在JPA 2.2之前,如果我想向Kotlin的ScrollableResults
发出Flow
,我必须这样做:
override fun findSomeUsers(batch: Int): Flow<User> {
return flow {
(em.delegate as Session).sessionFactory.openSession().use { session ->
val query = session.createQuery("select u from User u where ...")
query.fetchSize = batch
query.isReadOnly = true
query.scroll(ScrollMode.FORWARD_ONLY).use { results ->
while (results.next()) {
val u = results.get(0) as User
emit(u)
}
}
}
}
}
我必须将EntityManager
下放到Hibernate的Session
。
但是,由于JPA 2.2的Query
支持getResultStream
,因此应该有一种更清洁的方式实现这一目标:
@ExperimentalCoroutinesApi
override fun findSomeUsers(batchSize: Int): Flow<User> {
return channelFlow {
em.createQuery("select u from User u where ...")
.setHint(HINT_FETCH_SIZE,batchSize) // "org.hibernate.fetchSize"
.unwrap(javax.persistence.Query::class.java)
.resultStream
.asSequence()
.map { it as User }
.forEach { u ->
runBlocking {
send(u)
}
}
}
}
嗯,它运作良好,但是有点腥。
首先,为什么我不能只编码resultStream.asSequence.map {it as User}.asFlow()
? (客户端一无所获就结束了)
第二,runBlocking
块很丑。 runBlocking
仅应在测试中使用。但是我发现没有办法在代码中规避它。
有什么办法解决吗?
第三,这与问题无关。看来Spring-Data-JPA仍然不支持这种查询方法:
@Query("select u from User u where ...")
@MaybeSomeQueryHint(batchSize=:batchSize)
fun findSomeUsers(@Param("name="batchSize") batchSize: Int): Flow<User>
它将加载所有用户,然后抱怨重复的行...
客户端(测试)端代码是如此简单:
@ExperimentalCoroutinesApi
@Test
@Transactional
open fun testUsers() {
runBlocking {
userDao.findSomeUsers(100).collectIndexed { index,u: User ->
logger.info("[{}] {}",index,u)
}
}
}
对于@Marko来说,Stream
版本的效果很好:
override fun findSomeUserStream(batchSize: Int): Stream<User> {
return em.createQuery("select u from User u where ...")
.setHint(HINT_FETCH_SIZE,batchSize) // "org.hibernate.fetchSize"
.unwrap(javax.persistence.Query::class.java)
.resultStream
.map { it as User }
}
@Transactional // without this annotation,"Operation not allowed after ResultSet closed" will be thrown
@Test
open fun testUserStream() {
runBlocking {
userDao.findSomeUserStream(100).forEach { u ->
logger.info("{}",u)
}
}
}
// it works !!
@Transactional
@Test
open fun testUserStream2() {
runBlocking {
userDao.findSomeUserStream(100).asSequence().asFlow().collect { u ->
logger.info("{}",u)
}
}
}