For now, there are just 2 transactions with READ_COMMITTED isolation level. They are working with the same data in the same way: load data by id, get content, add a string to it and save the changed content.

The code shows, that if second transaction starts after first one and finishes before it, it is waiting until the first transaction is committed. Also it shows, that on READ_COMMITTED isolation level there could be easily a lost update situation. If you change at least to REPEATABLE_READ level, the exception will raise on 2nd transaction commit attempt.