Manpages - JSON.3pm

$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.

$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]}

$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 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… ]

$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

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!”

$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.

$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 = $json->allow_tags([$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.

$json->boolean_values([$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.

$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});

$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:

->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.

See SECURITY CONSIDERATIONS in JSON::XS for more info on why this is 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).

See SECURITY CONSIDERATIONS in JSON::XS for more info on why this is useful.

$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)

$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.

$boolean_object = JSON->boolean($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)).

$json->incr_parse( [$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.

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.

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.

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. 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 paclkage/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}, $self->{id}) }
2. In this case, the TO_JSON method of the object is invoked in scalar context. It must return a single scalar that can be directly encoded into JSON. This scalar replaces the object in the JSON text. For example, the following TO_JSON method will convert all URI objects to JSON strings when serialised. The fact that these values originally were URI objects is lost. sub URI::TO_JSON { my ($uri) = @_; $uri->as_string }
3. The object will be serialised as a JSON null value.
4. 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) }