Scala try-catch 没有捕获异常

Question

据我了解，以下代码 sn-p 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，它会崩溃并出现以下错误：

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，如果我进行以下任何更改，它将按预期运行：

• 将flatMap 替换为map
• 将"A" 替换为""

Answer 1

问题是来自Stream/Iterable 的评估延迟。如果你把它分解成更小的部分，这很容易看出。

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

将Stream映射到_.head意味着我们要把字符串当作集合，而空字符串作为集合没有head，所以会抛出异常。但它不会立即抛出异常，因为Iterable 仍然像Stream 一样懒惰，如上所示。仅评估第一个元素。如果我们让它非懒惰，它会抛出异常：

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

或者，如果将"A" 替换为""，也会出现异常，因为Stream 的头部被求值并映射到head，而head 没有。

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

当head 的Stream 或Iterable 抛出它时，您的代码中将捕获的唯一异常。好的，那么为什么 map 不抛出异常，而 flatMap 呢？好吧，这个map 实际上并没有对Stream 做任何事情：

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

所以它保持不变，有未评估的成员。但是flatMap 会尝试将Iterables 扁平化为一个集合，因此必须对它们进行评估以创建一个新集合。这样做会引发异常：

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