When should I call javax.jdo.Query.close(Object)?

I'm trying to understand when I should call Query.close(Object) or Query.closeAll();

From the docs I see that it will "close a query result" and "release all resources associated with it," but what does "close" mean? Does it mean I can't use objects that I get out of a Query after I've called close on them?

I ask becau开发者_运维问答se I'm trying to compartmentalize my code with functions like getUser(id), which will create a Query, fetch the User, and destroy the query. If I have to keep the Query around just to access the User, then I can't really do that compartmentalization.

What happens if I don't call close on an object? Will it get collected as garbage? Will I be able to access that object later?

Please let me know if I can further specify my question somehow.

You can't use the query results since you closed them (i.e the List object you got back from query.execute). You can access the objects in the results ... if you copied them to your own List, or made references to them in your code. Failure to call close can leak memory

When your query method returns a single object it is easy to simply close the query before returning the single object.

On the other hand, when your query method returns a collection the query method itself can not close the query before returning the result because the query needs to stay open while the caller is iterating through the results.

This puts the responsibility for closing a query that returns a collection on the caller and can introduce leaks if the caller neglects to close the query - I thought there must be a safer way and there is!

Guido, a long time DataNucleus user, created a 'auto closing' collection facade that wraps the collection returned by JDO's Query.execute method. Usage is extremely simple: Wrap the query result inside an instance of the auto closing collection object:

Instead of returning the Query result set like this:

return q.execute();

simply return an 'auto closing' wrapped version of it:

return new JDOQueryResultCollection(q, q.execute());

To the caller it appears like any other Collection but the wrapper keeps a reference to the query that created the collection result and auto closes it when the wrapper is disposed of by the GC.

Guido kindly gave us permission to include his clever auto closing code in our open source exPOJO library. The auto closing classes are completely independent of exPOJO and can be used in isolation. The classes of interest are in the expojo_jdo*.jar file that can be downloaded from:

http://www.expojo.com/

JDOQueryResultCollection is the only class used directly but it does have a few supporting classes.

Simply add the jar to your project and import com.sas.framework.expojo.jdo.JDOQueryResultCollection into any Java file that includes query methods that return results as a collection.

Alternatively you can extract the source files from the jar and include them in your project: They are:

JDOQueryResultCollection.java
Disposable.java
AutoCloseQueryIterator.java
ReadonlyIterator.java