I don't know whether it's totaly correct, but this is a possibility.
You have two beans City and State and two threads Country and Continent.
What is deadlock ?
A deadlock is a situation in which two or more competing actions are each waiting for the other to finish.
Deadlock
Thus, when you have Country trying acess to City (bean) and Continent trying acess to City too and Country don't finished yet then you have a deadlock.
In other words, you have two process trying acess to the same class, at the same time, it is a possibility.