Why Re-throw Exceptions?

Question

I\'ve seen the following code many times:

try
{
    ... // some code
}
catch (Exception ex)
{
    ... // Do something
    throw new CustomException(ex);

            


        

Answer 1

If you look at exceptions as on an alternative way to get a method result, then re-throwing an exception is like wrapping your result into some other object.

And this is a common operation in a non-exceptional world. Usually this happens on a border of two application layers - when a function from layer B calls a function from layer C, it transforms C's result into B's internal form.

A -- calls --> B -- calls --> C

If it doesn't, then at the layer A which calls the layer B there will be a full set of JDK exceptions to handle. :-)

As also the accepted answer points out, layer A might not even be aware of C's exception.

Example

Layer A, servlet: retrieves an image and it's meta information
Layer B, JPEG library: collects available DCIM tags to parse a JPEG file
Layer C, a simple DB: a class reading string records from a random access file. Some bytes are broken, so it throws an exception saying "can't read UTF-8 string for record 'bibliographicCitation'".

So A won't understand the meaning of 'bibliographicCitation'. So B should translate this exception for A into TagsReadingException which wraps the original.