Scala-如何使用ToolBox创建的动态case类提取Json4

Question

我使用Toolbox动态定义了case类。当我提取json4s时，我会得到以下异常：

import org.json4s._
import scala.reflect.runtime._
import scala.tools.reflect.ToolBox
implicit val formats = DefaultFormats
val cm = universe.runtimeMirror(getClass.getClassLoader)
val toolBox = cm.mkToolBox()
val parse =
toolBox.parse(
s"""
| case class Person( name:String, age:String)
| scala.reflect.classTag[ Person].runtimeClass
""".stripMargin)
val person = toolBox.compile( parse)().asInstanceOf[Class[_]]
val js = JsonMethods.parse("""{ "name":"Tom","age" : "28"}""")
val jv = js.extract[person.type ] //How do I pass the class type?
**"Exception in thread "main" org.json4s.MappingException: No constructor for type Class, JObject(List((name,JString(Tom)), (age,JString(28))))"** 

但是在创建动态创建的类的伪实例之后，然后传入那个伪类的类型，它就会被解析。

我不知道为什么。如何在不创建虚拟实例的情况下进行解析？

import org.json4s._
import scala.reflect.runtime._
import scala.tools.reflect.ToolBox
implicit val formats = DefaultFormats
val cm = universe.runtimeMirror(getClass.getClassLoader)
val toolBox = cm.mkToolBox()
val parse =
toolBox.parse(
s"""
| case class Person( name:String, age:String)
| scala.reflect.classTag[ Person].runtimeClass
""".stripMargin)
val person = toolBox.compile( parse)().asInstanceOf[Class[_]]
val dummy  = person.getConstructors.head.newInstance( "a", "b") //make dummy instance
val js = JsonMethods.parse("""{ "name":"Tom","age" : "28"}""")
println( js.extract[ dummy.type ] ) // Result: Person(Tom,28)

Answer 1

x.type是一个单例类型。所以person.type不可能是正确的，它是这个特定变量val person: Class[_]的单例类型。

幸运的是，由于运行时反射，dummy.type是正确的。这甚至适用于普通情况下的

import org.json4s._
import org.json4s.jackson.JsonMethods
implicit val formats = DefaultFormats
case class Person(name: String, age: String)
val js = JsonMethods.parse("""{ "name":"Tom","age" : "28"}""")
val dummy0: AnyRef = Person("a", "b")
val dummy: AnyRef = dummy0
js.extract[dummy.type] // Person(Tom,28)

实际上解决隐式后js.extract[Person]是

js.extract[Person](formats, ManifestFactory.classType(classOf[Person])

js.extract[dummy.type]是

js.extract[dummy.type](formats, ManifestFactory.singleType(dummy))

因此，对于工具箱生成的案例类，我们可以尝试

import org.json4s._
import org.json4s.jackson.JsonMethods
import scala.reflect.ManifestFactory
import scala.reflect.runtime.universe
import scala.reflect.runtime.universe._
import scala.tools.reflect.ToolBox
val cm = universe.runtimeMirror(getClass.getClassLoader)
val toolBox = cm.mkToolBox()
implicit val formats = DefaultFormats
val person = toolBox.eval(q"""
case class Person(name:String, age:String)
scala.reflect.classTag[Person].runtimeClass
""").asInstanceOf[Class[_]]
val js = JsonMethods.parse("""{ "name":"Tom","age" : "28"}""")
js.extract(formats, ManifestFactory.classType(person))
// java.lang.ClassCastException: __wrapper$1$6246735221dc4d64a9e372a9d0891e5e.__wrapper$1$6246735221dc4d64a9e372a9d0891e5e$Person$1 cannot be cast to scala.runtime.Nothing$

(toolBox.eval(tree)代替toolBox.compile(toolBox.parse(string))()(

但这行不通。

清单应从工具箱编译时中获取

import org.json4s._
import org.json4s.jackson.JsonMethods
import scala.reflect.runtime.universe
import scala.reflect.runtime.universe._
import scala.tools.reflect.ToolBox
val cm = universe.runtimeMirror(getClass.getClassLoader)
val toolBox = cm.mkToolBox()
implicit val formats = DefaultFormats
val person = toolBox.eval(q"""
case class Person(name:String, age:String)
val clazz = scala.reflect.classTag[Person].runtimeClass
scala.reflect.ManifestFactory.classType(clazz)
""").asInstanceOf[Manifest[_]]
val js = JsonMethods.parse("""{ "name":"Tom","age" : "28"}""")
js.extract(formats, person) // Person(Tom,28)

或者，您根本不需要java反射Class。你可以做

import scala.reflect.runtime
import scala.reflect.runtime.universe._
import scala.tools.reflect.ToolBox
val cm = runtime.currentMirror
val toolBox = cm.mkToolBox()
toolBox.eval(q"""
import org.json4s._
import org.json4s.jackson.JsonMethods
implicit val formats = DefaultFormats
case class Person(name: String, age: String)
val js = JsonMethods.parse(${"""{"name":"Tom","age" : "28"}"""})
js.extract[Person]
""") // Person(Tom,28)

或

import org.json4s._
import org.json4s.jackson.JsonMethods
import scala.reflect.runtime
import scala.reflect.runtime.universe._
import scala.tools.reflect.ToolBox
object Main extends App {
val cm = runtime.currentMirror
val toolBox = cm.mkToolBox()
implicit val formats = DefaultFormats
val person: ClassSymbol = toolBox.define(q"case class Person(name: String, age: String)")
val js = JsonMethods.parse("""{"name":"Tom","age" : "28"}""")
val jv = toolBox.eval(q"""
import Main._
js.extract[$person]
""")

println(jv)  // Person(Tom,28)
}