JAVA: an EntityManager object in a multithread environment

Question

if I have multiple threads, each use injector to get the EntityManager object, each use the em object to select a list of other class objects. Ready to be used in a for loop

Answer 1

You normally have transactions around what you do with database objects. What each given thread sees about the changes made by other threads is controlled by 'transaction isolation' settings.

Start learning about different isolation settings and apply the right setting according to your needs. There is a trade-off between accuracy and speed. http://en.wikipedia.org/wiki/Isolation_%28database_systems%29