Exception
Classes representing the components of exception handling.
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.(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 Catchs 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"))Catch behavior composition
Build Catch objects from exception lists and catch logic
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 returned Catch object has no specific logic defined and will simply
rethrow the exceptions it catches, you will typically want to call opt,
either or withTry on the return value, or assign custom logic by calling "withApply".
Note that Catch objects automatically rethrow ControlExceptions and others
which should only be caught in exceptional circumstances. If you really want
to catch exactly what you specify, use catchingPromiscuously instead.
Creates a Catch object which catches and ignores any of the supplied exceptions.
Creates a Catch object which catches and ignores any of the supplied exceptions.
Creates a Catch object which maps all the supplied exceptions to None.
Creates a Catch object which maps all the supplied exceptions to None.
Creates a Catch object which maps all the supplied exceptions to the given value.
Creates a Catch object which maps all the supplied exceptions to the given value.
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
Returns a partially constructed Catch object, which you must give
an exception handler function as an argument to by.
Returns a partially constructed Catch object, which you must give
an exception handler function as an argument to by.
- Example
handling(classOf[MalformedURLException], classOf[NullPointerException]) by (_.printStackTrace)
Promiscuous Catch behaviors
Useful if catching ControlThrowable or InterruptedException is required.
Logic Containers
Containers for catch and finally behavior.
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.
- Type Params
- T
result type of bodies used in try and catch blocks
- Value Params
- fin
Finally logic which if defined will be invoked after catch logic
- pf
Partial function used when applying catch logic to determine result value
- rethrow
Predicate on throwables determining when to rethrow a caught Throwable