Exception::Class - A module that allows you to declare real exception classes in Perl
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',
},
);
# try
eval { MyException->throw( error => 'I feel funny.' ) };
my $e;
# catch
if ( $e = Exception::Class->caught('MyException') )
{
warn $e->error, "\n", $e->trace->as_string, "\n";
warn join ' ', $e->euid, $e->egid, $e->uid, $e->gid, $e->pid, $e->time;
exit;
}
elsif ( $e = Exception::Class->caught('ExceptionWithFields') )
{
$e->quixotic ? do_something_wacky() : do_something_sane();
}
else
{
$e = Exception::Class->caught();
ref $e ? $e->rethrow : die $e;
}
# use an alias - without parens subroutine name is checked at
# compile time
throw_fields error => "No strawberry", grandiosity => "quite a bit";
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.
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:
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.
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.
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.
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.
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);
}
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.
Each Exception::Class::Base subclass can be set individually to
include a a stracktrace when the as_string method is called. The
default is to not include a stacktrace. Calling this method with a
value changes this behavior. It always returns the current value
(after any change is applied).
This value is inherited by any subclasses. However, if this value is
set for a subclass, it will thereafter be independent of the value in
Exception::Class::Base.
This is a class method, not an object method.
When a Devel::StackTrace object is created, it walks through the
stack and stores the arguments which were passed to each subroutine on
the stack. If any of these arguments are references, then that means
that the Devel::StackTrace ends up increasing the refcount of these
references, delaying their destruction.
Since Exception::Class::Base uses Devel::StackTrace internally,
this method provides a way to tell Devel::StackTrace not to store
these references. Instead, Devel::StackTrace replaces references
with their stringified representation.
This method defaults to true. As with Trace(), it is inherited by
subclasses but setting it in a subclass makes it independent
thereafter.
When a Devel::StackTrace object stringifies, by default it ignores
stringification overloading on any objects being dealt with.
Since Exception::Class::Base uses Devel::StackTrace internally,
this method provides a way to tell Devel::StackTrace to respect
overloading.
This method defaults to false. As with Trace(), it is inherited by
subclasses but setting it in a subclass makes it independent
thereafter.
When a Devel::StackTrace object stringifies, by default it displays
the full argument for each function. This parameter can be used to
limit the maximum length of each argument.
Since Exception::Class::Base uses Devel::StackTrace internally,
this method provides a way to tell Devel::StackTrace to limit the
length of arguments.
This method defaults to 0. As with Trace(), it is inherited by
subclasses but setting it in a subclass makes it independent
thereafter.
This method creates a new object with the given error message. If no
error message is given, $! is used. It then die's with this object
as its argument.
This method also takes a show_trace parameter which indicates
whether or not the particular exception object being created should
show a stacktrace when its as_string() method is called. This
overrides the value of Trace() for this class if it is given.
The frames included in the trace can be controlled by the ignore_class
and ignore_package parameters. These are passed directly to
Devel::Stacktrace's constructor. See Devel::Stacktrace for more details.
If only a single value is given to the constructor it is assumed to be the message parameter.
Additional keys corresponding to the fields defined for the particular exception subclass will also be accepted.
This method takes the same parameters as throw(), but instead of
dying simply returns a new exception object.
This method is always called when constructing a new exception object
via the throw() method.
Exception::Class::Base
subclass. The Exception::Class::Base class's description is
"Generic exception" (this may change in the future). This is also an
object method.
Simply dies with the object as its sole argument. It's just syntactic
sugar. This does not change any of the object's attribute values.
However, it will cause caller() to report the die as coming from
within the Exception::Class::Base class rather than where rethrow
was called.
Of course, you always have access to the original stacktrace for the exception object.
Carp::confess().
as_string() method to get the message. By default,
this is the same as calling the message() method, but may be
overridden by a subclass. See below for details.
The Exception::Class::Base object is overloaded so that
stringification produces a normal error message. It just calls the
as_string method described above. This means that you can just
print $@ after an eval and not worry about whether or not its an
actual object. It also means an application or module could do this:
$SIG{__DIE__} = sub { Exception::Class::Base->throw( error => join '', @_ ); };
and this would probably not break anything (unless someone was
expecting a different type of exception object from die()).
By default, the as_string() method simply returns the value
message or error param plus a stack trace, if the class's
Trace() method returns a true value or show_trace was set when
creating the exception.
However, once you add new fields to a subclass, you may want to include those fields in the stringified error.
Inside the as_string() method, the message (non-stack trace)
portion of the error is generated by calling the full_message()
method. This can be easily overridden. For example:
sub full_message
{
my $self = shift;
my $msg = $self->message;
$msg .= " and foo was " . $self->foo;
return $msg;
}
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.
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.
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.
If you are interested in adding try/catch/finally syntactic sugar to
your code then I recommend you check out U. Arun Kumar's Error.pm
module, which implements this syntax. It also includes its own base
exception class, Error::Simple.
If you would prefer to use the Exception::Class::Base class
included with this module, you'll have to add this to your code
somewhere:
push @Exception::Class::Base::ISA, 'Error'
unless Exception::Class::Base->isa('Error');
It's a hack but apparently it works.
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.
Dave Rolsky, <autarch@urth.org>
Copyright (c) 2000-2006 David Rolsky. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
The full text of the license can be found in the LICENSE file included with this module.