XS(3) User Contributed Perl Documentation XS(3)
NAME
JSON::XS - JSON serialising/deserialising, done correctly and fast
JSON::XS - <?><?><?><?><?><?><?> JSON
<?><?><?><?><?><?>/<?><?><?><?><?><?><?>
(http://fleur.hio.jp/perldoc/mix/lib/JSON/XS.html)
SYNOPSIS
use JSON::XS;
# exported functions, they croak on error
# and expect/generate UTF-8
$utf8_encoded_json_text = encode_json $perl_hash_or_arrayref;
$perl_hash_or_arrayref = decode_json $utf8_encoded_json_text;
# OO-interface
$coder = JSON::XS->new->ascii->pretty->allow_nonref;
$pretty_printed_unencoded = $coder->encode ($perl_scalar);
$perl_scalar = $coder->decode ($unicode_json_text);
# Note that JSON version 2.0 and above will automatically use JSON::XS
# if available, at virtually no speed overhead either, so you should
# be able to just:
use JSON;
# and do the same things, except that you have a pure-perl fallback now.
DESCRIPTION
This module converts Perl data structures to JSON and vice versa. Its
primary goal is to be correct and its secondary goal is to be fast. To
reach the latter goal it was written in C.
See MAPPING, below, on how JSON::XS maps perl values to JSON values and
vice versa.
FEATURES
o correct Unicode handling
This module knows how to handle Unicode, documents how and when it
does so, and even documents what "correct" means.
o 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
those.
o strict checking of JSON correctness
There is no guessing, no generating of illegal JSON texts by default,
and only JSON is accepted as input by default (the latter is a
security feature).
o fast
Compared to other JSON modules and other serialisers such as
Storable, this module usually compares favourably in terms of speed,
too.
o simple to use
This module has both a simple functional interface as well as an
object oriented interface.
o reasonably versatile output formats
You can choose between the most compact guaranteed-single-line format
possible (nice for simple line-based protocols), a pure-ASCII format
(for when your transport is not 8-bit clean, still supports the whole
Unicode range), or a pretty-printed format (for when you want to read
that stuff). Or you can combine those features in whatever way you
like.
FUNCTIONAL INTERFACE
The following convenience methods are provided by this module. They are
exported by default:
$json_text = encode_json $perl_scalar
Converts the given Perl data structure to a UTF-8 encoded, binary
string (that is, the string contains octets only). Croaks on error.
This function call is functionally identical to:
$json_text = JSON::XS->new->utf8->encode ($perl_scalar)
Except being faster.
$perl_scalar = decode_json $json_text
The opposite of "encode_json": expects a UTF-8 (binary) string and
tries to parse that as a 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.
A FEW NOTES ON UNICODE AND PERL
Since this often leads to confusion, here are a few very clear words on
how Unicode works in Perl, modulo bugs.
1. Perl strings can store characters with ordinal values > 255.
This enables you to store Unicode characters as single characters in
a Perl string - very natural.
2. Perl does not associate an encoding with your strings.
... 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
string.
It's a fact. Learn to live with it.
I hope this helps :)
OBJECT-ORIENTED INTERFACE
The object oriented interface lets you configure your own encoding or
decoding style, within the limits of supported formats.
$json = new JSON::XS
Creates a new JSON::XS object that can be used to de/encode JSON
strings. All boolean flags described below are by default disabled
(with the exception of "allow_nonref", which defaults to enabled
since version 4.0).
The mutators for flags all return the JSON object again and thus
calls can be chained:
my $json = JSON::XS->new->utf8->space_after->encode ({a => [1,2]})
=> {"a": [1, 2]}
$json = $json->ascii ([$enable])
$enabled = $json->get_ascii
If $enable is true (or missing), then the "encode" method will not
generate characters outside the code range 0..127 (which is ASCII).
Any Unicode characters outside that range will be escaped using
either a single \uXXXX (BMP characters) or a double \uHHHH\uLLLLL
escape sequence, as per RFC4627. The resulting encoded JSON text can
be treated as a native Unicode string, an ascii-encoded,
latin1-encoded or UTF-8 encoded string, or any other superset of
ASCII.
If $enable is false, then the "encode" method will not escape Unicode
characters unless required by the JSON syntax or other flags. This
results in a faster and more compact format.
See also the section ENCODING/CODESET FLAG NOTES later in this
document.
The main use for this flag is to produce JSON texts that can be
transmitted over a 7-bit channel, as the encoded JSON texts will not
contain any 8 bit characters.
JSON::XS->new->ascii (1)->encode ([chr 0x10401])
=> ["\ud801\udc01"]
$json = $json->latin1 ([$enable])
$enabled = $json->get_latin1
If $enable is true (or missing), then the "encode" method will encode
the resulting JSON text as latin1 (or iso-8859-1), escaping any
characters outside the code range 0..255. The resulting string can be
treated as a latin1-encoded JSON text or a native Unicode string. The
"decode" method will not be affected in any way by this flag, as
"decode" by default expects Unicode, which is a strict superset of
latin1.
If $enable is false, then the "encode" method will not escape Unicode
characters unless required by the JSON syntax or other flags.
See also the section ENCODING/CODESET FLAG NOTES later in this
document.
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 handed a 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
document.
Example, output UTF-16BE-encoded JSON:
use Encode;
$jsontext = encode "UTF-16BE", JSON::XS->new->encode ($object);
Example, decode UTF-32LE-encoded JSON:
use Encode;
$object = JSON::XS->new->decode (decode "UTF-32LE", $jsontext);
$json = $json->pretty ([$enable])
This enables (or disables) all of the "indent", "space_before" and
"space_after" (and in the future possibly more) flags in one call to
generate the most readable (or most compact) form possible.
Example, pretty-print some simple structure:
my $json = JSON::XS->new->pretty(1)->encode ({a => [1,2]})
=>
{
"a" : [
1,
2
]
}
$json = $json->indent ([$enable])
$enabled = $json->get_indent
If $enable is true (or missing), then the "encode" method will use a
multiline format as output, putting every array member or object/hash
key-value pair into its own line, indenting them properly.
If $enable is false, no newlines or indenting will be produced, and
the resulting JSON text is guaranteed not to contain any "newlines".
This setting has no effect when decoding JSON texts.
$json = $json->space_before ([$enable])
$enabled = $json->get_space_before
If $enable is true (or missing), then the "encode" method will add an
extra optional space before the ":" separating keys from values in
JSON objects.
If $enable is false, then the "encode" method will not add any extra
space at those places.
This setting has no effect when decoding JSON texts. You will also
most likely combine this setting with "space_after".
Example, space_before enabled, space_after and indent disabled:
{"key" :"value"}
$json = $json->space_after ([$enable])
$enabled = $json->get_space_after
If $enable is true (or missing), then the "encode" method will add an
extra optional space after the ":" separating keys from values in
JSON objects and extra whitespace after the "," separating key-value
pairs and array members.
If $enable is false, then the "encode" method will not add any extra
space at those places.
This setting has no effect when decoding JSON texts.
Example, space_before and indent disabled, space_after enabled:
{"key": "value"}
$json = $json->relaxed ([$enable])
$enabled = $json->get_relaxed
If $enable is true (or missing), then "decode" will accept some
extensions to normal JSON syntax (see below). "encode" will not be
affected in any way. Be aware that this option makes you accept
invalid JSON texts as if they were valid!. I suggest only to use this
option to parse application-specific files written by humans
(configuration files, resource files etc.)
If $enable is false (the default), then "decode" will only accept
valid JSON texts.
Currently accepted extensions are:
o list items can have an end-comma
JSON separates array elements and key-value pairs with commas.
This can be annoying if you write JSON texts manually and want to
be able to quickly append elements, so this extension accepts
comma at the end of such items not just between them:
[
1,
2, <- this comma not normally allowed
]
{
"k1": "v1",
"k2": "v2", <- this comma not normally allowed
}
o shell-style '#'-comments
Whenever JSON allows whitespace, shell-style comments are
additionally allowed. They are terminated by the first carriage-
return or line-feed character, after which more white-space and
comments are allowed.
[
1, # this comment not allowed in JSON
# neither this one...
]
o literal ASCII TAB characters in strings
Literal ASCII TAB characters are now allowed in strings (and
treated as "\t").
[
"Hello\tWorld",
"Hello<TAB>World", # literal <TAB> would not normally be allowed
]
$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
Unlike other boolean options, this opotion is enabled by default
beginning with version 4.0. See "SECURITY CONSIDERATIONS" for the
gory details.
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 without enabled
"allow_nonref", resulting in an error:
JSON::XS->new->allow_nonref (0)->encode ("Hello, World!")
=> hash- or arrayref expected...
$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
partner.
$json = $json->allow_blessed ([$enable])
$enabled = $json->get_allow_blessed
See "OBJECT SERIALISATION" for details.
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
object.
If $enable is false (the default), then "encode" will throw an
exception when it encounters a blessed object that it cannot convert
otherwise.
This setting has no effect on "decode".
$json = $json->convert_blessed ([$enable])
$enabled = $json->get_convert_blessed
See "OBJECT SERIALISATION" for details.
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.
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->get_allow_tags
See "OBJECT SERIALISATION" for details.
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.
$json->boolean_values ([$false, $true])
($false, $true) = $json->get_boolean_values
By default, JSON booleans will be decoded as overloaded
$Types::Serialiser::false and $Types::Serialiser::true objects.
With this method you can specify your own boolean values for decoding
- on decode, JSON "false" will be decoded as a copy of $false, and
JSON "true" will be decoded as $true ("copy" here is the same thing
as assigning a value to another variable, i.e. "$copy = $false").
Calling this method without any arguments will reset the booleans to
their default values.
"get_boolean_values" will return both $false and $true values, or the
empty list when they are set to the default.
$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 reference returns a single scalar
(which need not be a reference), this value (or rather a copy of it)
is inserted into the deserialised data structure. If it returns an
empty list (NOTE: not "undef", which is a valid scalar), the original
deserialised hash will be inserted. This setting can slow down
decoding considerably.
When $coderef is omitted or undefined, any existing callback will be
removed and "decode" will not change the deserialised hash in any
way.
Example, convert all JSON objects into the integer 5:
my $js = JSON::XS->new->filter_json_object (sub { 5 });
# returns [5]
$js->decode ('[{}]')
# throw an exception because allow_nonref is not enabled
# so a lone 5 is not allowed.
$js->decode ('{"a":1, "b":2}');
$json = $json->filter_json_single_key_object ($key [=>
$coderef->($value)])
Works remotely similar to "filter_json_object", but is only called
for JSON objects having a single key named $key.
This $coderef is called before the one specified via
"filter_json_object", if any. It gets passed the single value in the
JSON object. If it returns a single value, it will be inserted into
the data structure. If it returns nothing (not even "undef" but the
empty list), the callback from "filter_json_object" will be called
next, as if no single-key callback were specified.
If $coderef is omitted or undefined, the corresponding callback will
be disabled. There can only ever be one callback for a given key.
As this callback gets called less often then the "filter_json_object"
one, decoding speed will not usually suffer as much. Therefore,
single-key objects make excellent targets to serialise Perl objects
into, especially as single-key JSON objects are as close to the type-
tagged value concept as JSON gets (it's basically an ID/VALUE tuple).
Of course, JSON does not support this in any way, so you need to make
sure your data never looks like a serialised Perl hash.
Typical names for the single object key are "__class_whatever__", or
"$__dollars_are_rarely_used__$" or "}ugly_brace_placement", or even
things like "__class_md5sum(classname)__", to reduce the risk of
clashing with real hashes.
Example, decode JSON objects of the form "{ "__widget__" => <id> }"
into the corresponding $WIDGET{<id>} object:
# return whatever is in $WIDGET{5}:
JSON::XS
->new
->filter_json_single_key_object (__widget__ => sub {
$WIDGET{ $_[0] }
})
->decode ('{"__widget__": 5')
# this can be used with a TO_JSON method in some "widget" class
# for serialisation to json:
sub WidgetBase::TO_JSON {
my ($self) = @_;
unless ($self->{id}) {
$self->{id} = ..get..some..id..;
$WIDGET{$self->{id}} = $self;
}
{ __widget__ => $self->{id} }
}
$json = $json->shrink ([$enable])
$enabled = $json->get_shrink
Perl usually over-allocates memory a bit when allocating space for
strings. This flag optionally resizes strings generated by either
"encode" or "decode" to their minimum size possible. This can save
memory when your JSON texts are either very very long or you have
many short strings. It will also try to downgrade any strings to
octet-form if possible: perl stores strings internally either in an
encoding called UTF-X or in octet-form. The latter cannot store
everything but uses less space in general (and some buggy Perl or C
code might even rely on that internal representation being used).
The actual definition of what shrink does might change in future
versions, but it will always try to save space at the expense of
time.
If $enable is true (or missing), the string returned by "encode" will
be shrunk-to-fit, while all strings generated by "decode" will also
be shrunk-to-fit.
If $enable is false, then the normal perl allocation algorithms are
used. If you work with your data, then this is likely to be faster.
In the future, this setting might control other things, such as
converting strings that look like integers or floats into integers or
floats internally (there is no difference on the Perl level), saving
space.
$json = $json->max_depth ([$maximum_nesting_depth])
$max_depth = $json->get_max_depth
Sets the maximum nesting level (default 512) accepted while encoding
or decoding. If 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
useful.
$json = $json->max_size ([$maximum_string_size])
$max_size = $json->get_max_size
Set the maximum length a JSON text may have (in bytes) where decoding
is being attempted. The default is 0, meaning no limit. When "decode"
is called on a string 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
useful.
$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")
=> ([1], 3)
INCREMENTAL PARSING
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 calls).
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 (other than
whitespace) 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
them.
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 anything.
That means you can only use this function to look at or manipulate
text before or after complete JSON objects, not while the parser is
in the middle of parsing a JSON object.
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).
$json->incr_skip
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.
$json->incr_reset
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.
LIMITATIONS
The incremental parser is a non-exact parser: it works by gathering as
much text as possible that could be a valid JSON text, followed by trying
to decode it.
That means it sometimes needs to read more data than strictly necessary
to diagnose an invalid JSON text. For example, after parsing the
following fragment, the parser could stop with an error, as this fragment
cannot be the beginning of a valid JSON text:
[,
In reality, hopwever, the parser might continue to read data until a
length limit is exceeded or it finds a closing bracket.
EXAMPLES
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
manner):
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
example):
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* \[ //x;
}
# now we have the skipped the initial "[", so continue
# parsing all the elements.
for (;;) {
# in this loop we read data until we got a single JSON object
for (;;) {
if (my $obj = $json->incr_parse) {
# do something with $obj
last;
}
# add more data
sysread $fh, my $buf, 65536
or die "read error: $!";
$json->incr_parse ($buf); # void context, so no parsing
}
# in this loop we read data until we either found and parsed the
# separating "," between elements, or the final "]"
for (;;) {
# first skip whitespace
$json->incr_text =~ s/^\s*//;
# if we find "]", we are done
if ($json->incr_text =~ s/^\]//) {
print "finished.\n";
exit;
}
# if we find ",", we can continue with the next element
if ($json->incr_text =~ s/^,//) {
last;
}
# 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 :).
MAPPING
This section describes how JSON::XS maps Perl values to JSON values and
vice versa. These mappings are designed to "do the right thing" in most
circumstances automatically, preserving round-tripping characteristics
(what you put in comes out as something equivalent).
For the more enlightened: note that in the following descriptions,
lowercase perl refers to the Perl interpreter, while uppercase Perl
refers to the abstract Perl language itself.
JSON -> PERL
object
A JSON object becomes a reference to a hash in Perl. No ordering of
object keys is preserved (JSON does not preserve object key ordering
itself).
array
A JSON array becomes a reference to an array in Perl.
string
A JSON string becomes a string scalar in Perl - Unicode codepoints in
JSON are represented by the same codepoints in the Perl string, so no
manual decoding is necessary.
number
A JSON number becomes either an integer, numeric (floating point) or
string scalar in perl, depending on its range and any fractional
parts. On the Perl level, there is no difference between those as
Perl handles all the conversion details, but an integer may take
slightly less memory and might represent more values exactly than
floating point numbers.
If the number consists of digits only, JSON::XS will try to represent
it as an integer value. If that fails, it will try to represent it as
a numeric (floating point) value if that is possible without loss of
precision. Otherwise it will preserve the number as a string value
(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
number).
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).
null
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
arguments.
See "OBJECT SERIALISATION", below, for details.
PERL -> JSON
The mapping from Perl to JSON is slightly more difficult, as Perl is a
truly typeless language, so we can only guess which JSON type is meant by
a Perl value.
hash references
Perl hash references become JSON objects. As there is no inherent
ordering in hash keys (or JSON objects), they will usually be encoded
in a pseudo-random order. 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
readability.
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
$x *= 1; # same thing, the choice is yours.
You can not currently force the type in other, less obscure, ways.
Tell me if you need this capability (but don't forget to explain why
it's needed :).
Note that numerical precision has the same meaning as under Perl (so
binary to decimal conversion follows the same rules as in Perl, which
can differ to other languages). Also, your perl interpreter might
expose extensions to the floating point numbers of your platform,
such as infinities or NaN's - these cannot be represented in JSON,
and it is an error to pass those in.
OBJECT SERIALISATION
As JSON cannot directly represent Perl objects, you have to choose
between a pure JSON representation (without the ability to deserialise
the object automatically again), and a nonstandard extension to the JSON
syntax, tagged values.
SERIALISATION
What happens when "JSON::XS" encounters a Perl object depends on the
"allow_blessed", "convert_blessed" and "allow_tags" settings, which are
used in this order:
1. "allow_tags" is enabled and the object has a "FREEZE" method.
In this case, "JSON::XS" uses the Types::Serialiser object
serialisation protocol to create a tagged JSON value, using a
nonstandard extension to the JSON syntax.
This works by invoking the "FREEZE" method on the object, with the
first argument being the object to serialise, and the second argument
being the constant string "JSON" to distinguish it from other
serialisers.
The "FREEZE" method can return any number of values (i.e. zero or
more). These values and the paclkage/classname of the object will
then be encoded as a tagged JSON value in the following format:
("classname")[FREEZE return values...]
e.g.:
("URI")["http://www.google.com/"]
("MyDate")[2013,10,29]
("ImageData::JPEG")["Z3...VlCg=="]
For example, the hypothetical "My::Object" "FREEZE" method might use
the objects "type" and "id" members to encode the object:
sub My::Object::FREEZE {
my ($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) = @_;
$uri->as_string
}
3. "allow_blessed" is enabled.
The object will be serialised as a JSON null value.
4. none of the above
If none of the settings are enabled or the respective methods are
missing, "JSON::XS" throws an exception.
DESERIALISATION
For deserialisation there are only two cases to consider: either
nonstandard tagging was used, in which case "allow_tags" decides, or
objects cannot be automatically be deserialised, in which case you can
use postprocessing or the "filter_json_object" or
"filter_json_single_key_object" callbacks to get some real objects our of
your JSON.
This section only considers the tagged value case: I a tagged JSON object
is encountered during decoding and "allow_tags" is disabled, a parse
error will result (as if tagged values were not part of the grammar).
If "allow_tags" is enabled, "JSON::XS" will look up the "THAW" method of
the package/classname used during serialisation (it will not attempt to
load the package as a Perl module). If there is no such method, the
decoding will fail with an error.
Otherwise, the "THAW" method is invoked with the classname as first
argument, the constant string "JSON" as second argument, and all the
values from the JSON array (the values originally returned by the
"FREEZE" method) as remaining arguments.
The method must then return the object. While technically you can return
any Perl scalar, you might have to enable the "enable_nonref" setting to
make that work in all cases, so better return an actual blessed
reference.
As an example, let's implement a "THAW" function that regenerates the
"My::Object" from the "FREEZE" example earlier:
sub My::Object::THAW {
my ($class, $serialiser, $type, $id) = @_;
$class->new (type => $type, id => $id)
}
ENCODING/CODESET FLAG NOTES
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 a 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 Perl).
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
implement).
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.:
# DO NOT USE THIS!
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.
JSON and YAML
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 easier.
SPEED
It seems that JSON::XS is surprisingly fast, as shown in the following
tables. They have been generated with the help of the "eg/bench" program
in the JSON::XS distribution, to make it easy to compare on your own
system.
First comes a comparison between various modules using a very short
single-line JSON string (also available at
<http://dist.schmorp.de/misc/json/short.json>).
{"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 (<http://dist.schmorp.de/misc/json/long.json>).
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.
SECURITY CONSIDERATIONS
When you are using JSON in a protocol, talking to untrusted potentially
hostile creatures requires relatively few measures.
First of all, your JSON decoder should be secure, that is, should not
have any buffer overflows. Obviously, this module should ensure that and
I am trying hard on making that true, but you never know.
Second, you need to avoid resource-starving attacks. That means you
should limit the size of JSON texts you accept, or make sure then when
your resources run out, that's just fine (e.g. by using a separate
process that can crash safely). The size of a JSON text in octets or
characters is usually a good indication of the size of the resources
required to decode it into a Perl structure. While JSON::XS can check the
size of the JSON text, it might be too late when you already have it in
memory, so you might want to check the size before you accept the string.
Third, JSON::XS recurses using the C stack when decoding objects and
arrays. The C stack is a limited resource: for instance, on my amd64
machine with 8MB of stack size I can decode around 180k nested arrays but
only 14k nested JSON objects (due to perl itself recursing deeply on
croak to free the temporary). If that is exceeded, the program crashes.
To be conservative, the default nesting limit is set to 512. If your
process has a smaller stack, you should adjust this setting accordingly
with the "max_depth" method.
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
<http://blog.archive.jpsykes.com/47/practical-csrf-and-json-security/> to
see whether you are vulnerable to some common attack vectors (which
really are browser design bugs, but it is still you who will have to deal
with it, as major browser developers care only for features, not about
getting security right).
"OLD" VS. "NEW" JSON (RFC4627 VS. RFC7159)
JSON originally required JSON texts to represent an array or object -
scalar values were explicitly not allowed. This has changed, and versions
of JSON::XS beginning with 4.0 reflect this by allowing scalar values by
default.
One reason why one might not want this is that this removes a fundamental
property of JSON texts, namely that they are self-delimited and self-
contained, or in other words, you could take any number of "old" JSON
texts and paste them together, and the result would be unambiguously
parseable:
[1,3]{"k":5}[][null] # four JSON texts, without doubt
By allowing scalars, this property is lost: in the following example, is
this one JSON text (the number 12) or two JSON texts (the numbers 1 and
2):
12 # could be 12, or 1 and 2
Another lost property of "old" JSON is that no lookahead is required to
know the end of a JSON text, i.e. the JSON text definitely ended at the
last "]" or "}" character, there was no need to read extra characters.
For example, a viable network protocol with "old" JSON was to simply
exchange JSON texts without delimiter. For "new" JSON, you have to use a
suitable delimiter (such as a newline) after every JSON text or ensure
you never encode/decode scalar values.
Most protocols do work by only transferring arrays or objects, and the
easiest way to avoid problems with the "new" JSON definition is to
explicitly disallow scalar values in your encoder and decoder:
$json_coder = JSON::XS->new->allow_nonref (0)
This is a somewhat unhappy situation, and the blame can fully be put on
JSON's inmventor, Douglas Crockford, who unilaterally changed the format
in 2006 without consulting the IETF, forcing the IETF to either fork the
format or go with it (as I was told, the IETF wasn't amused).
RELATIONSHIP WITH I-JSON
JSON is a somewhat sloppily-defined format - it carries around obvious
Javascript baggage, such as not really defining number range, probably
because Javascript only has one type of numbers: IEEE 64 bit floats
("binary64").
For this reaosn, RFC7493 defines "Internet JSON", which is a restricted
subset of JSON that is supposedly more interoperable on the internet.
While "JSON::XS" does not offer specific support for I-JSON, it of course
accepts valid I-JSON and by default implements some of the limitations of
I-JSON, such as parsing numbers as perl numbers, which are usually a
superset of binary64 numbers.
To generate I-JSON, follow these rules:
o always generate UTF-8
I-JSON must be encoded in UTF-8, the default for "encode_json".
o numbers should be within IEEE 754 binary64 range
Basically all existing perl installations use binary64 to represent
floating point numbers, so all you need to do is to avoid large
integers.
o objects must not have duplicate keys
This is trivially done, as "JSON::XS" does not allow duplicate keys.
o do not generate scalar JSON texts, use "->allow_nonref (0)"
I-JSON strongly requests you to only encode arrays and objects into
JSON.
o times should be strings in ISO 8601 format
There are a myriad of modules on CPAN dealing with ISO 8601 - search
for "ISO8601" on CPAN and use one.
o encode binary data as base64
While it's tempting to just dump binary data as a string (and let
"JSON::XS" do the escaping), for I-JSON, it's recommended to encode
binary data as base64.
There are some other considerations - read RFC7493 for the details if
interested.
INTEROPERABILITY WITH OTHER MODULES
"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 other modules that do the same,
such as JSON::PP and CBOR::XS.
INTEROPERABILITY WITH OTHER JSON DECODERS
As long as you only serialise data that can be directly expressed in
JSON, "JSON::XS" is incapable of generating invalid JSON output (modulo
bugs, but "JSON::XS" has found more bugs in the official JSON testsuite
(1) than the official JSON testsuite has found in "JSON::XS" (0)).
When you have trouble decoding JSON generated by this module using other
decoders, then it is very likely that you have an encoding mismatch or
the other decoder is broken.
When decoding, "JSON::XS" is strict by default and will likely catch all
errors. There are currently two settings that change this: "relaxed"
makes "JSON::XS" accept (but not generate) some non-standard extensions,
and "allow_tags" will allow you to encode and decode Perl objects, at the
cost of not outputting valid JSON anymore.
TAGGED VALUE SYNTAX AND STANDARD JSON EN/DECODERS
When you use "allow_tags" to use the extended (and also nonstandard and
invalid) JSON syntax for serialised objects, and you still want to decode
the generated When you want to serialise objects, you can run a regex to
replace the tagged syntax by standard JSON arrays (it only works for
"normal" package names without comma, newlines or single colons). First,
the readable Perl version:
# if your FREEZE methods return no values, you need this replace first:
$json =~ s/\( \s* (" (?: [^\\":,]+|\\.|::)* ") \s* \) \s* \[\s*\]/[$1]/gx;
# this works for non-empty constructor arg lists:
$json =~ s/\( \s* (" (?: [^\\":,]+|\\.|::)* ") \s* \) \s* \[/[$1,/gx;
And here is a less readable version that is easy to adapt to other
languages:
$json =~ s/\(\s*("([^\\":,]+|\\.|::)*")\s*\)\s*\[/[$1,/g;
Here is an ECMAScript version (same regex):
json = json.replace (/\(\s*("([^\\":,]+|\\.|::)*")\s*\)\s*\[/g, "[$1,");
Since this syntax converts to standard JSON arrays, it might be hard to
distinguish serialised objects from normal arrays. You can prepend a
"magic number" as first array element to reduce chances of a collision:
$json =~ s/\(\s*("([^\\":,]+|\\.|::)*")\s*\)\s*\[/["XU1peReLzT4ggEllLanBYq4G9VzliwKF",$1,/g;
And after decoding the JSON text, you could walk the data structure
looking for arrays with a first element of
"XU1peReLzT4ggEllLanBYq4G9VzliwKF".
The same approach can be used to create the tagged format with another
encoder. First, you create an array with the magic string as first
member, the classname as second, and constructor arguments last, encode
it as part of your JSON structure, and then:
$json =~ s/\[\s*"XU1peReLzT4ggEllLanBYq4G9VzliwKF"\s*,\s*("([^\\":,]+|\\.|::)*")\s*,/($1)[/g;
Again, this has some limitations - the magic string must not be encoded
with character escapes, and the constructor arguments must be non-empty.
(I-)THREADS
This module is not guaranteed to be ithread (or MULTIPLICITY-) safe and
there are no plans to change this. Note that perl's builtin so-called
threads/ithreads are officially deprecated and should not be used.
THE PERILS OF SETLOCALE
Sometimes people avoid the Perl locale support and directly call the
system's setlocale function with "LC_ALL".
This breaks both perl and modules such as JSON::XS, as stringification of
numbers no longer works correctly (e.g. "$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
afterwards.
SOME HISTORY
At the time this module was created there already were a number of JSON
modules available on CPAN, so what was the reason to write yet another
JSON module? While it seems there are many JSON modules, none of them
correctly handled all corner cases, and in most cases their maintainers
are unresponsive, gone missing, or not listening to bug reports for other
reasons.
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 it and
doesn't require a C compiler when that is a problem.
Somewhere around version 3, this module was forked into
"Cpanel::JSON::XS", because its maintainer had serious trouble
understanding JSON and insisted on a fork with many bugs "fixed" that
weren't actually bugs, while spreading FUD about this module without
actually giving any details on his accusations. You be the judge, but in
my personal opinion, if you want quality, you will stay away from
dangerous forks like that.
BUGS
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 rt.cpan.org or any other bug reporting service.
I put the contact address into my modules for a reason.
SEE ALSO
The json_xs command line utility for quick experiments.
AUTHOR
Marc Lehmann <schmorp@schmorp.de>
http://home.schmorp.de/
perl v5.34.0 2020-10-27 XS(3)
json-xs 4.30.0 - Generated Sat Feb 11 06:12:32 CST 2023