriter.java:3453)
at org.apache.lucene.index.IndexWriter.commitInternal(IndexWriter.java:3524)
at org.apache.lucene.index.IndexWriter.closeInternal(IndexWriter.java:1879)
at org.apache.lucene.index.IndexWriter.close(IndexWriter.java:1822)
at org.apache.lucene.index.IndexWriter.close(IndexWriter.java:1786)

The code works properly on my local machine (I haven't added much of a code , just added some sample documents and did a indexwriter.close())

Has someone faced this problem before ?? and if so is there a workaround for it ??

The code where I am finding the problem is simple

RAMDirectory dir = new RAMDirectory();
IndexWriterConfig config = new IndexWriterConfig(Version.LUCENE_35, new StandardAnalyzer(Version.LUCENE_35));
IndexWriter writer = new IndexWriter(dir,config); 
Document doc;
doc = new Document();
doc.add(new Field("text","mary had a little lamb", Store.YES, Index.ANALYZED));
writer.addDocument(doc)
writer.close();

the exception is thrown when i am trying to close the writer in the last line

the problem is that for some reason Lucene tries to store os.version and os.arch in index.

I don't know why, however the solution is adding the properties to your appengine-web.xml:

<system-properties>
    <property name="os.version" value="1.0.GAE whatever" />
    <property name="os.arch" value="GAE whatever" />
</system-properties>

and it will work for you. Hope that helps :)