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I have a class ClassWithDates (let's call it "C") which contains a collection of dates. I have many objects of type C and thousands/millions of date objects, which often refer to the same time.

I'd like to have a pool of date objects, and return an already allocated one if one exists for the time requested. This is quite simple, but when using Hibernate I don't know how to do it.

If I do a "from ClassWithDates" query, and have lazy fetching off, Hibernates loads all the objects of the class ClassWithDates and all the child collections, but, as far as I know, it creates new date objects for each element in the collection, even when the objects are the same.

I'm interested in object pooling with Hibernate for immutable classes. How could I implement it?

The example given above is a simplification of the code I'm working on.

cdarwin
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  • I recently learned how to use Hibernate's user defined types. I suppose it would be possible to use one of those to 1) read in the date value (or representation of the date) and then 2) pull the immutable date object from a factory using the database value as a key. On save, reverse that process. Thing is, you'd still have to create an object for the date as you read it in, so you'd still have a lot of object creation. But they might get garbage collected soon enough. – Marvo Jun 15 '12 at 20:48
  • I hoped it could be done without using the low level Hibernate "architecture", however... I think you're right – cdarwin Jun 15 '12 at 21:18
  • I used it to convert blobs into strings, and to convert some dates-stored-as-integers (yeah, I know, not my idea) into date objects. I can totally envision where you'd pull the immutable dates from a pool. Here's a brief explanation of how I did the CLOB to string user type. Once you understand it, it's not too bad. http://www.bigmarv.net/blog/?p=251 – Marvo Jun 16 '12 at 00:03
  • I wonder if java.sql.Date's valueOf() is smart enough to "cache" the dates it creates in the same way that the Integer and Long valueOf methods do. Maybe that's your object pool right there. – Marvo Jun 16 '12 at 00:06

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