Re: Some same exceptions used in a given file

Lew <>
Sat, 23 Apr 2011 20:19:27 -0400
Merciadri Luca wrote:

Thanks for the example. And now if another method of the same class
throws also an IOException, but whose action needs not to be the same?

For example, how would I convert

public void foo
  // ...

  catch (IOException problem1)



public void bar
  // ...

  catch (IOException problem2)




I might do

public void foo throws IOException
  // ...

public void bar throws IOException
  // ...

and then catch the exception in the caller, but how do I differentiate
from problem1 to problem2? Sorry for this silly question, but it seems
not that habitual regarding Google answers on this.

There are a few approaches.

Right now you're starting from the leaf level of how to implement try-catch
and trying to glean an overall strategy. A better approach is the exact
opposite: Decide on your overall *application-domain* strategy first.

As you code in various layers of your application, think of a type as part of
the internal API for the application - a one-shot library. Good O-O
principles dictate that a compoment have a well-defined purpose and external
contract for interaction with other components. None of those strategic
conversations involve exceptions as such - just conditions and desired
responses and state transitions involving those conditions.

At the nitty-gritty detail level of implementation, OTOH, exceptions signal
out-of-band issues - things that stop the action. But their effect is local -
not strategic, but specific to a particular action in service of that
strategic effect. Since the strategic conversation is exception-agnostic, the
component must convert that exceptional condition into the appropriate
response that accords with its contract for interaction.

That contract varies with the layer involved. If the layer is a deeper one,
say a data-access layer (DAL), it functions exactly like an API. Again,
define its contract from the outside - what should API clients experience?
Sometimes that means that a particular API method has an 'Exception' subtype
as part of its contract. But that is dictated by the strategic purpose of the
API, not by knee-jerk, thoughtless reactions to lower-level events.

That upward view often comprises an application-specific exception, one that
tells the application "something lower broke", and then uses the 'cause'
attribute to say what.

How fine-grained does the top level view have to be, for Pete's sake? Why be
so obsessive-compulsive?

Usually the 'cause' chain and stack trace contains everything needed for
debugging, and nothing needed for the strategic upline viewpoint. Usually.

So here's how you convert low-level exceptions to higher-level ones, if (and
only if) that's what your strategy requires:

  public void actByContract( Foo stimulus ) throws LewsException
    BufferedReader reader;
      reader = new BufferedReader( new FileReader( stimulus.getFile() ));
    catch ( IOException exc )
      final String msg = "actByContract() broke";
      logger.error( msg, exc );
      LewsException lewx = new LewsException( msg, exc );
      throw lewx;

Key points:

  - Log and act upon exceptions at the lowest point they occur, if they aren't
one of your own wrapper exceptions like 'LewsException'.

  - Don't let exceptions walk all the way out of an application or lower-level
exceptions out of a component - convert them to some strategically valid,
contract-authorized interactions with outsiders.

  - Do determine the strategically and contractually valid interactions for
the application and every component, first, before implementation.

  - Do allow only strategically and contractually valid interactions to occur
and none other.

Honi soit qui mal y pense.

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