Provided by: libjson-xs-perl_4.030-2build3_amd64 
NAME
JSON::XS - JSON serialising/deserialising, done correctly and fast
JSON::XS - 正しくて高速な JSON シリアライザ/デシリアライザ
(http://fleur.hio.jp/perldoc/mix/lib/JSON/XS.html)
SYNOPSIS
use JSON::XS;
# exported functions, they croak on error
# and 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
$coder = JSON::XS->new->ascii->pretty->allow_nonref;
$pretty_printed_unencoded = $coder->encode ($perl_scalar);
$perl_scalar = $coder->decode ($unicode_json_text);
# Note that JSON version 2.0 and above will automatically use JSON::XS
# if available, at virtually no speed overhead either, so you should
# be able to just:
use JSON;
# and do the same things, except that you have a pure-perl fallback now.
DESCRIPTION
This module converts Perl data structures to JSON and vice versa. Its primary goal is to be correct and
its secondary goal is to be fast. To reach the latter goal it was written in C.
See MAPPING, below, on how JSON::XS maps perl values to JSON values and vice versa.
FEATURES
• correct Unicode handling
This module knows how to handle Unicode, documents how and when it does so, and even documents what
"correct" means.
• round-trip integrity
When you serialise a perl data structure using only data types supported by JSON and Perl, the
deserialised data structure is identical on the Perl level. (e.g. the string "2.0" doesn't suddenly
become "2" just because it looks like a number). There are minor exceptions to this, read the MAPPING
section below to learn about those.
• strict checking of JSON correctness
There is no guessing, no generating of illegal JSON texts by default, and only JSON is accepted as
input by default (the latter is a security feature).
• fast
Compared to other JSON modules and other serialisers such as Storable, this module usually compares
favourably in terms of speed, too.
• simple to use
This module has both a simple functional interface as well as an object oriented interface.
• reasonably versatile output formats
You can choose between the most compact guaranteed-single-line format possible (nice for simple line-
based protocols), a pure-ASCII format (for when your transport is not 8-bit clean, still supports the
whole Unicode range), or a pretty-printed format (for when you want to read that stuff). Or you can
combine those features in whatever way you like.
FUNCTIONAL INTERFACE
The following convenience methods are provided by this module. They are exported by default:
$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:
$json_text = JSON::XS->new->utf8->encode ($perl_scalar)
Except being faster.
$perl_scalar = decode_json $json_text
The opposite of "encode_json": expects a UTF-8 (binary) string and tries to parse that as a UTF-8
encoded JSON text, returning the resulting reference. Croaks on error.
This function call is functionally identical to:
$perl_scalar = JSON::XS->new->utf8->decode ($json_text)
Except being faster.
A FEW NOTES ON UNICODE AND PERL
Since this often leads to confusion, here are a few very clear words on how Unicode works in Perl, modulo
bugs.
1. Perl strings can store characters with ordinal values > 255.
This enables you to store Unicode characters as single characters in a Perl string - very natural.
2. Perl does not associate an encoding with your strings.
... until you force it to, e.g. when matching it against a regex, or printing the scalar to a file,
in which case Perl either interprets your string as locale-encoded text, octets/binary, or as
Unicode, depending on various settings. In no case is an encoding stored together with your data, it
is use that decides encoding, not any magical meta data.
3. The internal utf-8 flag has no meaning with regards to the encoding of your string.
Just ignore that flag unless you debug a Perl bug, a module written in XS or want to dive into the
internals of perl. Otherwise it will only confuse you, as, despite the name, it says nothing about
how your string is encoded. You can have Unicode strings with that flag set, with that flag clear,
and you can have binary data with that flag set and that flag clear. Other possibilities exist, too.
If you didn't know about that flag, just the better, pretend it doesn't exist.
4. A "Unicode String" is simply a string where each character can be validly interpreted as a Unicode
code point.
If you have UTF-8 encoded data, it is no longer a Unicode string, but a Unicode string encoded in
UTF-8, giving you a binary string.
5. A string containing "high" (> 255) character values is not a UTF-8 string.
It's a fact. Learn to live with it.
I hope this helps :)
OBJECT-ORIENTED INTERFACE
The object oriented interface lets you configure your own encoding or decoding style, within the limits
of supported formats.
$json = new JSON::XS
Creates a new JSON::XS 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 JSON object again and thus calls can be chained:
my $json = JSON::XS->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::XS->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::XS->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 handed a 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;
$jsontext = encode "UTF-16BE", JSON::XS->new->encode ($object);
Example, decode UTF-32LE-encoded JSON:
use Encode;
$object = JSON::XS->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.
Example, pretty-print some simple structure:
my $json = JSON::XS->new->pretty(1)->encode ({a => [1,2]})
=>
{
"a" : [
1,
2
]
}
$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...
]
• literal ASCII TAB characters in strings
Literal ASCII TAB characters are now allowed in strings (and treated as "\t").
[
"Hello\tWorld",
"Hello<TAB>World", # literal <TAB> would not normally be allowed
]
$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 opotion is enabled by default beginning with version 4.0. See
"SECURITY CONSIDERATIONS" for the gory details.
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 without enabled "allow_nonref", resulting in an error:
JSON::XS->new->allow_nonref (0)->encode ("Hello, World!")
=> hash- or arrayref expected...
$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 = $json->allow_tags ([$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 $Types::Serialiser::false and
$Types::Serialiser::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").
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->($hashref)])
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 reference 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::XS->new->filter_json_object (sub { 5 });
# returns [5]
$js->decode ('[{}]')
# 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 [=> $coderef->($value)])
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::XS
->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 = $json->shrink ([$enable])
$enabled = $json->get_shrink
Perl usually over-allocates memory a bit when allocating space for strings. This flag optionally
resizes strings generated by either "encode" or "decode" to their minimum size possible. This can
save memory when your JSON texts are either very very long or you have many short strings. It will
also try to downgrade any strings to octet-form if possible: perl stores strings internally either in
an encoding called UTF-X or in octet-form. The latter cannot store everything but uses less space in
general (and some buggy Perl or C code might even rely on that internal representation being used).
The actual definition of what shrink does might change in future versions, but it will always try to
save space at the expense of time.
If $enable is true (or missing), the string returned by "encode" will be shrunk-to-fit, while all
strings generated by "decode" will also be shrunk-to-fit.
If $enable is false, then the normal perl allocation algorithms are used. If you work with your
data, then this is likely to be faster.
In the future, this setting might control other things, such as converting strings that look like
integers or floats into integers or floats internally (there is no difference on the Perl level),
saving space.
$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.
Note that nesting is implemented by recursion in C. The default value has been chosen to be as large
as typical operating systems allow without crashing.
See SECURITY CONSIDERATIONS, below, 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, below, 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::XS->new->decode_prefix ("[1] the tail")
=> ([1], 3)
INCREMENTAL PARSING
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).
JSON::XS 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.
[void, scalar or list context] = $json->incr_parse ([$string])
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::XS->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.
LIMITATIONS
The incremental parser is a non-exact parser: it works by gathering as much text as possible that could
be a valid JSON text, followed by trying to decode it.
That means it sometimes needs to read more data than strictly necessary to diagnose an invalid JSON text.
For example, after parsing the following fragment, the parser could stop with an error, as this fragment
cannot be the beginning of a valid JSON text:
[,
In reality, hopwever, the parser might continue to read data until a length limit is exceeded or it finds
a closing bracket.
EXAMPLES
Some examples will make all this clearer. First, a simple example that works similarly to
"decode_prefix": We want to decode the JSON object at the start of a string and identify the portion
after the JSON object:
my $text = "[1,2,3] hello";
my $json = new JSON::XS;
my $obj = $json->incr_parse ($text)
or die "expected JSON object or array at beginning of string";
my $tail = $json->incr_text;
# $tail now contains " hello"
Easy, isn't it?
Now for a more complicated example: Imagine a hypothetical protocol where you read some requests from a
TCP stream, and each request is a JSON array, without any separation between them (in fact, it is often
useful to use newlines as "separators", as these get interpreted as whitespace at the start of the JSON
text, which makes it possible to test said protocol with "telnet"...).
Here is how you'd do it (it is trivial to write this in an event-based manner):
my $json = new JSON::XS;
# read some data from the socket
while (sysread $socket, my $buf, 4096) {
# split and decode as many requests as possible
for my $request ($json->incr_parse ($buf)) {
# act on the $request
}
}
Another complicated example: Assume you have a string with JSON objects or arrays, all separated by
(optional) comma characters (e.g. "[1],[2], [3]"). To parse them, we have to skip the commas between the
JSON texts, and here is where the lvalue-ness of "incr_text" comes in useful:
my $text = "[1],[2], [3]";
my $json = new JSON::XS;
# void context, so no parsing done
$json->incr_parse ($text);
# now extract as many objects as possible. note the
# use of scalar context so incr_text can be called.
while (my $obj = $json->incr_parse) {
# do something with $obj
# now skip the optional comma
$json->incr_text =~ s/^ \s* , //x;
}
Now lets go for a very complex example: Assume that you have a gigantic JSON array-of-objects, many
gigabytes in size, and you want to parse it, but you cannot load it into memory fully (this has actually
happened in the real world :).
Well, you lost, you have to implement your own JSON parser. But JSON::XS can still help you: You
implement a (very simple) array parser and let JSON decode the array elements, which are all full JSON
objects on their own (this wouldn't work if the array elements could be JSON numbers, for example):
my $json = new JSON::XS;
# open the monster
open my $fh, "<bigfile.json"
or die "bigfile: $!";
# first parse the initial "["
for (;;) {
sysread $fh, my $buf, 65536
or die "read error: $!";
$json->incr_parse ($buf); # void context, so no parsing
# Exit the loop once we found and removed(!) the initial "[".
# In essence, we are (ab-)using the $json object as a simple scalar
# we append data to.
last if $json->incr_text =~ s/^ \s* \[ //x;
}
# now we have the skipped the initial "[", so continue
# parsing all the elements.
for (;;) {
# in this loop we read data until we got a single JSON object
for (;;) {
if (my $obj = $json->incr_parse) {
# do something with $obj
last;
}
# add more data
sysread $fh, my $buf, 65536
or die "read error: $!";
$json->incr_parse ($buf); # void context, so no parsing
}
# in this loop we read data until we either found and parsed the
# separating "," between elements, or the final "]"
for (;;) {
# first skip whitespace
$json->incr_text =~ s/^\s*//;
# if we find "]", we are done
if ($json->incr_text =~ s/^\]//) {
print "finished.\n";
exit;
}
# if we find ",", we can continue with the next element
if ($json->incr_text =~ s/^,//) {
last;
}
# if we find anything else, we have a parse error!
if (length $json->incr_text) {
die "parse error near ", $json->incr_text;
}
# else add more data
sysread $fh, my $buf, 65536
or die "read error: $!";
$json->incr_parse ($buf); # void context, so no parsing
}
This is a complex example, but most of the complexity comes from the fact that we are trying to be
correct (bear with me if I am wrong, I never ran the above example :).
MAPPING
This section describes how JSON::XS maps 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 preserve 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, JSON::XS 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, JSON::XS only guarantees precision
up to but not including the least significant bit.
true, false
These JSON atoms become "Types::Serialiser::true" and "Types::Serialiser::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 "Types::Serialiser::is_bool" function (after "use Types::Serialier", of
course).
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. JSON::XS can optionally sort the
hash keys (determined by the canonical flag), so the same datastructure will serialise to the same
JSON text (given same settings and version of JSON::XS), 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.
Since "JSON::XS" uses the boolean model from Types::Serialiser, you can also "use Types::Serialiser"
and then use "Types::Serialiser::false" and "Types::Serialiser::true" to improve readability.
use Types::Serialiser;
encode_json [\0, Types::Serialiser::true] # yields [false,true]
Types::Serialiser::true, Types::Serialiser::false
These special values from the Types::Serialiser module 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:
JSON::XS 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 JSON 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 JSON number by numifying it:
my $x = "3"; # some variable containing a string
$x += 0; # numify it, ensuring it will be dumped as a number
$x *= 1; # same thing, the choice is yours.
You can not currently force the type in other, less obscure, ways. Tell me if you need this
capability (but don't forget to explain why it's needed :).
Note that numerical precision has the same meaning as under Perl (so binary to decimal conversion
follows the same rules as in Perl, which can differ to other languages). Also, your perl interpreter
might expose extensions to the floating point numbers of your platform, such as infinities or NaN's -
these cannot be represented in JSON, and it is an error to pass those in.
OBJECT SERIALISATION
As JSON cannot directly represent Perl objects, you have to choose between a pure JSON representation
(without the ability to deserialise the object automatically again), and a nonstandard extension to the
JSON syntax, tagged values.
SERIALISATION
What happens when "JSON::XS" encounters a Perl object depends on the "allow_blessed", "convert_blessed"
and "allow_tags" settings, which are used in this order:
1. "allow_tags" is enabled and the object has a "FREEZE" method.
In this case, "JSON::XS" uses the Types::Serialiser object serialisation protocol to create a tagged
JSON value, using a nonstandard extension to the JSON syntax.
This works by invoking the "FREEZE" method on the object, with the first argument being the object to
serialise, and the second argument being the constant string "JSON" to distinguish it from other
serialisers.
The "FREEZE" method can return any number of values (i.e. zero or more). These values and the
paclkage/classname of the object will then be encoded as a tagged JSON value in the following format:
("classname")[FREEZE return values...]
e.g.:
("URI")["http://www.google.com/"]
("MyDate")[2013,10,29]
("ImageData::JPEG")["Z3...VlCg=="]
For example, the hypothetical "My::Object" "FREEZE" method might use the objects "type" and "id"
members to encode the object:
sub My::Object::FREEZE {
my ($self, $serialiser) = @_;
($self->{type}, $self->{id})
}
2. "convert_blessed" is enabled and the object has a "TO_JSON" method.
In this case, the "TO_JSON" method of the object is invoked in scalar context. It must return a
single scalar that can be directly encoded into JSON. This scalar replaces the object in the JSON
text.
For example, the following "TO_JSON" method will convert all URI objects to JSON strings when
serialised. The fatc that these values originally were URI objects is lost.
sub URI::TO_JSON {
my ($uri) = @_;
$uri->as_string
}
3. "allow_blessed" is enabled.
The object will be serialised as a JSON null value.
4. none of the above
If none of the settings are enabled or the respective methods are missing, "JSON::XS" throws an
exception.
DESERIALISATION
For deserialisation there are only two cases to consider: either nonstandard tagging was used, in which
case "allow_tags" decides, or objects cannot be automatically be deserialised, in which case you can use
postprocessing or the "filter_json_object" or "filter_json_single_key_object" callbacks to get some real
objects our of your JSON.
This section only considers the tagged value case: I a tagged JSON object is encountered during decoding
and "allow_tags" is disabled, a parse error will result (as if tagged values were not part of the
grammar).
If "allow_tags" is enabled, "JSON::XS" will look up the "THAW" method of the package/classname used
during serialisation (it will not attempt to load the package as a Perl module). If there is no such
method, the decoding will fail with an error.
Otherwise, the "THAW" method is invoked with the classname as first argument, the constant string "JSON"
as second argument, and all the values from the JSON array (the values originally returned by the
"FREEZE" method) as remaining arguments.
The method must then return the object. While technically you can return any Perl scalar, you might have
to enable the "enable_nonref" setting to make that work in all cases, so better return an actual blessed
reference.
As an example, let's implement a "THAW" function that regenerates the "My::Object" from the "FREEZE"
example earlier:
sub My::Object::THAW {
my ($class, $serialiser, $type, $id) = @_;
$class->new (type => $type, id => $id)
}
ENCODING/CODESET FLAG NOTES
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 a 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.
JSON and ECMAscript
JSON syntax is based on how literals are represented in javascript (the not-standardised predecessor of
ECMAscript) which is presumably why it is called "JavaScript Object Notation".
However, JSON is not a subset (and also not a superset of course) of ECMAscript (the standard) or
javascript (whatever browsers actually implement).
If you want to use javascript's "eval" function to "parse" JSON, you might run into parse errors for
valid JSON texts, or the resulting data structure might not be queryable:
One of the problems is that U+2028 and U+2029 are valid characters inside JSON strings, but are not
allowed in ECMAscript string literals, so the following Perl fragment will not output something that can
be guaranteed to be parsable by javascript's "eval":
use JSON::XS;
print encode_json [chr 0x2028];
The right fix for this is to use a proper JSON parser in your javascript programs, and not rely on "eval"
(see for example Douglas Crockford's json2.js parser).
If this is not an option, you can, as a stop-gap measure, simply encode to ASCII-only JSON:
use JSON::XS;
print JSON::XS->new->ascii->encode ([chr 0x2028]);
Note that this will enlarge the resulting JSON text quite a bit if you have many non-ASCII characters.
You might be tempted to run some regexes to only escape U+2028 and U+2029, e.g.:
# DO NOT USE THIS!
my $json = JSON::XS->new->utf8->encode ([chr 0x2028]);
$json =~ s/\xe2\x80\xa8/\\u2028/g; # escape U+2028
$json =~ s/\xe2\x80\xa9/\\u2029/g; # escape U+2029
print $json;
Note that this is a bad idea: the above only works for U+2028 and U+2029 and thus only for fully
ECMAscript-compliant parsers. Many existing javascript implementations, however, have issues with other
characters as well - using "eval" naively simply will cause problems.
Another problem is that some javascript implementations reserve some property names for their own
purposes (which probably makes them non-ECMAscript-compliant). For example, Iceweasel reserves the
"__proto__" property name for its own purposes.
If that is a problem, you could parse try to filter the resulting JSON output for these property strings,
e.g.:
$json =~ s/"__proto__"\s*:/"__proto__renamed":/g;
This works because "__proto__" is not valid outside of strings, so every occurrence of ""__proto__"\s*:"
must be a string used as property name.
If you know of other incompatibilities, please let me know.
JSON and YAML
You often hear that JSON is a subset of YAML. This is, however, a mass hysteria(*) and very far from the
truth (as of the time of this writing), so let me state it clearly: in general, there is no way to
configure JSON::XS to output a data structure as valid YAML that works in all cases.
If you really must use JSON::XS to generate YAML, you should use this algorithm (subject to change in
future versions):
my $to_yaml = JSON::XS->new->utf8->space_after (1);
my $yaml = $to_yaml->encode ($ref) . "\n";
This will usually generate JSON texts that also parse as valid YAML. Please note that YAML has hardcoded
limits on (simple) object key lengths that JSON doesn't have and also has different and incompatible
unicode character escape syntax, so you should make sure that your hash keys are noticeably shorter than
the 1024 "stream characters" YAML allows and that you do not have characters with codepoint values
outside the Unicode BMP (basic multilingual page). YAML also does not allow "\/" sequences in strings
(which JSON::XS does not currently generate, but other JSON generators might).
There might be other incompatibilities that I am not aware of (or the YAML specification has been changed
yet again - it does so quite often). In general you should not try to generate YAML with a JSON generator
or vice versa, or try to parse JSON with a YAML parser or vice versa: chances are high that you will run
into severe interoperability problems when you least expect it.
(*) I have been pressured multiple times by Brian Ingerson (one of the authors of the YAML specification)
to remove this paragraph, despite him acknowledging that the actual incompatibilities exist. As I was
personally bitten by this "JSON is YAML" lie, I refused and said I will continue to educate people
about these issues, so others do not run into the same problem again and again. After this, Brian
called me a (quote)complete and worthless idiot(unquote).
In my opinion, instead of pressuring and insulting people who actually clarify issues with YAML and
the wrong statements of some of its proponents, I would kindly suggest reading the JSON spec (which
is not that difficult or long) and finally make YAML compatible to it, and educating users about the
changes, instead of spreading lies about the real compatibility for many years and trying to silence
people who point out that it isn't true.
Addendum/2009: the YAML 1.2 spec is still incompatible with JSON, even though the incompatibilities
have been documented (and are known to Brian) for many years and the spec makes explicit claims that
YAML is a superset of JSON. It would be so easy to fix, but apparently, bullying people and
corrupting userdata is so much easier.
SPEED
It seems that JSON::XS is surprisingly fast, as shown in the following tables. They have been generated
with the help of the "eg/bench" program in the JSON::XS distribution, to make it easy to compare on your
own system.
First comes a comparison between various modules using a very short single-line JSON string (also
available at <http://dist.schmorp.de/misc/json/short.json>).
{"method": "handleMessage", "params": ["user1",
"we were just talking"], "id": null, "array":[1,11,234,-5,1e5,1e7,
1, 0]}
It shows the number of encodes/decodes per second (JSON::XS uses the functional interface, while
JSON::XS/2 uses the OO interface with pretty-printing and hashkey sorting enabled, JSON::XS/3 enables
shrink. JSON::DWIW/DS uses the deserialise function, while JSON::DWIW::FJ uses the from_json method).
Higher is better:
module | encode | decode |
--------------|------------|------------|
JSON::DWIW/DS | 86302.551 | 102300.098 |
JSON::DWIW/FJ | 86302.551 | 75983.768 |
JSON::PP | 15827.562 | 6638.658 |
JSON::Syck | 63358.066 | 47662.545 |
JSON::XS | 511500.488 | 511500.488 |
JSON::XS/2 | 291271.111 | 388361.481 |
JSON::XS/3 | 361577.931 | 361577.931 |
Storable | 66788.280 | 265462.278 |
--------------+------------+------------+
That is, JSON::XS is almost six times faster than JSON::DWIW on encoding, about five times faster on
decoding, and over thirty to seventy times faster than JSON's pure perl implementation. It also compares
favourably to Storable for small amounts of data.
Using a longer test string (roughly 18KB, generated from Yahoo! Locals search API
(<http://dist.schmorp.de/misc/json/long.json>).
module | encode | decode |
--------------|------------|------------|
JSON::DWIW/DS | 1647.927 | 2673.916 |
JSON::DWIW/FJ | 1630.249 | 2596.128 |
JSON::PP | 400.640 | 62.311 |
JSON::Syck | 1481.040 | 1524.869 |
JSON::XS | 20661.596 | 9541.183 |
JSON::XS/2 | 10683.403 | 9416.938 |
JSON::XS/3 | 20661.596 | 9400.054 |
Storable | 19765.806 | 10000.725 |
--------------+------------+------------+
Again, JSON::XS leads by far (except for Storable which non-surprisingly decodes a bit faster).
On large strings containing lots of high Unicode characters, some modules (such as JSON::PC) seem to
decode faster than JSON::XS, but the result will be broken due to missing (or wrong) Unicode handling.
Others refuse to decode or encode properly, so it was impossible to prepare a fair comparison table for
that case.
SECURITY CONSIDERATIONS
When you are using JSON in a protocol, talking to untrusted potentially hostile creatures requires
relatively few measures.
First of all, your JSON decoder should be secure, that is, should not have any buffer overflows.
Obviously, this module should ensure that and I am trying hard on making that true, but you never know.
Second, you need to avoid resource-starving attacks. That means you should limit the size of JSON texts
you accept, or make sure then when your resources run out, that's just fine (e.g. by using a separate
process that can crash safely). The size of a JSON text in octets or characters is usually a good
indication of the size of the resources required to decode it into a Perl structure. While JSON::XS can
check the size of the JSON text, it might be too late when you already have it in memory, so you might
want to check the size before you accept the string.
Third, JSON::XS recurses using the C stack when decoding objects and arrays. The C stack is a limited
resource: for instance, on my amd64 machine with 8MB of stack size I can decode around 180k nested arrays
but only 14k nested JSON objects (due to perl itself recursing deeply on croak to free the temporary). If
that is exceeded, the program crashes. To be conservative, the default nesting limit is set to 512. If
your process has a smaller stack, you should adjust this setting accordingly with the "max_depth" method.
Something else could bomb you, too, that I forgot to think of. In that case, you get to keep the pieces.
I am always open for hints, though...
Also keep in mind that JSON::XS might leak contents of your Perl data structures in its error messages,
so when you serialise sensitive information you might want to make sure that exceptions thrown by
JSON::XS will not end up in front of untrusted eyes.
If you are using JSON::XS to return packets to consumption by JavaScript scripts in a browser you should
have a look at <http://blog.archive.jpsykes.com/47/practical-csrf-and-json-security/> to see whether you
are vulnerable to some common attack vectors (which really are browser design bugs, but it is still you
who will have to deal with it, as major browser developers care only for features, not about getting
security right).
"OLD" VS. "NEW" JSON (RFC4627 VS. RFC7159)
JSON originally required JSON texts to represent an array or object - scalar values were explicitly not
allowed. This has changed, and versions of JSON::XS beginning with 4.0 reflect this by allowing scalar
values by default.
One reason why one might not want this is that this removes a fundamental property of JSON texts, namely
that they are self-delimited and self-contained, or in other words, you could take any number of "old"
JSON texts and paste them together, and the result would be unambiguously parseable:
[1,3]{"k":5}[][null] # four JSON texts, without doubt
By allowing scalars, this property is lost: in the following example, is this one JSON text (the number
12) or two JSON texts (the numbers 1 and 2):
12 # could be 12, or 1 and 2
Another lost property of "old" JSON is that no lookahead is required to know the end of a JSON text, i.e.
the JSON text definitely ended at the last "]" or "}" character, there was no need to read extra
characters.
For example, a viable network protocol with "old" JSON was to simply exchange JSON texts without
delimiter. For "new" JSON, you have to use a suitable delimiter (such as a newline) after every JSON text
or ensure you never encode/decode scalar values.
Most protocols do work by only transferring arrays or objects, and the easiest way to avoid problems with
the "new" JSON definition is to explicitly disallow scalar values in your encoder and decoder:
$json_coder = JSON::XS->new->allow_nonref (0)
This is a somewhat unhappy situation, and the blame can fully be put on JSON's inmventor, Douglas
Crockford, who unilaterally changed the format in 2006 without consulting the IETF, forcing the IETF to
either fork the format or go with it (as I was told, the IETF wasn't amused).
RELATIONSHIP WITH I-JSON
JSON is a somewhat sloppily-defined format - it carries around obvious Javascript baggage, such as not
really defining number range, probably because Javascript only has one type of numbers: IEEE 64 bit
floats ("binary64").
For this reaosn, RFC7493 defines "Internet JSON", which is a restricted subset of JSON that is supposedly
more interoperable on the internet.
While "JSON::XS" does not offer specific support for I-JSON, it of course accepts valid I-JSON and by
default implements some of the limitations of I-JSON, such as parsing numbers as perl numbers, which are
usually a superset of binary64 numbers.
To generate I-JSON, follow these rules:
• always generate UTF-8
I-JSON must be encoded in UTF-8, the default for "encode_json".
• numbers should be within IEEE 754 binary64 range
Basically all existing perl installations use binary64 to represent floating point numbers, so all
you need to do is to avoid large integers.
• objects must not have duplicate keys
This is trivially done, as "JSON::XS" does not allow duplicate keys.
• do not generate scalar JSON texts, use "->allow_nonref (0)"
I-JSON strongly requests you to only encode arrays and objects into JSON.
• times should be strings in ISO 8601 format
There are a myriad of modules on CPAN dealing with ISO 8601 - search for "ISO8601" on CPAN and use
one.
• encode binary data as base64
While it's tempting to just dump binary data as a string (and let "JSON::XS" do the escaping), for
I-JSON, it's recommended to encode binary data as base64.
There are some other considerations - read RFC7493 for the details if interested.
INTEROPERABILITY WITH OTHER MODULES
"JSON::XS" uses the Types::Serialiser module to provide boolean constants. That means that the JSON true
and false values will be compatible to true and false values of other modules that do the same, such as
JSON::PP and CBOR::XS.
INTEROPERABILITY WITH OTHER JSON DECODERS
As long as you only serialise data that can be directly expressed in JSON, "JSON::XS" is incapable of
generating invalid JSON output (modulo bugs, but "JSON::XS" has found more bugs in the official JSON
testsuite (1) than the official JSON testsuite has found in "JSON::XS" (0)).
When you have trouble decoding JSON generated by this module using other decoders, then it is very likely
that you have an encoding mismatch or the other decoder is broken.
When decoding, "JSON::XS" is strict by default and will likely catch all errors. There are currently two
settings that change this: "relaxed" makes "JSON::XS" accept (but not generate) some non-standard
extensions, and "allow_tags" will allow you to encode and decode Perl objects, at the cost of not
outputting valid JSON anymore.
TAGGED VALUE SYNTAX AND STANDARD JSON EN/DECODERS
When you use "allow_tags" to use the extended (and also nonstandard and invalid) JSON syntax for
serialised objects, and you still want to decode the generated When you want to serialise objects, you
can run a regex to replace the tagged syntax by standard JSON arrays (it only works for "normal" package
names without comma, newlines or single colons). First, the readable Perl version:
# if your FREEZE methods return no values, you need this replace first:
$json =~ s/\( \s* (" (?: [^\\":,]+|\\.|::)* ") \s* \) \s* \[\s*\]/[$1]/gx;
# this works for non-empty constructor arg lists:
$json =~ s/\( \s* (" (?: [^\\":,]+|\\.|::)* ") \s* \) \s* \[/[$1,/gx;
And here is a less readable version that is easy to adapt to other languages:
$json =~ s/\(\s*("([^\\":,]+|\\.|::)*")\s*\)\s*\[/[$1,/g;
Here is an ECMAScript version (same regex):
json = json.replace (/\(\s*("([^\\":,]+|\\.|::)*")\s*\)\s*\[/g, "[$1,");
Since this syntax converts to standard JSON arrays, it might be hard to distinguish serialised objects
from normal arrays. You can prepend a "magic number" as first array element to reduce chances of a
collision:
$json =~ s/\(\s*("([^\\":,]+|\\.|::)*")\s*\)\s*\[/["XU1peReLzT4ggEllLanBYq4G9VzliwKF",$1,/g;
And after decoding the JSON text, you could walk the data structure looking for arrays with a first
element of "XU1peReLzT4ggEllLanBYq4G9VzliwKF".
The same approach can be used to create the tagged format with another encoder. First, you create an
array with the magic string as first member, the classname as second, and constructor arguments last,
encode it as part of your JSON structure, and then:
$json =~ s/\[\s*"XU1peReLzT4ggEllLanBYq4G9VzliwKF"\s*,\s*("([^\\":,]+|\\.|::)*")\s*,/($1)[/g;
Again, this has some limitations - the magic string must not be encoded with character escapes, and the
constructor arguments must be non-empty.
(I-)THREADS
This module is not guaranteed to be ithread (or MULTIPLICITY-) safe and there are no plans to change
this. Note that perl's builtin so-called threads/ithreads are officially deprecated and should not be
used.
THE PERILS OF SETLOCALE
Sometimes people avoid the Perl locale support and directly call the system's setlocale function with
"LC_ALL".
This breaks both perl and modules such as JSON::XS, as stringification of numbers no longer works
correctly (e.g. "$x = 0.1; print "$x"+1" might print 1, and JSON::XS might output illegal JSON as
JSON::XS relies on perl to stringify numbers).
The solution is simple: don't call "setlocale", or use it for only those categories you need, such as
"LC_MESSAGES" or "LC_CTYPE".
If you need "LC_NUMERIC", you should enable it only around the code that actually needs it (avoiding
stringification of numbers), and restore it afterwards.
SOME HISTORY
At the time this module was created there already were a number of JSON modules available on CPAN, so
what was the reason to write yet another JSON module? While it seems there are many JSON modules, none of
them correctly handled all corner cases, and in most cases their maintainers are unresponsive, gone
missing, or not listening to bug reports for other reasons.
Beginning with version 2.0 of the JSON module, when both JSON and JSON::XS are installed, then JSON will
fall back on JSON::XS (this can be overridden) with no overhead due to emulation (by inheriting
constructor and methods). If JSON::XS is not available, it will fall back to the compatible JSON::PP
module as backend, so using JSON instead of JSON::XS gives you a portable JSON API that can be fast when
you need it and doesn't require a C compiler when that is a problem.
Somewhere around version 3, this module was forked into "Cpanel::JSON::XS", because its maintainer had
serious trouble understanding JSON and insisted on a fork with many bugs "fixed" that weren't actually
bugs, while spreading FUD about this module without actually giving any details on his accusations. You
be the judge, but in my personal opinion, if you want quality, you will stay away from dangerous forks
like that.
BUGS
While the goal of this module is to be correct, that unfortunately does not mean it's bug-free, only that
I think its design is bug-free. If you keep reporting bugs they will be fixed swiftly, though.
Please refrain from using rt.cpan.org or any other bug reporting service. I put the contact address into
my modules for a reason.
SEE ALSO
The json_xs command line utility for quick experiments.
AUTHOR
Marc Lehmann <schmorp@schmorp.de>
http://home.schmorp.de/