Spray-json中的NPE由于递归隐式(上下文绑定问题?) [英] NPE in spray-json because of recursive implicits (context bound issue?)

问题描述

也许我在spray-json中发现了 bug .当我尝试获取具有自身类型字段的对象的json时，出现Null Pointer Exception.示例是:

Perhaps I discovered a bug in spray-json. I get Null Pointer Exception when I'm trying to get json of an object that has field of type of itself. Example is:

case class TestItem(subitems: Option[List[TestItem]])

object MyJsonProtocol extends DefaultJsonProtocol {
  implicit val testItemFormat: RootJsonFormat[TestItem] = jsonFormat(TestItem, "subitems")
}

import MyJsonProtocol._

object TestNPE {
  def main(args: Array[String]) {

    val subitems = List(TestItem(None))
    val item: TestItem = TestItem(Option(subitems))
    val jsonAst = item.toJson
    val json = jsonAst.prettyPrint
    println(json)
  }
}

这是调用栈

Exception in thread "main" java.lang.NullPointerException
    at spray.json.PimpedAny.toJson(package.scala:40)
    at spray.json.CollectionFormats$$anon$1$$anonfun$write$1.apply(CollectionFormats.scala:26)
    at spray.json.CollectionFormats$$anon$1$$anonfun$write$1.apply(CollectionFormats.scala:26)
    at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
    at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
    at scala.collection.immutable.List.foreach(List.scala:309)
    at scala.collection.TraversableLike$class.map(TraversableLike.scala:244)
    at scala.collection.AbstractTraversable.map(Traversable.scala:105)
    at spray.json.CollectionFormats$$anon$1.write(CollectionFormats.scala:26)
    at spray.json.CollectionFormats$$anon$1.write(CollectionFormats.scala:25)
    at spray.json.PimpedAny.toJson(package.scala:40)
    at spray.json.StandardFormats$OptionFormat.write(StandardFormats.scala:34)
    at spray.json.StandardFormats$OptionFormat.write(StandardFormats.scala:32)
    at spray.json.ProductFormats$class.productElement2Field(ProductFormats.scala:473)
    at spray.json.MyJsonProtocol$.productElement2Field(TestNPE.scala:5)
    at spray.json.ProductFormats$$anon$1.write(ProductFormats.scala:32)
    at spray.json.ProductFormats$$anon$1.write(ProductFormats.scala:30)
    at spray.json.PimpedAny.toJson(package.scala:40)
    at spray.json.TestNPE$.main(TestNPE.scala:18)
    at spray.json.TestNPE.main(TestNPE.scala)

Sooo我试图自己修复它，但是我对Scala的了解还不够强. NPE发生在此处尝试转换内部TestItem时.此时的功能参数write是null.

Sooo I tried to fix it myself but my knowledge of Scala is not strong enough yet. NPE happens here when it's attempting to convert inner TestItem. Function parameter write at that moment is null.

您能告诉我为什么它不使用我的隐式吗?我在调试器中看到，与其使用我的隐式write而不是包含某些魔术字段evidence$x$y的值，该字段随其变化深入了解通话链.我不知道那是什么我觉得这与上下文相关，但请阅读相关章节没有帮助.

Could you please explain to me why it doesn't use my implicit instead ? I see in the debugger that instead of using my implicit write contains value of some magic field evidence$x$y which changes as it goes deeper in calling chain. I don't know what it is. I feel like it's something related to context bound but reading relevant chapter didn't help.

推荐答案

嗯，我应该更加专心.我不得不使用lazyFormat包装器.

Well, I should have been more attentive. I had to use lazyFormat wrapper.

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