Handle an exception that occurs inside an aspect-proxied component call.

← Design Patterns and Idioms · Ref: Q1117

Exceptions propagate through aspects unchanged:

  calc as Calculator!
  try
    result <- calc.divide(0)
    stdout.println(`100 / 0 = ${result}`)
  catch
    -> ex as Exception
    stdout.println(`caught: ${ex.reason()}`)

A silent aspect does not catch or modify exceptions -- they pass straight through. In the example the component throws Exception("zero divisor") for a divisor of 0 and the caller's try/catch handles it exactly as if no aspect were present.

See Q1115 for basic aspect. See Q1116 for aspect with constructor.

Example

#!ek9
defines module qa.designpatterns.aspectexception

  defines component

    Calculator as abstract
      divide() as abstract
        -> divisor as Integer
        <- result as Integer?

    CheckedCalculator is Calculator
      override divide()
        -> divisor as Integer
        <- result as Integer?
        if divisor == 0
          throw Exception("zero divisor")
        result: 100 / divisor

  defines class

    AuditAspect extends Aspect
      default operator ?

  defines application

    ExceptionApp
      register CheckedCalculator() as Calculator with aspect of AuditAspect()

  defines program

    AspectExceptionDemo() with application of ExceptionApp
      stdout <- Stdout()
      calc as Calculator!

      //Normal call through aspect
      result <- calc.divide(10)
      stdout.println(`100 / 10 = ${result}`)

      //An exception thrown by the component passes through the aspect to this try/catch
      try
        failed <- calc.divide(0)
        stdout.println(`100 / 0 = ${failed}`)
      catch
        -> ex as Exception
        stdout.println(`caught: ${ex.reason()}`)

Other ways to ask this

I need to understand what happens when a proxied method throws through an aspect
In Spring AOP exceptions propagate through advice. Show the EK9 aspect exception path
Given a component that throws, verify the exception passes through the aspect to the caller
Test exception propagation through an aspect proxy

Coming from another language?

Spring AOP: exceptions propagate through @Around advice unless caught. AspectJ: same propagation. EK9: same — exceptions pass through the aspect proxy transparently.

Keywords: aspect, error, exception, propagate, proxy, try catch