Playframework JSON 解析 - 空指针异常 - 当存在数组时

Question

我正在使用 playframework 进行 JSON 解析，我面临以下 NullPointerException：

我的数据模型如下：

case class SearchLikeThisResult(total: Int, max_score: Double, hits: Seq[Hits])
case class Hits( index: String, typ: String, id: String, score: Double)

我的读者如下：

object SearchLikeThisHits {

  import play.api.libs.functional.syntax._

  implicit val searchLikeThisResult: Reads[SearchLikeThisResult] = (
    (JsPath \ "total").read[Int] and
    (JsPath \ "max_score").read[Double] and
    (JsPath \ "hits").read[Seq[Hits]]
    )(SearchLikeThisResult.apply _)


  implicit val hitsReads: Reads[Hits] = (
    (JsPath \ "_index").read[String] and
      (JsPath \ "_type").read[String] and
      (JsPath \ "_id").read[String] and
      (JsPath \ "_score").read[Double]
    )(Hits.apply _)

}

然后我有以下测试代码：

import play.api.libs.json.{JsValue, Json}

object Test extends App{

  val str = """{"total": 53, "max_score": 3.2948244, "hits": [
                                 {
                                     "_index": "hovno",
                                     "_type": "BYT",
                                     "_id": "3413569628",
                                     "_score": 3.2948244
                                 },
                                 {
                                      "_index": "hovno22",
                                      "_type": "BYT",
                                      "_id": "3413569628",
                                      "_score": 3.2948244
                                 }
                             ]
              }"""

  import SearchLikeThisHits.searchLikeThisResult

  val json = Json.parse(str)
  val r = json.as[SearchLikeThisResult]
}

这导致以下 NullPointerException：

Exception in thread "main" java.lang.NullPointerException
    at play.api.libs.json.Json$.fromJson(Json.scala:115)
    at play.api.libs.json.DefaultReads$$anon$$anonfun$reads.apply(Reads.scala:448)
    at play.api.libs.json.DefaultReads$$anon$$anonfun$reads.apply(Reads.scala:447)
    at scala.collection.LinearSeqOptimized$class.foldLeft(LinearSeqOptimized.scala:111)
    at scala.collection.immutable.List.foldLeft(List.scala:84)
    at scala.collection.generic.TraversableForwarder$class.foldLeft(TraversableForwarder.scala:41)
    at scala.collection.mutable.ListBuffer.foldLeft(ListBuffer.scala:45)
    at play.api.libs.json.DefaultReads$$anon.reads(Reads.scala:447)
    at play.api.libs.json.PathReads$$anonfun$at$$anonfun$apply.apply(JsConstraints.scala:36)
    at play.api.libs.json.PathReads$$anonfun$at$$anonfun$apply.apply(JsConstraints.scala:36)
    at play.api.libs.json.JsResult$class.flatMap(JsResult.scala:103)
    at play.api.libs.json.JsSuccess.flatMap(JsResult.scala:9)
    at play.api.libs.json.PathReads$$anonfun$at.apply(JsConstraints.scala:36)
    at play.api.libs.json.PathReads$$anonfun$at.apply(JsConstraints.scala:36)
    at play.api.libs.json.Reads$$anon.reads(Reads.scala:101)
    at play.api.libs.json.Reads$$anon$$anon.reads(Reads.scala:81)
    at play.api.libs.json.Reads$$anonfun$map.apply(Reads.scala:28)
    at play.api.libs.json.Reads$$anonfun$map.apply(Reads.scala:28)
    at play.api.libs.json.Reads$$anon.reads(Reads.scala:101)
    at play.api.libs.json.JsValue$class.as(JsValue.scala:65)
    at play.api.libs.json.JsObject.as(JsValue.scala:166)
    at models.data.Test$delayedInit$body.apply(Test.scala:33)
    at scala.Function0$class.apply$mcV$sp(Function0.scala:40)
    at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:12)
    at scala.App$$anonfun$main.apply(App.scala:71)
    at scala.App$$anonfun$main.apply(App.scala:71)
    at scala.collection.immutable.List.foreach(List.scala:318)
    at scala.collection.generic.TraversableForwarder$class.foreach(TraversableForwarder.scala:32)
    at scala.App$class.main(App.scala:71)
    at models.data.Test$.main(Test.scala:9)
    at models.data.Test.main(Test.scala)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:134)

如果我删除一个 json 数组，一切正常。我尝试了来自 playframework json 支持网页的类似示例，它可以正常工作。我在这里找不到问题。我基于调试的猜测是隐式可能存在问题但不知道如何帮助自己。我尝试了“-Xprint:typer”编译器选项，但没有发现任何可疑之处。

如有任何提示，我们将不胜感激

Answer 1

这是一个初始化顺序问题。您需要在 Reads[SearchLikeThisResult] 之前定义Reads[Hits] 。它编译是因为符号存在于代码中，当 Reads[SearchLikeThisResult] 被初始化时， Reads[Hits] 不是。在它尝试解析 Hits 的数组并命中 NPE 之前，它不会被注意到。

所以只需调换顺序。这与 this answer.
有关
implicit val hitsReads: Reads[Hits] = ( (JsPath \ "_index").read[String] and (JsPath \ "_type").read[String] and (JsPath \ "_id").read[String] and (JsPath \ "_score").read[Double] )(Hits.apply _) implicit val searchLikeThisResult: Reads[SearchLikeThisResult] = ( (JsPath \ "total").read[Int] and (JsPath \ "max_score").read[Double] and (JsPath \ "hits").read[Seq[Hits]] )(SearchLikeThisResult.apply _)

Playframework JSON 解析 - 空指针异常 - 当存在数组时

Playframework JSON parsing - Null pointer exception - when array is present

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playframework