The best replacement for datanucleus is eclipselink.

The best alternatives to datanucleus are rated as following:
★★★★★  eclipselink
★★★  spring data
★★  openjpa

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Speaking about datanucleus, one probably shouldn't forget about jdo.

Other search terms related to datanucleus and their ratings are:
★★★★★ jdo
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datanucleus
eclipselink
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