今天，我想谈谈 Spring 提供的@Transactional(readOnly = true)。之所以聊这个是因为我公司项目的代码里有很多@Transactional(readOnly = true)，用过的同学都说@Transactional(readOnly = true)提高了性能。先思考以下几点：
1.@Transactional(readOnly = true)是如何工作的，为什么使用它可以提高性能？

2.当我们使用 JPA 时，是否应该总是将@Transactional(readOnly = true)添加到服务层的只读方法？有什么取舍吗？

在开始之前，我们使用 Hibernate 来实现 JPA。
1. @Transactional(readOnly = true)是如何工作的，为什么使用它可以提高性能？
首先，让我们看一下事务接口。

/**
* 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;

我们可以看到 readOnly = true 选项允许优化。事务管理器将使用只读选项作为提示。让我们看看用于事务管理器的JpaTransactionManager。

@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()));
  //...
}

在JpaTransactionManager中，doBegin方法委托JpaDialect来开始实际的事务，并在JpaDialect中调用beginTransaction。让我们来看看HibernateJpaDialect类。

@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);
    }
   }
   //堆代码 duidaima.com
}

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;
}

在JpaDialect中，我们可以看到JpaDialect使用只读选项准备刷新模式。当 readOnly = true 时， JpaDialect 禁止刷新。此外，您还可以看到，在准备刷新模式后，session.setDefaultReadOnly(true)将session的readOnly属性设置为true。

/**
 * 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);

在Session接口中，通过将readOnly属性设置为true，将不会对只读实体进行脏检查，也不会维护持久状态的快照。此外，只读实体的更改也不会持久化。
总而言之，这些是在 Hibernate 中使用@Transactional(readOnly = true)所得到的结果:
性能改进：只读实体不进行脏检查
节省内存：不维护持久状态的快照
数据一致性：只读实体的更改不会持久化
当我们使用主从或读写副本集（或集群）时，@Transactional(readOnly = true)使我们能够连接到只读数据库

2.当我们使用 JPA 时，是否应该总是将@Transactional(readOnly = true)添加到服务层的只读方法？有什么取舍吗？
我看到，当使用@Transactional(readOnly = true)时，我们可以有很多优势。但是，将@Transactional(readOnly = true)添加到服务层的只读方法是否合适？以下是我担心的事情：
.无限制地使用事务可能会导致数据库死锁、性能和吞吐量下降。
.由于一个事务占用一个DB连接，所以@Transactional(readOnly = true)添加到Service层的方法可能会导致DB连接饥饿。
第一个问题很难重现，所以我做了一些测试来检查第二个问题。

@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;
}

我在服务层测试了两个方法，一个是@Transactional(readOnly = true)，另一个是存储库层中的@Transactional (readOnly = true)（在 SimpleJpaRepository 中，它是 Jpa Respitory 的默认实现，在类的顶部有@Transformational(ready Only)，因此 findAll()方法在默认情况下有@transactional(read only = True)）。

我从DB中获取userInfo并保持线程5秒钟，然后检查该方法何时释放连接。
结果如下：
对于服务层方法中的@Transactional(readOnly = true)，

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

对于存储库层方法中的@Transactional(readOnly = true)，

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

正如您所看到的，@Transactional(readOnly = true)一旦查询结果到达，存储库层就会释放连接。
然而，@Transactional(readOnly = true)在服务层的方法中直到服务层的方法结束才释放连接。
因此，当服务层的方法有需要大量时间的逻辑时要小心，因为它可以长时间持有数据库连接，这可能会导致数据库连接匮乏。

三. 总结
很明显，@Transactional(readOnly = true)有很多优点。
性能改进：只读实体不进行脏检查
节省内存：不维护持久状态的快照
数据一致性：只读实体的更改不会持久化
当我们使用主从或读写副本集（或集群）时，@Transactional(readOnly = true)使我们能够连接到只读数据库
但是，您还应该记住，@Transactional(readOnly = true)在服务层的方法中可能会导致数据库死锁、性能低下和数据库连接匮乏！
当您需要将只读查询仅仅作为一个事务执行时，请毫不犹豫选择的在服务层的方法中使用@Transactional(readOnly = true)，如果你的服务层的方法中有大量其他逻辑方法时，就要做取舍了！