JSON(3pm) User Contributed Perl Documentation JSON(3pm)
NAME
JSON - JSON (JavaScript Object Notation) encoder/decoder
SYNOPSIS
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
DESCRIPTION
This module is a thin wrapper for JSON::XS-compatible modules with a few
additional features. All the backend modules convert a Perl data
structure to a JSON text and vice versa. This module uses JSON::XS by
default, and when JSON::XS is not available, falls back on JSON::PP,
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
Cpanel::JSON::XS, 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.
CHOOSING BACKEND
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;
USING OPTIONAL FEATURES
There are a few options you can set when you "use" this module. These
historical options are only kept for backward compatibility, and should
not be used in a new application.
-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;
FUNCTIONAL INTERFACE
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.
encode_json
$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.
decode_json
$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.
to_json
$json_text = to_json($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)
from_json
$perl_scalar = from_json($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)
JSON::is_bool
$is_boolean = JSON::is_bool($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.
COMMON OBJECT-ORIENTED INTERFACE
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.
new
$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 (with the exception of "allow_nonref", which defaults to
enabled since version 4.0).
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]}
ascii
$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"]
latin1
$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)
utf8
$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);
pretty
$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.
indent
$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.
space_before
$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"}
space_after
$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"}
relaxed
$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 any way. 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...
]
canonical
$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.
allow_nonref
$json = $json->allow_nonref([$enable])
$enabled = $json->get_allow_nonref
Unlike other boolean options, this option is enabled by default
beginning with version 4.0.
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!"
allow_unknown
$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_blessed>.
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.
allow_blessed
$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".
convert_blessed
$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".
allow_tags (since version 3.0)
$json = $json->allow_tags([$enable])
$enabled = $json->get_allow_tags
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 "FREEZE" method
on the object's class. If found, it will be used to serialise the object
into a nonstandard tagged JSON value (that JSON decoders cannot decode).
It also causes "decode" to parse such tagged JSON values and deserialise
them via a call to the "THAW" method.
If $enable is false (the default), then "encode" will not consider this
type of conversion, and tagged JSON values will cause a parse error in
"decode", as if tags were not part of the grammar.
boolean_values (since version 4.0)
$json->boolean_values([$false, $true])
($false, $true) = $json->get_boolean_values
By default, JSON booleans will be decoded as overloaded $JSON::false and
$JSON::true objects.
With this method you can specify your own boolean values for decoding -
on decode, JSON "false" will be decoded as a copy of $false, and JSON
"true" will be decoded as $true ("copy" here is the same thing as
assigning a value to another variable, i.e. "$copy = $false").
This is useful when you want to pass a decoded data structure directly
to other serialisers like YAML, Data::MessagePack and so on.
Note that this works only when you "decode". You can set incompatible
boolean objects (like boolean), but when you "encode" a data structure
with such boolean objects, you still need to enable "convert_blessed"
(and add a "TO_JSON" method if necessary).
Calling this method without any arguments will reset the booleans to
their default values.
"get_boolean_values" will return both $false and $true values, or the
empty list when they are set to the default.
filter_json_object
$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 (or rather a copy of it) 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('[{}]');
# returns 5
$js->decode('{"a":1, "b":2}');
filter_json_single_key_object
$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} }
}
max_depth
$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.
See "SECURITY CONSIDERATIONS" in JSON::XS for more info on why this is
useful.
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 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).
See "SECURITY CONSIDERATIONS" in JSON::XS for more info on why this is
useful.
encode
$json_text = $json->encode($perl_scalar)
Converts the given Perl value or data structure to its JSON
representation. Croaks on error.
decode
$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.
decode_prefix
($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)
ADDITIONAL METHODS
The following methods are for this module only.
backend
$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.
is_xs
$boolean = $json->is_xs
Returns true if the backend inherits JSON::XS or Cpanel::JSON::XS.
is_pp
$boolean = $json->is_pp
Returns true if the backend inherits JSON::PP.
property
$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.
boolean
$boolean_object = JSON->boolean($scalar)
Returns $JSON::true if $scalar contains a true value, $JSON::false
otherwise. You can use this as a full-qualified function
("JSON::boolean($scalar)").
INCREMENTAL PARSING
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.
incr_parse
$json->incr_parse( [$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]");
incr_text
$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).
incr_skip
$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.
incr_reset
$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.
MAPPING
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.
JSON -> PERL
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.
tagged values ("(tag)value").
Another nonstandard extension to the JSON syntax, enabled with the
"allow_tags" setting, are tagged values. In this implementation, the
tag must be a perl package/class name encoded as a JSON string, and
the value must be a JSON array encoding optional constructor
arguments.
See "OBJECT SERIALISATION", below, for details.
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.
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 :).
Since version 2.91_01, JSON::PP uses a different number detection
logic that converts a scalar that is possible to turn into a number
safely. The new logic is slightly faster, and tends to help people
who use older perl or who want to encode complicated data structure.
However, this may results in a different JSON text from the one
JSON::XS encodes (and thus may break tests that compare entire JSON
texts). If you do need the previous behavior for better
compatibility or for finer control, set PERL_JSON_PP_USE_B
environmental variable to true before you "use" JSON.
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.
JSON.pm backend modules trust what you pass to "encode" method (or
"encode_json" function) is a clean, validated data structure with
values that can be represented as valid JSON values only, because
it's not from an external data source (as opposed to JSON texts you
pass to "decode" or "decode_json", which JSON backends consider
tainted and don't trust). As JSON backends don't know exactly what
you and consumers of your JSON texts want the unexpected values to
be (you may want to convert them into null, or to stringify them
with or without normalisation (string representation of
infinities/NaN may vary depending on platforms), or to croak without
conversion), you're advised to do what you and your consumers need
before you encode, and also not to numify values that may start with
values that look like a number (including infinities/NaN), without
validating.
OBJECT SERIALISATION
As JSON cannot directly represent Perl objects, you have to choose
between a pure JSON representation (without the ability to deserialise
the object automatically again), and a nonstandard extension to the JSON
syntax, tagged values.
SERIALISATION
What happens when this module encounters a Perl object depends on the
"allow_blessed", "convert_blessed" and "allow_tags" settings, which are
used in this order:
1. "allow_tags" is enabled and the object has a "FREEZE" method.
In this case, "JSON" creates a tagged JSON value, using a
nonstandard extension to the JSON syntax.
This works by invoking the "FREEZE" method on the object, with the
first argument being the object to serialise, and the second
argument being the constant string "JSON" to distinguish it from
other serialisers.
The "FREEZE" method can return any number of values (i.e. zero or
more). These values and the package/classname of the object will
then be encoded as a tagged JSON value in the following format:
("classname")[FREEZE return values...]
e.g.:
("URI")["http://www.google.com/"]
("MyDate")[2013,10,29]
("ImageData::JPEG")["Z3...VlCg=="]
For example, the hypothetical "My::Object" "FREEZE" method might use
the objects "type" and "id" members to encode the object:
sub My::Object::FREEZE {
my ($self, $serialiser) = @_;
($self->{type}, $self->{id})
}
2. "convert_blessed" is enabled and the object has a "TO_JSON" method.
In this case, the "TO_JSON" method of the object is invoked in
scalar context. It must return a single scalar that can be directly
encoded into JSON. This scalar replaces the object in the JSON text.
For example, the following "TO_JSON" method will convert all URI
objects to JSON strings when serialised. The fact that these values
originally were URI objects is lost.
sub URI::TO_JSON {
my ($uri) = @_;
$uri->as_string
}
3. "allow_blessed" is enabled.
The object will be serialised as a JSON null value.
4. none of the above
If none of the settings are enabled or the respective methods are
missing, this module throws an exception.
DESERIALISATION
For deserialisation there are only two cases to consider: either
nonstandard tagging was used, in which case "allow_tags" decides, or
objects cannot be automatically be deserialised, in which case you can
use postprocessing or the "filter_json_object" or
"filter_json_single_key_object" callbacks to get some real objects our
of your JSON.
This section only considers the tagged value case: a tagged JSON object
is encountered during decoding and "allow_tags" is disabled, a parse
error will result (as if tagged values were not part of the grammar).
If "allow_tags" is enabled, this module will look up the "THAW" method
of the package/classname used during serialisation (it will not attempt
to load the package as a Perl module). If there is no such method, the
decoding will fail with an error.
Otherwise, the "THAW" method is invoked with the classname as first
argument, the constant string "JSON" as second argument, and all the
values from the JSON array (the values originally returned by the
"FREEZE" method) as remaining arguments.
The method must then return the object. While technically you can return
any Perl scalar, you might have to enable the "allow_nonref" setting to
make that work in all cases, so better return an actual blessed
reference.
As an example, let's implement a "THAW" function that regenerates the
"My::Object" from the "FREEZE" example earlier:
sub My::Object::THAW {
my ($class, $serialiser, $type, $id) = @_;
$class->new (type => $type, id => $id)
}
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 - "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.
BACKWARD INCOMPATIBILITY
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.
BUGS
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>
As for bugs on a specific behavior, please report to the author of the
backend module you are using.
As for new features and requests to change common behaviors, please ask
the author of JSON::XS (Marc Lehmann, <schmorp[at]schmorp.de>) first, by
email (important!), to keep compatibility among JSON.pm backends.
SEE ALSO
JSON::XS, Cpanel::JSON::XS, JSON::PP for backends.
JSON::MaybeXS, an alternative that prefers Cpanel::JSON::XS.
"RFC4627"(<http://www.ietf.org/rfc/rfc4627.txt>)
RFC7159 (<http://www.ietf.org/rfc/rfc7159.txt>)
RFC8259 (<http://www.ietf.org/rfc/rfc8259.txt>)
AUTHOR
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.
CURRENT MAINTAINER
Kenichi Ishigaki, <ishigaki[at]cpan.org>
COPYRIGHT AND LICENSE
Copyright 2005-2013 by Makamaka Hannyaharamitu
Most of the documentation is taken from JSON::XS by Marc Lehmann
This library is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
perl v5.34.0 2022-10-11 JSON(3pm)
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