JSON - JSON (JavaScript Object Notation) encoder/decoder
use JSON; # imports encode_json, decode_json, to_json and from_json.
# simple and fast interfaces (expect/generate UTF-8)
$utf8_encoded_json_text = encode_json $perl_hash_or_arrayref;
$perl_hash_or_arrayref = decode_json $utf8_encoded_json_text;
# OO-interface
$json = JSON->new->allow_nonref;
$json_text = $json->encode( $perl_scalar );
$perl_scalar = $json->decode( $json_text );
$pretty_printed = $json->pretty->encode( $perl_scalar ); # pretty-printing
2.97001
This module is a thin wrapper for the JSON::XS manpage-compatible modules with a few
additional features. All the backend modules convert a Perl data structure
to a JSON text as of RFC4627 (which we know is obsolete but we still stick
to; see below for an option to support part of RFC7159) and vice versa.
This module uses the JSON::XS manpage by default, and when JSON::XS is not available,
this module falls back on the JSON::PP manpage, which is in the Perl core since 5.14.
If JSON::PP is not available either, this module then falls back on
JSON::backportPP (which is actually JSON::PP in a different .pm file)
bundled in the same distribution as this module. You can also explicitly
specify to use the Cpanel::JSON::XS manpage, a fork of JSON::XS by Reini Urban.
All these backend modules have slight incompatibilities between them,
including extra features that other modules don't support, but as long as you
use only common features (most important ones are described below), migration
from backend to backend should be reasonably easy. For details, see each
backend module you use.
This module respects an environmental variable called PERL_JSON_BACKEND
when it decides a backend module to use. If this environmental variable is
not set, it tries to load JSON::XS, and if JSON::XS is not available, it
falls back on JSON::PP, and then JSON::backportPP if JSON::PP is not available
either.
If you always don't want it to fall back on pure perl modules, set the
variable like this (export may be setenv , set and the likes,
depending on your environment):
> export PERL_JSON_BACKEND=JSON::XS
If you prefer Cpanel::JSON::XS to JSON::XS, then:
> export PERL_JSON_BACKEND=Cpanel::JSON::XS,JSON::XS,JSON::PP
You may also want to set this variable at the top of your test files, in order
not to be bothered with incompatibilities between backends (you need to wrap
this in BEGIN , and set before actually use -ing JSON module, as it decides
its backend as soon as it's loaded):
BEGIN { $ENV{PERL_JSON_BACKEND}='JSON::backportPP'; }
use JSON;
There are a few options you can set when you use this module:
- -support_by_pp
-
BEGIN { $ENV{PERL_JSON_BACKEND} = 'JSON::XS' }
use JSON -support_by_pp;
my $json = JSON->new;
# escape_slash is for JSON::PP only.
$json->allow_nonref->escape_slash->encode("/");
With this option, this module loads its pure perl backend along with
its XS backend (if available), and lets the XS backend to watch if you set
a flag only JSON::PP supports. When you do, the internal JSON::XS object
is replaced with a newly created JSON::PP object with the setting copied
from the XS object, so that you can use JSON::PP flags (and its slower
decode /encode methods) from then on. In other words, this is not
something that allows you to hook JSON::XS to change its behavior while
keeping its speed. JSON::XS and JSON::PP objects are quite different
(JSON::XS object is a blessed scalar reference, while JSON::PP object is
a blessed hash reference), and can't share their internals.
To avoid needless overhead (by copying settings), you are advised not
to use this option and just to use JSON::PP explicitly when you need
JSON::PP features.
- -convert_blessed_universally
-
use JSON -convert_blessed_universally;
my $json = JSON->new->allow_nonref->convert_blessed;
my $object = bless {foo => 'bar'}, 'Foo';
$json->encode($object); # => {"foo":"bar"}
JSON::XS-compatible backend modules don't encode blessed objects by
default (except for their boolean values, which are typically blessed
JSON::PP::Boolean objects). If you need to encode a data structure
that may contain objects, you usually need to look into the structure
and replace objects with alternative non-blessed values, or enable
convert_blessed and provide a TO_JSON method for each object's
(base) class that may be found in the structure, in order to let the
methods replace the objects with whatever scalar values the methods
return.
If you need to serialise data structures that may contain arbitrary
objects, it's probably better to use other serialisers (such as
Sereal or Storable for example), but if you do want to use
this module for that purpose, -convert_blessed_universally option
may help, which tweaks encode method of the backend to install
UNIVERSAL::TO_JSON method (locally) before encoding, so that
all the objects that don't have their own TO_JSON method can
fall back on the method in the UNIVERSAL namespace. Note that you
still need to enable convert_blessed flag to actually encode
objects in a data structure, and UNIVERSAL::TO_JSON method
installed by this option only converts blessed hash/array references
into their unblessed clone (including private keys/values that are
not supposed to be exposed). Other blessed references will be
converted into null.
This feature is experimental and may be removed in the future.
- -no_export
-
When you don't want to import functional interfaces from a module, you
usually supply
() to its use statement.
use JSON (); # no functional interfaces
If you don't want to import functional interfaces, but you also want to
use any of the above options, add -no_export to the option list.
# no functional interfaces, while JSON::PP support is enabled.
use JSON -support_by_pp, -no_export;
This section is taken from JSON::XS. encode_json and decode_json
are exported by default.
This module also exports to_json and from_json for backward
compatibility. These are slower, and may expect/generate different stuff
from what encode_json and decode_json do, depending on their
options. It's better just to use Object-Oriented interfaces than using
these two functions.
$json_text = encode_json $perl_scalar
Converts the given Perl data structure to a UTF-8 encoded, binary string
(that is, the string contains octets only). Croaks on error.
This function call is functionally identical to:
$json_text = JSON->new->utf8->encode($perl_scalar)
Except being faster.
$perl_scalar = decode_json $json_text
The opposite of encode_json : expects an UTF-8 (binary) string and tries
to parse that as an UTF-8 encoded JSON text, returning the resulting
reference. Croaks on error.
This function call is functionally identical to:
$perl_scalar = JSON->new->utf8->decode($json_text)
Except being faster.
$json_text = to_json($perl_scalar[, $optional_hashref])
Converts the given Perl data structure to a Unicode string by default.
Croaks on error.
Basically, this function call is functionally identical to:
$json_text = JSON->new->encode($perl_scalar)
Except being slower.
You can pass an optional hash reference to modify its behavior, but
that may change what to_json expects/generates (see
ENCODING/CODESET FLAG NOTES for details).
$json_text = to_json($perl_scalar, {utf8 => 1, pretty => 1})
# => JSON->new->utf8(1)->pretty(1)->encode($perl_scalar)
$perl_scalar = from_json($json_text[, $optional_hashref])
The opposite of to_json : expects a Unicode string and tries
to parse it, returning the resulting reference. Croaks on error.
Basically, this function call is functionally identical to:
$perl_scalar = JSON->new->decode($json_text)
You can pass an optional hash reference to modify its behavior, but
that may change what from_json expects/generates (see
ENCODING/CODESET FLAG NOTES for details).
$perl_scalar = from_json($json_text, {utf8 => 1})
# => JSON->new->utf8(1)->decode($json_text)
$is_boolean = JSON::is_bool($scalar)
Returns true if the passed scalar represents either JSON::true or
JSON::false, two constants that act like 1 and 0 respectively
and are also used to represent JSON true and false in Perl strings.
See MAPPING, below, for more information on how JSON values are mapped to
Perl.
This section is also taken from JSON::XS.
The object oriented interface lets you configure your own encoding or
decoding style, within the limits of supported formats.
$json = JSON->new
Creates a new JSON::XS-compatible backend object that can be used to de/encode JSON
strings. All boolean flags described below are by default disabled.
The mutators for flags all return the backend object again and thus calls can
be chained:
my $json = JSON->new->utf8->space_after->encode({a => [1,2]})
=> {"a": [1, 2]}
$json = $json->ascii([$enable])
$enabled = $json->get_ascii
If $enable is true (or missing), then the encode method will not
generate characters outside the code range 0..127 (which is ASCII). Any
Unicode characters outside that range will be escaped using either a
single \uXXXX (BMP characters) or a double \uHHHH\uLLLLL escape sequence,
as per RFC4627. The resulting encoded JSON text can be treated as a native
Unicode string, an ascii-encoded, latin1-encoded or UTF-8 encoded string,
or any other superset of ASCII.
If $enable is false, then the encode method will not escape Unicode
characters unless required by the JSON syntax or other flags. This results
in a faster and more compact format.
See also the section ENCODING/CODESET FLAG NOTES later in this document.
The main use for this flag is to produce JSON texts that can be
transmitted over a 7-bit channel, as the encoded JSON texts will not
contain any 8 bit characters.
JSON->new->ascii(1)->encode([chr 0x10401])
=> ["\ud801\udc01"]
$json = $json->latin1([$enable])
$enabled = $json->get_latin1
If $enable is true (or missing), then the encode method will encode
the resulting JSON text as latin1 (or iso-8859-1), escaping any characters
outside the code range 0..255 . The resulting string can be treated as a
latin1-encoded JSON text or a native Unicode string. The decode method
will not be affected in any way by this flag, as decode by default
expects Unicode, which is a strict superset of latin1.
If $enable is false, then the encode method will not escape Unicode
characters unless required by the JSON syntax or other flags.
See also the section ENCODING/CODESET FLAG NOTES later in this document.
The main use for this flag is efficiently encoding binary data as JSON
text, as most octets will not be escaped, resulting in a smaller encoded
size. The disadvantage is that the resulting JSON text is encoded
in latin1 (and must correctly be treated as such when storing and
transferring), a rare encoding for JSON. It is therefore most useful when
you want to store data structures known to contain binary data efficiently
in files or databases, not when talking to other JSON encoders/decoders.
JSON->new->latin1->encode (["\x{89}\x{abc}"]
=> ["\x{89}\\u0abc"] # (perl syntax, U+abc escaped, U+89 not)
$json = $json->utf8([$enable])
$enabled = $json->get_utf8
If $enable is true (or missing), then the encode method will encode
the JSON result into UTF-8, as required by many protocols, while the
decode method expects to be handled an UTF-8-encoded string. Please
note that UTF-8-encoded strings do not contain any characters outside the
range 0..255 , they are thus useful for bytewise/binary I/O. In future
versions, enabling this option might enable autodetection of the UTF-16
and UTF-32 encoding families, as described in RFC4627.
If $enable is false, then the encode method will return the JSON
string as a (non-encoded) Unicode string, while decode expects thus a
Unicode string. Any decoding or encoding (e.g. to UTF-8 or UTF-16) needs
to be done yourself, e.g. using the Encode module.
See also the section ENCODING/CODESET FLAG NOTES later in this document.
Example, output UTF-16BE-encoded JSON:
use Encode;
$jsontext = encode "UTF-16BE", JSON->new->encode ($object);
Example, decode UTF-32LE-encoded JSON:
use Encode;
$object = JSON->new->decode (decode "UTF-32LE", $jsontext);
$json = $json->pretty([$enable])
This enables (or disables) all of the indent , space_before and
space_after (and in the future possibly more) flags in one call to
generate the most readable (or most compact) form possible.
$json = $json->indent([$enable])
$enabled = $json->get_indent
If $enable is true (or missing), then the encode method will use a multiline
format as output, putting every array member or object/hash key-value pair
into its own line, indenting them properly.
If $enable is false, no newlines or indenting will be produced, and the
resulting JSON text is guaranteed not to contain any newlines .
This setting has no effect when decoding JSON texts.
$json = $json->space_before([$enable])
$enabled = $json->get_space_before
If $enable is true (or missing), then the encode method will add an extra
optional space before the : separating keys from values in JSON objects.
If $enable is false, then the encode method will not add any extra
space at those places.
This setting has no effect when decoding JSON texts. You will also
most likely combine this setting with space_after .
Example, space_before enabled, space_after and indent disabled:
{"key" :"value"}
$json = $json->space_after([$enable])
$enabled = $json->get_space_after
If $enable is true (or missing), then the encode method will add an extra
optional space after the : separating keys from values in JSON objects
and extra whitespace after the , separating key-value pairs and array
members.
If $enable is false, then the encode method will not add any extra
space at those places.
This setting has no effect when decoding JSON texts.
Example, space_before and indent disabled, space_after enabled:
{"key": "value"}
$json = $json->relaxed([$enable])
$enabled = $json->get_relaxed
If $enable is true (or missing), then decode will accept some
extensions to normal JSON syntax (see below). encode will not be
affected in anyway. Be aware that this option makes you accept invalid
JSON texts as if they were valid!. I suggest only to use this option to
parse application-specific files written by humans (configuration files,
resource files etc.)
If $enable is false (the default), then decode will only accept
valid JSON texts.
Currently accepted extensions are:
- list items can have an end-comma
JSON separates array elements and key-value pairs with commas. This
can be annoying if you write JSON texts manually and want to be able to
quickly append elements, so this extension accepts comma at the end of
such items not just between them:
[
1,
2, <- this comma not normally allowed
]
{
"k1": "v1",
"k2": "v2", <- this comma not normally allowed
}
- shell-style '#'-comments
Whenever JSON allows whitespace, shell-style comments are additionally
allowed. They are terminated by the first carriage-return or line-feed
character, after which more white-space and comments are allowed.
[
1, # this comment not allowed in JSON
# neither this one...
]
$json = $json->canonical([$enable])
$enabled = $json->get_canonical
If $enable is true (or missing), then the encode method will output JSON objects
by sorting their keys. This is adding a comparatively high overhead.
If $enable is false, then the 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.
$json = $json->allow_nonref([$enable])
$enabled = $json->get_allow_nonref
If $enable is true (or missing), then the encode method can convert a
non-reference into its corresponding string, number or null JSON value,
which is an extension to RFC4627. Likewise, decode will accept those JSON
values instead of croaking.
If $enable is false, then the encode method will croak if it isn't
passed an arrayref or hashref, as JSON texts must either be an object
or array. Likewise, decode will croak if given something that is not a
JSON object or array.
Example, encode a Perl scalar as JSON value with enabled allow_nonref ,
resulting in an invalid JSON text:
JSON->new->allow_nonref->encode ("Hello, World!")
=> "Hello, World!"
$json = $json->allow_unknown ([$enable])
$enabled = $json->get_allow_unknown
If $enable is true (or missing), then encode will not throw an
exception when it encounters values it cannot represent in JSON (for
example, filehandles) but instead will encode a JSON null value. Note
that blessed objects are not included here and are handled separately by
c<allow_nonref>.
If $enable is false (the default), then encode will throw an
exception when it encounters anything it cannot encode as JSON.
This option does not affect decode in any way, and it is recommended to
leave it off unless you know your communications partner.
$json = $json->allow_blessed([$enable])
$enabled = $json->get_allow_blessed
See OBJECT SERIALISATION for details.
If $enable is true (or missing), then the encode method will not
barf when it encounters a blessed reference that it cannot convert
otherwise. Instead, a JSON null value is encoded instead of the object.
If $enable is false (the default), then encode will throw an
exception when it encounters a blessed object that it cannot convert
otherwise.
This setting has no effect on decode .
$json = $json->convert_blessed([$enable])
$enabled = $json->get_convert_blessed
See OBJECT SERIALISATION for details.
If $enable is true (or missing), then encode , upon encountering a
blessed object, will check for the availability of the 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 TO_JSON method may safely call die if it wants. If TO_JSON
returns other blessed objects, those will be handled in the same
way. TO_JSON must take care of not causing an endless recursion cycle
(== crash) in this case. The name of 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 to_json
function or method.
If $enable is false (the default), then encode will not consider
this type of conversion.
This setting has no effect on decode .
$json = $json->filter_json_object([$coderef])
When $coderef is specified, it will be called from 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: not undef , which is a valid scalar), the
original deserialised hash will be inserted. This setting can slow down
decoding considerably.
When $coderef is omitted or undefined, any existing callback will
be removed and 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}');
$json = $json->filter_json_single_key_object($key [=> $coderef])
Works remotely similar to filter_json_object , but is only called for
JSON objects having a single key named $key .
This $coderef is called before the one specified via
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 undef but the empty list),
the callback from filter_json_object will be called next, as if no
single-key callback were specified.
If $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 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 __class_whatever__ , or
$__dollars_are_rarely_used__$ or }ugly_brace_placement , or even
things like __class_md5sum(classname)__ , to reduce the risk of clashing
with real hashes.
Example, decode JSON objects of the form { "__widget__" => <id> }
into the corresponding $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} }
}
$json = $json->max_depth([$maximum_nesting_depth])
$max_depth = $json->get_max_depth
Sets the maximum nesting level (default 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 { or [
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.
$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 0 , meaning no limit. When 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 encode (yet).
If no argument is given, the limit check will be deactivated (same as when
0 is specified).
$json_text = $json->encode($perl_scalar)
Converts the given Perl value or data structure to its JSON
representation. Croaks on error.
$perl_scalar = $json->decode($json_text)
The opposite of encode : expects a JSON text and tries to parse it,
returning the resulting simple scalar or reference. Croaks on error.
($perl_scalar, $characters) = $json->decode_prefix($json_text)
This works like the 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)
The following methods are for this module only.
$backend = $json->backend
Since 2.92, 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 isa , instead of string comparison.
$boolean = $json->is_xs
Returns true if the backend inherits JSON::XS or Cpanel::JSON::XS.
$boolean = $json->is_pp
Returns true if the backend inherits JSON::PP.
$settings = $json->property()
Returns a reference to a hash that holds all the common flag settings.
$json = $json->property('utf8' => 1)
$value = $json->property('utf8') # 1
You can use this to get/set a value of a particular flag.
This section is also taken from JSON::XS.
In some cases, there is the need for incremental parsing of JSON
texts. While this module always has to keep both JSON text and resulting
Perl data structure in memory at one time, it does allow you to parse a
JSON stream incrementally. It does so by accumulating text until it has
a full JSON object, which it then can decode. This process is similar to
using decode_prefix to see if a full JSON object is available, but
is much more efficient (and can be implemented with a minimum of method
calls).
This module will only attempt to parse the JSON text once it is sure it
has enough text to get a decisive result, using a very simple but
truly incremental parser. This means that it sometimes won't stop as
early as the full parser, for example, it doesn't detect mismatched
parentheses. The only thing it guarantees is that it starts decoding as
soon as a syntactically valid JSON text has been seen. This means you need
to set resource limits (e.g. max_size ) to ensure the parser will stop
parsing in the presence if syntax errors.
The following methods implement this incremental parser.
$json->incr_parse( [$string] ) # void context
$obj_or_undef = $json->incr_parse( [$string] ) # scalar context
@obj_or_empty = $json->incr_parse( [$string] ) # list context
This is the central parsing function. It can both append new text and
extract objects from the stream accumulated so far (both of these
functions are optional).
If $string is given, then this string is appended to the already
existing JSON fragment stored in the $json object.
After that, if the function is called in void context, it will simply
return without doing anything further. This can be used to add more text
in as many chunks as you want.
If the method is called in scalar context, then it will try to extract
exactly one JSON object. If that is successful, it will return this
object, otherwise it will return undef . If there is a parse error,
this method will croak just as decode would do (one can then use
incr_skip to skip the erroneous part). This is the most common way of
using the method.
And finally, in list context, it will try to extract as many objects
from the stream as it can find and return them, or the empty list
otherwise. For this to work, there must be no separators (other than
whitespace) between the JSON objects or arrays, instead they must be
concatenated back-to-back. If an error occurs, an exception will be
raised as in the scalar context case. Note that in this case, any
previously-parsed JSON texts will be lost.
Example: Parse some JSON arrays/objects in a given string and return
them.
my @objs = JSON->new->incr_parse ("[5][7][1,2]");
$lvalue_string = $json->incr_text
This method returns the currently stored JSON fragment as an lvalue, that
is, you can manipulate it. This only works when a preceding call to
incr_parse in scalar context successfully returned an object. Under
all other circumstances you must not call this function (I mean it.
although in simple tests it might actually work, it will fail under
real world conditions). As a special exception, you can also call this
method before having parsed anything.
That means you can only use this function to look at or manipulate text
before or after complete JSON objects, not while the parser is in the
middle of parsing a JSON object.
This function is useful in two cases: a) finding the trailing text after a
JSON object or b) parsing multiple JSON objects separated by non-JSON text
(such as commas).
$json->incr_skip
This will reset the state of the incremental parser and will remove
the parsed text from the input buffer so far. This is useful after
incr_parse died, in which case the input buffer and incremental parser
state is left unchanged, to skip the text parsed so far and to reset the
parse state.
The difference to incr_reset is that only text until the parse error
occurred is removed.
$json->incr_reset
This completely resets the incremental parser, that is, after this call,
it will be as if the parser had never parsed anything.
This is useful if you want to repeatedly parse JSON objects and want to
ignore any trailing data, which means you have to reset the parser after
each successful decode.
Most of this section is also taken from JSON::XS.
This section describes how the backend modules map Perl values to JSON values and
vice versa. These mappings are designed to ``do the right thing'' in most
circumstances automatically, preserving round-tripping characteristics
(what you put in comes out as something equivalent).
For the more enlightened: note that in the following descriptions,
lowercase perl refers to the Perl interpreter, while uppercase Perl
refers to the abstract Perl language itself.
- object
-
A JSON object becomes a reference to a hash in Perl. No ordering of object
keys is preserved (JSON does not preserver object key ordering itself).
- array
-
A JSON array becomes a reference to an array in Perl.
- string
-
A JSON string becomes a string scalar in Perl - Unicode codepoints in JSON
are represented by the same codepoints in the Perl string, so no manual
decoding is necessary.
- number
-
A JSON number becomes either an integer, numeric (floating point) or
string scalar in perl, depending on its range and any fractional parts. On
the Perl level, there is no difference between those as Perl handles all
the conversion details, but an integer may take slightly less memory and
might represent more values exactly than floating point numbers.
If the number consists of digits only, this module will try to represent
it as an integer value. If that fails, it will try to represent it as
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.
- true, false
-
These JSON atoms become
JSON::true and JSON::false ,
respectively. They are overloaded to act almost exactly like the numbers
1 and 0 . You can check whether a scalar is a JSON boolean by using
the JSON::is_bool function.
- null
-
A JSON null atom becomes
undef in Perl.
- shell-style comments (# text)
-
As a nonstandard extension to the JSON syntax that is enabled by the
relaxed setting, shell-style comments are allowed. They can start
anywhere outside strings and go till the end of the line.
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.
- 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 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.
- array references
-
Perl array references become JSON arrays.
- other references
-
Other unblessed references are generally not allowed and will cause an
exception to be thrown, except for references to the integers
0 and
1 , which get turned into false and true atoms in JSON. You can
also use JSON::false and JSON::true to improve readability.
encode_json [\0,JSON::true] # yields [false,true]
- JSON::true, JSON::false, JSON::null
-
These special values become JSON true and JSON false values,
respectively. You can also use
\1 and \0 directly if you want.
- blessed objects
-
Blessed objects are not directly representable in JSON, but
JSON::XS
allows various ways of handling objects. See OBJECT SERIALISATION,
below, for details.
- 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
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
$x *= 1; # same thing, the choice is yours.
You can not currently force the type in other, less obscure, ways. Tell me
if you need this capability (but don't forget to explain why it's needed
:).
Note that numerical precision has the same meaning as under Perl (so
binary to decimal conversion follows the same rules as in Perl, which
can differ to other languages). Also, your perl interpreter might expose
extensions to the floating point numbers of your platform, such as
infinities or NaN's - these cannot be represented in JSON, and it is an
error to pass those in.
As for Perl objects, this module only supports a pure JSON representation
(without the ability to deserialise the object automatically again).
What happens when this module encounters a Perl object depends on the
allow_blessed and convert_blessed settings, which are used in
this order:
- convert_blessed is enabled and the object has a TO_JSON method.
In this case, the TO_JSON method of the object is invoked in scalar
context. It must return a single scalar that can be directly encoded into
JSON. This scalar replaces the object in the JSON text.
For example, the following TO_JSON method will convert all URI
objects to JSON strings when serialised. The fact that these values
originally were URI objects is lost.
sub URI::TO_JSON {
my ($uri) = @_;
$uri->as_string
}
- allow_blessed is enabled.
The object will be serialised as a JSON null value.
- none of the above
If none of the settings are enabled or the respective methods are missing,
this module throws an exception.
This section is taken from JSON::XS.
The interested reader might have seen a number of flags that signify
encodings or codesets - utf8 , latin1 and ascii . There seems to be
some confusion on what these do, so here is a short comparison:
utf8 controls whether the JSON text created by encode (and expected
by decode ) is UTF-8 encoded or not, while latin1 and ascii only
control whether 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
encode and 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 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 and encodings at
the same time, which can be confusing.
- utf8 flag disabled
-
When
utf8 is disabled (the default), then encode /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).
- utf8 flag enabled
-
If the
utf8 -flag is enabled, encode /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 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.
- latin1 or ascii flags enabled
-
With
latin1 (or ascii ) enabled, encode will escape characters
with ordinal values > 255 (> 127 with ascii ) and encode the remaining
characters as specified by the utf8 flag.
If utf8 is disabled, then the result is also correctly encoded in those
character sets (as both are proper subsets of Unicode, meaning that a
Unicode string with all character values < 256 is the same thing as a
ISO-8859-1 string, and a Unicode string with all character values < 128 is
the same thing as an ASCII string in Perl).
If utf8 is enabled, you still get a correct UTF-8-encoded string,
regardless of these flags, just some more characters will be escaped using
\uXXXX then before.
Note that ISO-8859-1-encoded strings are not compatible with UTF-8
encoding, while ASCII-encoded strings are. That is because the ISO-8859-1
encoding is NOT a subset of UTF-8 (despite the ISO-8859-1 codeset being
a subset of Unicode), while ASCII is.
Surprisingly, decode will ignore these flags and so treat all input
values as governed by the utf8 flag. If it is disabled, this allows you
to decode ISO-8859-1- and ASCII-encoded strings, as both strict subsets of
Unicode. If it is enabled, you can correctly decode UTF-8 encoded strings.
So neither latin1 nor ascii are incompatible with the utf8 flag -
they only govern when the JSON output engine escapes a character or not.
The main use for latin1 is to relatively efficiently store binary data
as JSON, at the expense of breaking compatibility with most JSON decoders.
The main use for ascii is to force the output to not contain characters
with values > 127, which means you can interpret the resulting string
as UTF-8, ISO-8859-1, ASCII, KOI8-R or most about any character set and
8-bit-encoding, and still get the same data structure back. This is useful
when your channel for JSON transfer is not 8-bit clean or the encoding
might be mangled in between (e.g. in mail), and works because ASCII is a
proper subset of most 8-bit and multibyte encodings in use in the world.
Since version 2.90, stringification (and string comparison) for
JSON::true and JSON::false has not been overloaded. It shouldn't
matter as long as you treat them as boolean values, but a code that
expects they are stringified as ``true'' or ``false'' doesn't work as
you have expected any more.
if (JSON::true eq 'true') { # now fails
print "The result is $JSON::true now."; # => The result is 1 now.
And now these boolean values don't inherit JSON::Boolean, either.
When you need to test a value is a JSON boolean value or not, use
JSON::is_bool function, instead of testing the value inherits
a particular boolean class or not.
Please report bugs on backend selection and additional features
this module provides to RT or GitHub issues for this module:
- https://rt.cpan.org/Public/Dist/Display.html?Queue=JSON
-
- https://github.com/makamaka/JSON/issues
-
Please report bugs and feature requests on decoding/encoding
and boolean behaviors to the author of the backend module you
are using.
the JSON::XS manpage, the Cpanel::JSON::XS manpage, the JSON::PP manpage for backends.
the JSON::MaybeXS manpage, an alternative that prefers Cpanel::JSON::XS.
RFC4627 (http://www.ietf.org/rfc/rfc4627.txt)
Makamaka Hannyaharamitu, <makamaka[at]cpan.org>
JSON::XS was written by Marc Lehmann <schmorp[at]schmorp.de>
The release of this new version owes to the courtesy of Marc Lehmann.
Copyright 2005-2013 by Makamaka Hannyaharamitu
This library is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.
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