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：
-
@Transactional(readOnly = true)
是如何工作的，为什么使用它可以提高性能？
-
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。
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.
* Defaults to {@code false}.
*
This just serves as a hint for the actual transaction subsystem;
* it will not necessarily cause failure of write access attempts.
* A transaction manager which cannot interpret the read-only hint will
* not 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);
}
}
// ...
}
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
-
-
-
-
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
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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 transactionalReadOnlyOnService(){
List userDtos = userRepository.findAll().stream()
.map(userMapper::toDto)
.toList();
timeSleepAndPrintConnection();
return userDtos;
}
public List transactionalReadOnlyOnRepository(){
List 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
