What is the role of transaction isolation levels in the @Transactional annotation?

Transaction isolation levels in the @Transactional annotation determine the degree of visibility of data changes made by one transaction to other concurrently executing transactions. They help control concurrency issues such as dirty reads, non-repeatable reads, and phantom reads.

In Spring, you can specify the isolation level using the isolation parameter, for example:

@Transactional(isolation = Isolation.READ_COMMITTED)
public void someMethod() {
    // business logic
}

Main isolation levels:

• READ_UNCOMMITTED — allows reading uncommitted changes, possible dirty reads.
• READ_COMMITTED — prevents dirty reads but allows non-repeatable reads.
• REPEATABLE_READ — prevents non-repeatable reads but phantom reads are possible.
• SERIALIZABLE — the strictest level, provides full transaction isolation but reduces performance.

Choosing an isolation level affects the balance between data integrity and application performance.