What are transaction isolation levels? Which ones do you know?

Transaction isolation levels determine how much one transaction can see the changes made by other concurrent transactions and help control issues with concurrent data access. The main isolation levels:

• READ UNCOMMITTED — the lowest level, a transaction can see uncommitted changes made by others (dirty read).
• READ COMMITTED — a transaction only sees committed changes, preventing dirty reads.
• REPEATABLE READ — guarantees that repeated reads of the same data within a transaction will yield the same result, preventing non-repeatable reads.
• SERIALIZABLE — the strictest level, transactions are executed as if they were sequential, preventing phantom reads.

In Java, when working with databases via JDBC or JPA, you can set the transaction isolation level, for example:

connection.setTransactionIsolation(Connection.TRANSACTION_REPEATABLE_READ);