[英]How to write parametric/generic functions in kotlin
我正在尝试找到一种解决方法,使Spring Reactive Webclient
与JDBC
工作。 这是我的想法: https : //gitorko.github.io/2019/04/02/Spring-Webflux-Reactive-JDBC/ 。
我正在编写一个调用 jdbc 存储库接口的service
,而不是返回我的域对象的类型MyClass
返回一个Mono<MyClass>
像这样:
//other specific imports here
import org.springframework.stereotype.Service
import reactor.core.publisher.Mono
import reactor.core.scheduler.Scheduler
import reactor.core.scheduler.Schedulers
import java.util.concurrent.Callable
@Service
class MyClassService(val repo: MyClassRepository, val jdbcScheduler: Scheduler){
fun save(obj: MyClass?): Mono<MyClass?>? {
return asyncCallable { repo.save(obj) }
}
protected fun <S> asyncCallable(callable: Callable<S>?): Mono<S>? {
return Mono.fromCallable(callable).subscribeOn(Schedulers.parallel()).publishOn(jdbcScheduler)
}
}
//this is a jdbc repository
interface MyClassRepository : CrudRepository<MyClass, UUID> {}
现在的问题是调用asyncCallable { repo.save(obj) }
返回编译错误inferred type is MyClass? but TypeVariable(S) was expected
inferred type is MyClass? but TypeVariable(S) was expected
并且Mono.fromCallable(callable).subscribeOn(Schedulers.parallel()).publishOn(jdbcScheduler)
返回编译错误inferred type is Callable<S>? but Callable<out TypeVariable(T)!> was expected
inferred type is Callable<S>? but Callable<out TypeVariable(T)!> was expected
。 通过阅读有关 kotlin 泛型的信息,我了解到这与方差有关。 如果我没有错,函数asyncCallable
在泛型类型S
上是不变的,在这种情况下需要协方差吗?
我认为您需要的语法是asyncCallable(Callable { repo.save(obj) })
。
完整示例:
@Service
class MyClassService(val repo: MyClassRepository, val jdbcScheduler: Scheduler){
fun save(obj: MyClass): Mono<MyClass?>? {
return asyncCallable(Callable { repo.save(obj) })
}
protected fun <S> asyncCallable(callable: Callable<S>): Mono<S>? {
return Mono.fromCallable(callable).subscribeOn(Schedulers.parallel()).publishOn(jdbcScheduler)
}
}
我也会删除?
s,但我留下了它们以使其尽可能接近您的代码。
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