Q:

Highest Performance Database in Java

I need ideas to implement a (really) high performance in-memory Database/Storage Mechanism in Java.  In the range of storing 20,000+ java objects, updated every 5 or so seconds.
Some options I am open to:
Pure JDBC/database combination
JDO
JPA/ORM/database combination
An Object Database
Other Storage Mechanisms
What is my best option?  What are your experiences?
EDIT:  I also need like to be able to Query these objects

A:

You could try something like Prevayler (basically an in-memory cache that handles serialization and backup for you so data persists and is transactionally safe). There are other similar projects.
I've used it for a large project, it's safe and extremely fast.
If it's the same set of 20,000 objects, or at least not 20,000 new objects every 5 seconds but lots of changes, you might be better off cacheing the changes and periodically writing the changes in batch mode (jdbc batch updates are much faster than individual row updates). Depends on whether you need each write to be transactionally wrapped, and whether you'll need a record of the change logs or just aggregate changes. 
Edit: as other posts have mentioned Prevayler I thought I'd leave a note on what it does: 
Basically you create a searchable/serializable object (typically a Map  of some sort) which is wrapped in a Prevayler instance, which is serialized to disk. Rather than making changes directly to your map, you make changes by sending your Prevayler instance a serializable record of your change (just an object that contains the change instruction). Prevayler's version of a transaction is to write your serialization changes to disk so that in the event of failure it can load the last complete backup and then replay the changes against that. It's safe, although you do have to have enough memory to load all of your data, and it's a fairly old API, so no generic interfaces, unfortunately. But definitely stable and works as advertised. 

A:

I highly recommend H2.  This is a kind of "second generation" version of HSQLDB done by one of the original authors.  H2 allows us to unit-test our DAO layer without requiring an actual PostgreSQL database, which is awesome.
There is an active net group and mailing list, and the author Thomas Mueller is very responsive to queries (hah, little pun there.)  

A:

I don't know if it is the fastest option, but I've been very satisfied with H2 whenever I've used it.  It's written by the same person who originally wrote Hypersonic (which later became HSQLDB).
Another option that is allegedly very fast is Prevayler.

