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Under GNU General Public License
2012-05-24 14:54 @38.107.179.236 Crawled by CCBot/1.0 (+http://www.commoncrawl.org/bot.html)
XS(3pm) User Contributed Perl Documentation XS(3pm)
NAME
JSON::XS - JSON serialising/deserialising, done correctly and fast
JSON::XS - aeXa~XXa~XXa~XXeXXeXXa~XX JSON
a~XXa~XXa~XXa~XXa~XXa~XX/a~XXa~XXa~XXa~XXa~XXa~XXa~XX
(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
o correct Unicode handling
This module knows how to handle Unicode, and even documents how and when it does so.
o 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).
o 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).
o fast
Compared to other JSON modules, this module compares favourably in terms of speed,
too.
o simple to use
This module has both a simple functional interface as well as an OO interface.
o 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:
o 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
}
o 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 AT schmorp.de>
http://home.schmorp.de/
perl v5.10.0 2007-12-05 XS(3pm)
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