object Exception
Classes representing the components of exception handling.
Each class is independently composable.
This class differs from scala.util.Try in that it focuses on composing exception handlers rather than
composing behavior. All behavior should be composed first and fed to a Catch object using one of the
opt
, either
or withTry
methods. Taken together the classes provide a DSL for composing catch and finally
behaviors.
Examples
Create a Catch
which handles specified exceptions.
import scala.util.control.Exception._ import java.net._ val s = "https://www.scala-lang.org/" // Some(https://www.scala-lang.org/) val x1: Option[URL] = catching(classOf[MalformedURLException]) opt new URL(s) // Right(https://www.scala-lang.org/) val x2: Either[Throwable,URL] = catching(classOf[MalformedURLException], classOf[NullPointerException]) either new URL(s) // Success(https://www.scala-lang.org/) val x3: Try[URL] = catching(classOf[MalformedURLException], classOf[NullPointerException]) withTry new URL(s) val defaultUrl = new URL("http://example.com") // URL(http://example.com) because htt/xx throws MalformedURLException val x4: URL = failAsValue(classOf[MalformedURLException])(defaultUrl)(new URL("htt/xx"))
Create a Catch
which logs exceptions using handling
and by
.
def log(t: Throwable): Unit = t.printStackTrace val withThrowableLogging: Catch[Unit] = handling(classOf[MalformedURLException]) by (log) def printUrl(url: String) : Unit = { val con = new URL(url) openConnection() val source = scala.io.Source.fromInputStream(con.getInputStream()) source.getLines().foreach(println) } val badUrl = "htt/xx" // Prints stacktrace, // java.net.MalformedURLException: no protocol: htt/xx // at java.net.URL.<init>(URL.java:586) withThrowableLogging { printUrl(badUrl) } val goodUrl = "https://www.scala-lang.org/" // Prints page content, // <!DOCTYPE html> // <html> withThrowableLogging { printUrl(goodUrl) }
Use unwrapping
to create a Catch
that unwraps exceptions before rethrowing.
class AppException(cause: Throwable) extends RuntimeException(cause) val unwrappingCatch: Catch[Nothing] = unwrapping(classOf[AppException]) def calcResult: Int = throw new AppException(new NullPointerException) // Throws NPE not AppException, // java.lang.NullPointerException // at .calcResult(<console>:17) val result = unwrappingCatch(calcResult)
Use failAsValue
to provide a default when a specified exception is caught.
val inputDefaulting: Catch[Int] = failAsValue(classOf[NumberFormatException])(0) val candidatePick = "seven" // scala.io.StdIn.readLine() // Int = 0 val pick = inputDefaulting(candidatePick.toInt)
Compose multiple Catch
s with or
to build a Catch
that provides default values varied by exception.
val formatDefaulting: Catch[Int] = failAsValue(classOf[NumberFormatException])(0) val nullDefaulting: Catch[Int] = failAsValue(classOf[NullPointerException])(-1) val otherDefaulting: Catch[Int] = nonFatalCatch withApply(_ => -100) val combinedDefaulting: Catch[Int] = formatDefaulting or nullDefaulting or otherDefaulting def p(s: String): Int = s.length * s.toInt // Int = 0 combinedDefaulting(p("tenty-nine")) // Int = -1 combinedDefaulting(p(null: String)) // Int = -100 combinedDefaulting(throw new IllegalStateException) // Int = 22 combinedDefaulting(p("11"))
- Source
- Exception.scala
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Catch behavior composition
Build Catch objects from exception lists and catch logic
- def catching[T](exceptions: Class[_]*): Catch[T]
Creates a
Catch
object which will catch any of the supplied exceptions.Creates a
Catch
object which will catch any of the supplied exceptions. Since the returnedCatch
object has no specific logic defined and will simply rethrow the exceptions it catches, you will typically want to callopt
,either
orwithTry
on the return value, or assign custom logic by calling "withApply".Note that
Catch
objects automatically rethrowControlExceptions
and others which should only be caught in exceptional circumstances. If you really want to catch exactly what you specify, usecatchingPromiscuously
instead. - def failAsValue[T](exceptions: Class[_]*)(value: => T): Catch[T]
Creates a
Catch
object which maps all the supplied exceptions to the given value. - def failing[T](exceptions: Class[_]*): Catch[Option[T]]
Creates a
Catch
object which maps all the supplied exceptions toNone
. - def ignoring(exceptions: Class[_]*): Catch[Unit]
Creates a
Catch
object which catches and ignores any of the supplied exceptions. - def unwrapping[T](exceptions: Class[_]*): Catch[T]
Creates a
Catch
object which unwraps any of the supplied exceptions.
Finally behavior composition
Build Catch objects from finally logic
General purpose catch objects
Catch objects with predefined behavior. Use combinator methods to compose additional behavior.
DSL behavior composition
Expressive Catch behavior composition
- def handling[T](exceptions: Class[_]*): By[(Throwable) => T, Catch[T]]
Returns a partially constructed
Catch
object, which you must give an exception handler function as an argument toby
.Returns a partially constructed
Catch
object, which you must give an exception handler function as an argument toby
.handling(classOf[MalformedURLException], classOf[NullPointerException]) by (_.printStackTrace)
Example:
Promiscuous Catch behaviors
Useful if catching ControlThrowable
or InterruptedException
is required.
- def catchingPromiscuously[T](exceptions: Class[_]*): Catch[T]
Creates a
Catch
object which will catch any of the supplied exceptions.Creates a
Catch
object which will catch any of the supplied exceptions. Unlike "catching" which filters out those in shouldRethrow, this one will catch whatever you ask of it includingControlThrowable
orInterruptedException
.
Logic Containers
Containers for catch and finally behavior.
- class Catch[+T] extends Described
A container class for catch/finally logic.
A container class for catch/finally logic.
Pass a different value for rethrow if you want to probably unwisely allow catching control exceptions and other throwables which the rest of the world may expect to get through.
- T
result type of bodies used in try and catch blocks
- class Finally extends Described
A container class for finally code.
Ungrouped
- final def allCatcher[T]: Catcher[T]
- def catching[T](c: Catcher[T]): Catch[T]
- def catchingPromiscuously[T](c: Catcher[T]): Catch[T]
- def mkCatcher[Ex <: Throwable, T](isDef: (Ex) => Boolean, f: (Ex) => T)(implicit arg0: ClassTag[Ex]): PartialFunction[Throwable, T]
- def mkThrowableCatcher[T](isDef: (Throwable) => Boolean, f: (Throwable) => T): PartialFunction[Throwable, T]
- final def nonFatalCatcher[T]: Catcher[T]
- final val nothingCatcher: Catcher[Nothing]
- def shouldRethrow(x: Throwable): Boolean
!!! Not at all sure of every factor which goes into this, and/or whether we need multiple standard variations.
!!! Not at all sure of every factor which goes into this, and/or whether we need multiple standard variations.
- returns
true if
x
isControlThrowable
orInterruptedException
otherwise false.
- implicit def throwableSubtypeToCatcher[Ex <: Throwable, T](pf: PartialFunction[Ex, T])(implicit arg0: ClassTag[Ex]): Catcher[T]
This is the documentation for the Scala standard library.
Package structure
The scala package contains core types like
Int
,Float
,Array
orOption
which are accessible in all Scala compilation units without explicit qualification or imports.Notable packages include:
scala.collection
and its sub-packages contain Scala's collections frameworkscala.collection.immutable
- Immutable, sequential data-structures such asVector
,List
,Range
,HashMap
orHashSet
scala.collection.mutable
- Mutable, sequential data-structures such asArrayBuffer
,StringBuilder
,HashMap
orHashSet
scala.collection.concurrent
- Mutable, concurrent data-structures such asTrieMap
scala.concurrent
- Primitives for concurrent programming such asFutures
andPromises
scala.io
- Input and output operationsscala.math
- Basic math functions and additional numeric types likeBigInt
andBigDecimal
scala.sys
- Interaction with other processes and the operating systemscala.util.matching
- Regular expressionsOther packages exist. See the complete list on the right.
Additional parts of the standard library are shipped as separate libraries. These include:
scala.reflect
- Scala's reflection API (scala-reflect.jar)scala.xml
- XML parsing, manipulation, and serialization (scala-xml.jar)scala.collection.parallel
- Parallel collections (scala-parallel-collections.jar)scala.util.parsing
- Parser combinators (scala-parser-combinators.jar)scala.swing
- A convenient wrapper around Java's GUI framework called Swing (scala-swing.jar)Automatic imports
Identifiers in the scala package and the
scala.Predef
object are always in scope by default.Some of these identifiers are type aliases provided as shortcuts to commonly used classes. For example,
List
is an alias forscala.collection.immutable.List
.Other aliases refer to classes provided by the underlying platform. For example, on the JVM,
String
is an alias forjava.lang.String
.