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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
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.
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 overridden) with no overhead due to emulation (by inheriting
constructor and methods). If JSON::XS is not available, it will fall
back to the compatible JSON::PP module as backend, so using JSON instead
of JSON::XS gives you a portable JSON API that can be fast when you need
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 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, documents how and when it
does so, and even documents what "correct" means.
* round-trip integrity
When you serialise a perl data structure using only data types
supported by JSON and Perl, the deserialised data structure is
identical on the Perl level. (e.g. the string "2.0" doesn't suddenly
become "2" just because it looks like a number). There *are* minor
exceptions to this, read the MAPPING section below to learn about
* strict checking of JSON correctness
There is no guessing, no generating of illegal JSON texts by
default, and only JSON is accepted as input by default (the latter
is a security feature).
* fast
Compared to other JSON modules and other serialisers such as
Storable, this module usually compares favourably in terms of speed,
* simple to use
This module has both a simple functional interface as well as an
object oriented interface.
* reasonably versatile output formats
You can choose between the most compact guaranteed-single-line
format possible (nice for simple line-based protocols), a pure-ASCII
format (for when your transport is not 8-bit clean, still supports
the whole Unicode range), or a pretty-printed format (for when you
want to read that stuff). Or you can combine those features in
whatever way you like.
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.
Since this often leads to confusion, here are a few very clear words on
how Unicode works in Perl, modulo bugs.
1. Perl strings can store characters with ordinal values > 255.
This enables you to store Unicode characters as single characters in
a Perl string - very natural.
2. Perl does *not* associate an encoding with your strings.
... until you force it to, e.g. when matching it against a regex, or
printing the scalar to a file, in which case Perl either interprets
your string as locale-encoded text, octets/binary, or as Unicode,
depending on various settings. In no case is an encoding stored
together with your data, it is *use* that decides encoding, not any
magical meta data.
3. The internal utf-8 flag has no meaning with regards to the encoding
of your string.
Just ignore that flag unless you debug a Perl bug, a module written
in XS or want to dive into the internals of perl. Otherwise it will
only confuse you, as, despite the name, it says nothing about how
your string is encoded. You can have Unicode strings with that flag
set, with that flag clear, and you can have binary data with that
flag set and that flag clear. Other possibilities exist, too.
If you didn't know about that flag, just the better, pretend it
doesn't exist.
4. A "Unicode String" is simply a string where each character can be
validly interpreted as a Unicode code point.
If you have UTF-8 encoded data, it is no longer a Unicode string,
but a Unicode string encoded in UTF-8, giving you a binary string.
5. A string containing "high" (> 255) character values is *not* a UTF-8
It's 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
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
If $enable is false, then the "encode" method will not escape
Unicode characters unless required by the JSON syntax or other
flags. This results in a faster and more compact format.
See also the section *ENCODING/CODESET FLAG NOTES* later in this
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
See also the section *ENCODING/CODESET FLAG NOTES* later in this
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.
See also the section *ENCODING/CODESET FLAG NOTES* later in this
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])
$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
If $enable is false, no newlines or indenting will be produced, and
the resulting JSON text is guaranteed not to contain any "newlines".
This setting has no effect when decoding JSON texts.
$json = $json->space_before ([$enable])
$enabled = $json->get_space_before
If $enable is true (or missing), then the "encode" method will add
an extra optional space before the ":" separating keys from values
in JSON objects.
If $enable is false, then the "encode" method will not add any extra
space at those places.
This setting has no effect when decoding JSON texts. You will also
most likely combine this setting with "space_after".
Example, space_before enabled, space_after and indent disabled:
{"key" :"value"}
$json = $json->space_after ([$enable])
$enabled = $json->get_space_after
If $enable is true (or missing), then the "encode" method will add
an extra optional space after the ":" separating keys from values in
JSON objects and extra whitespace after the "," separating key-value
pairs and array members.
If $enable is false, then the "encode" method will not add any extra
space at those places.
This setting has no effect when decoding JSON texts.
Example, space_before and indent disabled, space_after enabled:
{"key": "value"}
$json = $json->relaxed ([$enable])
$enabled = $json->get_relaxed
If $enable is true (or missing), then "decode" will accept some
extensions to normal JSON syntax (see below). "encode" will not be
affected in anyway. *Be aware that this option makes you accept
invalid JSON texts as if they were valid!*. I suggest only to use
this option to parse application-specific files written by humans
(configuration files, resource files etc.)
If $enable is false (the default), then "decode" will only accept
valid JSON texts.
Currently accepted extensions are:
* list items can have an end-comma
JSON *separates* array elements and key-value pairs with commas.
This can be annoying if you write JSON texts manually and want
to be able to quickly append elements, so this extension accepts
comma at the end of such items not just between them:
2, <- this comma not normally allowed
"k1": "v1",
"k2": "v2", <- this comma not normally allowed
* shell-style '#'-comments
Whenever JSON allows whitespace, shell-style comments are
additionally allowed. They are terminated by the first
carriage-return or line-feed character, after which more
white-space and comments are allowed.
1, # this comment not allowed in JSON
# neither this one...
$json = $json->canonical ([$enable])
$enabled = $json->get_canonical
If $enable is true (or missing), then the "encode" method will
output JSON objects by sorting their keys. This is adding a
comparatively high overhead.
If $enable is false, then the "encode" method will output key-value
pairs in the order Perl stores them (which will likely change
between runs of the same script, and can change even within the same
run from 5.18 onwards).
This option is useful if you want the same data structure to be
encoded as the same JSON text (given the same overall settings). If
it is disabled, the same hash might be encoded differently even if
contains the same data, as key-value pairs have no inherent ordering
in Perl.
This setting has no effect when decoding JSON texts.
This setting has currently no effect on tied hashes.
$json = $json->allow_nonref ([$enable])
$enabled = $json->get_allow_nonref
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 = $json->allow_unknown ([$enable])
$enabled = $json->get_allow_unknown
If $enable is true (or missing), then "encode" will *not* throw an
exception when it encounters values it cannot represent in JSON (for
example, filehandles) but instead will encode a JSON "null" value.
Note that blessed objects are not included here and are handled
separately by c<allow_nonref>.
If $enable is false (the default), then "encode" will throw an
exception when it encounters anything it cannot encode as JSON.
This option does not affect "decode" in any way, and it is
recommended to leave it off unless you know your communications
$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 that it cannot convert
otherwise. Instead, a JSON "null" value is encoded instead of the
If $enable is false (the default), then "encode" will throw an
exception when it encounters a blessed object that it cannot convert
This setting has no effect on "decode".
$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
The "TO_JSON" method may safely call die if it wants. If "TO_JSON"
returns other blessed objects, those will be handled in the same
way. "TO_JSON" must take care of not causing an endless recursion
cycle (== crash) in this case. The name of "TO_JSON" was chosen
because other methods called by the Perl core (== not by the user of
the object) are usually in upper case letters and to avoid
collisions with any "to_json" function or method.
If $enable is false (the default), then "encode" will not consider
this type of conversion.
This setting has no effect on "decode".
$json = $json->allow_tags ([$enable])
$enabled = $json->allow_tags
If $enable is true (or missing), then "encode", upon encountering a
blessed object, will check for the availability of the "FREEZE"
method on the object's class. If found, it will be used to serialise
the object into a nonstandard tagged JSON value (that JSON decoders
cannot decode).
It also causes "decode" to parse such tagged JSON values and
deserialise them via a call to the "THAW" method.
If $enable is false (the default), then "encode" will not consider
this type of conversion, and tagged JSON values will cause a parse
error in "decode", as if tags were not part of the grammar.
$json = $json->filter_json_object ([$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
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 [=>
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}:
->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} =;
$WIDGET{$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
If $enable is true (or missing), the string returned by "encode"
will be shrunk-to-fit, while all strings generated by "decode" will
also be shrunk-to-fit.
If $enable is false, then the normal perl allocation algorithms are
used. If you work with your data, then this is likely to be faster.
In the future, this setting might control other things, such as
converting strings that look like integers or floats into integers
or floats internally (there is no difference on the Perl level),
saving space.
$json = $json->max_depth ([$maximum_nesting_depth])
$max_depth = $json->get_max_depth
Sets the maximum nesting level (default 512) accepted while encoding
or decoding. If a higher nesting level is detected in JSON text or a
Perl data structure, then the encoder and decoder will stop and
croak at that point.
Nesting level is defined by number of hash- or arrayrefs that the
encoder needs to traverse to reach a given point or the number of
"{" or "[" characters without their matching closing parenthesis
crossed to reach a given character in a string.
Setting the maximum depth to one disallows any nesting, so that
ensures that the object is only a single hash/object or array.
If no argument is given, the highest possible setting will be used,
which is rarely useful.
Note that nesting is implemented by recursion in C. The default
value has been chosen to be as large as typical operating systems
allow without crashing.
See SECURITY CONSIDERATIONS, below, for more info on why this is
$json = $json->max_size ([$maximum_string_size])
$max_size = $json->get_max_size
Set the maximum length a JSON text may have (in bytes) where
decoding is being attempted. The default is 0, meaning no limit.
When "decode" is called on a string that is longer then this many
bytes, it will not attempt to decode the string but throw an
exception. This setting has no effect on "encode" (yet).
If no argument is given, the limit check will be deactivated (same
as when 0 is specified).
See SECURITY CONSIDERATIONS, below, for more info on why this is
$json_text = $json->encode ($perl_scalar)
Converts the given Perl value or data structure to its JSON
representation. Croaks on error.
$perl_scalar = $json->decode ($json_text)
The opposite of "encode": expects a JSON text and tries to parse it,
returning the resulting simple scalar or reference. Croaks on error.
($perl_scalar, $characters) = $json->decode_prefix ($json_text)
This works like the "decode" method, but instead of raising an
exception when there is trailing garbage after the first JSON
object, it will silently stop parsing there and return the number of
characters consumed so far.
This is useful if your JSON texts are not delimited by an outer
protocol and you need to know where the JSON text ends.
JSON::XS->new->decode_prefix ("[1] the tail")
=> ([], 3)
In some cases, there is the need for incremental parsing of JSON texts.
While this module always has to keep both JSON text and resulting Perl
data structure in memory at one time, it does allow you to parse a JSON
stream incrementally. It does so by accumulating text until it has a
full JSON object, which it then can decode. This process is similar to
using "decode_prefix" to see if a full JSON object is available, but is
much more efficient (and can be implemented with a minimum of method
JSON::XS will only attempt to parse the JSON text once it is sure it has
enough text to get a decisive result, using a very simple but truly
incremental parser. This means that it sometimes won't stop as early as
the full parser, for example, it doesn't detect mismatched parentheses.
The only thing it guarantees is that it starts decoding as soon as a
syntactically valid JSON text has been seen. This means you need to set
resource limits (e.g. "max_size") to ensure the parser will stop parsing
in the presence if syntax errors.
The following methods implement this incremental parser.
[void, scalar or list context] = $json->incr_parse ([$string])
This is the central parsing function. It can both append new text
and extract objects from the stream accumulated so far (both of
these functions are optional).
If $string is given, then this string is appended to the already
existing JSON fragment stored in the $json object.
After that, if the function is called in void context, it will
simply return without doing anything further. This can be used to
add more text in as many chunks as you want.
If the method is called in scalar context, then it will try to
extract exactly *one* JSON object. If that is successful, it will
return this object, otherwise it will return "undef". If there is a
parse error, this method will croak just as "decode" would do (one
can then use "incr_skip" to skip the erroneous part). This is the
most common way of using the method.
And finally, in list context, it will try to extract as many objects
from the stream as it can find and return them, or the empty list
otherwise. For this to work, there must be no separators between the
JSON objects or arrays, instead they must be concatenated
back-to-back. If an error occurs, an exception will be raised as in
the scalar context case. Note that in this case, any
previously-parsed JSON texts will be lost.
Example: Parse some JSON arrays/objects in a given string and return
my @objs = JSON::XS->new->incr_parse ("[5][7][1,2]");
$lvalue_string = $json->incr_text
This method returns the currently stored JSON fragment as an lvalue,
that is, you can manipulate it. This *only* works when a preceding
call to "incr_parse" in *scalar context* successfully returned an
object. Under all other circumstances you must not call this
function (I mean it. although in simple tests it might actually
work, it *will* fail under real world conditions). As a special
exception, you can also call this method before having parsed
This function is useful in two cases: a) finding the trailing text
after a JSON object or b) parsing multiple JSON objects separated by
non-JSON text (such as commas).
This will reset the state of the incremental parser and will remove
the parsed text from the input buffer so far. This is useful after
"incr_parse" died, in which case the input buffer and incremental
parser state is left unchanged, to skip the text parsed so far and
to reset the parse state.
The difference to "incr_reset" is that only text until the parse
error occurred is removed.
This completely resets the incremental parser, that is, after this
call, it will be as if the parser had never parsed anything.
This is useful if you want to repeatedly parse JSON objects and want
to ignore any trailing data, which means you have to reset the
parser after each successful decode.
All options that affect decoding are supported, except "allow_nonref".
The reason for this is that it cannot be made to work sensibly: JSON
objects and arrays are self-delimited, i.e. you can concatenate them
back to back and still decode them perfectly. This does not hold true
for JSON numbers, however.
For example, is the string 1 a single JSON number, or is it simply the
start of 12? Or is 12 a single JSON number, or the concatenation of 1
and 2? In neither case you can tell, and this is why JSON::XS takes the
conservative route and disallows this case.
Some examples will make all this clearer. First, a simple example that
works similarly to "decode_prefix": We want to decode the JSON object at
the start of a string and identify the portion after the JSON object:
my $text = "[1,2,3] hello";
my $json = new JSON::XS;
my $obj = $json->incr_parse ($text)
or die "expected JSON object or array at beginning of string";
my $tail = $json->incr_text;
# $tail now contains " hello"
Easy, isn't it?
Now for a more complicated example: Imagine a hypothetical protocol
where you read some requests from a TCP stream, and each request is a
JSON array, without any separation between them (in fact, it is often
useful to use newlines as "separators", as these get interpreted as
whitespace at the start of the JSON text, which makes it possible to
test said protocol with "telnet"...).
Here is how you'd do it (it is trivial to write this in an event-based
my $json = new JSON::XS;
# read some data from the socket
while (sysread $socket, my $buf, 4096) {
# split and decode as many requests as possible
for my $request ($json->incr_parse ($buf)) {
# act on the $request
Another complicated example: Assume you have a string with JSON objects
or arrays, all separated by (optional) comma characters (e.g. "[1],[2],
[3]"). To parse them, we have to skip the commas between the JSON texts,
and here is where the lvalue-ness of "incr_text" comes in useful:
my $text = "[1],[2], [3]";
my $json = new JSON::XS;
# void context, so no parsing done
$json->incr_parse ($text);
# now extract as many objects as possible. note the
# use of scalar context so incr_text can be called.
while (my $obj = $json->incr_parse) {
# do something with $obj
# now skip the optional comma
$json->incr_text =~ s/^ \s* , //x;
Now lets go for a very complex example: Assume that you have a gigantic
JSON array-of-objects, many gigabytes in size, and you want to parse it,
but you cannot load it into memory fully (this has actually happened in
the real world :).
Well, you lost, you have to implement your own JSON parser. But JSON::XS
can still help you: You implement a (very simple) array parser and let
JSON decode the array elements, which are all full JSON objects on their
own (this wouldn't work if the array elements could be JSON numbers, for
my $json = new JSON::XS;
# open the monster
open my $fh, "<bigfile.json"
or die "bigfile: $!";
# first parse the initial "["
for (;;) {
sysread $fh, my $buf, 65536
or die "read error: $!";
$json->incr_parse ($buf); # void context, so no parsing
# Exit the loop once we found and removed(!) the initial "[".
# In essence, we are (ab-)using the $json object as a simple scalar
# we append data to.
last if $json->incr_text =~ s/^ \s* \[ //x;
# now we have the skipped the initial "[", so continue
# parsing all the elements.
for (;;) {
# in this loop we read data until we got a single JSON object
for (;;) {
if (my $obj = $json->incr_parse) {
# do something with $obj
# add more data
sysread $fh, my $buf, 65536
or die "read error: $!";
$json->incr_parse ($buf); # void context, so no parsing
# in this loop we read data until we either found and parsed the
# separating "," between elements, or the final "]"
for (;;) {
# first skip whitespace
$json->incr_text =~ s/^\s*//;
# if we find "]", we are done
if ($json->incr_text =~ s/^\]//) {
print "finished.\n";
# if we find ",", we can continue with the next element
if ($json->incr_text =~ s/^,//) {
# if we find anything else, we have a parse error!
if (length $json->incr_text) {
die "parse error near ", $json->incr_text;
# else add more data
sysread $fh, my $buf, 65536
or die "read error: $!";
$json->incr_parse ($buf); # void context, so no parsing
This is a complex example, but most of the complexity comes from the
fact that we are trying to be correct (bear with me if I am wrong, I
never ran the above example :).
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.
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
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 (in which case you lose roundtripping ability, as the
JSON number will be re-encoded to a JSON string).
Numbers containing a fractional or exponential part will always be
represented as numeric (floating point) values, possibly at a loss
of precision (in which case you might lose perfect roundtripping
ability, but the JSON number will still be re-encoded as a JSON
Note that precision is not accuracy - binary floating point values
cannot represent most decimal fractions exactly, and when converting
from and to floating point, JSON::XS only guarantees precision up to
but not including the least significant bit.
true, false
These JSON atoms become "Types::Serialiser::true" and
"Types::Serialiser::false", respectively. They are overloaded to act
almost exactly like the numbers 1 and 0. You can check whether a
scalar is a JSON boolean by using the "Types::Serialiser::is_bool"
function (after "use Types::Serialier", of course).
A JSON null atom becomes "undef" in Perl.
shell-style comments ("# *text*")
As a nonstandard extension to the JSON syntax that is enabled by the
"relaxed" setting, shell-style comments are allowed. They can start
anywhere outside strings and go till the end of the line.
tagged values ("(*tag*)*value*").
Another nonstandard extension to the JSON syntax, enabled with the
"allow_tags" setting, are tagged values. In this implementation, the
*tag* must be a perl package/class name encoded as a JSON string,
and the *value* must be a JSON array encoding optional constructor
See "OBJECT SERIALISATION", below, for details.
The mapping from Perl to JSON is slightly more difficult, as Perl is a
truly typeless language, so we can only guess which JSON type is meant
by a Perl value.
hash references
Perl hash references become JSON objects. As there is no inherent
ordering in hash keys (or JSON objects), they will usually be
encoded in a pseudo-random order. JSON::XS can optionally sort the
hash keys (determined by the *canonical* flag), so the same
datastructure will serialise to the same JSON text (given same
settings and version of JSON::XS), but this incurs a runtime
overhead and is only rarely useful, e.g. when you want to compare
some JSON text against another for equality.
array references
Perl array references become JSON arrays.
other references
Other unblessed references are generally not allowed and will cause
an exception to be thrown, except for references to the integers 0
and 1, which get turned into "false" and "true" atoms in JSON.
Since "JSON::XS" uses the boolean model from Types::Serialiser, you
can also "use Types::Serialiser" and then use
"Types::Serialiser::false" and "Types::Serialiser::true" to improve
use Types::Serialiser;
encode_json [\0, Types::Serialiser::true] # yields [false,true]
Types::Serialiser::true, Types::Serialiser::false
These special values from the Types::Serialiser module become JSON
true and JSON false values, respectively. You can also use "\1" and
"\0" directly if you want.
blessed objects
Blessed objects are not directly representable in JSON, but
"JSON::XS" allows various ways of handling objects. See "OBJECT
SERIALISATION", below, for details.
simple scalars
Simple Perl scalars (any scalar that is not a reference) are the
most difficult objects to encode: JSON::XS will encode undefined
scalars as JSON "null" values, scalars that have last been used in a
string context before encoding as JSON strings, and anything else as
number value:
# dump as number
encode_json [2] # yields [2]
encode_json [-3.0e17] # yields [-3e+17]
my $value = 5; encode_json [$value] # yields [5]
# used as string, so dump as string
print $value;
encode_json [$value] # yields ["5"]
# undef becomes null
encode_json [undef] # yields [null]
You can force the type to be a JSON string by stringifying it:
my $x = 3.1; # some variable containing a number
"$x"; # stringified
$x .= ""; # another, more awkward way to stringify
print $x; # perl does it for you, too, quite often
You can force the type to be a JSON number by numifying it:
my $x = "3"; # some variable containing a string
$x += 0; # numify it, ensuring it will be dumped as a number
$x *= 1; # same thing, the choice is yours.
You can not currently force the type in other, less obscure, ways.
Tell me if you need this capability (but don't forget to explain why
it's needed :).
Note that numerical precision has the same meaning as under Perl (so
binary to decimal conversion follows the same rules as in Perl,
which can differ to other languages). Also, your perl interpreter
might expose extensions to the floating point numbers of your
platform, such as infinities or NaN's - these cannot be represented
in JSON, and it is an error to pass those in.
As JSON cannot directly represent Perl objects, you have to choose
between a pure JSON representation (without the ability to deserialise
the object automatically again), and a nonstandard extension to the JSON
syntax, tagged values.
What happens when "JSON::XS" encounters a Perl object depends on the
"allow_blessed", "convert_blessed" and "allow_tags" settings, which are
used in this order:
1. "allow_tags" is enabled and the object has a "FREEZE" method.
In this case, "JSON::XS" uses the Types::Serialiser object
serialisation protocol to create a tagged JSON value, using a
nonstandard extension to the JSON syntax.
This works by invoking the "FREEZE" method on the object, with the
first argument being the object to serialise, and the second
argument being the constant string "JSON" to distinguish it from
other serialisers.
The "FREEZE" method can return any number of values (i.e. zero or
more). These values and the paclkage/classname of the object will
then be encoded as a tagged JSON value in the following format:
("classname")[FREEZE return values...]
For example, the hypothetical "My::Object" "FREEZE" method might use
the objects "type" and "id" members to encode the object:
sub My::Object::FREEZE {
my ($self, $serialiser) = @_;
($self->{type}, $self->{id})
2. "convert_blessed" is enabled and the object has a "TO_JSON" method.
In this case, the "TO_JSON" method of the object is invoked in
scalar context. It must return a single scalar that can be directly
encoded into JSON. This scalar replaces the object in the JSON text.
For example, the following "TO_JSON" method will convert all URI
objects to JSON strings when serialised. The fatc that these values
originally were URI objects is lost.
sub URI::TO_JSON {
my ($uri) = @_;
3. "allow_blessed" is enabled.
The object will be serialised as a JSON null value.
4. none of the above
If none of the settings are enabled or the respective methods are
missing, "JSON::XS" throws an exception.
For deserialisation there are only two cases to consider: either
nonstandard tagging was used, in which case "allow_tags" decides, or
objects cannot be automatically be deserialised, in which case you can
use postprocessing or the "filter_json_object" or
"filter_json_single_key_object" callbacks to get some real objects our
of your JSON.
This section only considers the tagged value case: I a tagged JSON
object is encountered during decoding and "allow_tags" is disabled, a
parse error will result (as if tagged values were not part of the
If "allow_tags" is enabled, "JSON::XS" will look up the "THAW" method of
the package/classname used during serialisation (it will not attempt to
load the package as a Perl module). If there is no such method, the
decoding will fail with an error.
Otherwise, the "THAW" method is invoked with the classname as first
argument, the constant string "JSON" as second argument, and all the
values from the JSON array (the values originally returned by the
"FREEZE" method) as remaining arguments.
The method must then return the object. While technically you can return
any Perl scalar, you might have to enable the "enable_nonref" setting to
make that work in all cases, so better return an actual blessed
As an example, let's implement a "THAW" function that regenerates the
"My::Object" from the "FREEZE" example earlier:
sub My::Object::THAW {
my ($class, $serialiser, $type, $id) = @_;
$class->new (type => $type, id => $id)
The interested reader might have seen a number of flags that signify
encodings or codesets - "utf8", "latin1" and "ascii". There seems to be
some confusion on what these do, so here is a short comparison:
"utf8" controls whether the JSON text created by "encode" (and expected
by "decode") is UTF-8 encoded or not, while "latin1" and "ascii" only
control whether "encode" escapes character values outside their
respective codeset range. Neither of these flags conflict with each
other, although some combinations make less sense than others.
Care has been taken to make all flags symmetrical with respect to
"encode" and "decode", that is, texts encoded with any combination of
these flag values will be correctly decoded when the same flags are used
- in general, if you use different flag settings while encoding vs. when
decoding you likely have a bug somewhere.
Below comes a verbose discussion of these flags. Note that a "codeset"
is simply an abstract set of character-codepoint pairs, while an
encoding takes those codepoint numbers and *encodes* them, in our case
into octets. Unicode is (among other things) a codeset, UTF-8 is an
encoding, and ISO-8859-1 (= latin 1) and ASCII are both codesets *and*
encodings at the same time, which can be confusing.
"utf8" flag disabled
When "utf8" is disabled (the default), then "encode"/"decode"
generate and expect Unicode strings, that is, characters with high
ordinal Unicode values (> 255) will be encoded as such characters,
and likewise such characters are decoded as-is, no changes to them
will be done, except "(re-)interpreting" them as Unicode codepoints
or Unicode characters, respectively (to Perl, these are the same
thing in strings unless you do funny/weird/dumb stuff).
This is useful when you want to do the encoding yourself (e.g. when
you want to have UTF-16 encoded JSON texts) or when some other layer
does the encoding for you (for example, when printing to a terminal
using a filehandle that transparently encodes to UTF-8 you certainly
do NOT want to UTF-8 encode your data first and have Perl encode it
another time).
"utf8" flag enabled
If the "utf8"-flag is enabled, "encode"/"decode" will encode all
characters using the corresponding UTF-8 multi-byte sequence, and
will expect your input strings to be encoded as UTF-8, that is, no
"character" of the input string must have any value > 255, as UTF-8
does not allow that.
The "utf8" flag therefore switches between two modes: disabled means
you will get a Unicode string in Perl, enabled means you get an
UTF-8 encoded octet/binary string in Perl.
"latin1" or "ascii" flags enabled
With "latin1" (or "ascii") enabled, "encode" will escape characters
with ordinal values > 255 (> 127 with "ascii") and encode the
remaining characters as specified by the "utf8" flag.
If "utf8" is disabled, then the result is also correctly encoded in
those character sets (as both are proper subsets of Unicode, meaning
that a Unicode string with all character values < 256 is the same
thing as a ISO-8859-1 string, and a Unicode string with all
character values < 128 is the same thing as an ASCII string in
If "utf8" is enabled, you still get a correct UTF-8-encoded string,
regardless of these flags, just some more characters will be escaped
using "\uXXXX" then before.
Note that ISO-8859-1-*encoded* strings are not compatible with UTF-8
encoding, while ASCII-encoded strings are. That is because the
ISO-8859-1 encoding is NOT a subset of UTF-8 (despite the ISO-8859-1
*codeset* being a subset of Unicode), while ASCII is.
Surprisingly, "decode" will ignore these flags and so treat all
input values as governed by the "utf8" flag. If it is disabled, this
allows you to decode ISO-8859-1- and ASCII-encoded strings, as both
strict subsets of Unicode. If it is enabled, you can correctly
decode UTF-8 encoded strings.
So neither "latin1" nor "ascii" are incompatible with the "utf8"
flag - they only govern when the JSON output engine escapes a
character or not.
The main use for "latin1" is to relatively efficiently store binary
data as JSON, at the expense of breaking compatibility with most
JSON decoders.
The main use for "ascii" is to force the output to not contain
characters with values > 127, which means you can interpret the
resulting string as UTF-8, ISO-8859-1, ASCII, KOI8-R or most about
any character set and 8-bit-encoding, and still get the same data
structure back. This is useful when your channel for JSON transfer
is not 8-bit clean or the encoding might be mangled in between (e.g.
in mail), and works because ASCII is a proper subset of most 8-bit
and multibyte encodings in use in the world.
JSON and ECMAscript
JSON syntax is based on how literals are represented in javascript (the
not-standardised predecessor of ECMAscript) which is presumably why it
is called "JavaScript Object Notation".
However, JSON is not a subset (and also not a superset of course) of
ECMAscript (the standard) or javascript (whatever browsers actually
If you want to use javascript's "eval" function to "parse" JSON, you
might run into parse errors for valid JSON texts, or the resulting data
structure might not be queryable:
One of the problems is that U+2028 and U+2029 are valid characters
inside JSON strings, but are not allowed in ECMAscript string literals,
so the following Perl fragment will not output something that can be
guaranteed to be parsable by javascript's "eval":
use JSON::XS;
print encode_json [chr 0x2028];
The right fix for this is to use a proper JSON parser in your javascript
programs, and not rely on "eval" (see for example Douglas Crockford's
json2.js parser).
If this is not an option, you can, as a stop-gap measure, simply encode
to ASCII-only JSON:
use JSON::XS;
print JSON::XS->new->ascii->encode ([chr 0x2028]);
Note that this will enlarge the resulting JSON text quite a bit if you
have many non-ASCII characters. You might be tempted to run some regexes
to only escape U+2028 and U+2029, e.g.:
my $json = JSON::XS->new->utf8->encode ([chr 0x2028]);
$json =~ s/\xe2\x80\xa8/\\u2028/g; # escape U+2028
$json =~ s/\xe2\x80\xa9/\\u2029/g; # escape U+2029
print $json;
Note that *this is a bad idea*: the above only works for U+2028 and
U+2029 and thus only for fully ECMAscript-compliant parsers. Many
existing javascript implementations, however, have issues with other
characters as well - using "eval" naively simply *will* cause problems.
Another problem is that some javascript implementations reserve some
property names for their own purposes (which probably makes them
non-ECMAscript-compliant). For example, Iceweasel reserves the
"__proto__" property name for its own purposes.
If that is a problem, you could parse try to filter the resulting JSON
output for these property strings, e.g.:
$json =~ s/"__proto__"\s*:/"__proto__renamed":/g;
This works because "__proto__" is not valid outside of strings, so every
occurrence of ""__proto__"\s*:" must be a string used as property name.
If you know of other incompatibilities, please let me know.
You often hear that JSON is a subset of YAML. This is, however, a mass
hysteria(*) and very far from the truth (as of the time of this
writing), so let me state it clearly: *in general, there is no way to
configure JSON::XS to output a data structure as valid YAML* that works
in all cases.
If you really must use JSON::XS to generate YAML, you should use this
algorithm (subject to change in future versions):
my $to_yaml = JSON::XS->new->utf8->space_after (1);
my $yaml = $to_yaml->encode ($ref) . "\n";
This will *usually* generate JSON texts that also parse as valid YAML.
Please note that YAML has hardcoded limits on (simple) object key
lengths that JSON doesn't have and also has different and incompatible
unicode character escape syntax, so you should make sure that your hash
keys are noticeably shorter than the 1024 "stream characters" YAML
allows and that you do not have characters with codepoint values outside
the Unicode BMP (basic multilingual page). YAML also does not allow "\/"
sequences in strings (which JSON::XS does not *currently* generate, but
other JSON generators might).
There might be other incompatibilities that I am not aware of (or the
YAML specification has been changed yet again - it does so quite often).
In general you should not try to generate YAML with a JSON generator or
vice versa, or try to parse JSON with a YAML parser or vice versa:
chances are high that you will run into severe interoperability problems
when you least expect it.
(*) I have been pressured multiple times by Brian Ingerson (one of the
authors of the YAML specification) to remove this paragraph, despite
him acknowledging that the actual incompatibilities exist. As I was
personally bitten by this "JSON is YAML" lie, I refused and said I
will continue to educate people about these issues, so others do not
run into the same problem again and again. After this, Brian called
me a (quote)*complete and worthless idiot*(unquote).
In my opinion, instead of pressuring and insulting people who
actually clarify issues with YAML and the wrong statements of some
of its proponents, I would kindly suggest reading the JSON spec
(which is not that difficult or long) and finally make YAML
compatible to it, and educating users about the changes, instead of
spreading lies about the real compatibility for many *years* and
trying to silence people who point out that it isn't true.
Addendum/2009: the YAML 1.2 spec is still incompatible with JSON,
even though the incompatibilities have been documented (and are
known to Brian) for many years and the spec makes explicit claims
that YAML is a superset of JSON. It would be so easy to fix, but
apparently, bullying people and corrupting userdata is so much
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
First comes a comparison between various modules using a very short
single-line JSON string (also available at
{"method": "handleMessage", "params": ["user1",
"we were just talking"], "id": null, "array":[1,11,234,-5,1e5,1e7,
1, 0]}
It shows the number of encodes/decodes per second (JSON::XS uses the
functional interface, while JSON::XS/2 uses the OO interface with
pretty-printing and hashkey sorting enabled, JSON::XS/3 enables shrink.
JSON::DWIW/DS uses the deserialise function, while JSON::DWIW::FJ uses
the from_json method). Higher is better:
module | encode | decode |
JSON::DWIW/DS | 86302.551 | 102300.098 |
JSON::DWIW/FJ | 86302.551 | 75983.768 |
JSON::PP | 15827.562 | 6638.658 |
JSON::Syck | 63358.066 | 47662.545 |
JSON::XS | 511500.488 | 511500.488 |
JSON::XS/2 | 291271.111 | 388361.481 |
JSON::XS/3 | 361577.931 | 361577.931 |
Storable | 66788.280 | 265462.278 |
That is, JSON::XS is almost six times faster than JSON::DWIW on
encoding, about five times faster on decoding, and over thirty to
seventy times faster than JSON's pure perl implementation. It also
compares favourably to Storable for small amounts of data.
Using a longer test string (roughly 18KB, generated from Yahoo! Locals
search API (<>).
module | encode | decode |
JSON::DWIW/DS | 1647.927 | 2673.916 |
JSON::DWIW/FJ | 1630.249 | 2596.128 |
JSON::PP | 400.640 | 62.311 |
JSON::Syck | 1481.040 | 1524.869 |
JSON::XS | 20661.596 | 9541.183 |
JSON::XS/2 | 10683.403 | 9416.938 |
JSON::XS/3 | 20661.596 | 9400.054 |
Storable | 19765.806 | 10000.725 |
Again, JSON::XS leads by far (except for Storable which non-surprisingly
decodes a bit faster).
On large strings containing lots of high Unicode characters, some
modules (such as JSON::PC) seem to decode faster than JSON::XS, but the
result will be broken due to missing (or wrong) Unicode handling. Others
refuse to decode or encode properly, so it was impossible to prepare a
fair comparison table for that case.
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
Third, JSON::XS recurses using the C stack when decoding objects and
arrays. The C stack is a limited resource: for instance, on my amd64
machine with 8MB of stack size I can decode around 180k nested arrays
but only 14k nested JSON objects (due to perl itself recursing deeply on
croak to free the temporary). If that is exceeded, the program crashes.
To be conservative, the default nesting limit is set to 512. If your
process has a smaller stack, you should adjust this setting accordingly
with the "max_depth" method.
Something else could bomb you, too, that I forgot to think of. In that
case, you get to keep the pieces. I am always open for hints, though...
Also keep in mind that JSON::XS might leak contents of your Perl data
structures in its error messages, so when you serialise sensitive
information you might want to make sure that exceptions thrown by
JSON::XS will not end up in front of untrusted eyes.
If you are using JSON::XS to return packets to consumption by JavaScript
scripts in a browser you should have a look at
to see whether you are vulnerable to some common attack vectors (which
really are browser design bugs, but it is still you who will have to
deal with it, as major browser developers care only for features, not
about getting security right).
"JSON::XS" uses the Types::Serialiser module to provide boolean
constants. That means that the JSON true and false values will be
comaptible to true and false values of iother modules that do the same,
such as JSON::PP and CBOR::XS.
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, it's *much* faster, cheaper, better).
(It might actually work, but you have been warned).
Sometimes people avoid the Perl locale support and directly call the
system's setlocale function with "LC_ALL".
This breaks both perl and modules such as JSON::XS, as stringification
of numbers no longer works correctly (e.g. "$x = 0.1; print "$x"+1"
might print 1, and JSON::XS might output illegal JSON as JSON::XS relies
on perl to stringify numbers).
The solution is simple: don't call "setlocale", or use it for only those
categories you need, such as "LC_MESSAGES" or "LC_CTYPE".
If you need "LC_NUMERIC", you should enable it only around the code that
actually needs it (avoiding stringification of numbers), and restore it
While the goal of this module is to be correct, that unfortunately does
not mean it's bug-free, only that I think its design is bug-free. If you
keep reporting bugs they will be fixed swiftly, though.
Please refrain from using or any other bug reporting
service. I put the contact address into my modules for a reason.
The json_xs command line utility for quick experiments.
Marc Lehmann <>