I'm trying to build a json to case class en/decoder using json4s Using generics icw Manifest seems to work for ordinary types/classes, but more complex configs seem to fail.
How can I use json4s combined with extracting more complex types from json strings?
import org.json4s._
import org.json4s.native.JsonMethods._
implicit val formats = org.json4s.DefaultFormats
case class User(name:String)
case class Product(id:String)
case class Meta(count:Int)
case class ResultList[T: Manifest](meta: Meta, result: List[T])
// Without generics
case class ResultListUser(meta: Meta, result: List[User])
case class ResultListProduct(meta: Meta, result: List[Product])
// general decode method
def decode[T: Manifest](jsonStr: String): T = {
parse(jsonStr).extract[T]
}
// data
val userJson = """{"meta":{"count":2},"result":[{"name":"Tom"},{"name":"Lucas"}]}"""
val productJson = """{"meta":{"count":2},"result":[{"id":"123"},{"id":"456"}]}"""
val resultListUser = decode[ResultListUser](userJson)
resultListUser: ResultListUser = ResultListUser(Meta(2),List(User(Tom), User(Lucas)))
val resultListProduct = decode[ResultListProduct](productJson)
resultListProduct: ResultListProduct = ResultListProduct(Meta(2),List(Product(123), Product(456)))
val resultListUser2 = decode[ResultList[User]](userJson)
org.json4s.package$MappingException: No usable value for evidence$1
No constructor for type Manifest[User], JNothing
...
val resultListProduct2 = decode[ResultList[Product]](productJson)
org.json4s.package$MappingException: No usable value for evidence$1 No constructor for type Manifest[Product], JNothing at org.json4s.reflect.package$.fail(/Users/tomlous/Development/Scala/testjes/src/main/scala/json4sgenerics.sc:91) at org.json4s.Extraction$ClassInstanceBuilder.org$json4s$Extraction$ClassInstanceBuilder$$buildCtorArg(/Users/tomlous/Development/Scala/testjes/src/main/scala/json4sgenerics.sc:522) at org.json4s.Extraction$ClassInstanceBuilder$$anonfun$15.apply(/Users/tomlous/Development/Scala/testjes/src/main/scala/json4sgenerics.sc:542) at org.json4s.Extraction$ClassInstanceBuilder$$anonfun$15.apply(/Users/tomlous/Development/Scala/testjes/src/main/scala/json4sgenerics.sc:542) at scala.collection.TraversableLike$$anonfun$map$1.apply(/Users/tomlous/Development/Scala/testjes/src/main/scala/json4sgenerics.sc:230) at scala.collection.TraversableLike$$anonfun$map$1.apply(/Users/tomlous/Development/Scala/testjes/src/main/scala/json4sgenerics.sc:230) at scala.collection.mutable.ResizableArray$class.foreach(/Users/tomlous/Development/Scala/testjes/src/main/scala/json4sgenerics.sc:55) at scala.collection.mutable.ArrayBuffer.foreach(/Users/tomlous/Development/Scala/testjes/src/main/scala/json4sgenerics.sc:44) at scala.collection.TraversableLike$class.map(/Users/tomlous/Development/Scala/testjes/src/main/scala/json4sgenerics.sc:230) at scala.collection.AbstractTraversable.map(/Users/tomlous/Development/Scala/testjes/src/main/scala/json4sgenerics.sc:100) at org.json4s.Extraction$ClassInstanceBuilder.instantiate(/Users/tomlous/Development/Scala/testjes/src/main/scala/json4sgenerics.sc:542) at org.json4s.Extraction$ClassInstanceBuilder.result(/Users/tomlous/Development/Scala/testjes/src/main/scala/json4sgenerics.sc:593) at org.json4s.Extraction$$anonfun$extract$6.apply(/Users/tomlous/Development/Scala/testjes/src/main/scala/json4sgenerics.sc:396) at org.json4s.Extraction$$anonfun$extract$6.apply(/Users/tomlous/Development/Scala/testjes/src/main/scala/json4sgenerics.sc:388) at org.json4s.Extraction$.customOrElse(/Users/tomlous/Development/Scala/testjes/src/main/scala/json4sgenerics.sc:602) at org.json4s.Extraction$.extract(/Users/tomlous/Development/Scala/testjes/src/main/scala/json4sgenerics.sc:388) at worksheet.worksheet(/Users/tomlous/Development/Scala/testjes/src/main/scala/json4sgenerics.sc:35)
Caused by: org.json4s.package$MappingException: No constructor for type Manifest[Product], JNothing at org.json4s.reflect.package$.fail(package.scala:95) at org.json4s.Extraction$ClassInstanceBuilder$$anonfun$org$json4s$Extraction$ClassInstanceBuilder$$constructor$1.apply(Extraction.scala:477) at org.json4s.Extraction$ClassInstanceBuilder$$anonfun$org$json4s$Extraction$ClassInstanceBuilder$$constructor$1.apply(Extraction.scala:477) at scala.Option.getOrElse(Option.scala:121) at org.json4s.Extraction$ClassInstanceBuilder.org$json4s$Extraction$ClassInstanceBuilder$$constructor(Extraction.scala:477) at org.json4s.Extraction$ClassInstanceBuilder.instantiate(Extraction.scala:532) at org.json4s.Extraction$ClassInstanceBuilder.result(Extraction.scala:597) at org.json4s.Extraction$$anonfun$extract$6.apply(Extraction.scala:400) at org.json4s.Extraction$$anonfun$extract$6.apply(Extraction.scala:392) at org.json4s.Extraction$.customOrElse(Extraction.scala:606) at org.json4s.Extraction$.extract(Extraction.scala:392) at org.json4s.Extraction$ClassInstanceBuilder.org$json4s$Extraction$ClassInstanceBuilder$$buildCtorArg(Extraction.scala:514) at org.json4s.Extraction$ClassInstanceBuilder$$anonfun$15.apply(Extraction.scala:546) at org.json4s.Extraction$ClassInstanceBuilder$$anonfun$15.apply(Extraction.scala:546) at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234) at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234) at scala.collection.mutable.ResizableArray$class.foreach(ResizableArray.scala:59) at scala.collection.mutable.ArrayBuffer.foreach(ArrayBuffer.scala:48) at scala.collection.TraversableLike$class.map(TraversableLike.scala:234) at scala.collection.AbstractTraversable.map(Traversable.scala:104) at org.json4s.Extraction$ClassInstanceBuilder.instantiate(Extraction.scala:546) at org.json4s.Extraction$ClassInstanceBuilder.result(Extraction.scala:597) at org.json4s.Extraction$$anonfun$extract$6.apply(Extraction.scala:400) at org.json4s.Extraction$$anonfun$extract$6.apply(Extraction.scala:392) at org.json4s.Extraction$.customOrElse(Extraction.scala:606) at org.json4s.Extraction$.extract(Extraction.scala:392) at org.json4s.Extraction$.extract(Extraction.scala:39) at org.json4s.ExtractableJsonAstNode.extract(ExtractableJsonAstNode.scala:21) at A$A7$A$A7.decode(json4sgenerics.sc:24) at A$A7$A$A7.resultListProduct2$lzycompute(json4sgenerics.sc:37) at A$A7$A$A7.resultListProduct2(json4sgenerics.sc:37) at A$A7$A$A7.get$$instance$$resultListProduct2(json4sgenerics.sc:36) at A$A7$.main(json4sgenerics.sc:92) at A$A7.main(json4sgenerics.sc) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:497) at org.jetbrains.plugins.scala.worksheet.MyWorksheetRunner.main(MyWorksheetRunner.java:22)