Scala try-catch没有捕获异常

Question

据我所知,以下代码片段应该可以正常执行.Stream("A", "").toIterable.map(_.head)抛出异常,应该被try块捕获.

import org.scalatest.FunSuite

class TryCatchTest extends FunSuite {
  test("Try failure") {
    List(1).flatMap {
      _ =>
        try {
          Stream("A", "").toIterable.map(_.head)
        } catch {
          case e: Throwable => Nil
        }
    }
  }
}

但是当我运行它时,崩溃时出现以下错误:

next on empty iterator
java.util.NoSuchElementException: next on empty iterator
    at scala.collection.Iterator$$anon$2.next(Iterator.scala:39)
    at scala.collection.Iterator$$anon$2.next(Iterator.scala:37)
    at scala.collection.IndexedSeqLike$Elements.next(IndexedSeqLike.scala:63)
    at scala.collection.IterableLike$class.head(IterableLike.scala:107)
    at scala.collection.immutable.StringOps.scala$collection$IndexedSeqOptimized$$super$head(StringOps.scala:30)
    at scala.collection.IndexedSeqOptimized$class.head(IndexedSeqOptimized.scala:126)
    at scala.collection.immutable.StringOps.head(StringOps.scala:30)
    at TryCatchTest$$anonfun$1$$anonfun$apply$mcV$sp$1$$anonfun$apply$1.apply(TryCatchTest.scala:8)
    at TryCatchTest$$anonfun$1$$anonfun$apply$mcV$sp$1$$anonfun$apply$1.apply(TryCatchTest.scala:8)
    at scala.collection.immutable.Stream$$anonfun$map$1.apply(Stream.scala:418)
    at scala.collection.immutable.Stream$$anonfun$map$1.apply(Stream.scala:418)
    at scala.collection.immutable.Stream$Cons.tail(Stream.scala:1222)
    at scala.collection.immutable.Stream$Cons.tail(Stream.scala:1212)
    at scala.collection.immutable.Stream.foreach(Stream.scala:595)
    at scala.collection.immutable.List.flatMap(List.scala:327)
    at TryCatchTest$$anonfun$1.apply$mcV$sp(TryCatchTest.scala:5)
    at TryCatchTest$$anonfun$1.apply(TryCatchTest.scala:5)
    at TryCatchTest$$anonfun$1.apply(TryCatchTest.scala:5)
    at org.scalatest.Transformer$$anonfun$apply$1.apply$mcV$sp(Transformer.scala:22)
    at org.scalatest.OutcomeOf$class.outcomeOf(OutcomeOf.scala:85)
    at org.scalatest.OutcomeOf$.outcomeOf(OutcomeOf.scala:104)
    at org.scalatest.Transformer.apply(Transformer.scala:22)
    at org.scalatest.Transformer.apply(Transformer.scala:20)
    at org.scalatest.FunSuiteLike$$anon$1.apply(FunSuiteLike.scala:166)
    at org.scalatest.Suite$class.withFixture(Suite.scala:1122)
    at org.scalatest.FunSuite.withFixture(FunSuite.scala:1555)
    at org.scalatest.FunSuiteLike$class.invokeWithFixture$1(FunSuiteLike.scala:163)
    at org.scalatest.FunSuiteLike$$anonfun$runTest$1.apply(FunSuiteLike.scala:175)
    at org.scalatest.FunSuiteLike$$anonfun$runTest$1.apply(FunSuiteLike.scala:175)
    at org.scalatest.SuperEngine.runTestImpl(Engine.scala:306)
    at org.scalatest.FunSuiteLike$class.runTest(FunSuiteLike.scala:175)
    at org.scalatest.FunSuite.runTest(FunSuite.scala:1555)
    at org.scalatest.FunSuiteLike$$anonfun$runTests$1.apply(FunSuiteLike.scala:208)
    at org.scalatest.FunSuiteLike$$anonfun$runTests$1.apply(FunSuiteLike.scala:208)
    at org.scalatest.SuperEngine$$anonfun$traverseSubNodes$1$1.apply(Engine.scala:413)
    at org.scalatest.SuperEngine$$anonfun$traverseSubNodes$1$1.apply(Engine.scala:401)
    at scala.collection.immutable.List.foreach(List.scala:381)
    at org.scalatest.SuperEngine.traverseSubNodes$1(Engine.scala:401)
    at org.scalatest.SuperEngine.org$scalatest$SuperEngine$$runTestsInBranch(Engine.scala:396)
    at org.scalatest.SuperEngine.runTestsImpl(Engine.scala:483)
    at org.scalatest.FunSuiteLike$class.runTests(FunSuiteLike.scala:208)
    at org.scalatest.FunSuite.runTests(FunSuite.scala:1555)
    at org.scalatest.Suite$class.run(Suite.scala:1424)
    at org.scalatest.FunSuite.org$scalatest$FunSuiteLike$$super$run(FunSuite.scala:1555)
    at org.scalatest.FunSuiteLike$$anonfun$run$1.apply(FunSuiteLike.scala:212)
    at org.scalatest.FunSuiteLike$$anonfun$run$1.apply(FunSuiteLike.scala:212)
    at org.scalatest.SuperEngine.runImpl(Engine.scala:545)
    at org.scalatest.FunSuiteLike$class.run(FunSuiteLike.scala:212)
    at org.scalatest.FunSuite.run(FunSuite.scala:1555)
    at org.scalatest.tools.SuiteRunner.run(SuiteRunner.scala:55)
    at org.scalatest.tools.Runner$$anonfun$doRunRunRunDaDoRunRun$3.apply(Runner.scala:2563)
    at org.scalatest.tools.Runner$$anonfun$doRunRunRunDaDoRunRun$3.apply(Runner.scala:2557)
    at scala.collection.immutable.List.foreach(List.scala:381)
    at org.scalatest.tools.Runner$.doRunRunRunDaDoRunRun(Runner.scala:2557)
    at org.scalatest.tools.Runner$$anonfun$runOptionallyWithPassFailReporter$2.apply(Runner.scala:1044)
    at org.scalatest.tools.Runner$$anonfun$runOptionallyWithPassFailReporter$2.apply(Runner.scala:1043)
    at org.scalatest.tools.Runner$.withClassLoaderAndDispatchReporter(Runner.scala:2722)
    at org.scalatest.tools.Runner$.runOptionallyWithPassFailReporter(Runner.scala:1043)
    at org.scalatest.tools.Runner$.run(Runner.scala:883)
    at org.scalatest.tools.Runner.run(Runner.scala)
    at org.jetbrains.plugins.scala.testingSupport.scalaTest.ScalaTestRunner.runScalaTest2(ScalaTestRunner.java:138)
    at org.jetbrains.plugins.scala.testingSupport.scalaTest.ScalaTestRunner.main(ScalaTestRunner.java:28)
    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:140)

有趣的是,如果我做出以下任何更改,它将按预期运行:

• 替换flatMap为map
• 替换"A"为""

Answer 1

问题是来自Stream/的延迟评估Iterable.如果你将它分解成更小的部分,这很容易看出来.

scala> Stream("A", "").toIterable.map(_.head)
res7: Iterable[Char] = Stream(A, ?)

映射Stream到_.head意味着我们要处理的字符串作为收藏品,而空字符串作为一个集合没有head,所以它会抛出异常.但是它并没有立即抛出异常,因为你Iterable仍然Stream可以像上面那样懒散.只评估第一个元素.如果我们使它非惰性,它会抛出异常:

scala> Stream("A", "").toIterable.map(_.head)
java.util.NoSuchElementException: next on empty iterator

或者,如果你替换了"A"with "",你也会得到一个异常,因为它的头部Stream被计算并映射到head,而没有.

scala> Stream("", "").toIterable.map(_.head).toList
java.util.NoSuchElementException: next on empty iterator

将在您的代码捕获唯一的例外是当head的Stream还是Iterable把它扔了.好的,那么为什么不map抛出异常,但是flatMap呢？嗯,这map实际上并没有对Stream:

scala> List(1).map { _ => Stream("A", "").toIterable.map(_.head) }
res11: List[Iterable[Char]] = List(Stream(A, ?))

因此,未被评估的成员保持不变.但是flatMap会尝试将Iterables 压缩成一个集合,因此必须对它们进行评估以制作新的集合.在这样做时,抛出异常:

scala> List(1).flatMap { _ => Stream("A", "").toIterable.map(_.head) }
java.util.NoSuchElementException: next on empty iterator