Scala模式匹配泛型类型擦除问题

Posted 2021-04-25 过往记忆大数据

　　在中一个很强大的功能就是模式匹配，本文并不打算介绍模式匹配的概念以及如何使用。本文的主要内容是讨论模式匹配泛型类型擦除问题。先来看看泛型类型擦除是什么情况：

 
   
   
 

  
    
    
  
scala> def test(a:Any) = a match { 
  
    
    
  
 | case a :List[String] => println("iteblog is ok"); 
  
    
    
  
 | case _ => 
  
    
    
  
 |} 
 
   
   
 

　　按照代码的意思应该是匹配List[String]类型的数据，但是这个test(List("Iteblog")) 和 test(List(2015))都是可以通过的。

1	scala> test(List("Iteblog"))

2	iteblog is ok

3

4	scala> test(List(2015))

5	iteblog is ok

　　这是因为类型被擦除了，你在编译这个代码的时候也会看到编译器给的提示：

1	<console>:14: warning: non-variable type argument String in typepattern List[String] is unchecked since it is eliminated by erasure

2	casea :List[String] => println("iteblog is ok");

　　上面测试结果都得到了结果，虽然不是我们要的，但是并没有出现什么异常。下面的代码却会出现异常：

 
   
   
 

  
    
    
  
scala> case class Container[+A](value: A)
  
    
    
  
defined class Container
  
    
    
  
 
  
    
    
  
scala> val double = Container(3.3) 
  
    
    
  
double: Container[Double] = Container(3.3)
  
    
    
  
 
  
    
    
  
scala> double match {
  
    
    
  
 | case c: Container[String] => println(c.value.toUpperCase)
  
    
    
  
 | case c: Container[Double] => println(math.sqrt(c.value))
  
    
    
  
 | case _ => println("_")
  
    
    
  
 | }
 
   
   
 

　　因为类型信息被编译器擦除了，所以你的double虽然是Container[Double]类型的，但是只能匹配到第一个case，而你想把一个double转换成String，所以会出现以下的异常：

01	java.lang.ClassCastException: java.lang.Double cannot be cast to java.lang.String

02	at $iwC$$iwC$$iwC$$iwC$$iwC$$iwC.<init>(<console>:19)

03	at $iwC$$iwC$$iwC$$iwC$$iwC.<init>(<console>:27)

04	at $iwC$$iwC$$iwC$$iwC.<init>(<console>:29)

05	at $iwC$$iwC$$iwC.<init>(<console>:31)

06	at $iwC$$iwC.<init>(<console>:33)

07	at $iwC.<init>(<console>:35)

08	at <init>(<console>:37)

09	at .<init>(<console>:41)

10	at .<clinit>(<console>)

11	at .<init>(<console>:7)

12	at .<clinit>(<console>)

13	at $print(<console>)

14	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

15	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)

16	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)

17	at java.lang.reflect.Method.invoke(Method.java:606)

18	at org.apache.spark.repl.SparkIMain$ReadEvalPrint.call(SparkIMain.scala:1065)

19	at org.apache.spark.repl.SparkIMain$Request.loadAndRun(SparkIMain.scala:1340)

20	at org.apache.spark.repl.SparkIMain.loadAndRunReq$1(SparkIMain.scala:840)

21	at org.apache.spark.repl.SparkIMain.interpret(SparkIMain.scala:871)

22	at org.apache.spark.repl.SparkIMain.interpret(SparkIMain.scala:819)

23	at org.apache.spark.repl.SparkILoop.reallyInterpret$1(SparkILoop.scala:857)

24	at org.apache.spark.repl.SparkILoop.interpretStartingWith(SparkILoop.scala:902)

25	at org.apache.spark.repl.SparkILoop.reallyInterpret$1(SparkILoop.scala:875)

26	at org.apache.spark.repl.SparkILoop.interpretStartingWith(SparkILoop.scala:902)

27	at org.apache.spark.repl.SparkILoop.reallyInterpret$1(SparkILoop.scala:875)

28	at org.apache.spark.repl.SparkILoop.interpretStartingWith(SparkILoop.scala:902)

29	at org.apache.spark.repl.SparkILoop.reallyInterpret$1(SparkILoop.scala:875)

30	at org.apache.spark.repl.SparkILoop.interpretStartingWith(SparkILoop.scala:902)

31	at org.apache.spark.repl.SparkILoop.reallyInterpret$1(SparkILoop.scala:875)

32	at org.apache.spark.repl.SparkILoop.interpretStartingWith(SparkILoop.scala:902)

33	at org.apache.spark.repl.SparkILoop.command(SparkILoop.scala:814)

34	at org.apache.spark.repl.SparkILoop.processLine$1(SparkILoop.scala:657)

35	at org.apache.spark.repl.SparkILoop.innerLoop$1(SparkILoop.scala:665)

36	at org.apache.spark.repl.SparkILoop.org$apache$spark$repl$SparkILoop$$loop(SparkILoop.scala:670)

37	at org.apache.spark.repl.SparkILoop$$anonfun$org$apache$spark$repl$SparkILoop$$process$1.apply$mcZ$sp(SparkILoop.scala:997)

38	at org.apache.spark.repl.SparkILoop$$anonfun$org$apache$spark$repl$SparkILoop$$process$1.apply(SparkILoop.scala:945)

39	at org.apache.spark.repl.SparkILoop$$anonfun$org$apache$spark$repl$SparkILoop$$process$1.apply(SparkILoop.scala:945)

40	at scala.tools.nsc.util.ScalaClassLoader$.savingContextLoader(ScalaClassLoader.scala:135)

41	at org.apache.spark.repl.SparkILoop.org$apache$spark$repl$SparkILoop$$process(SparkILoop.scala:945)

42	at org.apache.spark.repl.SparkILoop.process(SparkILoop.scala:1059)

43	at org.apache.spark.repl.Main$.main(Main.scala:31)

44	at org.apache.spark.repl.Main.main(Main.scala)

45	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

46	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)

47	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)

48	at java.lang.reflect.Method.invoke(Method.java:606)

49	at org.apache.spark.deploy.SparkSubmit$.org$apache$spark$deploy$SparkSubmit$$runMain(SparkSubmit.scala:673)

50	at org.apache.spark.deploy.SparkSubmit$.doRunMain$1(SparkSubmit.scala:180)

51	at org.apache.spark.deploy.SparkSubmit$.submit(SparkSubmit.scala:205)

52	at org.apache.spark.deploy.SparkSubmit$.main(SparkSubmit.scala:120)

53	at org.apache.spark.deploy.SparkSubmit.main(SparkSubmit.scala)

那如何来处理这种问题呢？其实你可以用下面的代码解决：

1	def matchContainer[A: Manifest](c: Container[A]) = c match {

2	case c: Container[String] ifmanifest <:< manifest[String] => println(c.value.toUpperCase)

3	case c: Container[Double] ifmanifest <:< manifest[Double] => println(math.sqrt(c.value))

4	case c: Container[_] => println("other")

5

}

它可以判断出c的类型，但是Manifest已经被标记为deprecated，我们可以使用TypeTag替代Manifest：

1	import reflect.runtime.universe._

2	def matchContainer[A: TypeTag](c: Container[A]) = c match {

3	case c: Container[String] if typeOf[A] <:< typeOf[String] => println(c.value.toUpperCase)

4	case c: Container[Double] if typeOf[A] <:< typeOf[Double] => println(math.sqrt(c.value))

5	case c: Container[_] => println("other")

6

}

不过TypeTags 是线程不安全的，可以参见：http://docs.scala-lang.org/overviews/reflection/thread-safety.html。

如果在多线程环境下，我们不能使用上面的代码。我们可以用下面代码解决：

1	var container: Container[Any] = double

2	container match {

3	case Container(x: String) => println("string")

4	case Container(x: Double) => println("double")

5	case _ => println("iteblog")

6

}