best open source ODBMS for java

Question

Could you advise the best open source ODMBS (object oriented database management system) for Java application?

I understand that usually there is no one definite answer for such questions. So the main points I look for are:

1. good documentation/ support / community
2. reliability
3. performance

UPDATE:

I see db4o has a long history, the biggest java ODBMS community and dual-licensing. I consider this to be a good sign of good documentation/ support/ maturity.

Prevayler looks really awesome but only if you are ok with storing (caching) all your DB in the memory. Which can be not suitable for big DBs with a lot of rarely queried data.

Answer 1

Sun's gotten behind MySQL. It's not a bad choice. I think the docs are good.

I use MySQL, but my preference is PostgreSQL. I like the features, the docs, the admin console. For the life of me, I can't understand why Sun paid $1B for MySQL when they had PostgreSQL. I guess they thought the brand was better.

My third choice would be Hypersonic.

Fourth is Derby, simply because it ships with JDK 6.

Answer 2

You can take a look at prevayler, which is an interesting concept.

For more mainstream take a look at db4o. You can alsa find good comparison here.

Answer 3

I do some work on the jdbm project project - it's not super-full featured, but I find that it often provides exactly the level of functionality that I'm looking for in small apps.

Answer 4

Why do you want an object DBMS per se? If you're looking for the database functions in particular, there are lots of free Object-Relational mapping frameworks.

Otherwise Wikipedia has a nice comparison list.