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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.
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 overriden) with no overhead due to emulation (by inheritign
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
and doesn't require a C compiler when that is a problem.
As this is the n-th-something JSON module on CPAN, what was the reason
to write yet another JSON module? While it seems there are many JSON
modules, none of them correctly handle all corner cases, and in most
cases their maintainers are unresponsive, gone missing, or not listening
to bug reports for other reasons.
See COMPARISON, below, for a comparison to some other JSON modules.
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, and even documents how and
when it does so.
* round-trip integrity
When you serialise a perl data structure using only datatypes
supported by JSON, 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).
* 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, this module compares favourably in
terms of speed, too.
* simple to use
This module has both a simple functional interface as well as an OO
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 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:
$perl_scalar = JSON::XS->new->utf8->decode ($json_text)
except being faster.
$is_boolean = JSON::XS::is_bool $scalar
Returns true if the passed scalar represents either JSON::XS::true
or JSON::XS::false, two constants that act like 1 and 0,
respectively and are used to represent JSON "true" and "false"
values in Perl.
See MAPPING, below, for more information on how JSON values are
mapped to Perl.
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.
Unless 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 metadata.
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 codepoint.
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*.
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.
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.
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 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.
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 anyway. *Be aware that this option makes you accept
invalid JSON texts as if they were valid!*. I suggest only to use
this option to parse application-specific files written by humans
(configuration files, resource files etc.)
If $enable is false (the default), then "decode" will only accept
valid JSON texts.
Currently accepted extensions are:
* list items can have an end-comma
JSON *separates* array elements and key-value pairs with commas.
This can be annoying if you write JSON texts manually and want
to be able to quickly append elements, so this extension accepts
comma at the end of such items not just between them:
[
1,
2, <- this comma not normally allowed
]
{
"k1": "v1",
"k2": "v2", <- this comma not normally allowed
}
* shell-style '#'-comments
Whenever JSON allows whitespace, shell-style comments are
additionally allowed. They are terminated by the first
carriage-return or line-feed character, after which more
white-space and comments are allowed.
[
1, # this comment not allowed in JSON
# neither this one...
]
$json = $json->canonical ([$enable])
$enabled = $json->get_canonical
If $enable is true (or missing), then the "encode" method will
output JSON objects by sorting their keys. This is adding a
comparatively high overhead.
If $enable is false, then the "encode" method will output key-value
pairs in the order Perl stores them (which will likely change
between runs of the same script).
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.
$json = $json->allow_nonref ([$enable])
$enabled = $json->get_allow_nonref
If $enable is true (or missing), then the "encode" method can
convert a non-reference into its corresponding string, number or
null JSON value, which is an extension to RFC4627. Likewise,
"decode" will accept those JSON values instead of croaking.
If $enable is false, then the "encode" method will croak if it isn't
passed an arrayref or hashref, as JSON texts must either be an
object or array. Likewise, "decode" will croak if given something
that is not a JSON object or array.
Example, encode a Perl scalar as JSON value with enabled
"allow_nonref", resulting in an invalid JSON text:
JSON::XS->new->allow_nonref->encode ("Hello, World!")
=> "Hello, World!"
$json = $json->allow_blessed ([$enable])
$enabled = $json->get_allow_blessed
If $enable is true (or missing), then the "encode" method will not
barf when it encounters a blessed reference. Instead, the value of
the convert_blessed option will decide whether "null"
("convert_blessed" disabled or no "TO_JSON" method found) or a
representation of the object ("convert_blessed" enabled and
"TO_JSON" method found) is being encoded. Has no effect on "decode".
If $enable is false (the default), then "encode" will throw an
exception when it encounters a blessed object.
$json = $json->convert_blessed ([$enable])
$enabled = $json->get_convert_blessed
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. If no "TO_JSON" method is found, the value of
"allow_blessed" will decide what to do.
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.
This setting does not yet influence "decode" in any way, but in the
future, global hooks might get installed that influence "decode" and
are enabled by this setting.
If $enable is false, then the "allow_blessed" setting will decide
what to do when a blessed object is found.
$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 references returns a single
scalar (which need not be a reference), this value (i.e. a copy of
that scalar to avoid aliasing) is inserted into the deserialised
data structure. If it returns an empty list (NOTE: *not* "undef",
which is a valid scalar), the original deserialised hash will be
inserted. This setting can slow down decoding considerably.
When $coderef is omitted or undefined, any existing callback will be
removed and "decode" will not change the deserialised hash in any
way.
Example, convert all JSON objects into the integer 5:
my $js = JSON::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 the JSON text or Perl data structure has an equal or
higher nesting level then this limit, 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.
The argument to "max_depth" will be rounded up to the next highest
power of two. If no argument is given, the highest possible setting
will be used, which is rarely useful.
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 longer then this number of
characters it will not attempt to decode the string but throw an
exception. This setting has no effect on "encode" (yet).
The argument to "max_size" will be rounded up to the next highest
power of two (so may be more than requested). 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 data structure (a simple scalar or a
reference to a hash or array) to its JSON representation. Simple
scalars will be converted into JSON string or number sequences,
while references to arrays become JSON arrays and references to
hashes become JSON objects. Undefined Perl values (e.g. "undef")
become JSON "null" values. Neither "true" nor "false" values will be
generated.
$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.
JSON numbers and strings become simple Perl scalars. JSON arrays
become Perl arrayrefs and JSON objects become Perl hashrefs. "true"
becomes 1, "false" becomes 0 and "null" becomes "undef".
($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 (which is not the brightest thing to do in the first place)
and you need to know where the JSON text ends.
JSON::XS->new->decode_prefix ("[1] the tail")
=> ([], 3)
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.
Numbers containing a fractional or exponential part will always be
represented as numeric (floating point) values, possibly at a loss
of precision.
This might create round-tripping problems as numbers might become
strings, but as Perl is typeless there is no other way to do it.
true, false
These JSON atoms become "JSON::XS::true" and "JSON::XS::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::XS::is_bool" function.
null
A JSON null atom becomes "undef" in Perl.
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 that can change between runs of the
same program but stays generally the same within a single run of a
program. 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. You
can also use "JSON::XS::false" and "JSON::XS::true" to improve
readability.
encode_json [\0,JSON::XS::true] # yields [false,true]
JSON::XS::true, JSON::XS::false
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 allowed. JSON::XS currently tries to encode
their underlying representation (hash- or arrayref), but this
behaviour might change in future versions.
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 value, 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.
COMPARISON
As already mentioned, this module was created because none of the
existing JSON modules could be made to work correctly. First I will
describe the problems (or pleasures) I encountered with various existing
JSON modules, followed by some benchmark values. JSON::XS was designed
not to suffer from any of these problems or limitations.
JSON 1.07
Slow (but very portable, as it is written in pure Perl).
Undocumented/buggy Unicode handling (how JSON handles Unicode values
is undocumented. One can get far by feeding it Unicode strings and
doing en-/decoding oneself, but Unicode escapes are not working
properly).
No round-tripping (strings get clobbered if they look like numbers,
e.g. the string 2.0 will encode to 2.0 instead of "2.0", and that
will decode into the number 2.
JSON::PC 0.01
Very fast.
Undocumented/buggy Unicode handling.
No round-tripping.
Has problems handling many Perl values (e.g. regex results and other
magic values will make it croak).
Does not even generate valid JSON ("{1,2}" gets converted to "{1:2}"
which is not a valid JSON text.
Unmaintained (maintainer unresponsive for many months, bugs are not
getting fixed).
JSON::Syck 0.21
Very buggy (often crashes).
Very inflexible (no human-readable format supported, format pretty
much undocumented. I need at least a format for easy reading by
humans and a single-line compact format for use in a protocol, and
preferably a way to generate ASCII-only JSON texts).
Completely broken (and confusingly documented) Unicode handling
(Unicode escapes are not working properly, you need to set
ImplicitUnicode to *different* values on en- and decoding to get
symmetric behaviour).
No round-tripping (simple cases work, but this depends on whether
the scalar value was used in a numeric context or not).
Dumping hashes may skip hash values depending on iterator state.
Unmaintained (maintainer unresponsive for many months, bugs are not
getting fixed).
Does not check input for validity (i.e. will accept non-JSON input
and return "something" instead of raising an exception. This is a
security issue: imagine two banks transferring money between each
other using JSON. One bank might parse a given non-JSON request and
deduct money, while the other might reject the transaction with a
syntax error. While a good protocol will at least recover, that is
extra unnecessary work and the transaction will still not succeed).
JSON::DWIW 0.04
Very fast. Very natural. Very nice.
Undocumented Unicode handling (but the best of the pack. Unicode
escapes still don't get parsed properly).
Very inflexible.
No round-tripping.
Does not generate valid JSON texts (key strings are often unquoted,
empty keys result in nothing being output)
Does not check input for validity.
JSON and YAML
You often hear that JSON is a subset (or a close subset) of YAML. This
is, however, a mass hysteria and very far from the truth. In general,
there is no way to configure JSON::XS to output a data structure as
valid YAML.
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, so you should make sure that your hash
keys are noticeably shorter than the 1024 characters YAML allows.
There might be other incompatibilities that I am not aware of. 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.
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:
{"method": "handleMessage", "params": ["user1", "we were just talking"], \
"id": null, "array":[1,11,234,-5,1e5,1e7, true, false]}
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).
Higher is better:
module | encode | decode |
-----------|------------|------------|
JSON 1.x | 4990.842 | 4088.813 |
JSON::DWIW | 51653.990 | 71575.154 |
JSON::PC | 65948.176 | 74631.744 |
JSON::PP | 8931.652 | 3817.168 |
JSON::Syck | 24877.248 | 27776.848 |
JSON::XS | 388361.481 | 227951.304 |
JSON::XS/2 | 227951.304 | 218453.333 |
JSON::XS/3 | 338250.323 | 218453.333 |
Storable | 16500.016 | 135300.129 |
-----------+------------+------------+
That is, JSON::XS is about five times faster than JSON::DWIW on
encoding, about three times faster on decoding, and over forty times
faster than JSON, even with pretty-printing and key sorting. 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://nanoref.com/yahooapis/mgPdGg):
module | encode | decode |
-----------|------------|------------|
JSON 1.x | 55.260 | 34.971 |
JSON::DWIW | 825.228 | 1082.513 |
JSON::PC | 3571.444 | 2394.829 |
JSON::PP | 210.987 | 32.574 |
JSON::Syck | 552.551 | 787.544 |
JSON::XS | 5780.463 | 4854.519 |
JSON::XS/2 | 3869.998 | 4798.975 |
JSON::XS/3 | 5862.880 | 4798.975 |
Storable | 4445.002 | 5235.027 |
-----------+------------+------------+
Again, JSON::XS leads by far (except for Storable which non-surprisingly
decodes 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.
And last but least, something else could bomb you that I forgot to think
of. In that case, you get to keep the pieces. I am always open for
hints, though...
If you are using JSON::XS to return packets to consumption by JavaScript
scripts in a browser you should have a look at
<http://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 doing
security right).
THREADS
This module is *not* guaranteed to be thread safe and there are no plans
to change this until Perl gets thread support (as opposed to the
horribly slow so-called "threads" which are simply slow and bloated
process simulations - use fork, its *much* faster, cheaper, better).
(It might actually work, but you have been warned).
BUGS
While the goal of this module is to be correct, that unfortunately does
not mean its bug-free, only that I think its design is bug-free. It is
still relatively early in its development. 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.
AUTHOR
Marc Lehmann <schmorp@schmorp.de>
http://home.schmorp.de/
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