Commit(), Rollback(), Record Locking

I am having some problems with this, actually just fell confuse and need some advice.

I have read some book regarding all this rollback and commit issues.

This is the sample programs that I read.

...
...
con.setAutoCommit(false);
try {

PreparedStatement updateSales = con.prepareStatement("UPDATE Book SET SALES = ? WHERE Title LIKE ?");
updateSales.setInt(1, 50);
updateSales.setString(2, "ABC");
updateSales.executeUpdate();

PreparedStatement updateInvoice = con.prepareStatement("UPDATE Bill SET Amount = ? WHERE Title LIKE ?");
updateInvoice.setInt(1, 2500);
updateInvoice.setString(2, "LMN");
updateInvoice.executeUpdate();

con.commit();
con.setAutoCommit(true);
}
catch (SQLException e) {
    con.rollback();
    System.out.print(e.printStackTrace());
}

...
...


How is this work? So far I know, con.setAutoCommit() will set autocommit to false first. After that, it'll commit the statement after the 2 prepared Statement. And if the exception thrown, it'll rollback, con.rollback().
 

Here is the question.

1. How this program will know where to rollback and how it'll know which one can be recover back and up to which level it'll help me to rollback? Aren't should I set something like a "mark" to tell them until where they should start and when should stop?

2. Is it these 2 method, commit() and rollback() is including the Record Locking problems? For example, if User A just right after press the "Submit" button, what happend if another user, User B trying to press the sample program's "submit" button, will this cause the Record Locking? When at the same time 2 user is accessing the same file at the same time. Actually I have this concet because my legacy system is running RPG and RPG do take this in concern seriously. But so far I know, Java can handle multithread quite well. SO, just want to confirm any potential problem for me to hanbdle this record locking?

3. Any other recommandation reading material for me to read? I have tried the www.javaworld.com to find som eresources but ended with dissapointment. Please share some URL on this rollback, commit and recordlocking issues with me.

Thank You.