播放2.3隐式json转换导致空指针异常

我试图将json解析到我的case类

DealFormMap

case class DealFormMap(limit: Option[Int], filter: Option[DealFormFilterMap])
case class DealFormFilterMap(date: Option[String], code: Option[String])

implicit val dealFormMapReads: Reads[DealFormMap] = (
    (JsPath \ "limit").readNullable[Int] and
    (JsPath \ "filter").readNullable[DealFormFilterMap]
)(DealFormMap)

implicit val dealFormFilterMapReads: Reads[DealFormFilterMap] = (
    (JsPath \ "date").readNullable[String] and
    (JsPath \ "code").readNullable[String]
)(DealFormFilterMap)

有问题的JSON和解析尝试

val str = """{"limit":10,"filter":{"date":"2014-10-27"}}"""
val frm = Json.parse(str).as[DealFormMap]

导致一个我似乎无法破解的神秘错误堆栈

play.api.Application$$anon$1: Execution exception[[NullPointerException: null]]
    at play.api.Application$class.handleError(Application.scala:296) ~[play_2.11-2.3.5.jar:2.3.5]
    at play.api.DefaultApplication.handleError(Application.scala:402) [play_2.11-2.3.5.jar:2.3.5]
    at play.core.server.netty.PlayDefaultUpstreamHandler$$anonfun$14$$anonfun$apply$1.applyOrElse(PlayDefaultUpstreamHandler.scala:205) [play_2.11-2.3.5.jar:2.3.5]
    at play.core.server.netty.PlayDefaultUpstreamHandler$$anonfun$14$$anonfun$apply$1.applyOrElse(PlayDefaultUpstreamHandler.scala:202) [play_2.11-2.3.5.jar:2.3.5]
    at scala.runtime.AbstractPartialFunction.apply(AbstractPartialFunction.scala:36) [scala-library-2.11.2.jar:na]
Caused by: java.lang.NullPointerException: null
    at play.api.libs.json.PathReads$$anonfun$nullable$1$$anonfun$apply$7$$anonfun$apply$9.apply(JsConstraints.scala:65) ~[play-json_2.11-2.3.5.jar:2.3.5]
    at play.api.libs.json.PathReads$$anonfun$nullable$1$$anonfun$apply$7$$anonfun$apply$9.apply(JsConstraints.scala:63) ~[play-json_2.11-2.3.5.jar:2.3.5]
    at play.api.libs.json.JsResult$class.fold(JsResult.scala:76) ~[play-json_2.11-2.3.5.jar:2.3.5]
    at play.api.libs.json.JsSuccess.fold(JsResult.scala:9) ~[play-json_2.11-2.3.5.jar:2.3.5]
    at play.api.libs.json.PathReads$$anonfun$nullable$1$$anonfun$apply$7.apply(JsConstraints.scala:61) ~[play-json_2.11-2.3.5.jar:2.3.5]

---

我这里没有什么想法了，有什么问题吗？

问题是初始化顺序

dealFormMapReads

依赖于隐式的

DealFormFilterPreads

，它在之后才定义。它将进行编译，因为找到了隐式，即使它尚未初始化，因此

dealFormMapReads

被读取为

null

，这最终导致NPE

延迟加载将修复此问题：

implicit val dealFormMapReads: Reads[DealFormMap] = (
     (JsPath \ "limit").readNullable[Int] and
     (JsPath \ "filter").lazyReadNullable[DealFormFilterMap](dealFormFilterMapReads)
)(DealFormMap)

或者，您可以交换

读取的定义顺序


此处抛出的NPE类似于此示例：

case class A(i: Int)

object Test {
     val test = a.toString
     val a = A(1)
}

// Compiles up to here

Test.test // throws NPE, because `a` was not initialized before `test`

问题是LimbSoup提到的顺序。但是值得一提的是，使用它，你可以达到同样的效果

import play.api.libs.json._

implicit val dealFormFilterMapReads: Reads[DealFormFilterMap] = Json.reads[DealFormFilterMap]

implicit val dealFormMapReads: Reads[DealFormMap] = Json.reads[DealFormMap]

请注意，通过使用此宏，如果更改顺序

implicit val dealFormMapReads: Reads[DealFormMap] = Json.reads[DealFormMap]

implicit val dealFormFilterMapReads: Reads[DealFormFilterMap] = Json.reads[DealFormFilterMap]   

您将得到以下有用的警告：

 Reference to uninitialized value dealFormFilterMapReads
[warn] implicit val dealFormMapReads: Reads[DealFormMap] = Json.reads[DealFormMap]
[warn]                                                                 ^
[warn] one warning found

您可以通过使
dealFormFilterMapReads
lazy或重新排序来修复（同样如LimpSoup所述），这在本例中更有意义

免责声明
在播放中使用Json宏inception需要Scala
2.10
，并且在
2.1.0