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45 non-PAUSE users.

Marc A. Lehmann


JSON::XS - JSON serialising/deserialising, done correctly and fast


 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;

 # objToJson and jsonToObj aliases to to_json and from_json
 # are exported for compatibility to the JSON module,
 # but should not be used in new code.

 # 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 (a simple scalar or a reference to a hash or array) 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 simple scalar or reference. Croaks on error.

This function call is functionally identical to:

   $perl_scalar = JSON::XS->new->utf8->decode ($json_text)

except being faster.


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.

If $enable is false, then the encode method will not escape Unicode characters unless required by the JSON syntax. This results in a faster and more compact format.

  JSON::XS->new->ascii (1)->encode ([chr 0x10401])
  => ["\ud801\udc01"]
$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 = $json->canonical ([$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 = $json->shrink ([$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 nearest power of two.

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.


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 or numeric (floating point) 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.

true, false

These JSON atoms become 0, 1, respectively. Information is lost in this process. Future versions might represent those values differently, but they will be guarenteed to act like these integers would normally in Perl.


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]
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.


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 JSON string:

   {"method": "handleMessage", "params": ["user1", "we were just talking"], "id": null}

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). Higher is better:

   module     |     encode |     decode |
   JSON       |  11488.516 |   7823.035 |
   JSON::DWIW |  94708.054 | 129094.260 |
   JSON::PC   |  63884.157 | 128528.212 |
   JSON::Syck |  34898.677 |  42096.911 |
   JSON::XS   | 654027.064 | 396423.669 |
   JSON::XS/2 | 371564.190 | 371725.613 |

That is, JSON::XS is more than six times faster than JSON::DWIW on encoding, more than three times faster on decoding, and about thirty times faster than JSON, even with pretty-printing and key sorting.

Using a longer test string (roughly 18KB, generated from Yahoo! Locals search API (http://nanoref.com/yahooapis/mgPdGg):

   module     |     encode |     decode |
   JSON       |    273.023 |     44.674 |
   JSON::DWIW |   1089.383 |   1145.704 |
   JSON::PC   |   3097.419 |   2393.921 |
   JSON::Syck |    514.060 |    843.053 |
   JSON::XS   |   6479.668 |   3636.364 |
   JSON::XS/2 |   3774.221 |   3599.124 |

Again, JSON::XS leads by far.

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.

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 alway sopen for hints, though...


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.


 Marc Lehmann <schmorp@schmorp.de>