Play框架JSON解析 - 当数组存在时，出现空指针异常

Question

我正在使用Play框架进行JSON解析，但遇到了以下空指针异常：

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)

我的读者如下所示：

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 _)

}

接下来是我的测试代码：

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]
}

这会导致以下空指针异常：

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json数组那就一切正常了。我尝试了来自playframework的json支持网页上的类似示例，它们可以正确运行。我无法发现问题所在。根据我的调试猜测，可能存在隐式转换的问题，但是不知道如何进一步帮助自己。我尝试了编译器选项“-Xprint:typer”，但没有发现任何可疑的东西。任何提示将不胜感激。

- jaksky

Answer 1

这是一个初始化顺序的问题。在定义Reads [SearchLikeThisResult]之前，需要先定义Reads [Hits]。代码能编译通过是因为该符号存在，但当初始化Reads [SearchLikeThisResult]时，Reads [Hits]还没有被初始化。直到尝试解析Hits数组时才会出现NPE问题。

只需交换顺序即可。这与此答案有关。

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 _)

- Michael Zajac

---

我为什么需要在一开始就声明Writes[T]以便在后面关注顺序呢？我只需要默认行为，就像Jackson的writeAsString（Any）一样，不需要隐式声明。 - Macchiatow