@Transactional(readOnly=true) 真的是提高性能的灵丹妙药吗？

/**
* A boolean flag that can be set to {@code true} if the transaction is
* effectively read-only, allowing for corresponding optimizations at runtime.
* <p>Defaults to {@code false}.
* <p>This just serves as a hint for the actual transaction subsystem;
* it will <i>not necessarily</i> cause failure of write access attempts.
* A transaction manager which cannot interpret the read-only hint will
* <i>not</i> throw an exception when asked for a read-only transaction
* but rather silently ignore the hint.
* @see org.springframework.transaction.interceptor.TransactionAttribute#isReadOnly()
* @see org.springframework.transaction.support.TransactionSynchronizationManager#isCurrentTransactionReadOnly()
*/
boolean readOnly() default false;

@Override
protected void doBegin(Object transaction, TransactionDefinition definition) {
 JpaTransactionObject txObject = (JpaTransactionObject) transaction;
  // .
  // Delegate to JpaDialect for actual transaction begin.
  int timeoutToUse = determineTimeout(definition);
  Object transactionData = getJpaDialect().beginTransaction(em,
    new JpaTransactionDefinition(definition, timeoutToUse, txObject.isNewEntityManagerHolder()));
  //...
}

@Override
public Object beginTransaction(EntityManager entityManager, TransactionDefinition definition)
  throws PersistenceException, SQLException, TransactionException {
   // ...
   // Adapt flush mode and store previous isolation level, if any.
   FlushMode previousFlushMode = prepareFlushMode(session, definition.isReadOnly());
   if (definition instanceof ResourceTransactionDefinition &&
     ((ResourceTransactionDefinition) definition).isLocalResource()) {
    // As of 5.1, we explicitly optimize for a transaction-local EntityManager,
    // aligned with native HibernateTransactionManager behavior.
    previousFlushMode = null;
    if (definition.isReadOnly()) {
     session.setDefaultReadOnly(true);
    }
   }
   // ...
}

protected FlushMode prepareFlushMode(Session session, boolean readOnly) throws PersistenceException {
    FlushMode flushMode = session.getHibernateFlushMode();
    if (readOnly) {
     // We should suppress flushing for a read-only transaction.
     if (!flushMode.equals(FlushMode.MANUAL)) {
      session.setHibernateFlushMode(Flusode.MANUAL);
      return flushMode;
     }
    }
    else {
     // We need AUTO or COMMIT for a non-read-only transaction.
     if (flushMode.lessThan(FlushMode.COMMIT)) {
      session.setHibernateFlushMode(FlushMode.AUTO);
      return flushMode;
     }
    }
    // No FlushMode change needed...
    return null;
}

/**
 * Change the default for entities and proxies loaded into this session
 * from modifiable to read-only mode, or from modifiable to read-only mode.
 *
 * Read-only entities are not dirty-checked and snapshots of persistent
 * state are not maintained. Read-only entities can be modified, but
 * changes are not persisted.
 *
 * When a proxy is initialized, the loaded entity will have the same
 * read-only/modifiable setting as the uninitialized
 * proxy has, regardless of the session's current setting.
 *
 * To change the read-only/modifiable setting for a particular entity
 * or proxy that is already in this session:
 * @see Session#setReadOnly(Object,boolean)
 *
 * To override this session's read-only/modifiable setting for entities
 * and proxies loaded by a Query:
 * @see Query#setReadOnly(boolean)
 *
 * @param readOnly true, the default for loaded entities/proxies is read-only;
 *                 false, the default for loaded entities/proxies is modifiable
 */
void setDefaultReadOnly(boolean readOnly);

@Transactional(readOnly = true)
public List<UserDto> transactionalReadOnlyOnService(){
    List<UserDto> userDtos = userRepository.findAll().stream()
            .map(userMapper::toDto)
            .toList();
    timeSleepAndPrintConnection();
    return userDtos;
}

public List<UserDto> transactionalReadOnlyOnRepository(){
    List<UserDto> userDtos = userRepository.findAll().stream()
            .map(userMapper::toDto)
            .toList();
    timeSleepAndPrintConnection();
    return userDtos;
}

activeConnections:0, IdleConnections:10, TotalConnections:10
start transactionalReadOnlyOnService!!
Hibernate: 
    select
        u1_0.id,
        u1_0.email,
        u1_0.name,
        u1_0.profile_file_name 
    from
        users u1_0
activeConnections:1, IdleConnections:9, TotalConnections:10
activeConnections:1, IdleConnections:9, TotalConnections:10
activeConnections:1, IdleConnections:9, TotalConnections:10
activeConnections:1, IdleConnections:9, TotalConnections:10
activeConnections:1, IdleConnections:9, TotalConnections:10
end transactionalReadOnlyOnService!!
activeConnections:0, IdleConnections:10, TotalConnections:10

activeConnections:0, IdleConnections:10, TotalConnections:10
start transactionalReadOnlyOnRepository!!
Hibernate: 
    select
        u1_0.id,
        u1_0.email,
        u1_0.name,
        u1_0.profile_file_name 
    from
        users u1_0
activeConnections:0, IdleConnections:10, TotalConnections:10
activeConnections:0, IdleConnections:10, TotalConnections:10
activeConnections:0, IdleConnections:10, TotalConnections:10
activeConnections:0, IdleConnections:10, TotalConnections:10
activeConnections:0, IdleConnections:10, TotalConnections:10
end transactionalReadOnlyOnRepository!!
activeConnections:0, IdleConnections:10, TotalConnections:10