XS(3pm) User Contributed Perl Documentation XS(3pm)
NAME
JSON::XS - JSON serialising/deserialising, done correctly and fast
JSON::XS - XXXXXXX JSON XXXXXX/XXXXXXX
(http://fleur.hio.jp/perldoc/mix/lib/JSON/XS.html)
SYNOPSIS
use JSON::XS;
# exported functions, they croak on error
# and expect/generate UTF-8
$utf8_encoded_json_text = encode_json $perl_hash_or_arrayref;
$perl_hash_or_arrayref = decode_json $utf8_encoded_json_text;
# OO-interface
$coder = JSON::XS->new->ascii->pretty->allow_nonref;
$pretty_printed_unencoded = $coder->encode ($perl_scalar);
$perl_scalar = $coder->decode ($unicode_json_text);
# Note that JSON version 2.0 and above will automatically use JSON::XS
# if available, at virtually no speed overhead either, so you should
# be able to just:
use JSON;
# and do the same things, except that you have a pure-perl fallback now.
DESCRIPTION
This module converts Perl data structures to JSON and vice versa. Its primary goal is to be correct and its secondary goal is to be fast.
To reach the latter goal it was written in C.
Beginning with version 2.0 of the JSON module, when both JSON and JSON::XS are installed, then JSON will fall back on JSON::XS (this can be
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.
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 interface.
o reasonably versatile output formats
You can choose between the most compact guaranteed-single-line format possible (nice for simple line-based protocols), a pure-ASCII
format (for when your transport is not 8-bit clean, still supports the whole Unicode range), or a pretty-printed format (for when you
want to read that stuff). Or you can combine those features in whatever way you like.
FUNCTIONAL INTERFACE
The following convenience methods are provided by this module. They are exported by default:
$json_text = encode_json $perl_scalar
Converts the given Perl data structure to a UTF-8 encoded, binary string (that is, the string contains octets only). Croaks on error.
This function call is functionally identical to:
$json_text = JSON::XS->new->utf8->encode ($perl_scalar)
Except being faster.
$perl_scalar = decode_json $json_text
The opposite of "encode_json": expects an UTF-8 (binary) string and tries to parse that as an UTF-8 encoded JSON text, returning the
resulting reference. Croaks on error.
This function call is functionally identical to:
$perl_scalar = JSON::XS->new->utf8->decode ($json_text)
Except being faster.
$is_boolean = JSON::XS::is_bool $scalar
Returns true if the passed scalar represents either JSON::XS::true or JSON::XS::false, two constants that act like 1 and 0,
respectively and are used to represent JSON "true" and "false" values in Perl.
See MAPPING, below, for more information on how JSON values are mapped to Perl.
A FEW NOTES ON UNICODE AND PERL
Since this often leads to confusion, here are a few very clear words on how Unicode works in Perl, modulo bugs.
1. Perl strings can store characters with ordinal values > 255.
This enables you to store Unicode characters as single characters in a Perl string - very natural.
2. Perl does not associate an encoding with your strings.
... 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.
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 uHHHHuLLLLL
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])
=> ["ud801udc01"]
$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 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 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 anyway. Be aware that this option makes you accept invalid JSON texts as if they were valid!. I suggest only to use this
option to parse application-specific files written by humans (configuration files, resource files etc.)
If $enable is false (the default), then "decode" will only accept valid JSON texts.
Currently accepted extensions are:
o list items can have an end-comma
JSON separates array elements and key-value pairs with commas. This can be annoying if you write JSON texts manually and want to be
able to quickly append elements, so this extension accepts comma at the end of such items not just between them:
[
1,
2, <- this comma not normally allowed
]
{
"k1": "v1",
"k2": "v2", <- this comma not normally allowed
}
o shell-style '#'-comments
Whenever JSON allows whitespace, shell-style comments are additionally allowed. They are terminated by the first carriage-return or
line-feed character, after which more white-space and comments are allowed.
[
1, # this comment not allowed in JSON
# neither this one...
]
$json = $json->canonical ([$enable])
$enabled = $json->get_canonical
If $enable is true (or missing), then the "encode" method will output JSON objects by sorting their keys. This is adding a
comparatively high overhead.
If $enable is false, then the "encode" method will output key-value pairs in the order Perl stores them (which will likely change
between runs of the same script).
This option is useful if you want the same data structure to be encoded as the same JSON text (given the same overall settings). If it
is disabled, the same hash might be encoded differently even if contains the same data, as key-value pairs have no inherent ordering in
Perl.
This setting has no effect when decoding JSON texts.
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 partner.
$json = $json->allow_blessed ([$enable])
$enabled = $json->get_allow_blessed
If $enable is true (or missing), then the "encode" method will not barf when it encounters a blessed reference. Instead, the value of
the convert_blessed option will decide whether "null" ("convert_blessed" disabled or no "TO_JSON" method found) or a representation of
the object ("convert_blessed" enabled and "TO_JSON" method found) is being encoded. Has no effect on "decode".
If $enable is false (the default), then "encode" will throw an exception when it encounters a blessed object.
$json = $json->convert_blessed ([$enable])
$enabled = $json->get_convert_blessed
If $enable is true (or missing), then "encode", upon encountering a blessed object, will check for the availability of the "TO_JSON"
method on the object's class. If found, it will be called in scalar context and the resulting scalar will be encoded instead of the
object. If no "TO_JSON" method is found, the value of "allow_blessed" will decide what to do.
The "TO_JSON" method may safely call die if it wants. If "TO_JSON" returns other blessed objects, those will be handled in the same
way. "TO_JSON" must take care of not causing an endless recursion cycle (== crash) in this case. The name of "TO_JSON" was chosen
because other methods called by the Perl core (== not by the user of the object) are usually in upper case letters and to avoid
collisions with any "to_json" function or method.
This setting does not yet influence "decode" in any way, but in the future, global hooks might get installed that influence "decode"
and are enabled by this setting.
If $enable is false, then the "allow_blessed" setting will decide what to do when a blessed object is found.
$json = $json->filter_json_object ([$coderef->($hashref)])
When $coderef is specified, it will be called from "decode" each time it decodes a JSON object. The only argument is a reference to the
newly-created hash. If the code references returns a single scalar (which need not be a reference), this value (i.e. a copy of that
scalar to avoid aliasing) is inserted into the deserialised data structure. If it returns an empty list (NOTE: not "undef", which is a
valid scalar), the original deserialised hash will be inserted. This setting can slow down decoding considerably.
When $coderef is omitted or undefined, any existing callback will be removed and "decode" will not change the deserialised hash in any
way.
Example, convert all JSON objects into the integer 5:
my $js = JSON::XS->new->filter_json_object (sub { 5 });
# returns [5]
$js->decode ('[{}]')
# throw an exception because allow_nonref is not enabled
# so a lone 5 is not allowed.
$js->decode ('{"a":1, "b":2}');
$json = $json->filter_json_single_key_object ($key [=> $coderef->($value)])
Works remotely similar to "filter_json_object", but is only called for JSON objects having a single key named $key.
This $coderef is called before the one specified via "filter_json_object", if any. It gets passed the single value in the JSON object.
If it returns a single value, it will be inserted into the data structure. If it returns nothing (not even "undef" but the empty list),
the callback from "filter_json_object" will be called next, as if no single-key callback were specified.
If $coderef is omitted or undefined, the corresponding callback will be disabled. There can only ever be one callback for a given key.
As this callback gets called less often then the "filter_json_object" one, decoding speed will not usually suffer as much. Therefore,
single-key objects make excellent targets to serialise Perl objects into, especially as single-key JSON objects are as close to the
type-tagged value concept as JSON gets (it's basically an ID/VALUE tuple). Of course, JSON does not support this in any way, so you
need to make sure your data never looks like a serialised Perl hash.
Typical names for the single object key are "__class_whatever__", or "$__dollars_are_rarely_used__$" or "}ugly_brace_placement", or
even things like "__class_md5sum(classname)__", to reduce the risk of clashing with real hashes.
Example, decode JSON objects of the form "{ "__widget__" => <id> }" into the corresponding $WIDGET{<id>} object:
# return whatever is in $WIDGET{5}:
JSON::XS
->new
->filter_json_single_key_object (__widget__ => sub {
$WIDGET{ $_[0] }
})
->decode ('{"__widget__": 5')
# this can be used with a TO_JSON method in some "widget" class
# for serialisation to json:
sub WidgetBase::TO_JSON {
my ($self) = @_;
unless ($self->{id}) {
$self->{id} = ..get..some..id..;
$WIDGET{$self->{id}} = $self;
}
{ __widget__ => $self->{id} }
}
$json = $json->shrink ([$enable])
$enabled = $json->get_shrink
Perl usually over-allocates memory a bit when allocating space for strings. This flag optionally resizes strings generated by either
"encode" or "decode" to their minimum size possible. This can save memory when your JSON texts are either very very long or you have
many short strings. It will also try to downgrade any strings to octet-form if possible: perl stores strings internally either in an
encoding called UTF-X or in octet-form. The latter cannot store everything but uses less space in general (and some buggy Perl or C
code might even rely on that internal representation being used).
The actual definition of what shrink does might change in future versions, but it will always try to save space at the expense of time.
If $enable is true (or missing), the string returned by "encode" will be shrunk-to-fit, while all strings generated by "decode" will
also be shrunk-to-fit.
If $enable is false, then the normal perl allocation algorithms are used. If you work with your data, then this is likely to be
faster.
In the future, this setting might control other things, such as converting strings that look like integers or floats into integers or
floats internally (there is no difference on the Perl level), saving space.
$json = $json->max_depth ([$maximum_nesting_depth])
$max_depth = $json->get_max_depth
Sets the maximum nesting level (default 512) accepted while encoding or decoding. If 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 data structure (a simple scalar or a reference to a hash or array) to its JSON representation. Simple scalars
will be converted into JSON string or number sequences, while references to arrays become JSON arrays and references to hashes become
JSON objects. Undefined Perl values (e.g. "undef") become JSON "null" values. Neither "true" nor "false" values will be generated.
$perl_scalar = $json->decode ($json_text)
The opposite of "encode": expects a JSON text and tries to parse it, returning the resulting simple scalar or reference. Croaks on
error.
JSON numbers and strings become simple Perl scalars. JSON arrays become Perl arrayrefs and JSON objects become Perl hashrefs. "true"
becomes JSON::XS::true (equals 1 numerically and as a string), "false" becomes JSON::XS::false (equals 0) and "null" becomes "undef".
($perl_scalar, $characters) = $json->decode_prefix ($json_text)
This works like the "decode" method, but instead of raising an exception when there is trailing garbage after the first JSON object, it
will silently stop parsing there and return the number of characters consumed so far.
This is useful if your JSON texts are not delimited by an outer protocol (which is not the brightest thing to do in the first place)
and you need to know where the JSON text ends.
JSON::XS->new->decode_prefix ("[1] the tail")
=> ([], 3)
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 errornous 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 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.
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 occured 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
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.
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.
";
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 toa 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 leats significant bit.
true, false
These JSON atoms become "JSON::XS::true" and "JSON::XS::false", respectively. They are overloaded to act almost exactly like the
numbers 1 and 0. You can check whether a scalar is a JSON boolean by using the "JSON::XS::is_bool" function.
null
A JSON null atom becomes "undef" in Perl.
PERL -> JSON
The mapping from Perl to JSON is slightly more difficult, as Perl is a truly typeless language, so we can only guess which JSON type is
meant by a Perl value.
hash references
Perl hash references become JSON objects. As there is no inherent ordering in hash keys (or JSON objects), they will usually be encoded
in a pseudo-random order that can change between runs of the same program but stays generally the same within a single run of a
program. JSON::XS can optionally sort the hash keys (determined by the canonical flag), so the same datastructure will serialise to the
same JSON text (given same settings and version of JSON::XS), but this incurs a runtime overhead and is only rarely useful, e.g. when
you want to compare some JSON text against another for equality.
array references
Perl array references become JSON arrays.
other references
Other unblessed references are generally not allowed and will cause an exception to be thrown, except for references to the integers 0
and 1, which get turned into "false" and "true" atoms in JSON. You can also use "JSON::XS::false" and "JSON::XS::true" to improve
readability.
encode_json [ , JSON::XS::true] # yields [false,true]
JSON::XS::true, JSON::XS::false
These special values become JSON true and JSON false values, respectively. You can also use "1" and "