Badger::Exception
use Badger::Exception; # create exception object my $exception = Badger::Exception->new({ type => $type, info => $info, }); # query exception type and info fields $type = $exception->type(); $info = $exception->info(); ($type, $info) = $exception->type_info(); # print string summarising exception print $exception->text(); # use automagic stringification print $exception; # throw exception $exception->throw;
This module defines an object class for representing exceptions. These are simple objects that store various bits of information about an error condition.
The type
denotes what kind of error occurred (e.g.
'file
', 'parser
', 'database
',
etc.). The info
field provides further information about the
error (e.g. 'foo/bar.html not found
', 'parser error at
line 42
', 'server is on fire
', etc). Other optional
fields include file
and line
for specifying the
location of the error.
In most cases you wouldn't generate and/or throw an exception object
directly from your code. A better approach is to define a
throw()
method in a base class which does this for you.
The Badger::Base module is an example of just such a module. You can use this as a base class for your modules to inherit the throw() method.
Here's an example of a module that implements a method which expects an
argument. If if doesn't get the argument it's looking for then it throws
an exception via the inherited throw() method. The exception type is
example
and the additional information is No argument
specified
.
package Your::Module; use base 'Badger::Base'; sub example_method { my $self = shift; my $arg = shift || self->throw( example => 'No argument specified' ); # ...do something with ... }
The error()
provides a higher level of abstraction. You provide the error message
(which becomes the exception info
) and it will generate an
exception type based on the package name of your module.
package Your::Module; use base 'Badger::Base'; sub example_method { my $self = shift; my $arg = shift || self->error('No argument specified' ); # ...do something with ... }
In the example above, an exception will be thrown with a
type
defined as your.module
. The module name is
converted to lower case and the package delimiters are replaced with
dots. There are configuration options that allow you to define other
exceptions types. Consult the Badger::Base documentation for further information.
You can choose any values you like for type
and
info
. The type
is used to identify what kind of
error occurred and should be a short word like "example
", or
a dot-separated sequence of words like
"example.file.missing"
. In the latter case, dotted exception
types are assumed to represent a hierarchy where
example.file.missing
error is a more specialised kind of
example.file
error, which in turn is a more specialised kind
of example
error. The match_type() method takes this into account
when matching exception types.
eval { # some code that throws an exception }; if ($@) { if ($@->match_type('example')) { # caught 'example' or 'example.*' error # ...now do something } else { # re-throw any other exception types $@->throw; } }
The info
field should provide a more detailed error message
in a format suitable for human consumption.
The Badger::Exception
module also has a tracing mode which
will automatically save the caller stack at the point at which the error
is thrown. This allows you to inspect the full code path which led to the
error from the comfort of you exception catching code, rather than having
to deal with it at the point where the error is throw.
# deep in your code somewhere.... in a class derived from Badger::Base $self->throw( database => 'The database is made of cheese', trace => 1, );
The text()
method (which is called whenever the object is
stringified) will then append a stack track to the end of the generated
message.
# high up in your calling code: eval { $object->do_something_gnarly }; if ($err = $@) { print $err; exit; }
You can also call the stack() method to return the stored call stack information, or the stack_trace() method to see a textual summary.
You can enable the tracing behaviour for all exception objects by setting
the $TRACE
package variable.
use Badger::Exception; $Badger::Exception::TRACE = 1;
The trace import hook is provided as a short-cut for this.
use Badger::Exception trace => 1;
This import hook can be used to set the $TRACE
package
variable to enable stack tracing for the Badger::Exception module.
use Badger::Exception trace => 1
When stack tracing is enabled, the exception will store information about the calling stack at the point at which it is thrown. This information will be displayed by the text() method. It is also available in raw form via the stack() method.
Constructor method for creating a new exception.
my $exception = Badger::Exception->new( type => 'database', info => 'could not connect', file => '/path/to/file.pm', line => 420, );
When called without arguments, this method returns the exception type, as
defined by the first argument passed to the new()
constructor method.
my $type = $exception->type();
It can also be called with an argument to set a new type for the exception.
$exception->type('database');
When called without arguments, this method returns the information field for the exception.
my $info = $exception->info();
It can also be called with an argument to define new information for the exception.
$exception->info('could not connect');
Method to get or set the name of the file in which the exception was raised.
$exception->file('path/to/file.pm'); print $exception->file; # /path/to/file.pm
Method to get or set the line number at which the exception was raised.
$exception->line(420); print $exception->line; # 420
This method returns a text representation of the exception object. The
string returned is formatted as $type error - $info
.
print $exception->text(); # database error - could not connect
This method is also bound to the stringification operator, allowing you
to simple print
the exception object to get the same result
as calling text()
explicitly.
print $exception; # database error - could not connect
Method to get or set the flag which determines if the exception captures
a stack backtrace at the point at which it is thrown. It can be called as
an object method to affect an individual exception object, or as a class
method to get or set the $TRACE
package variable which
provides the default value for any exceptions created from then on.
$exception->trace(1); # object method print $exception->trace; # 1 Badger::Exception->trace(1); # class method - sets $TRACE print Badger::Exception->trace; # 1
This method selects and returns a type string from the arguments passed that is the nearest correct match for the current exception type. This is used to select the most appropriate handler for the exception.
my $match = $exception->match_type('file', 'parser', 'database') || die "no match for exception\n";
In this example, the exception will return one of the values
file
, parser
or database
, if and
only if its type is one of those values. Otherwise it will return undef;
Exception types can be organised into a hierarchical structure by
delimiting each part of the type with a period. For example, the
database
exception type might be further divided into the
more specific database.connection
,
database.query
and database.server_on_fire
exception types.
An exception of type database.connection
will match a
handler type of database.connection
or more generally,
database
. The longer (more specific) handler name will
always match in preference to a shorter (more general) handler as shown
in the next example:
$exception->type('database.connection'); my $match = $exception->match_type('database', 'database.connection') || die "no match for exception\n"; print $match; # database.connection
When there is no exact match, the match_type()
method will
return something more general that matches. In the following example,
there is no specific handler type for database.exploded
, but
the more general database
type still matches.
$exception->type('database.exploded'); my $match = $exception->match_type('database', 'database.connection') || die "no match for exception\n"; print $match; # database
You can also specify multiple exception types using a reference to a list.
if ($exception->match_type(['warp.drive', 'shields'])) { ... }
Or using a single string of whitespace delimited exception types.
if ($exception->match_type('warp.drive shields')) { ... }
You can also pass a reference to a hash array in which the keys are exception types. The corresponding value for a matching type will be returned.
my $type_map = { 'warp.drive' => 'propulsion', 'impulse.drive' => 'propulsion', 'shields' => 'defence', 'phasers' => 'defence' }; if ($exception->match_type($type_map)) { ... }
This method throws the exception by calling die()
with the
exception object as an argument. If the $TRACE
flag is set
to a true value then the method will first save the pertinent details
from a stack backtrace into the exception object before throwing it.
If stack tracing is enabled then this method will return a reference to a
list of information from the caller stack at the point at which the
exception was thrown. Each item in the list is a reference to a list
containing the information returned by the inbuilt caller()
method. See perldoc -f caller
for further information.
use Badger::Exception trace => 1; eval { # some code that throws an exception object $exception->throw(); }; my $catch = $@; # exception object my $stack = $catch->stack; foreach my $caller (@$stack) { my ($pkg, $file, $line, @other_stuff) = @$caller; # do something }
The first set of information relates to the immediate caller of the throw() method. The next item is the caller of that method, and so on.
If stack tracing is enabled then this method returns a text string summarising the caller stack at the point at which the exception was thrown.
use Badger::Exception trace => 1; eval { # some code that throws an exception object $exception->throw(); }; if ($@) { print $@->stack_trace; }
Andy Wardley http://wardley.org/
Copyright (C) 1996-2009 Andy Wardley. All Rights Reserved.
This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself.