播放框架形式的映射递归项目

Question

我有在它的形式映射以下案例类的同伴对象：

case class ContractItem(description: String, price: Option[BigDecimal], durationMonths: Option[Int], subItems: List[ContractItem]) 
object ContractItem { 
    val mapping: Mapping[ContractItem] = Forms.mapping(
    "description" -> text, 
    "price" -> optional(bigDecimal), 
    "durationMonths" -> optional(number), 
    "subItems" -> Forms.list(ContractItem.mapping) 
)(ContractItem.apply)(ContractItem.unapply) 
} 

正如你所看到的情形类是递归的，因为它包含相同类型的列表，并且我试图在映射中包含这个。这一切都ok了编译，但是当我尝试运行这个项目中，我得到以下几点：

java.lang.NullPointerException 
at play.api.data.RepeatedMapping.<init>(Form.scala:636) 
at play.api.data.Forms$.list(Forms.scala:389) 
at models.Installation$ContractItem$.<init>(Installation.scala:17) 
at models.Installation$ContractItem$.<clinit>(Installation.scala) 
at models.Installation$Contract$.<init>(Installation.scala:23) 
at models.Installation$Contract$.<clinit>(Installation.scala) 
at models.Installation$.<init>(Installation.scala:31) 
at models.Installation$.<clinit>(Installation.scala) 
at Temp.<init>(Temp.scala:82) 
at .<init>(<console>:7) 
at .<clinit>(<console>) 
at .<init>(<console>:7) 
at .<clinit>(<console>) 
at $print(<console>) 
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) 
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) 
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) 
at java.lang.reflect.Method.invoke(Method.java:597) 
at scala.tools.nsc.interpreter.IMain$ReadEvalPrint.call(IMain.scala:734) 
at scala.tools.nsc.interpreter.IMain$Request.loadAndRun(IMain.scala:983) 
at scala.tools.nsc.interpreter.IMain.loadAndRunReq$1(IMain.scala:573) 
at scala.tools.nsc.interpreter.IMain.interpret(IMain.scala:604) 
at scala.tools.nsc.interpreter.IMain.interpret(IMain.scala:568) 
at scala.tools.nsc.interpreter.ILoop.reallyInterpret$1(ILoop.scala:745) 
at scala.tools.nsc.interpreter.ILoop.interpretStartingWith(ILoop.scala:790) 
at scala.tools.nsc.interpreter.ILoop.command(ILoop.scala:702) 
at scala.tools.nsc.interpreter.ILoop.processLine$1(ILoop.scala:566) 
at scala.tools.nsc.interpreter.ILoop.innerLoop$1(ILoop.scala:573) 
at scala.tools.nsc.interpreter.ILoop.loop(ILoop.scala:576) 
at scala.tools.nsc.interpreter.ILoop$$anonfun$process$1.apply$mcZ$sp(ILoop.scala:867) 
at scala.tools.nsc.interpreter.ILoop$$anonfun$process$1.apply(ILoop.scala:822) 
at scala.tools.nsc.interpreter.ILoop$$anonfun$process$1.apply(ILoop.scala:822) 
at scala.tools.nsc.util.ScalaClassLoader$.savingContextLoader(ScalaClassLoader.scala:135) 
at scala.tools.nsc.interpreter.ILoop.process(ILoop.scala:822) 
at scala.tools.nsc.interpreter.ILoop.main(ILoop.scala:889) 
at xsbt.ConsoleInterface.run(ConsoleInterface.scala:69) 
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) 
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) 
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) 
at java.lang.reflect.Method.invoke(Method.java:597) 
at sbt.compiler.AnalyzingCompiler.call(AnalyzingCompiler.scala:102) 
at sbt.compiler.AnalyzingCompiler.console(AnalyzingCompiler.scala:77) 
at sbt.Console.sbt$Console$$console0$1(Console.scala:23) 
at sbt.Console$$anonfun$apply$2$$anonfun$apply$1.apply$mcV$sp(Console.scala:24) 
at sbt.TrapExit$.sbt$TrapExit$$executeMain$1(TrapExit.scala:33) 
at sbt.TrapExit$$anon$1.run(TrapExit.scala:42) 

任何想法，如果这样的事情，甚至有可能？应该是我会想到的！或解决方法的任何想法？

谢谢！ NFV

更新：

在后续的使用

lazy val mapping 

结果：

java.lang.StackOverflowError 
at play.api.data.Mapping$class.$init$(Form.scala:416) 
at play.api.data.OptionalMapping.<init>(Form.scala:704) 
at play.api.data.Forms$.optional(Forms.scala:363) 
at models.Installation$ContractItem$.mapping$lzycompute(Installation.scala:15) 
at models.Installation$ContractItem$.mapping(Installation.scala:13) 
at models.Installation$ContractItem$.mapping$lzycompute(Installation.scala:17) 
at models.Installation$ContractItem$.mapping(Installation.scala:13) 
at models.Installation$ContractItem$.mapping$lzycompute(Installation.scala:17) 
at models.Installation$ContractItem$.mapping(Installation.scala:13) 
at models.Installation$ContractItem$.mapping$lzycompute(Installation.scala:17) 
at models.Installation$ContractItem$.mapping(Installation.scala:13) 
at models.Installation$ContractItem$.mapping$lzycompute(Installation.scala:17) 
at models.Installation$ContractItem$.mapping(Installation.scala:13) 

Answer 1

变化：

val mapping: Mapping[ContractItem] 

到：

lazy val mapping: Mapping[ContractItem] 

通过制作mapping懒惰，它能够引用本身而不会抛出NPE。