Exception::Class(3) User Contributed Perl Documentation Exception::Class(3)NAMEException::Class - A module that allows you to declare real exception
classes in Perl
VERSION
version 1.37
SYNOPSIS
use Exception::Class (
'MyException',
'AnotherException' => { isa => 'MyException' },
'YetAnotherException' => {
isa => 'AnotherException',
description => 'These exceptions are related to IPC'
},
'ExceptionWithFields' => {
isa => 'YetAnotherException',
fields => [ 'grandiosity', 'quixotic' ],
alias => 'throw_fields',
},
);
use Scalar::Util qw( blessed );
use Try::Tiny;
try {
MyException->throw( error => 'I feel funny.' );
}
catch {
die $_ unless blessed $_ && $_->can('rethrow');
if ( $_->isa('Exception::Class') ) {
warn $_->error, "\n", $_->trace->as_string, "\n";
warn join ' ', $_->euid, $_->egid, $_->uid, $_->gid, $_->pid, $_->time;
exit;
}
elsif ( $_->isa('ExceptionWithFields') ) {
if ( $_->quixotic ) {
handle_quixotic_exception();
}
else {
handle_non_quixotic_exception();
}
}
else {
$_->rethrow;
}
};
# without Try::Tiny
eval { ... };
if ( my $e = Exception::Class->caught() ) { ... }
# use an alias - without parens subroutine name is checked at
# compile time
throw_fields error => "No strawberry", grandiosity => "quite a bit";
DESCRIPTION
RECOMMENDATION 1: If you are writing modern Perl code with Moose or Moo
I highly recommend using Throwable instead of this module.
RECOMMENDATION 2: Whether or not you use Throwable, you should use
Try::Tiny.
Exception::Class allows you to declare exception hierarchies in your
modules in a "Java-esque" manner.
It features a simple interface allowing programmers to 'declare'
exception classes at compile time. It also has a base exception class,
Exception::Class::Base, that can be easily extended.
It is designed to make structured exception handling simpler and better
by encouraging people to use hierarchies of exceptions in their
applications, as opposed to a single catch-all exception class.
This module does not implement any try/catch syntax. Please see the
"OTHER EXCEPTION MODULES (try/catch syntax)" section for more
information on how to get this syntax.
You will also want to look at the documentation for
Exception::Class::Base, which is the default base class for all
exception objects created by this module.
DECLARING EXCEPTION CLASSES
Importing "Exception::Class" allows you to automagically create
Exception::Class::Base subclasses. You can also create subclasses via
the traditional means of defining your own subclass with @ISA. These
two methods may be easily combined, so that you could subclass an
exception class defined via the automagic import, if you desired this.
The syntax for the magic declarations is as follows:
'MANDATORY CLASS NAME' => \%optional_hashref
The hashref may contain the following options:
· isa
This is the class's parent class. If this isn't provided then the
class name in $Exception::Class::BASE_EXC_CLASS is assumed to be
the parent (see below).
This parameter lets you create arbitrarily deep class hierarchies.
This can be any other Exception::Class::Base subclass in your
declaration or a subclass loaded from a module.
To change the default exception class you will need to change the
value of $Exception::Class::BASE_EXC_CLASS before calling
"import()". To do this simply do something like this:
BEGIN { $Exception::Class::BASE_EXC_CLASS = 'SomeExceptionClass'; }
If anyone can come up with a more elegant way to do this please let
me know.
CAVEAT: If you want to automagically subclass an
Exception::Class::Base subclass loaded from a file, then you must
compile the class (via use or require or some other magic) before
you import "Exception::Class" or you'll get a compile time error.
· fields
This allows you to define additional attributes for your exception
class. Any field you define can be passed to the "throw()" or
"new()" methods as additional parameters for the constructor. In
addition, your exception object will have an accessor method for
the fields you define.
This parameter can be either a scalar (for a single field) or an
array reference if you need to define multiple fields.
Fields will be inherited by subclasses.
· alias
Specifying an alias causes this class to create a subroutine of the
specified name in the caller's namespace. Calling this subroutine
is equivalent to calling "<class>->throw(@_)" for the given
exception class.
Besides convenience, using aliases also allows for additional
compile time checking. If the alias is called without parentheses,
as in "throw_fields "an error occurred"", then Perl checks for the
existence of the "throw_fields()" subroutine at compile time. If
instead you do "ExceptionWithFields->throw(...)", then Perl checks
the class name at runtime, meaning that typos may sneak through.
· description
Each exception class has a description method that returns a fixed
string. This should describe the exception class (as opposed to
any particular exception object). This may be useful for debugging
if you start catching exceptions you weren't expecting
(particularly if someone forgot to document them) and you don't
understand the error messages.
The "Exception::Class" magic attempts to detect circular class
hierarchies and will die if it finds one. It also detects missing
links in a chain, for example if you declare Bar to be a subclass of
Foo and never declare Foo.
Try::Tiny
If you are interested in adding try/catch/finally syntactic sugar to
your code then I recommend you check out Try::Tiny. This is a great
module that helps you ignore some of the weirdness with "eval" and $@.
Here's an example of how the two modules work together:
use Exception::Class ( 'My::Exception' );
use Scalar::Util qw( blessed );
use Try::Tiny;
try {
might_throw();
}
catch {
if ( blessed $_ && $_->isa('My::Exception') ) {
handle_it();
}
else {
die $_;
}
};
Note that you cannot use "Exception::Class->caught()" with Try::Tiny.
Catching Exceptions Without Try::Tiny
"Exception::Class" provides some syntactic sugar for catching
exceptions in a safe manner:
eval {...};
if ( my $e = Exception::Class->caught('My::Error') ) {
cleanup();
do_something_with_exception($e);
}
The "caught()" method takes a class name and returns an exception
object if the last thrown exception is of the given class, or a
subclass of that class. If it is not given any arguments, it simply
returns $@.
You should always make a copy of the exception object, rather than
using $@ directly. This is necessary because if your "cleanup()"
function uses "eval", or calls something which uses it, then $@ is
overwritten. Copying the exception preserves it for the call to
"do_something_with_exception()".
Exception objects also provide a caught method so you can write:
if ( my $e = My::Error->caught() ) {
cleanup();
do_something_with_exception($e);
}
Uncatchable Exceptions
Internally, the "caught()" method will call "isa()" on the exception
object. You could make an exception "uncatchable" by overriding
"isa()" in that class like this:
package Exception::Uncatchable;
sub isa { shift->rethrow }
Of course, this only works if you always call
"Exception::Class->caught()" after an "eval".
USAGE RECOMMENDATION
If you're creating a complex system that throws lots of different types
of exceptions, consider putting all the exception declarations in one
place. For an app called Foo you might make a "Foo::Exceptions" module
and use that in all your code. This module could just contain the code
to make "Exception::Class" do its automagic class creation. Doing this
allows you to more easily see what exceptions you have, and makes it
easier to keep track of them.
This might look something like this:
package Foo::Bar::Exceptions;
use Exception::Class (
Foo::Bar::Exception::Senses =>
{ description => 'sense-related exception' },
Foo::Bar::Exception::Smell => {
isa => 'Foo::Bar::Exception::Senses',
fields => 'odor',
description => 'stinky!'
},
Foo::Bar::Exception::Taste => {
isa => 'Foo::Bar::Exception::Senses',
fields => [ 'taste', 'bitterness' ],
description => 'like, gag me with a spoon!'
},
...
);
You may want to create a real module to subclass Exception::Class::Base
as well, particularly if you want your exceptions to have more methods.
Subclassing Exception::Class::Base
As part of your usage of "Exception::Class", you may want to create
your own base exception class which subclasses Exception::Class::Base.
You should feel free to subclass any of the methods documented above.
For example, you may want to subclass "new()" to add additional
information to your exception objects.
Exception::Class FUNCTIONS
The "Exception::Class" method offers one function, "Classes()", which
is not exported. This method returns a list of the classes that have
been created by calling the "Exception::Class" import() method. Note
that this is all the subclasses that have been created, so it may
include subclasses created by things like CPAN modules, etc. Also note
that if you simply define a subclass via the normal Perl method of
setting @ISA or "use base", then your subclass will not be included.
SUPPORT
Please submit bugs to the CPAN RT system at
http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Exception%3A%3AClass or
via email at bug-exception-class@rt.cpan.org.
DONATIONS
If you'd like to thank me for the work I've done on this module, please
consider making a "donation" to me via PayPal. I spend a lot of free
time creating free software, and would appreciate any support you'd
care to offer.
Please note that I am not suggesting that you must do this in order for
me to continue working on this particular software. I will continue to
do so, inasmuch as I have in the past, for as long as it interests me.
Similarly, a donation made in this way will probably not make me work
on this software much more, unless I get so many donations that I can
consider working on free software full time, which seems unlikely at
best.
To donate, log into PayPal and send money to autarch@urth.org or use
the button on this page:
<http://www.urth.org/~autarch/fs-donation.html>
AUTHOR
Dave Rolsky <autarch@urth.org>
COPYRIGHT AND LICENSE
This software is copyright (c) 2013 by Dave Rolsky.
This is free software; you can redistribute it and/or modify it under
the same terms as the Perl 5 programming language system itself.
perl v5.18.2 2013-02-24 Exception::Class(3)
