App-Kritika
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character, after which more white-space and comments are allowed.
[
1, # this comment not allowed in JSON
# neither this one...
]
=back
=head2 canonical
$json = $json->canonical([$enable])
$enabled = $json->get_canonical
If C<$enable> is true (or missing), then the C<encode> method will output JSON objects
by sorting their keys. This is adding a comparatively high overhead.
If C<$enable> is false, then the C<encode> method will output key-value
pairs in the order Perl stores them (which will likely change between runs
of the same script, and can change even within the same run from 5.18
onwards).
This option is useful if you want the same data structure to be encoded as
the same JSON text (given the same overall settings). If it is disabled,
the same hash might be encoded differently even if contains the same data,
as key-value pairs have no inherent ordering in Perl.
This setting has no effect when decoding JSON texts.
This setting has currently no effect on tied hashes.
=head2 allow_nonref
$json = $json->allow_nonref([$enable])
$enabled = $json->get_allow_nonref
If C<$enable> is true (or missing), then the C<encode> method can convert a
non-reference into its corresponding string, number or null JSON value,
which is an extension to RFC4627. Likewise, C<decode> will accept those JSON
values instead of croaking.
If C<$enable> is false, then the C<encode> method will croak if it isn't
passed an arrayref or hashref, as JSON texts must either be an object
or array. Likewise, C<decode> will croak if given something that is not a
JSON object or array.
Example, encode a Perl scalar as JSON value with enabled C<allow_nonref>,
resulting in an invalid JSON text:
JSON->new->allow_nonref->encode ("Hello, World!")
=> "Hello, World!"
=head2 allow_unknown
$json = $json->allow_unknown ([$enable])
$enabled = $json->get_allow_unknown
If C<$enable> is true (or missing), then C<encode> will I<not> throw an
exception when it encounters values it cannot represent in JSON (for
example, filehandles) but instead will encode a JSON C<null> value. Note
that blessed objects are not included here and are handled separately by
c<allow_nonref>.
If C<$enable> is false (the default), then C<encode> will throw an
exception when it encounters anything it cannot encode as JSON.
This option does not affect C<decode> in any way, and it is recommended to
leave it off unless you know your communications partner.
=head2 allow_blessed
$json = $json->allow_blessed([$enable])
$enabled = $json->get_allow_blessed
See L<OBJECT SERIALISATION> for details.
If C<$enable> is true (or missing), then the C<encode> method will not
barf when it encounters a blessed reference that it cannot convert
otherwise. Instead, a JSON C<null> value is encoded instead of the object.
If C<$enable> is false (the default), then C<encode> will throw an
exception when it encounters a blessed object that it cannot convert
otherwise.
This setting has no effect on C<decode>.
=head2 convert_blessed
$json = $json->convert_blessed([$enable])
$enabled = $json->get_convert_blessed
See L<OBJECT SERIALISATION> for details.
If C<$enable> is true (or missing), then C<encode>, upon encountering a
blessed object, will check for the availability of the C<TO_JSON> method
on the object's class. If found, it will be called in scalar context and
the resulting scalar will be encoded instead of the object.
The C<TO_JSON> method may safely call die if it wants. If C<TO_JSON>
returns other blessed objects, those will be handled in the same
way. C<TO_JSON> must take care of not causing an endless recursion cycle
(== crash) in this case. The name of C<TO_JSON> was chosen because other
methods called by the Perl core (== not by the user of the object) are
usually in upper case letters and to avoid collisions with any C<to_json>
function or method.
If C<$enable> is false (the default), then C<encode> will not consider
this type of conversion.
This setting has no effect on C<decode>.
=head2 filter_json_object
$json = $json->filter_json_object([$coderef])
When C<$coderef> is specified, it will be called from C<decode> each
time it decodes a JSON object. The only argument is a reference to the
newly-created hash. If the code references returns a single scalar (which
need not be a reference), this value (i.e. a copy of that scalar to avoid
aliasing) is inserted into the deserialised data structure. If it returns
an empty list (NOTE: I<not> C<undef>, which is a valid scalar), the
original deserialised hash will be inserted. This setting can slow down
decoding considerably.
When C<$coderef> is omitted or undefined, any existing callback will
be removed and C<decode> will not change the deserialised hash in any
way.
Example, convert all JSON objects into the integer 5:
my $js = JSON->new->filter_json_object (sub { 5 });
# returns [5]
$js->decode ('[{}]'); # the given subroutine takes a hash reference.
# throw an exception because allow_nonref is not enabled
# so a lone 5 is not allowed.
$js->decode ('{"a":1, "b":2}');
=head2 filter_json_single_key_object
$json = $json->filter_json_single_key_object($key [=> $coderef])
Works remotely similar to C<filter_json_object>, but is only called for
JSON objects having a single key named C<$key>.
This C<$coderef> is called before the one specified via
C<filter_json_object>, if any. It gets passed the single value in the JSON
object. If it returns a single value, it will be inserted into the data
structure. If it returns nothing (not even C<undef> but the empty list),
the callback from C<filter_json_object> will be called next, as if no
single-key callback were specified.
If C<$coderef> is omitted or undefined, the corresponding callback will be
disabled. There can only ever be one callback for a given key.
As this callback gets called less often then the C<filter_json_object>
one, decoding speed will not usually suffer as much. Therefore, single-key
objects make excellent targets to serialise Perl objects into, especially
as single-key JSON objects are as close to the type-tagged value concept
as JSON gets (it's basically an ID/VALUE tuple). Of course, JSON does not
support this in any way, so you need to make sure your data never looks
like a serialised Perl hash.
Typical names for the single object key are C<__class_whatever__>, or
C<$__dollars_are_rarely_used__$> or C<}ugly_brace_placement>, or even
things like C<__class_md5sum(classname)__>, to reduce the risk of clashing
with real hashes.
Example, decode JSON objects of the form C<< { "__widget__" => <id> } >>
into the corresponding C<< $WIDGET{<id>} >> object:
# return whatever is in $WIDGET{5}:
JSON
->new
->filter_json_single_key_object (__widget__ => sub {
$WIDGET{ $_[0] }
})
->decode ('{"__widget__": 5')
# this can be used with a TO_JSON method in some "widget" class
# for serialisation to json:
sub WidgetBase::TO_JSON {
my ($self) = @_;
unless ($self->{id}) {
$self->{id} = ..get..some..id..;
$WIDGET{$self->{id}} = $self;
}
{ __widget__ => $self->{id} }
}
=head2 max_depth
$json = $json->max_depth([$maximum_nesting_depth])
$max_depth = $json->get_max_depth
Sets the maximum nesting level (default C<512>) accepted while encoding
or decoding. If a higher nesting level is detected in JSON text or a Perl
data structure, then the encoder and decoder will stop and croak at that
point.
Nesting level is defined by number of hash- or arrayrefs that the encoder
needs to traverse to reach a given point or the number of C<{> or C<[>
characters without their matching closing parenthesis crossed to reach a
given character in a string.
Setting the maximum depth to one disallows any nesting, so that ensures
that the object is only a single hash/object or array.
If no argument is given, the highest possible setting will be used, which
is rarely useful.
=head2 max_size
$json = $json->max_size([$maximum_string_size])
$max_size = $json->get_max_size
Set the maximum length a JSON text may have (in bytes) where decoding is
being attempted. The default is C<0>, meaning no limit. When C<decode>
is called on a string that is longer then this many bytes, it will not
attempt to decode the string but throw an exception. This setting has no
effect on C<encode> (yet).
If no argument is given, the limit check will be deactivated (same as when
C<0> is specified).
=head2 encode
$json_text = $json->encode($perl_scalar)
Converts the given Perl value or data structure to its JSON
representation. Croaks on error.
=head2 decode
$perl_scalar = $json->decode($json_text)
The opposite of C<encode>: expects a JSON text and tries to parse it,
returning the resulting simple scalar or reference. Croaks on error.
=head2 decode_prefix
($perl_scalar, $characters) = $json->decode_prefix($json_text)
This works like the C<decode> method, but instead of raising an exception
when there is trailing garbage after the first JSON object, it will
silently stop parsing there and return the number of characters consumed
so far.
This is useful if your JSON texts are not delimited by an outer protocol
and you need to know where the JSON text ends.
JSON->new->decode_prefix ("[1] the tail")
=> ([1], 3)
=head1 ADDITIONAL METHODS
The following methods are for this module only.
=head2 backend
$backend = $json->backend
Since 2.92, C<backend> method returns an abstract backend module used currently,
which should be JSON::Backend::XS (which inherits JSON::XS or Cpanel::JSON::XS),
or JSON::Backend::PP (which inherits JSON::PP), not to monkey-patch the actual
backend module globally.
If you need to know what is used actually, use C<isa>, instead of string comparison.
=head2 is_xs
$boolean = $json->is_xs
Returns true if the backend inherits JSON::XS or Cpanel::JSON::XS.
=head2 is_pp
$boolean = $json->is_pp
Returns true if the backend inherits JSON::PP.
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a numeric (floating point) value if that is possible without loss of
precision. Otherwise it will preserve the number as a string value (in
which case you lose roundtripping ability, as the JSON number will be
re-encoded to a JSON string).
Numbers containing a fractional or exponential part will always be
represented as numeric (floating point) values, possibly at a loss of
precision (in which case you might lose perfect roundtripping ability, but
the JSON number will still be re-encoded as a JSON number).
Note that precision is not accuracy - binary floating point values cannot
represent most decimal fractions exactly, and when converting from and to
floating point, this module only guarantees precision up to but not including
the least significant bit.
=item true, false
These JSON atoms become C<JSON::true> and C<JSON::false>,
respectively. They are overloaded to act almost exactly like the numbers
C<1> and C<0>. You can check whether a scalar is a JSON boolean by using
the C<JSON::is_bool> function.
=item null
A JSON null atom becomes C<undef> in Perl.
=item shell-style comments (C<< # I<text> >>)
As a nonstandard extension to the JSON syntax that is enabled by the
C<relaxed> setting, shell-style comments are allowed. They can start
anywhere outside strings and go till the end of the line.
=back
=head2 PERL -> JSON
The mapping from Perl to JSON is slightly more difficult, as Perl is a
truly typeless language, so we can only guess which JSON type is meant by
a Perl value.
=over 4
=item hash references
Perl hash references become JSON objects. As there is no inherent
ordering in hash keys (or JSON objects), they will usually be encoded
in a pseudo-random order. This module can optionally sort the hash keys
(determined by the I<canonical> flag), so the same data structure will
serialise to the same JSON text (given same settings and version of
the same backend), but this incurs a runtime overhead and is only rarely useful,
e.g. when you want to compare some JSON text against another for equality.
=item array references
Perl array references become JSON arrays.
=item other references
Other unblessed references are generally not allowed and will cause an
exception to be thrown, except for references to the integers C<0> and
C<1>, which get turned into C<false> and C<true> atoms in JSON. You can
also use C<JSON::false> and C<JSON::true> to improve readability.
encode_json [\0,JSON::true] # yields [false,true]
=item JSON::true, JSON::false, JSON::null
These special values become JSON true and JSON false values,
respectively. You can also use C<\1> and C<\0> directly if you want.
=item blessed objects
Blessed objects are not directly representable in JSON, but C<JSON::XS>
allows various ways of handling objects. See L<OBJECT SERIALISATION>,
below, for details.
=item simple scalars
Simple Perl scalars (any scalar that is not a reference) are the most
difficult objects to encode: this module will encode undefined scalars as
JSON C<null> values, scalars that have last been used in a string context
before encoding as JSON strings, and anything else as number value:
# dump as number
encode_json [2] # yields [2]
encode_json [-3.0e17] # yields [-3e+17]
my $value = 5; encode_json [$value] # yields [5]
# used as string, so dump as string
print $value;
encode_json [$value] # yields ["5"]
# undef becomes null
encode_json [undef] # yields [null]
You can force the type to be a string by stringifying it:
my $x = 3.1; # some variable containing a number
"$x"; # stringified
$x .= ""; # another, more awkward way to stringify
print $x; # perl does it for you, too, quite often
You can force the type to be a number by numifying it:
my $x = "3"; # some variable containing a string
$x += 0; # numify it, ensuring it will be dumped as a number
$x *= 1; # same thing, the choice is yours.
You can not currently force the type in other, less obscure, ways. Tell me
if you need this capability (but don't forget to explain why it's needed
:).
Note that numerical precision has the same meaning as under Perl (so
binary to decimal conversion follows the same rules as in Perl, which
can differ to other languages). Also, your perl interpreter might expose
extensions to the floating point numbers of your platform, such as
infinities or NaN's - these cannot be represented in JSON, and it is an
error to pass those in.
=back
=head2 OBJECT SERIALISATION
As for Perl objects, this module only supports a pure JSON representation
(without the ability to deserialise the object automatically again).
=head3 SERIALISATION
What happens when this module encounters a Perl object depends on the
C<allow_blessed> and C<convert_blessed> settings, which are used in
this order:
=over 4
=item 1. C<convert_blessed> is enabled and the object has a C<TO_JSON> method.
In this case, the C<TO_JSON> method of the object is invoked in scalar
context. It must return a single scalar that can be directly encoded into
JSON. This scalar replaces the object in the JSON text.
For example, the following C<TO_JSON> method will convert all L<URI>
objects to JSON strings when serialised. The fact that these values
originally were L<URI> objects is lost.
sub URI::TO_JSON {
my ($uri) = @_;
$uri->as_string
}
=item 2. C<allow_blessed> is enabled.
The object will be serialised as a JSON null value.
=item 3. none of the above
If none of the settings are enabled or the respective methods are missing,
this module throws an exception.
=back
=head1 ENCODING/CODESET FLAG NOTES
This section is taken from JSON::XS.
The interested reader might have seen a number of flags that signify
encodings or codesets - C<utf8>, C<latin1> and C<ascii>. There seems to be
some confusion on what these do, so here is a short comparison:
C<utf8> controls whether the JSON text created by C<encode> (and expected
by C<decode>) is UTF-8 encoded or not, while C<latin1> and C<ascii> only
control whether C<encode> escapes character values outside their respective
codeset range. Neither of these flags conflict with each other, although
some combinations make less sense than others.
Care has been taken to make all flags symmetrical with respect to
C<encode> and C<decode>, that is, texts encoded with any combination of
these flag values will be correctly decoded when the same flags are used
- in general, if you use different flag settings while encoding vs. when
decoding you likely have a bug somewhere.
Below comes a verbose discussion of these flags. Note that a "codeset" is
simply an abstract set of character-codepoint pairs, while an encoding
takes those codepoint numbers and I<encodes> them, in our case into
octets. Unicode is (among other things) a codeset, UTF-8 is an encoding,
and ISO-8859-1 (= latin 1) and ASCII are both codesets I<and> encodings at
the same time, which can be confusing.
=over 4
=item C<utf8> flag disabled
When C<utf8> is disabled (the default), then C<encode>/C<decode> generate
and expect Unicode strings, that is, characters with high ordinal Unicode
values (> 255) will be encoded as such characters, and likewise such
characters are decoded as-is, no changes to them will be done, except
"(re-)interpreting" them as Unicode codepoints or Unicode characters,
respectively (to Perl, these are the same thing in strings unless you do
funny/weird/dumb stuff).
This is useful when you want to do the encoding yourself (e.g. when you
want to have UTF-16 encoded JSON texts) or when some other layer does
the encoding for you (for example, when printing to a terminal using a
filehandle that transparently encodes to UTF-8 you certainly do NOT want
to UTF-8 encode your data first and have Perl encode it another time).
=item C<utf8> flag enabled
If the C<utf8>-flag is enabled, C<encode>/C<decode> will encode all
characters using the corresponding UTF-8 multi-byte sequence, and will
expect your input strings to be encoded as UTF-8, that is, no "character"
of the input string must have any value > 255, as UTF-8 does not allow
that.
The C<utf8> flag therefore switches between two modes: disabled means you
will get a Unicode string in Perl, enabled means you get an UTF-8 encoded
octet/binary string in Perl.
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