Play 文档中的此JSON自动映射示例失败。为什么?
https://www.playframework.com/documentation/2.5.x/ScalaJsonAutomated
libraryDependencies += "com.typesafe.play" %% "play" % "2.5.0"
---
import play.api.libs.json._
case class Resident(name: String, age: Int, role: Option[String])
implicit val residentWrites = Json.writes[Resident]
println(Json.toJson(Resident("john", 33, None)))
---
Error: No unapply or unapplySeq function found
implicit val residentWrites = Json.writes[Resident]
最佳答案
有问题的代码或多或少看起来像这样:
import play.api.libs.json._
object Test {
def main(args: Array[String]): Unit = {
case class Resident(name: String, age: Int, role: Option[String])
implicit val residentWrites = Json.writes[Resident]
println(Json.toJson(Resident("john", 33, None)))
}
}
这里的问题是,该宏显然不适用于在方法内部定义的类。这并不是一个令人不安的限制,尽管我们宁愿不做这种事情。
要解决问题,可以将类def移到其他位置,例如对象级别
object Test {
case class Resident(name: String, age: Int, role: Option[String])
def main(args: Array[String]): Unit = {
implicit val residentWrites = Json.writes[Resident]
println(Json.toJson(Resident("john", 33, None)))
}
}
或文件级别
case class Resident(name: String, age: Int, role: Option[String])
object Test {
def main(args: Array[String]): Unit = {
implicit val residentWrites = Json.writes[Resident]
println(Json.toJson(Resident("john", 33, None)))
}
}
我知道这只是为了测试目的,以查看最小的示例,但是我仍然会提到我们通常如何使用
Writes
定义类。object Resident {
implicit val residentWrites = Json.writes[Resident]
}
case class Resident(name: String, age: Int, role: Option[String])
这样,无论何时导入
Resident
,其写操作都将在隐式范围内。
关于scala play json未找到unapply或unapplySeq函数,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/36926225/