webapp - how request processing work

Before I ask question, I should probably mention that I'm a newbie to webapp.  
so, here I go:
 
1) When multiple users send request to a wepapp (Created using jsp, struts... 
and uses Tomcat as the servlet container) at the same time, how does each of 
them get handle?  FIFO or the servlet container just spawn of a child job for 
each request and process them at the same time?
 
2) This is the main problem that I'm dealing with
    2.1) I've a DAO class that uses datasource to get a connection object.  
Connection object is declared as static.  See below:
          private static Connection conn = null;
            public DAO() throws DAOException {
            if (conn == null) {
                setConn(); 
            }
    2.2) All database DAO objects extends DAO class to get connection object to 
do file io.  I'm trying to add commitment control to this webapp.  I can do 
this by doing conn.setAutoCommit(false).  My question is that conn is a static 
object, as I understand it, that means there will only be one instance per jvm. 
 If multiple user send request to this webapp, how can I handle the commit and 
rollback to make sure that it doesn't interface with each request.
 
thanks.