If you think you are going to be doing a lot of this kind of concurrent processing, you might want to look at using Akka rather than threads as actors are a heck of a lot easier to manage than threads.
What is the real reason behind to run two JPA queries in parallel? You should also be cautious when creating your own threads especially when your app is running inside a JEE container. Like Chris mentions in his post above, you would be better off using Akka.
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Did a rm -R / to find out that I lost my entire Linux installation!
Thanks for your answers.
The reason to run the queries in parallel is to make the web application run faster.
When the user searches for some data, the result is displayed in a new page.
This new page contains the data inside a datagrid (first query) and various sums in the footer (second query).