Question

我正在使用playframework进行JSON解析，我正面临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$$anon$2$$anonfun$reads$6.apply(Reads.scala:448)
    at play.api.libs.json.DefaultReads$$anon$2$$anonfun$reads$6.apply(Reads.scala:447)
    at scala.collection.LinearSeqOptimized$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$$anon$2.reads(Reads.scala:447)
    at play.api.libs.json.PathReads$$anonfun$at$1$$anonfun$apply$2.apply(JsConstraints.scala:36)
    at play.api.libs.json.PathReads$$anonfun$at$1$$anonfun$apply$2.apply(JsConstraints.scala:36)
    at play.api.libs.json.JsResult$class.flatMap(JsResult.scala:103)
    at play.api.libs.json.JsSuccess.flatMap(JsResult.scala:9)
    at play.api.libs.json.PathReads$$anonfun$at$1.apply(JsConstraints.scala:36)
    at play.api.libs.json.PathReads$$anonfun$at$1.apply(JsConstraints.scala:36)
    at play.api.libs.json.Reads$$anon$8.reads(Reads.scala:101)
    at play.api.libs.json.Reads$$anon$3$$anon$4.reads(Reads.scala:81)
    at play.api.libs.json.Reads$$anonfun$map$1.apply(Reads.scala:28)
    at play.api.libs.json.Reads$$anonfun$map$1.apply(Reads.scala:28)
    at play.api.libs.json.Reads$$anon$8.reads(Reads.scala:101)
    at play.api.libs.json.JsValue$class.as(JsValue.scala:65)
    at play.api.libs.json.JsObject.as(JsValue.scala:166)
    at models.data.Test$delayedInit$body.apply(Test.scala:33)
    at scala.Function0$class.apply$mcV$sp(Function0.scala:40)
    at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:12)
    at scala.App$$anonfun$main$1.apply(App.scala:71)
    at scala.App$$anonfun$main$1.apply(App.scala:71)
    at scala.collection.immutable.List.foreach(List.scala:318)
    at scala.collection.generic.TraversableForwarder$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)

如果我删除一个全部有效的json数组。我尝试了类似的例子来自playframework json支持网页，它正常工作。我无法在这里发现问题。我基于调试的猜测是，可能存在implicits的问题，但不知道如何更多地帮助自己。我尝试了“-Xprint：typer”编译器选项，但没有发现任何可疑的东西。

任何提示都将受到赞赏

Answer 1

这是一个初始化顺序问题。您需要在 Reads[Hits]之前定义Reads[SearchLikeThisResult] 。它编译是因为符号存在于代码中，当初始化Reads[SearchLikeThisResult]时，Reads[Hits]不是。在尝试解析Hits数组并点击NPE之前，它不会被注意到。

所以只需交换订单。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 - 当数组存在时

1 个答案: