Lock mysql database, when insert data more than one user at one time

Question

I'm creating an application using Java RMI. In my case, more than 10 users are adding data to the database. My application has a class GenerateID and it's on server side. For every user's id generation process is going through this class. It has a method named generateID() I need to know, if more than one users are inserting data to the database at same time, What will be happening?

Thanks in Advance!