++ed by:

63 PAUSE users
45 non-PAUSE users.

Marc A. Lehmann


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)


 use JSON::XS;

 # exported functions, they croak on error
 # and expect/generate UTF-8

 $utf8_encoded_json_text = to_json $perl_hash_or_arrayref;
 $perl_hash_or_arrayref  = from_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);


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.

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.


  • 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 guarenteed 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.


The following convinience methods are provided by this module. They are exported by default:

$json_text = to_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 = from_json $json_text

The opposite of to_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.


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.

Its a fact. Learn to live with it.

I hope this helps :)


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])

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])

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 transfering), 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])

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" : [
$json = $json->indent ([$enable])

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, identing them properly.

If $enable is false, no newlines or indenting will be produced, and the resulting JSON text is guarenteed not to contain any newlines.

This setting has no effect when decoding JSON texts.

$json = $json->space_before ([$enable])

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])

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])

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:

          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])

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 migh 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])

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])

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 wether 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])

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 the to_json function.

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 (its 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}:
      ->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])

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])

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])

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)


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 uppcercase Perl refers to the abstract Perl language itself.



A JSON object becomes a reference to a hash in Perl. No ordering of object keys is preserved (JSON does not preserver object key ordering itself).


A JSON array becomes a reference to an array in Perl.


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.


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 wether a scalar is a JSON boolean by using the JSON::XS::is_bool function.


A JSON null atom becomes undef in Perl.


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.

   to_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
   to_json [2]                      # yields [2]
   to_json [-3.0e17]                # yields [-3e+17]
   my $value = 5; to_json [$value]  # yields [5]

   # used as string, so dump as string
   print $value;
   to_json [$value]                 # yields ["5"]

   # undef becomes null
   to_json [undef]                  # yields [null]

You can force the type to be a string by stringifying it:

   my $x = 3.1; # some variable containing a number
   "$x";        # stringified
   $x .= "";    # another, more awkward way to stringify
   print $x;    # perl does it for you, too, quite often

You can force the type to be a number by numifying it:

   my $x = "3"; # some variable containing a string
   $x += 0;     # numify it, ensuring it will be dumped as a number
   $x *= 1;     # same thing, the choise is yours.

You can not currently output JSON booleans or force the type in other, less obscure, ways. Tell me if you need this capability.


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 roundtripping (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 roundtripping.

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 roundtripping (simple cases work, but this depends on wether 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 transfering 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).


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 roundtripping.

Does not generate valid JSON texts (key strings are often unquoted, empty keys result in nothing being output)

Does not check input for validity.


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 noticably 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.


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:

   Storable   |  15779.925 |  14169.946 |
   module     |     encode |     decode |
   JSON       |   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 fourty 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       |     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.


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, thats 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 wether 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).


This module is not guarenteed 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 ahve ben warned).


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.


 Marc Lehmann <schmorp@schmorp.de>

1 POD Error

The following errors were encountered while parsing the POD:

Around line 5:

Non-ASCII character seen before =encoding in '正しくて高速な'. Assuming UTF-8