Badger::Exception - structured exception for error handling
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.
Badger::Base