我想支持Json和Parquet文件格式。客户端不应该关心它们的实现,但是必须传递一种类型来标识格式。
到目前为止,我有两个具有此类签名的类:
class ParquetFileWriter[T](val path: String)(implicit val writer: ParquetWriter[T]) extends FileWriter[T]
和
class JsonFileWriter[T](val path: String)(implicit writer: JsonWriter[T]) extends FileWriter[T]
它们扩展的特征:
trait FileWriter[T] {
def write(items: Seq[T]): Unit
}
我想创建一个工厂类来通过参数构建类:
class Factory {
def of[T](format: Format): FileWriter[T] = {
format match {
case ParquetSpark =>
new ParquetFileWriter[T](defaultPath)
case Json =>
new JsonFileWriter[T](defaultPath)
}
}
}
问题是ParquetFileWriter和JsonFileWriter需要隐式(当它们来自spray.json和com.github.mjakubowski84.parquet4s库时,它们不在我的控制范围之内。
如果格式依赖于不同的隐式格式,如何实现工厂格式?我在编译时收到“找不到隐式值”。
如果同时具有两个隐式仅添加上下文边界就足够了
class Factory {
def of[T: ParquetWriter : JsonWriter](format: Format): FileWriter[T] = {
format match {
case ParquetSpark =>
new ParquetFileWriter[T]("defaultPath")
case Json =>
new JsonFileWriter[T]("defaultPath")
}
}
}
如果可能只有这两个隐式变量之一尝试使Factory
成为类型类
trait Factory[T] {
def of(format: Format): FileWriter[T]
}
trait LowPriorityFactories {
implicit def parquet[T: ParquetWriter]: Factory[T] = {
case ParquetSpark => new ParquetFileWriter[T]("defaultPath")
case _ => throw new Exception
}
implicit def json[T: JsonWriter]: Factory[T] = {
case Json => new JsonFileWriter[T]("defaultPath")
case _ => throw new Exception
}
}
object Factory extends LowPriorityFactories {
def of[T](format: Format)(implicit factory: Factory[T]): FileWriter[T] = factory.of(format)
implicit def jsonParquet[T: JsonWriter : ParquetWriter]: Factory[T] = {
case ParquetSpark => new ParquetFileWriter[T]("defaultPath")
case Json => new JsonFileWriter[T]("defaultPath")
}
}
现在,如果您同时具有两个隐式,则适用于Json
和ParquetSpark
implicit val jsonWriter: JsonWriter[Int] = null
implicit val parketWriter: ParquetWriter[Int] = null
Factory.of[Int](Json)
Factory.of[Int](ParquetSpark)
如果只有隐式JsonWriter
,则仅适用于Json
implicit val jsonWriter: JsonWriter[Int] = null
Factory.of[Int](Json)
Factory.of[Int](ParquetSpark) // Exception
如果只有隐式ParquetWriter
,则仅适用于ParquetSpark
implicit val parketWriter: ParquetWriter[Int] = null
Factory.of[Int](Json) // Exception
Factory.of[Int](ParquetSpark)
另一个选择是使选择更具类型级别将其从运行时转移到编译时(然后,您根本不需要模式匹配,并且编译错误代替了运行时异常)。
sealed trait Format
case object ParquetSpark extends Format
type ParquetSpark = ParquetSpark.type
case object Json extends Format
type Json = Json.type
trait Factory[T, F <: Format] {
def of: FileWriter[T]
}
object Factory {
def of[T, F <: Format](implicit factory: Factory[T, F]): FileWriter[T] = factory.of
implicit def parquet[T: ParquetWriter]: Factory[T, ParquetSpark] = new Factory[T, ParquetSpark] {
override def of: FileWriter[T] = new ParquetFileWriter[T]("defaultPath")
}
implicit def json[T: JsonWriter]: Factory[T, Json] = new Factory[T, Json] {
override def of: FileWriter[T] = new JsonFileWriter[T]("defaultPath")
}
}
{
implicit val jsonWriter: JsonWriter[Int] = null
Factory.of[Int, Json]
// Factory.of[Int, ParquetSpark] // doesn't compile
}
{
implicit val parketWriter: ParquetWriter[Int] = null
// Factory.of[Int, Json] // doesn't compile
Factory.of[Int, ParquetSpark]
}
{
implicit val jsonWriter: JsonWriter[Int] = null
implicit val parketWriter: ParquetWriter[Int] = null
Factory.of[Int, Json]
Factory.of[Int, ParquetSpark]
}