Exception Handling – Where to Handle Fatal Exceptions

exception handlingexceptions

Suppose I have a controller that loads a file and hands it over to the processing.

Should I handle the exception in the file loader and return Null if something is wrong, or should I throw the exception and handle it in the controller?
Without the file the rest of the program can´t work. Where should I handle a exception that shuts down the program properly?

I want to shut down an Android application properly.

Best Answer

In layman's words:

You should not return null.
The exception should be handled in the top-most layer.
The program should not terminate, but give feedback to the user, unless it is a Unix style command line program where it's ok to terminate after showing an error message.
By definition no exception is fatal and each one can be handled.
Errors are fatal and cannot be recovered from.
Exceptions and errors are not the same.

Related Solutions

Python – When and How to Use Exceptions

The recommendation in Python is to use exceptions to indicate failure. This is true even if you expect failure on a regular basis.

Look at it from the perspective of the caller of your code:

my_status = get_abe_status(my_url)

What if we return None? If the caller doesn't specifically handle the case that get_abe_status failed, it will simply try to continue on with my_stats being None. That may produce a difficult to diagnose bug later on. Even if you do check for None, this code has no clue why get_abe_status() failed.

But what if we raise an exception? If the caller doesn't specifically handle the case, the exception will propagate upward eventually hitting the default exception handler. That may not be the what you want, but its better then introducing a subtle bug elsewhere in the program. Additionally, the exception gives information about what went wrong which is lost in the first version.

From the caller's perspective, its simply more convenient to get an exception than a return value. And that's the python style, to use exceptions to indicate failure conditions not return values.

Some will take a different perspective and argue that you should only use exceptions for cases you never really expect to happen. They argue that normally running running could should not raise any exceptions. One reason that is given for this is that exceptions are grossly inefficient, but that's not actually true for Python.

A couple of points on your code:

try:
    hits[0]
except IndexError:
    raise NotFoundError("No mentions found.")

That's a really confusing way to check for an empty list. Don't induce an exception just to check something. Use an if.

# say we expect four hits...
if len(hits) != 4:
    raise Warning("An unexpected number of hits.")
    logger.warning("An unexpected number of hits.")

You do realize that the logger.warning line will never run right?

Java – Handling DomainExceptions using REST endpoint

I prefer to do something like this:

The rest end point will receive an request with data then,
Business service class is called with the data
Validate the data if invalid throw an exception with proper description, e.g.: throw new InvalidArgumentException("invalid objet Id");
On controller if above exception is thrown then return 400 Bad Request with error description e.g.: {"ok":false, "error":"invalid_argument","msg":"invalid objet Id"}

By doing this way the business class can be used anywhere and response can be handled accordingly.

Best Answer

Related Solutions

Python – When and How to Use Exceptions

Java – Handling DomainExceptions using REST endpoint

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