In Spring, @Transactional(readOnly = true) is used to optimize transactions that only perform read operations. It has several effects on the transaction behavior, the persistence context, and the database interaction. Here’s a detailed explanation:

1. Primary Purpose

• Marks a method (or class) as read-only for the transaction manager.
• Signals that no data modifications will be performed.
• Helps optimize performance by reducing unnecessary overhead for reads.

2. Impact on Hibernate / JPA

When using Hibernate with Spring:

1. Flush mode is set to FlushMode.MANUAL:
   • Hibernate does not track changes for automatic flushing.
   • Reduces unnecessary dirty checking and SQL UPDATE statements.
   @Transactional(readOnly = true) public List<Product> getProducts() { return productRepository.findAll(); // no flush will occur }
2. Persistence context is read-only:
   • Any attempt to modify an entity inside this transaction may not be persisted, depending on the JPA provider.
   • In Hibernate, updates may be ignored or cause exceptions if flush occurs.
3. Performance optimization:
   • Less memory usage because Hibernate skips dirty checking.
   • Avoids unnecessary locks in some databases.

3. Impact on the Database

• Some databases can optimize read-only transactions: DatabaseEffectPostgreSQLCan use snapshot isolation efficientlyOracleMay avoid certain locks for readsMySQL / InnoDBRead-only flag is advisory; depends on engine
• No writes are allowed: If a write operation occurs, behavior depends on the database and JPA provider.

4. Behavior in Spring

• Read-only transactions are propagated to nested methods if they participate in the same transaction.
• Using @Transactional(readOnly = true) does not prevent calling save() in the repository, but the changes may not be flushed until the transaction ends. This can lead to subtle bugs if you expect modifications to persist.

5. When to Use

• Methods that only perform SELECT queries.
• Example:

@Service
public class ProductService {

    @Transactional(readOnly = true)
    public List<Product> getAllProducts() {
        return productRepository.findAll();
    }
}

• Avoid using readOnly = true for methods that insert, update, or delete entities.

6. Key Notes and Best Practices

1. Performance: Reduces Hibernate dirty checking overhead.
2. Intent: Signals developers and Spring that this method is read-only, improving clarity.
3. Propagation: Read-only behavior propagates to transactional calls unless overridden.
4. Caution: Calling a write operation in a read-only transaction can lead to ignored updates or unexpected behavior depending on JPA provider and database.