Why null pointer exception with Jena TDB getDefaultModel?

Question

Hi All,

I cobbled together some Jena TDB code based on stuff I found on the Web. It's basic boilerplate.

In the code below, if the dataset directory does not exist, then everything works well. However, if the dataset exists, then I get an NPE that began life at the call to dataset.getDefaultModel().

public static void main(String[] args)
{
 String directory = "c:\\semantic web\\jena\\tdb";
 Dataset dataset = TDBFactory.createDataset(directory);
 Model tdb = dataset.getDefaultModel();

Stack trace of the NPE looks like this:

Exception in thread "main" java.lang.NullPointerException
    at com.hp.hpl.jena.tdb.store.DatasetPrefixesTDB.readPrefixMap(DatasetPrefixesTDB.java:173)
    at com.hp.hpl.jena.sparql.graph.GraphPrefixesProjection.getNsPrefixMap(GraphPrefixesProjection.java:62)
    at com.hp.hpl.jena.tdb.store.DatasetPrefixesTDB.getPrefixMapping(DatasetPrefixesTDB.java:222)
    at com.hp.hpl.jena.tdb.store.DatasetPrefixesTDB.getPrefixMapping(DatasetPrefixesTDB.java:213)
    at com.hp.hpl.jena.tdb.store.GraphTriplesTDB.createPrefixMapping(GraphTriplesTDB.java:92)
    at com.hp.hpl.jena.sparql.graph.GraphBase2.getPrefixMapping(GraphBase2.java:194)
    at com.hp.hpl.jena.rdf.model.impl.ModelCom.getPrefixMapping(ModelCom.java:908)
    at com.hp.hpl.jena.rdf.model.impl.ModelCom.withDefaultMappings(ModelCom.java:952)
    at com.hp.hpl.jena.rdf.model.impl.ModelCom.<init>(ModelCom.java:66)
    at com.hp.hpl.jena.rdf.model.impl.ModelCom.<init>(ModelCom.java:62)
    at com.hp.hpl.jena.rdf.model.ModelFactory.createModelForGraph(ModelFactory.java:166)
    at com.hp.hpl.jena.sparql.core.DatasetImpl.graph2model(DatasetImpl.java:266)
    at com.hp.hpl.jena.sparql.core.DatasetImpl.getDefaultModel(DatasetImpl.java:102)
    at query.TestTdb.main(TestTdb.java:26)

Can someone explain what's causing this? I'm stumped. Oh, Jena version is 2.7.4.

Thanks.

-Saul

Answer 1

1

At a guess, it looks like the previous use of the directory was not shutting the store down properly, leaving the on disk files inconsistent.

What is the rest of the code doing, when the store is new?

Call TDB.sync(dataset) before exiting or, ideally, use transactions.

link

answered 28 Nov '12, 04:28

AndyS ♦
10.0k●2●7
accept rate: 33%

Thanks, Andy. I will soon check the code and get back to you.

(28 Nov '12, 10:30) SaulTocsin

Andy,

Sorry, but I am today unable to reproduce this problem. I restored code to its original state and ran it several times both with and without an extant TDB. Works fine.

It is entirely likely that I was closing only the QueryExecutionFactory and not the dataset. I certainly didn't have the TDB.sync(dataset) that you recommended. I've now added these to the code.

Thanks for your help.

-Saul

(28 Nov '12, 12:33) SaulTocsin

It will be to do with update and adding data - not query. Query does not change the database.

(28 Nov '12, 13:44) AndyS ♦

Understood, thanks.

(28 Nov '12, 14:56) SaulTocsin

Why null pointer exception with Jena TDB getDefaultModel?

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