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1NAME 1NAME
2 JSON::XS - JSON serialising/deserialising, done correctly and fast 2 JSON::XS - JSON serialising/deserialising, done correctly and fast
3 3
4 JSON::XS - 正しくて高速な JSON シリアライザ/デシリアライザ
5 (http://fleur.hio.jp/perldoc/mix/lib/JSON/XS.html)
6
4SYNOPSIS 7SYNOPSIS
5 use JSON::XS; 8 use JSON::XS;
9
10 # exported functions, they croak on error
11 # and expect/generate UTF-8
12
13 $utf8_encoded_json_text = encode_json $perl_hash_or_arrayref;
14 $perl_hash_or_arrayref = decode_json $utf8_encoded_json_text;
15
16 # OO-interface
17
18 $coder = JSON::XS->new->ascii->pretty->allow_nonref;
19 $pretty_printed_unencoded = $coder->encode ($perl_scalar);
20 $perl_scalar = $coder->decode ($unicode_json_text);
21
22 # Note that JSON version 2.0 and above will automatically use JSON::XS
23 # if available, at virtually no speed overhead either, so you should
24 # be able to just:
25
26 use JSON;
27
28 # and do the same things, except that you have a pure-perl fallback now.
6 29
7DESCRIPTION 30DESCRIPTION
8 This module converts Perl data structures to JSON and vice versa. Its 31 This module converts Perl data structures to JSON and vice versa. Its
9 primary goal is to be *correct* and its secondary goal is to be *fast*. 32 primary goal is to be *correct* and its secondary goal is to be *fast*.
10 To reach the latter goal it was written in C. 33 To reach the latter goal it was written in C.
11 34
12 As this is the n-th-something JSON module on CPAN, what was the reason 35 See MAPPING, below, on how JSON::XS maps perl values to JSON values and
13 to write yet another JSON module? While it seems there are many JSON 36 vice versa.
14 modules, none of them correctly handle all corner cases, and in most
15 cases their maintainers are unresponsive, gone missing, or not listening
16 to bug reports for other reasons.
17
18 See COMPARISON, below, for a comparison to some other JSON modules.
19 37
20 FEATURES 38 FEATURES
21 * correct handling of unicode issues 39 * correct Unicode handling
40
22 This module knows how to handle Unicode, and even documents how it 41 This module knows how to handle Unicode, documents how and when it
23 does so. 42 does so, and even documents what "correct" means.
24 43
25 * round-trip integrity 44 * round-trip integrity
45
26 When you serialise a perl data structure using only datatypes 46 When you serialise a perl data structure using only data types
27 supported by JSON, the deserialised data structure is identical on 47 supported by JSON and Perl, the deserialised data structure is
28 the Perl level. (e.g. the string "2.0" doesn't suddenly become "2"). 48 identical on the Perl level. (e.g. the string "2.0" doesn't suddenly
49 become "2" just because it looks like a number). There *are* minor
50 exceptions to this, read the MAPPING section below to learn about
51 those.
29 52
30 * strict checking of JSON correctness 53 * strict checking of JSON correctness
54
31 There is no guessing, no generating of illegal JSON strings by 55 There is no guessing, no generating of illegal JSON texts by
32 default, and only JSON is accepted as input (the latter is a 56 default, and only JSON is accepted as input by default (the latter
33 security feature). 57 is a security feature).
34 58
35 * fast 59 * fast
36 compared to other JSON modules, this module compares favourably.
37 60
61 Compared to other JSON modules and other serialisers such as
62 Storable, this module usually compares favourably in terms of speed,
63 too.
64
38 * simple to use 65 * simple to use
66
39 This module has both a simple functional interface as well as an OO 67 This module has both a simple functional interface as well as an
40 interface. 68 object oriented interface.
41 69
42 * reasonably versatile output formats 70 * reasonably versatile output formats
71
43 You can choose between the most compact format possible, a 72 You can choose between the most compact guaranteed-single-line
44 pure-ascii format, or a pretty-printed format. Or you can combine 73 format possible (nice for simple line-based protocols), a pure-ASCII
74 format (for when your transport is not 8-bit clean, still supports
75 the whole Unicode range), or a pretty-printed format (for when you
76 want to read that stuff). Or you can combine those features in
45 those features in whatever way you like. 77 whatever way you like.
46 78
47FUNCTIONAL INTERFACE 79FUNCTIONAL INTERFACE
48 The following convinience methods are provided by this module. They are 80 The following convenience methods are provided by this module. They are
49 exported by default: 81 exported by default:
50 82
51 $json_string = to_json $perl_scalar 83 $json_text = encode_json $perl_scalar
52 Converts the given Perl data structure (a simple scalar or a 84 Converts the given Perl data structure to a UTF-8 encoded, binary
53 reference to a hash or array) to a UTF-8 encoded, binary string
54 (that is, the string contains octets only). Croaks on error. 85 string (that is, the string contains octets only). Croaks on error.
55 86
56 This function call is functionally identical to "JSON::XS->new->utf8 87 This function call is functionally identical to:
57 (1)->encode ($perl_scalar)".
58 88
89 $json_text = JSON::XS->new->utf8->encode ($perl_scalar)
90
91 Except being faster.
92
59 $perl_scalar = from_json $json_string 93 $perl_scalar = decode_json $json_text
60 The opposite of "to_json": expects an UTF-8 (binary) string and 94 The opposite of "encode_json": expects a UTF-8 (binary) string and
61 tries to parse that as an UTF-8 encoded JSON string, returning the 95 tries to parse that as a UTF-8 encoded JSON text, returning the
62 resulting simple scalar or reference. Croaks on error. 96 resulting reference. Croaks on error.
63 97
64 This function call is functionally identical to "JSON::XS->new->utf8 98 This function call is functionally identical to:
65 (1)->decode ($json_string)". 99
100 $perl_scalar = JSON::XS->new->utf8->decode ($json_text)
101
102 Except being faster.
103
104A FEW NOTES ON UNICODE AND PERL
105 Since this often leads to confusion, here are a few very clear words on
106 how Unicode works in Perl, modulo bugs.
107
108 1. Perl strings can store characters with ordinal values > 255.
109 This enables you to store Unicode characters as single characters in
110 a Perl string - very natural.
111
112 2. Perl does *not* associate an encoding with your strings.
113 ... until you force it to, e.g. when matching it against a regex, or
114 printing the scalar to a file, in which case Perl either interprets
115 your string as locale-encoded text, octets/binary, or as Unicode,
116 depending on various settings. In no case is an encoding stored
117 together with your data, it is *use* that decides encoding, not any
118 magical meta data.
119
120 3. The internal utf-8 flag has no meaning with regards to the encoding
121 of your string.
122 Just ignore that flag unless you debug a Perl bug, a module written
123 in XS or want to dive into the internals of perl. Otherwise it will
124 only confuse you, as, despite the name, it says nothing about how
125 your string is encoded. You can have Unicode strings with that flag
126 set, with that flag clear, and you can have binary data with that
127 flag set and that flag clear. Other possibilities exist, too.
128
129 If you didn't know about that flag, just the better, pretend it
130 doesn't exist.
131
132 4. A "Unicode String" is simply a string where each character can be
133 validly interpreted as a Unicode code point.
134 If you have UTF-8 encoded data, it is no longer a Unicode string,
135 but a Unicode string encoded in UTF-8, giving you a binary string.
136
137 5. A string containing "high" (> 255) character values is *not* a UTF-8
138 string.
139 It's a fact. Learn to live with it.
140
141 I hope this helps :)
66 142
67OBJECT-ORIENTED INTERFACE 143OBJECT-ORIENTED INTERFACE
68 The object oriented interface lets you configure your own encoding or 144 The object oriented interface lets you configure your own encoding or
69 decoding style, within the limits of supported formats. 145 decoding style, within the limits of supported formats.
70 146
71 $json = new JSON::XS 147 $json = new JSON::XS
72 Creates a new JSON::XS object that can be used to de/encode JSON 148 Creates a new JSON::XS object that can be used to de/encode JSON
73 strings. All boolean flags described below are by default 149 strings. All boolean flags described below are by default *disabled*
74 *disabled*. 150 (with the exception of "allow_nonref", which defaults to *enabled*
151 since version 4.0).
75 152
76 The mutators for flags all return the JSON object again and thus 153 The mutators for flags all return the JSON object again and thus
77 calls can be chained: 154 calls can be chained:
78 155
79 my $json = JSON::XS->new->utf8(1)->space_after(1)->encode ({a => [1,2]}) 156 my $json = JSON::XS->new->utf8->space_after->encode ({a => [1,2]})
80 => {"a": [1, 2]} 157 => {"a": [1, 2]}
81 158
82 $json = $json->ascii ($enable) 159 $json = $json->ascii ([$enable])
160 $enabled = $json->get_ascii
83 If $enable is true, then the "encode" method will not generate 161 If $enable is true (or missing), then the "encode" method will not
84 characters outside the code range 0..127. Any unicode characters 162 generate characters outside the code range 0..127 (which is ASCII).
85 outside that range will be escaped using either a single \uXXXX (BMP 163 Any Unicode characters outside that range will be escaped using
86 characters) or a double \uHHHH\uLLLLL escape sequence, as per 164 either a single \uXXXX (BMP characters) or a double \uHHHH\uLLLLL
87 RFC4627. 165 escape sequence, as per RFC4627. The resulting encoded JSON text can
166 be treated as a native Unicode string, an ascii-encoded,
167 latin1-encoded or UTF-8 encoded string, or any other superset of
168 ASCII.
88 169
89 If $enable is false, then the "encode" method will not escape 170 If $enable is false, then the "encode" method will not escape
90 Unicode characters unless necessary. 171 Unicode characters unless required by the JSON syntax or other
172 flags. This results in a faster and more compact format.
91 173
174 See also the section *ENCODING/CODESET FLAG NOTES* later in this
175 document.
176
177 The main use for this flag is to produce JSON texts that can be
178 transmitted over a 7-bit channel, as the encoded JSON texts will not
179 contain any 8 bit characters.
180
92 JSON::XS->new->ascii (1)->encode (chr 0x10401) 181 JSON::XS->new->ascii (1)->encode ([chr 0x10401])
93 => \ud801\udc01 182 => ["\ud801\udc01"]
94 183
184 $json = $json->latin1 ([$enable])
185 $enabled = $json->get_latin1
186 If $enable is true (or missing), then the "encode" method will
187 encode the resulting JSON text as latin1 (or iso-8859-1), escaping
188 any characters outside the code range 0..255. The resulting string
189 can be treated as a latin1-encoded JSON text or a native Unicode
190 string. The "decode" method will not be affected in any way by this
191 flag, as "decode" by default expects Unicode, which is a strict
192 superset of latin1.
193
194 If $enable is false, then the "encode" method will not escape
195 Unicode characters unless required by the JSON syntax or other
196 flags.
197
198 See also the section *ENCODING/CODESET FLAG NOTES* later in this
199 document.
200
201 The main use for this flag is efficiently encoding binary data as
202 JSON text, as most octets will not be escaped, resulting in a
203 smaller encoded size. The disadvantage is that the resulting JSON
204 text is encoded in latin1 (and must correctly be treated as such
205 when storing and transferring), a rare encoding for JSON. It is
206 therefore most useful when you want to store data structures known
207 to contain binary data efficiently in files or databases, not when
208 talking to other JSON encoders/decoders.
209
210 JSON::XS->new->latin1->encode (["\x{89}\x{abc}"]
211 => ["\x{89}\\u0abc"] # (perl syntax, U+abc escaped, U+89 not)
212
95 $json = $json->utf8 ($enable) 213 $json = $json->utf8 ([$enable])
214 $enabled = $json->get_utf8
96 If $enable is true, then the "encode" method will encode the JSON 215 If $enable is true (or missing), then the "encode" method will
97 string into UTF-8, as required by many protocols, while the "decode" 216 encode the JSON result into UTF-8, as required by many protocols,
98 method expects to be handled an UTF-8-encoded string. Please note 217 while the "decode" method expects to be handed a UTF-8-encoded
99 that UTF-8-encoded strings do not contain any characters outside the 218 string. Please note that UTF-8-encoded strings do not contain any
100 range 0..255, they are thus useful for bytewise/binary I/O. 219 characters outside the range 0..255, they are thus useful for
220 bytewise/binary I/O. In future versions, enabling this option might
221 enable autodetection of the UTF-16 and UTF-32 encoding families, as
222 described in RFC4627.
101 223
102 If $enable is false, then the "encode" method will return the JSON 224 If $enable is false, then the "encode" method will return the JSON
103 string as a (non-encoded) unicode string, while "decode" expects 225 string as a (non-encoded) Unicode string, while "decode" expects
104 thus a unicode string. Any decoding or encoding (e.g. to UTF-8 or 226 thus a Unicode string. Any decoding or encoding (e.g. to UTF-8 or
105 UTF-16) needs to be done yourself, e.g. using the Encode module. 227 UTF-16) needs to be done yourself, e.g. using the Encode module.
106 228
229 See also the section *ENCODING/CODESET FLAG NOTES* later in this
230 document.
231
232 Example, output UTF-16BE-encoded JSON:
233
234 use Encode;
235 $jsontext = encode "UTF-16BE", JSON::XS->new->encode ($object);
236
237 Example, decode UTF-32LE-encoded JSON:
238
239 use Encode;
240 $object = JSON::XS->new->decode (decode "UTF-32LE", $jsontext);
241
107 $json = $json->pretty ($enable) 242 $json = $json->pretty ([$enable])
108 This enables (or disables) all of the "indent", "space_before" and 243 This enables (or disables) all of the "indent", "space_before" and
109 "space_after" (and in the future possibly more) flags in one call to 244 "space_after" (and in the future possibly more) flags in one call to
110 generate the most readable (or most compact) form possible. 245 generate the most readable (or most compact) form possible.
246
247 Example, pretty-print some simple structure:
111 248
112 my $json = JSON::XS->new->pretty(1)->encode ({a => [1,2]}) 249 my $json = JSON::XS->new->pretty(1)->encode ({a => [1,2]})
113 => 250 =>
114 { 251 {
115 "a" : [ 252 "a" : [
116 1, 253 1,
117 2 254 2
118 ] 255 ]
119 } 256 }
120 257
121 $json = $json->indent ($enable) 258 $json = $json->indent ([$enable])
259 $enabled = $json->get_indent
122 If $enable is true, then the "encode" method will use a multiline 260 If $enable is true (or missing), then the "encode" method will use a
123 format as output, putting every array member or object/hash 261 multiline format as output, putting every array member or
124 key-value pair into its own line, identing them properly. 262 object/hash key-value pair into its own line, indenting them
263 properly.
125 264
126 If $enable is false, no newlines or indenting will be produced, and 265 If $enable is false, no newlines or indenting will be produced, and
127 the resulting JSON strings is guarenteed not to contain any 266 the resulting JSON text is guaranteed not to contain any "newlines".
128 "newlines".
129 267
130 This setting has no effect when decoding JSON strings. 268 This setting has no effect when decoding JSON texts.
131 269
132 $json = $json->space_before ($enable) 270 $json = $json->space_before ([$enable])
271 $enabled = $json->get_space_before
133 If $enable is true, then the "encode" method will add an extra 272 If $enable is true (or missing), then the "encode" method will add
134 optional space before the ":" separating keys from values in JSON 273 an extra optional space before the ":" separating keys from values
135 objects. 274 in JSON objects.
136 275
137 If $enable is false, then the "encode" method will not add any extra 276 If $enable is false, then the "encode" method will not add any extra
138 space at those places. 277 space at those places.
139 278
140 This setting has no effect when decoding JSON strings. You will also 279 This setting has no effect when decoding JSON texts. You will also
141 most likely combine this setting with "space_after". 280 most likely combine this setting with "space_after".
142 281
282 Example, space_before enabled, space_after and indent disabled:
283
284 {"key" :"value"}
285
143 $json = $json->space_after ($enable) 286 $json = $json->space_after ([$enable])
287 $enabled = $json->get_space_after
144 If $enable is true, then the "encode" method will add an extra 288 If $enable is true (or missing), then the "encode" method will add
145 optional space after the ":" separating keys from values in JSON 289 an extra optional space after the ":" separating keys from values in
146 objects and extra whitespace after the "," separating key-value 290 JSON objects and extra whitespace after the "," separating key-value
147 pairs and array members. 291 pairs and array members.
148 292
149 If $enable is false, then the "encode" method will not add any extra 293 If $enable is false, then the "encode" method will not add any extra
150 space at those places. 294 space at those places.
151 295
152 This setting has no effect when decoding JSON strings. 296 This setting has no effect when decoding JSON texts.
153 297
298 Example, space_before and indent disabled, space_after enabled:
299
300 {"key": "value"}
301
302 $json = $json->relaxed ([$enable])
303 $enabled = $json->get_relaxed
304 If $enable is true (or missing), then "decode" will accept some
305 extensions to normal JSON syntax (see below). "encode" will not be
306 affected in any way. *Be aware that this option makes you accept
307 invalid JSON texts as if they were valid!*. I suggest only to use
308 this option to parse application-specific files written by humans
309 (configuration files, resource files etc.)
310
311 If $enable is false (the default), then "decode" will only accept
312 valid JSON texts.
313
314 Currently accepted extensions are:
315
316 * list items can have an end-comma
317
318 JSON *separates* array elements and key-value pairs with commas.
319 This can be annoying if you write JSON texts manually and want
320 to be able to quickly append elements, so this extension accepts
321 comma at the end of such items not just between them:
322
323 [
324 1,
325 2, <- this comma not normally allowed
326 ]
327 {
328 "k1": "v1",
329 "k2": "v2", <- this comma not normally allowed
330 }
331
332 * shell-style '#'-comments
333
334 Whenever JSON allows whitespace, shell-style comments are
335 additionally allowed. They are terminated by the first
336 carriage-return or line-feed character, after which more
337 white-space and comments are allowed.
338
339 [
340 1, # this comment not allowed in JSON
341 # neither this one...
342 ]
343
344 * literal ASCII TAB characters in strings
345
346 Literal ASCII TAB characters are now allowed in strings (and
347 treated as "\t").
348
349 [
350 "Hello\tWorld",
351 "Hello<TAB>World", # literal <TAB> would not normally be allowed
352 ]
353
154 $json = $json->canonical ($enable) 354 $json = $json->canonical ([$enable])
355 $enabled = $json->get_canonical
155 If $enable is true, then the "encode" method will output JSON 356 If $enable is true (or missing), then the "encode" method will
156 objects by sorting their keys. This is adding a comparatively high 357 output JSON objects by sorting their keys. This is adding a
157 overhead. 358 comparatively high overhead.
158 359
159 If $enable is false, then the "encode" method will output key-value 360 If $enable is false, then the "encode" method will output key-value
160 pairs in the order Perl stores them (which will likely change 361 pairs in the order Perl stores them (which will likely change
161 between runs of the same script). 362 between runs of the same script, and can change even within the same
363 run from 5.18 onwards).
162 364
163 This option is useful if you want the same data structure to be 365 This option is useful if you want the same data structure to be
164 encoded as the same JSON string (given the same overall settings). 366 encoded as the same JSON text (given the same overall settings). If
165 If it is disabled, the same hash migh be encoded differently even if 367 it is disabled, the same hash might be encoded differently even if
166 contains the same data, as key-value pairs have no inherent ordering 368 contains the same data, as key-value pairs have no inherent ordering
167 in Perl. 369 in Perl.
168 370
169 This setting has no effect when decoding JSON strings. 371 This setting has no effect when decoding JSON texts.
170 372
373 This setting has currently no effect on tied hashes.
374
171 $json = $json->allow_nonref ($enable) 375 $json = $json->allow_nonref ([$enable])
376 $enabled = $json->get_allow_nonref
377 Unlike other boolean options, this opotion is enabled by default
378 beginning with version 4.0. See "SECURITY CONSIDERATIONS" for the
379 gory details.
380
172 If $enable is true, then the "encode" method can convert a 381 If $enable is true (or missing), then the "encode" method can
173 non-reference into its corresponding string, number or null JSON 382 convert a non-reference into its corresponding string, number or
174 value, which is an extension to RFC4627. Likewise, "decode" will 383 null JSON value, which is an extension to RFC4627. Likewise,
175 accept those JSON values instead of croaking. 384 "decode" will accept those JSON values instead of croaking.
176 385
177 If $enable is false, then the "encode" method will croak if it isn't 386 If $enable is false, then the "encode" method will croak if it isn't
178 passed an arrayref or hashref, as JSON strings must either be an 387 passed an arrayref or hashref, as JSON texts must either be an
179 object or array. Likewise, "decode" will croak if given something 388 object or array. Likewise, "decode" will croak if given something
180 that is not a JSON object or array. 389 that is not a JSON object or array.
181 390
391 Example, encode a Perl scalar as JSON value without enabled
392 "allow_nonref", resulting in an error:
393
394 JSON::XS->new->allow_nonref (0)->encode ("Hello, World!")
395 => hash- or arrayref expected...
396
397 $json = $json->allow_unknown ([$enable])
398 $enabled = $json->get_allow_unknown
399 If $enable is true (or missing), then "encode" will *not* throw an
400 exception when it encounters values it cannot represent in JSON (for
401 example, filehandles) but instead will encode a JSON "null" value.
402 Note that blessed objects are not included here and are handled
403 separately by c<allow_nonref>.
404
405 If $enable is false (the default), then "encode" will throw an
406 exception when it encounters anything it cannot encode as JSON.
407
408 This option does not affect "decode" in any way, and it is
409 recommended to leave it off unless you know your communications
410 partner.
411
412 $json = $json->allow_blessed ([$enable])
413 $enabled = $json->get_allow_blessed
414 See "OBJECT SERIALISATION" for details.
415
416 If $enable is true (or missing), then the "encode" method will not
417 barf when it encounters a blessed reference that it cannot convert
418 otherwise. Instead, a JSON "null" value is encoded instead of the
419 object.
420
421 If $enable is false (the default), then "encode" will throw an
422 exception when it encounters a blessed object that it cannot convert
423 otherwise.
424
425 This setting has no effect on "decode".
426
427 $json = $json->convert_blessed ([$enable])
428 $enabled = $json->get_convert_blessed
429 See "OBJECT SERIALISATION" for details.
430
431 If $enable is true (or missing), then "encode", upon encountering a
432 blessed object, will check for the availability of the "TO_JSON"
433 method on the object's class. If found, it will be called in scalar
434 context and the resulting scalar will be encoded instead of the
435 object.
436
437 The "TO_JSON" method may safely call die if it wants. If "TO_JSON"
438 returns other blessed objects, those will be handled in the same
439 way. "TO_JSON" must take care of not causing an endless recursion
440 cycle (== crash) in this case. The name of "TO_JSON" was chosen
441 because other methods called by the Perl core (== not by the user of
442 the object) are usually in upper case letters and to avoid
443 collisions with any "to_json" function or method.
444
445 If $enable is false (the default), then "encode" will not consider
446 this type of conversion.
447
448 This setting has no effect on "decode".
449
450 $json = $json->allow_tags ([$enable])
451 $enabled = $json->get_allow_tags
452 See "OBJECT SERIALISATION" for details.
453
454 If $enable is true (or missing), then "encode", upon encountering a
455 blessed object, will check for the availability of the "FREEZE"
456 method on the object's class. If found, it will be used to serialise
457 the object into a nonstandard tagged JSON value (that JSON decoders
458 cannot decode).
459
460 It also causes "decode" to parse such tagged JSON values and
461 deserialise them via a call to the "THAW" method.
462
463 If $enable is false (the default), then "encode" will not consider
464 this type of conversion, and tagged JSON values will cause a parse
465 error in "decode", as if tags were not part of the grammar.
466
467 $json->boolean_values ([$false, $true])
468 ($false, $true) = $json->get_boolean_values
469 By default, JSON booleans will be decoded as overloaded
470 $Types::Serialiser::false and $Types::Serialiser::true objects.
471
472 With this method you can specify your own boolean values for
473 decoding - on decode, JSON "false" will be decoded as a copy of
474 $false, and JSON "true" will be decoded as $true ("copy" here is the
475 same thing as assigning a value to another variable, i.e. "$copy =
476 $false").
477
478 Calling this method without any arguments will reset the booleans to
479 their default values.
480
481 "get_boolean_values" will return both $false and $true values, or
482 the empty list when they are set to the default.
483
484 $json = $json->filter_json_object ([$coderef->($hashref)])
485 When $coderef is specified, it will be called from "decode" each
486 time it decodes a JSON object. The only argument is a reference to
487 the newly-created hash. If the code reference returns a single
488 scalar (which need not be a reference), this value (or rather a copy
489 of it) is inserted into the deserialised data structure. If it
490 returns an empty list (NOTE: *not* "undef", which is a valid
491 scalar), the original deserialised hash will be inserted. This
492 setting can slow down decoding considerably.
493
494 When $coderef is omitted or undefined, any existing callback will be
495 removed and "decode" will not change the deserialised hash in any
496 way.
497
498 Example, convert all JSON objects into the integer 5:
499
500 my $js = JSON::XS->new->filter_json_object (sub { 5 });
501 # returns [5]
502 $js->decode ('[{}]')
503 # throw an exception because allow_nonref is not enabled
504 # so a lone 5 is not allowed.
505 $js->decode ('{"a":1, "b":2}');
506
507 $json = $json->filter_json_single_key_object ($key [=>
508 $coderef->($value)])
509 Works remotely similar to "filter_json_object", but is only called
510 for JSON objects having a single key named $key.
511
512 This $coderef is called before the one specified via
513 "filter_json_object", if any. It gets passed the single value in the
514 JSON object. If it returns a single value, it will be inserted into
515 the data structure. If it returns nothing (not even "undef" but the
516 empty list), the callback from "filter_json_object" will be called
517 next, as if no single-key callback were specified.
518
519 If $coderef is omitted or undefined, the corresponding callback will
520 be disabled. There can only ever be one callback for a given key.
521
522 As this callback gets called less often then the
523 "filter_json_object" one, decoding speed will not usually suffer as
524 much. Therefore, single-key objects make excellent targets to
525 serialise Perl objects into, especially as single-key JSON objects
526 are as close to the type-tagged value concept as JSON gets (it's
527 basically an ID/VALUE tuple). Of course, JSON does not support this
528 in any way, so you need to make sure your data never looks like a
529 serialised Perl hash.
530
531 Typical names for the single object key are "__class_whatever__", or
532 "$__dollars_are_rarely_used__$" or "}ugly_brace_placement", or even
533 things like "__class_md5sum(classname)__", to reduce the risk of
534 clashing with real hashes.
535
536 Example, decode JSON objects of the form "{ "__widget__" => <id> }"
537 into the corresponding $WIDGET{<id>} object:
538
539 # return whatever is in $WIDGET{5}:
540 JSON::XS
541 ->new
542 ->filter_json_single_key_object (__widget__ => sub {
543 $WIDGET{ $_[0] }
544 })
545 ->decode ('{"__widget__": 5')
546
547 # this can be used with a TO_JSON method in some "widget" class
548 # for serialisation to json:
549 sub WidgetBase::TO_JSON {
550 my ($self) = @_;
551
552 unless ($self->{id}) {
553 $self->{id} = ..get..some..id..;
554 $WIDGET{$self->{id}} = $self;
555 }
556
557 { __widget__ => $self->{id} }
558 }
559
560 $json = $json->shrink ([$enable])
561 $enabled = $json->get_shrink
562 Perl usually over-allocates memory a bit when allocating space for
563 strings. This flag optionally resizes strings generated by either
564 "encode" or "decode" to their minimum size possible. This can save
565 memory when your JSON texts are either very very long or you have
566 many short strings. It will also try to downgrade any strings to
567 octet-form if possible: perl stores strings internally either in an
568 encoding called UTF-X or in octet-form. The latter cannot store
569 everything but uses less space in general (and some buggy Perl or C
570 code might even rely on that internal representation being used).
571
572 The actual definition of what shrink does might change in future
573 versions, but it will always try to save space at the expense of
574 time.
575
576 If $enable is true (or missing), the string returned by "encode"
577 will be shrunk-to-fit, while all strings generated by "decode" will
578 also be shrunk-to-fit.
579
580 If $enable is false, then the normal perl allocation algorithms are
581 used. If you work with your data, then this is likely to be faster.
582
583 In the future, this setting might control other things, such as
584 converting strings that look like integers or floats into integers
585 or floats internally (there is no difference on the Perl level),
586 saving space.
587
588 $json = $json->max_depth ([$maximum_nesting_depth])
589 $max_depth = $json->get_max_depth
590 Sets the maximum nesting level (default 512) accepted while encoding
591 or decoding. If a higher nesting level is detected in JSON text or a
592 Perl data structure, then the encoder and decoder will stop and
593 croak at that point.
594
595 Nesting level is defined by number of hash- or arrayrefs that the
596 encoder needs to traverse to reach a given point or the number of
597 "{" or "[" characters without their matching closing parenthesis
598 crossed to reach a given character in a string.
599
600 Setting the maximum depth to one disallows any nesting, so that
601 ensures that the object is only a single hash/object or array.
602
603 If no argument is given, the highest possible setting will be used,
604 which is rarely useful.
605
606 Note that nesting is implemented by recursion in C. The default
607 value has been chosen to be as large as typical operating systems
608 allow without crashing.
609
610 See SECURITY CONSIDERATIONS, below, for more info on why this is
611 useful.
612
613 $json = $json->max_size ([$maximum_string_size])
614 $max_size = $json->get_max_size
615 Set the maximum length a JSON text may have (in bytes) where
616 decoding is being attempted. The default is 0, meaning no limit.
617 When "decode" is called on a string that is longer then this many
618 bytes, it will not attempt to decode the string but throw an
619 exception. This setting has no effect on "encode" (yet).
620
621 If no argument is given, the limit check will be deactivated (same
622 as when 0 is specified).
623
624 See SECURITY CONSIDERATIONS, below, for more info on why this is
625 useful.
626
182 $json_string = $json->encode ($perl_scalar) 627 $json_text = $json->encode ($perl_scalar)
183 Converts the given Perl data structure (a simple scalar or a 628 Converts the given Perl value or data structure to its JSON
184 reference to a hash or array) to its JSON representation. Simple 629 representation. Croaks on error.
185 scalars will be converted into JSON string or number sequences,
186 while references to arrays become JSON arrays and references to
187 hashes become JSON objects. Undefined Perl values (e.g. "undef")
188 become JSON "null" values. Neither "true" nor "false" values will be
189 generated.
190 630
191 $perl_scalar = $json->decode ($json_string) 631 $perl_scalar = $json->decode ($json_text)
192 The opposite of "encode": expects a JSON string and tries to parse 632 The opposite of "encode": expects a JSON text and tries to parse it,
193 it, returning the resulting simple scalar or reference. Croaks on 633 returning the resulting simple scalar or reference. Croaks on error.
194 error.
195 634
196 JSON numbers and strings become simple Perl scalars. JSON arrays 635 ($perl_scalar, $characters) = $json->decode_prefix ($json_text)
197 become Perl arrayrefs and JSON objects become Perl hashrefs. "true" 636 This works like the "decode" method, but instead of raising an
198 becomes 1, "false" becomes 0 and "null" becomes "undef". 637 exception when there is trailing garbage after the first JSON
638 object, it will silently stop parsing there and return the number of
639 characters consumed so far.
199 640
200COMPARISON 641 This is useful if your JSON texts are not delimited by an outer
201 As already mentioned, this module was created because none of the 642 protocol and you need to know where the JSON text ends.
202 existing JSON modules could be made to work correctly. First I will
203 describe the problems (or pleasures) I encountered with various existing
204 JSON modules, followed by some benchmark values. JSON::XS was designed
205 not to suffer from any of these problems or limitations.
206 643
207 JSON 644 JSON::XS->new->decode_prefix ("[1] the tail")
208 Slow (but very portable, as it is written in pure Perl). 645 => ([1], 3)
209 646
210 Undocumented/buggy Unicode handling (how JSON handles unicode values 647INCREMENTAL PARSING
211 is undocumented. One can get far by feeding it unicode strings and 648 In some cases, there is the need for incremental parsing of JSON texts.
212 doing en-/decoding oneself, but unicode escapes are not working 649 While this module always has to keep both JSON text and resulting Perl
650 data structure in memory at one time, it does allow you to parse a JSON
651 stream incrementally. It does so by accumulating text until it has a
652 full JSON object, which it then can decode. This process is similar to
653 using "decode_prefix" to see if a full JSON object is available, but is
654 much more efficient (and can be implemented with a minimum of method
655 calls).
656
657 JSON::XS will only attempt to parse the JSON text once it is sure it has
658 enough text to get a decisive result, using a very simple but truly
659 incremental parser. This means that it sometimes won't stop as early as
660 the full parser, for example, it doesn't detect mismatched parentheses.
661 The only thing it guarantees is that it starts decoding as soon as a
662 syntactically valid JSON text has been seen. This means you need to set
663 resource limits (e.g. "max_size") to ensure the parser will stop parsing
664 in the presence if syntax errors.
665
666 The following methods implement this incremental parser.
667
668 [void, scalar or list context] = $json->incr_parse ([$string])
669 This is the central parsing function. It can both append new text
670 and extract objects from the stream accumulated so far (both of
671 these functions are optional).
672
673 If $string is given, then this string is appended to the already
674 existing JSON fragment stored in the $json object.
675
676 After that, if the function is called in void context, it will
677 simply return without doing anything further. This can be used to
678 add more text in as many chunks as you want.
679
680 If the method is called in scalar context, then it will try to
681 extract exactly *one* JSON object. If that is successful, it will
682 return this object, otherwise it will return "undef". If there is a
683 parse error, this method will croak just as "decode" would do (one
684 can then use "incr_skip" to skip the erroneous part). This is the
685 most common way of using the method.
686
687 And finally, in list context, it will try to extract as many objects
688 from the stream as it can find and return them, or the empty list
689 otherwise. For this to work, there must be no separators (other than
690 whitespace) between the JSON objects or arrays, instead they must be
691 concatenated back-to-back. If an error occurs, an exception will be
692 raised as in the scalar context case. Note that in this case, any
693 previously-parsed JSON texts will be lost.
694
695 Example: Parse some JSON arrays/objects in a given string and return
696 them.
697
698 my @objs = JSON::XS->new->incr_parse ("[5][7][1,2]");
699
700 $lvalue_string = $json->incr_text
701 This method returns the currently stored JSON fragment as an lvalue,
702 that is, you can manipulate it. This *only* works when a preceding
703 call to "incr_parse" in *scalar context* successfully returned an
704 object. Under all other circumstances you must not call this
705 function (I mean it. although in simple tests it might actually
706 work, it *will* fail under real world conditions). As a special
707 exception, you can also call this method before having parsed
708 anything.
709
710 That means you can only use this function to look at or manipulate
711 text before or after complete JSON objects, not while the parser is
712 in the middle of parsing a JSON object.
713
714 This function is useful in two cases: a) finding the trailing text
715 after a JSON object or b) parsing multiple JSON objects separated by
716 non-JSON text (such as commas).
717
718 $json->incr_skip
719 This will reset the state of the incremental parser and will remove
720 the parsed text from the input buffer so far. This is useful after
721 "incr_parse" died, in which case the input buffer and incremental
722 parser state is left unchanged, to skip the text parsed so far and
723 to reset the parse state.
724
725 The difference to "incr_reset" is that only text until the parse
726 error occurred is removed.
727
728 $json->incr_reset
729 This completely resets the incremental parser, that is, after this
730 call, it will be as if the parser had never parsed anything.
731
732 This is useful if you want to repeatedly parse JSON objects and want
733 to ignore any trailing data, which means you have to reset the
734 parser after each successful decode.
735
736 LIMITATIONS
737 The incremental parser is a non-exact parser: it works by gathering as
738 much text as possible that *could* be a valid JSON text, followed by
739 trying to decode it.
740
741 That means it sometimes needs to read more data than strictly necessary
742 to diagnose an invalid JSON text. For example, after parsing the
743 following fragment, the parser *could* stop with an error, as this
744 fragment *cannot* be the beginning of a valid JSON text:
745
746 [,
747
748 In reality, hopwever, the parser might continue to read data until a
749 length limit is exceeded or it finds a closing bracket.
750
751 EXAMPLES
752 Some examples will make all this clearer. First, a simple example that
753 works similarly to "decode_prefix": We want to decode the JSON object at
754 the start of a string and identify the portion after the JSON object:
755
756 my $text = "[1,2,3] hello";
757
758 my $json = new JSON::XS;
759
760 my $obj = $json->incr_parse ($text)
761 or die "expected JSON object or array at beginning of string";
762
763 my $tail = $json->incr_text;
764 # $tail now contains " hello"
765
766 Easy, isn't it?
767
768 Now for a more complicated example: Imagine a hypothetical protocol
769 where you read some requests from a TCP stream, and each request is a
770 JSON array, without any separation between them (in fact, it is often
771 useful to use newlines as "separators", as these get interpreted as
772 whitespace at the start of the JSON text, which makes it possible to
773 test said protocol with "telnet"...).
774
775 Here is how you'd do it (it is trivial to write this in an event-based
776 manner):
777
778 my $json = new JSON::XS;
779
780 # read some data from the socket
781 while (sysread $socket, my $buf, 4096) {
782
783 # split and decode as many requests as possible
784 for my $request ($json->incr_parse ($buf)) {
785 # act on the $request
786 }
787 }
788
789 Another complicated example: Assume you have a string with JSON objects
790 or arrays, all separated by (optional) comma characters (e.g. "[1],[2],
791 [3]"). To parse them, we have to skip the commas between the JSON texts,
792 and here is where the lvalue-ness of "incr_text" comes in useful:
793
794 my $text = "[1],[2], [3]";
795 my $json = new JSON::XS;
796
797 # void context, so no parsing done
798 $json->incr_parse ($text);
799
800 # now extract as many objects as possible. note the
801 # use of scalar context so incr_text can be called.
802 while (my $obj = $json->incr_parse) {
803 # do something with $obj
804
805 # now skip the optional comma
806 $json->incr_text =~ s/^ \s* , //x;
807 }
808
809 Now lets go for a very complex example: Assume that you have a gigantic
810 JSON array-of-objects, many gigabytes in size, and you want to parse it,
811 but you cannot load it into memory fully (this has actually happened in
812 the real world :).
813
814 Well, you lost, you have to implement your own JSON parser. But JSON::XS
815 can still help you: You implement a (very simple) array parser and let
816 JSON decode the array elements, which are all full JSON objects on their
817 own (this wouldn't work if the array elements could be JSON numbers, for
818 example):
819
820 my $json = new JSON::XS;
821
822 # open the monster
823 open my $fh, "<bigfile.json"
824 or die "bigfile: $!";
825
826 # first parse the initial "["
827 for (;;) {
828 sysread $fh, my $buf, 65536
829 or die "read error: $!";
830 $json->incr_parse ($buf); # void context, so no parsing
831
832 # Exit the loop once we found and removed(!) the initial "[".
833 # In essence, we are (ab-)using the $json object as a simple scalar
834 # we append data to.
835 last if $json->incr_text =~ s/^ \s* \[ //x;
836 }
837
838 # now we have the skipped the initial "[", so continue
839 # parsing all the elements.
840 for (;;) {
841 # in this loop we read data until we got a single JSON object
842 for (;;) {
843 if (my $obj = $json->incr_parse) {
844 # do something with $obj
845 last;
846 }
847
848 # add more data
849 sysread $fh, my $buf, 65536
850 or die "read error: $!";
851 $json->incr_parse ($buf); # void context, so no parsing
852 }
853
854 # in this loop we read data until we either found and parsed the
855 # separating "," between elements, or the final "]"
856 for (;;) {
857 # first skip whitespace
858 $json->incr_text =~ s/^\s*//;
859
860 # if we find "]", we are done
861 if ($json->incr_text =~ s/^\]//) {
862 print "finished.\n";
863 exit;
864 }
865
866 # if we find ",", we can continue with the next element
867 if ($json->incr_text =~ s/^,//) {
868 last;
869 }
870
871 # if we find anything else, we have a parse error!
872 if (length $json->incr_text) {
873 die "parse error near ", $json->incr_text;
874 }
875
876 # else add more data
877 sysread $fh, my $buf, 65536
878 or die "read error: $!";
879 $json->incr_parse ($buf); # void context, so no parsing
880 }
881
882 This is a complex example, but most of the complexity comes from the
883 fact that we are trying to be correct (bear with me if I am wrong, I
884 never ran the above example :).
885
886MAPPING
887 This section describes how JSON::XS maps Perl values to JSON values and
888 vice versa. These mappings are designed to "do the right thing" in most
889 circumstances automatically, preserving round-tripping characteristics
890 (what you put in comes out as something equivalent).
891
892 For the more enlightened: note that in the following descriptions,
893 lowercase *perl* refers to the Perl interpreter, while uppercase *Perl*
894 refers to the abstract Perl language itself.
895
896 JSON -> PERL
897 object
898 A JSON object becomes a reference to a hash in Perl. No ordering of
899 object keys is preserved (JSON does not preserve object key ordering
900 itself).
901
902 array
903 A JSON array becomes a reference to an array in Perl.
904
905 string
906 A JSON string becomes a string scalar in Perl - Unicode codepoints
907 in JSON are represented by the same codepoints in the Perl string,
908 so no manual decoding is necessary.
909
910 number
911 A JSON number becomes either an integer, numeric (floating point) or
912 string scalar in perl, depending on its range and any fractional
913 parts. On the Perl level, there is no difference between those as
914 Perl handles all the conversion details, but an integer may take
915 slightly less memory and might represent more values exactly than
916 floating point numbers.
917
918 If the number consists of digits only, JSON::XS will try to
919 represent it as an integer value. If that fails, it will try to
920 represent it as a numeric (floating point) value if that is possible
921 without loss of precision. Otherwise it will preserve the number as
922 a string value (in which case you lose roundtripping ability, as the
923 JSON number will be re-encoded to a JSON string).
924
925 Numbers containing a fractional or exponential part will always be
926 represented as numeric (floating point) values, possibly at a loss
927 of precision (in which case you might lose perfect roundtripping
928 ability, but the JSON number will still be re-encoded as a JSON
929 number).
930
931 Note that precision is not accuracy - binary floating point values
932 cannot represent most decimal fractions exactly, and when converting
933 from and to floating point, JSON::XS only guarantees precision up to
934 but not including the least significant bit.
935
936 true, false
937 These JSON atoms become "Types::Serialiser::true" and
938 "Types::Serialiser::false", respectively. They are overloaded to act
939 almost exactly like the numbers 1 and 0. You can check whether a
940 scalar is a JSON boolean by using the "Types::Serialiser::is_bool"
941 function (after "use Types::Serialier", of course).
942
943 null
944 A JSON null atom becomes "undef" in Perl.
945
946 shell-style comments ("# *text*")
947 As a nonstandard extension to the JSON syntax that is enabled by the
948 "relaxed" setting, shell-style comments are allowed. They can start
949 anywhere outside strings and go till the end of the line.
950
951 tagged values ("(*tag*)*value*").
952 Another nonstandard extension to the JSON syntax, enabled with the
953 "allow_tags" setting, are tagged values. In this implementation, the
954 *tag* must be a perl package/class name encoded as a JSON string,
955 and the *value* must be a JSON array encoding optional constructor
956 arguments.
957
958 See "OBJECT SERIALISATION", below, for details.
959
960 PERL -> JSON
961 The mapping from Perl to JSON is slightly more difficult, as Perl is a
962 truly typeless language, so we can only guess which JSON type is meant
963 by a Perl value.
964
965 hash references
966 Perl hash references become JSON objects. As there is no inherent
967 ordering in hash keys (or JSON objects), they will usually be
968 encoded in a pseudo-random order. JSON::XS can optionally sort the
969 hash keys (determined by the *canonical* flag), so the same
970 datastructure will serialise to the same JSON text (given same
971 settings and version of JSON::XS), but this incurs a runtime
972 overhead and is only rarely useful, e.g. when you want to compare
973 some JSON text against another for equality.
974
975 array references
976 Perl array references become JSON arrays.
977
978 other references
979 Other unblessed references are generally not allowed and will cause
980 an exception to be thrown, except for references to the integers 0
981 and 1, which get turned into "false" and "true" atoms in JSON.
982
983 Since "JSON::XS" uses the boolean model from Types::Serialiser, you
984 can also "use Types::Serialiser" and then use
985 "Types::Serialiser::false" and "Types::Serialiser::true" to improve
986 readability.
987
988 use Types::Serialiser;
989 encode_json [\0, Types::Serialiser::true] # yields [false,true]
990
991 Types::Serialiser::true, Types::Serialiser::false
992 These special values from the Types::Serialiser module become JSON
993 true and JSON false values, respectively. You can also use "\1" and
994 "\0" directly if you want.
995
996 blessed objects
997 Blessed objects are not directly representable in JSON, but
998 "JSON::XS" allows various ways of handling objects. See "OBJECT
999 SERIALISATION", below, for details.
1000
1001 simple scalars
1002 Simple Perl scalars (any scalar that is not a reference) are the
1003 most difficult objects to encode: JSON::XS will encode undefined
1004 scalars as JSON "null" values, scalars that have last been used in a
1005 string context before encoding as JSON strings, and anything else as
1006 number value:
1007
1008 # dump as number
1009 encode_json [2] # yields [2]
1010 encode_json [-3.0e17] # yields [-3e+17]
1011 my $value = 5; encode_json [$value] # yields [5]
1012
1013 # used as string, so dump as string
1014 print $value;
1015 encode_json [$value] # yields ["5"]
1016
1017 # undef becomes null
1018 encode_json [undef] # yields [null]
1019
1020 You can force the type to be a JSON string by stringifying it:
1021
1022 my $x = 3.1; # some variable containing a number
1023 "$x"; # stringified
1024 $x .= ""; # another, more awkward way to stringify
1025 print $x; # perl does it for you, too, quite often
1026
1027 You can force the type to be a JSON number by numifying it:
1028
1029 my $x = "3"; # some variable containing a string
1030 $x += 0; # numify it, ensuring it will be dumped as a number
1031 $x *= 1; # same thing, the choice is yours.
1032
1033 You can not currently force the type in other, less obscure, ways.
1034 Tell me if you need this capability (but don't forget to explain why
1035 it's needed :).
1036
1037 Note that numerical precision has the same meaning as under Perl (so
1038 binary to decimal conversion follows the same rules as in Perl,
1039 which can differ to other languages). Also, your perl interpreter
1040 might expose extensions to the floating point numbers of your
1041 platform, such as infinities or NaN's - these cannot be represented
1042 in JSON, and it is an error to pass those in.
1043
1044 OBJECT SERIALISATION
1045 As JSON cannot directly represent Perl objects, you have to choose
1046 between a pure JSON representation (without the ability to deserialise
1047 the object automatically again), and a nonstandard extension to the JSON
1048 syntax, tagged values.
1049
1050 SERIALISATION
1051 What happens when "JSON::XS" encounters a Perl object depends on the
1052 "allow_blessed", "convert_blessed" and "allow_tags" settings, which are
1053 used in this order:
1054
1055 1. "allow_tags" is enabled and the object has a "FREEZE" method.
1056 In this case, "JSON::XS" uses the Types::Serialiser object
1057 serialisation protocol to create a tagged JSON value, using a
1058 nonstandard extension to the JSON syntax.
1059
1060 This works by invoking the "FREEZE" method on the object, with the
1061 first argument being the object to serialise, and the second
1062 argument being the constant string "JSON" to distinguish it from
1063 other serialisers.
1064
1065 The "FREEZE" method can return any number of values (i.e. zero or
1066 more). These values and the paclkage/classname of the object will
1067 then be encoded as a tagged JSON value in the following format:
1068
1069 ("classname")[FREEZE return values...]
1070
1071 e.g.:
1072
1073 ("URI")["http://www.google.com/"]
1074 ("MyDate")[2013,10,29]
1075 ("ImageData::JPEG")["Z3...VlCg=="]
1076
1077 For example, the hypothetical "My::Object" "FREEZE" method might use
1078 the objects "type" and "id" members to encode the object:
1079
1080 sub My::Object::FREEZE {
1081 my ($self, $serialiser) = @_;
1082
1083 ($self->{type}, $self->{id})
1084 }
1085
1086 2. "convert_blessed" is enabled and the object has a "TO_JSON" method.
1087 In this case, the "TO_JSON" method of the object is invoked in
1088 scalar context. It must return a single scalar that can be directly
1089 encoded into JSON. This scalar replaces the object in the JSON text.
1090
1091 For example, the following "TO_JSON" method will convert all URI
1092 objects to JSON strings when serialised. The fatc that these values
1093 originally were URI objects is lost.
1094
1095 sub URI::TO_JSON {
1096 my ($uri) = @_;
1097 $uri->as_string
1098 }
1099
1100 3. "allow_blessed" is enabled.
1101 The object will be serialised as a JSON null value.
1102
1103 4. none of the above
1104 If none of the settings are enabled or the respective methods are
1105 missing, "JSON::XS" throws an exception.
1106
1107 DESERIALISATION
1108 For deserialisation there are only two cases to consider: either
1109 nonstandard tagging was used, in which case "allow_tags" decides, or
1110 objects cannot be automatically be deserialised, in which case you can
1111 use postprocessing or the "filter_json_object" or
1112 "filter_json_single_key_object" callbacks to get some real objects our
1113 of your JSON.
1114
1115 This section only considers the tagged value case: I a tagged JSON
1116 object is encountered during decoding and "allow_tags" is disabled, a
1117 parse error will result (as if tagged values were not part of the
1118 grammar).
1119
1120 If "allow_tags" is enabled, "JSON::XS" will look up the "THAW" method of
1121 the package/classname used during serialisation (it will not attempt to
1122 load the package as a Perl module). If there is no such method, the
1123 decoding will fail with an error.
1124
1125 Otherwise, the "THAW" method is invoked with the classname as first
1126 argument, the constant string "JSON" as second argument, and all the
1127 values from the JSON array (the values originally returned by the
1128 "FREEZE" method) as remaining arguments.
1129
1130 The method must then return the object. While technically you can return
1131 any Perl scalar, you might have to enable the "enable_nonref" setting to
1132 make that work in all cases, so better return an actual blessed
1133 reference.
1134
1135 As an example, let's implement a "THAW" function that regenerates the
1136 "My::Object" from the "FREEZE" example earlier:
1137
1138 sub My::Object::THAW {
1139 my ($class, $serialiser, $type, $id) = @_;
1140
1141 $class->new (type => $type, id => $id)
1142 }
1143
1144ENCODING/CODESET FLAG NOTES
1145 The interested reader might have seen a number of flags that signify
1146 encodings or codesets - "utf8", "latin1" and "ascii". There seems to be
1147 some confusion on what these do, so here is a short comparison:
1148
1149 "utf8" controls whether the JSON text created by "encode" (and expected
1150 by "decode") is UTF-8 encoded or not, while "latin1" and "ascii" only
1151 control whether "encode" escapes character values outside their
1152 respective codeset range. Neither of these flags conflict with each
1153 other, although some combinations make less sense than others.
1154
1155 Care has been taken to make all flags symmetrical with respect to
1156 "encode" and "decode", that is, texts encoded with any combination of
1157 these flag values will be correctly decoded when the same flags are used
1158 - in general, if you use different flag settings while encoding vs. when
1159 decoding you likely have a bug somewhere.
1160
1161 Below comes a verbose discussion of these flags. Note that a "codeset"
1162 is simply an abstract set of character-codepoint pairs, while an
1163 encoding takes those codepoint numbers and *encodes* them, in our case
1164 into octets. Unicode is (among other things) a codeset, UTF-8 is an
1165 encoding, and ISO-8859-1 (= latin 1) and ASCII are both codesets *and*
1166 encodings at the same time, which can be confusing.
1167
1168 "utf8" flag disabled
1169 When "utf8" is disabled (the default), then "encode"/"decode"
1170 generate and expect Unicode strings, that is, characters with high
1171 ordinal Unicode values (> 255) will be encoded as such characters,
1172 and likewise such characters are decoded as-is, no changes to them
1173 will be done, except "(re-)interpreting" them as Unicode codepoints
1174 or Unicode characters, respectively (to Perl, these are the same
1175 thing in strings unless you do funny/weird/dumb stuff).
1176
1177 This is useful when you want to do the encoding yourself (e.g. when
1178 you want to have UTF-16 encoded JSON texts) or when some other layer
1179 does the encoding for you (for example, when printing to a terminal
1180 using a filehandle that transparently encodes to UTF-8 you certainly
1181 do NOT want to UTF-8 encode your data first and have Perl encode it
1182 another time).
1183
1184 "utf8" flag enabled
1185 If the "utf8"-flag is enabled, "encode"/"decode" will encode all
1186 characters using the corresponding UTF-8 multi-byte sequence, and
1187 will expect your input strings to be encoded as UTF-8, that is, no
1188 "character" of the input string must have any value > 255, as UTF-8
1189 does not allow that.
1190
1191 The "utf8" flag therefore switches between two modes: disabled means
1192 you will get a Unicode string in Perl, enabled means you get a UTF-8
1193 encoded octet/binary string in Perl.
1194
1195 "latin1" or "ascii" flags enabled
1196 With "latin1" (or "ascii") enabled, "encode" will escape characters
1197 with ordinal values > 255 (> 127 with "ascii") and encode the
1198 remaining characters as specified by the "utf8" flag.
1199
1200 If "utf8" is disabled, then the result is also correctly encoded in
1201 those character sets (as both are proper subsets of Unicode, meaning
1202 that a Unicode string with all character values < 256 is the same
1203 thing as a ISO-8859-1 string, and a Unicode string with all
1204 character values < 128 is the same thing as an ASCII string in
213 properly). 1205 Perl).
214 1206
215 No roundtripping (strings get clobbered if they look like numbers, 1207 If "utf8" is enabled, you still get a correct UTF-8-encoded string,
216 e.g. the string 2.0 will encode to 2.0 instead of "2.0", and that 1208 regardless of these flags, just some more characters will be escaped
217 will decode into the number 2. 1209 using "\uXXXX" then before.
218 1210
219 JSON::PC 1211 Note that ISO-8859-1-*encoded* strings are not compatible with UTF-8
220 Very fast. 1212 encoding, while ASCII-encoded strings are. That is because the
1213 ISO-8859-1 encoding is NOT a subset of UTF-8 (despite the ISO-8859-1
1214 *codeset* being a subset of Unicode), while ASCII is.
221 1215
222 Undocumented/buggy Unicode handling. 1216 Surprisingly, "decode" will ignore these flags and so treat all
1217 input values as governed by the "utf8" flag. If it is disabled, this
1218 allows you to decode ISO-8859-1- and ASCII-encoded strings, as both
1219 strict subsets of Unicode. If it is enabled, you can correctly
1220 decode UTF-8 encoded strings.
223 1221
224 No roundtripping. 1222 So neither "latin1" nor "ascii" are incompatible with the "utf8"
1223 flag - they only govern when the JSON output engine escapes a
1224 character or not.
225 1225
226 Has problems handling many Perl values (e.g. regex results and other 1226 The main use for "latin1" is to relatively efficiently store binary
227 magic values will make it croak). 1227 data as JSON, at the expense of breaking compatibility with most
1228 JSON decoders.
228 1229
229 Does not even generate valid JSON ("{1,2}" gets converted to "{1:2}" 1230 The main use for "ascii" is to force the output to not contain
230 which is not a valid JSON string. 1231 characters with values > 127, which means you can interpret the
1232 resulting string as UTF-8, ISO-8859-1, ASCII, KOI8-R or most about
1233 any character set and 8-bit-encoding, and still get the same data
1234 structure back. This is useful when your channel for JSON transfer
1235 is not 8-bit clean or the encoding might be mangled in between (e.g.
1236 in mail), and works because ASCII is a proper subset of most 8-bit
1237 and multibyte encodings in use in the world.
231 1238
232 Unmaintained (maintainer unresponsive for many months, bugs are not 1239 JSON and ECMAscript
233 getting fixed). 1240 JSON syntax is based on how literals are represented in javascript (the
1241 not-standardised predecessor of ECMAscript) which is presumably why it
1242 is called "JavaScript Object Notation".
234 1243
235 JSON::Syck 1244 However, JSON is not a subset (and also not a superset of course) of
236 Very buggy (often crashes). 1245 ECMAscript (the standard) or javascript (whatever browsers actually
1246 implement).
237 1247
238 Very inflexible (no human-readable format supported, format pretty 1248 If you want to use javascript's "eval" function to "parse" JSON, you
239 much undocumented. I need at least a format for easy reading by 1249 might run into parse errors for valid JSON texts, or the resulting data
240 humans and a single-line compact format for use in a protocol, and 1250 structure might not be queryable:
241 preferably a way to generate ASCII-only JSON strings).
242 1251
243 Completely broken (and confusingly documented) Unicode handling 1252 One of the problems is that U+2028 and U+2029 are valid characters
244 (unicode escapes are not working properly, you need to set 1253 inside JSON strings, but are not allowed in ECMAscript string literals,
245 ImplicitUnicode to *different* values on en- and decoding to get 1254 so the following Perl fragment will not output something that can be
246 symmetric behaviour). 1255 guaranteed to be parsable by javascript's "eval":
247 1256
248 No roundtripping (simple cases work, but this depends on wether the 1257 use JSON::XS;
249 scalar value was used in a numeric context or not).
250 1258
251 Dumping hashes may skip hash values depending on iterator state. 1259 print encode_json [chr 0x2028];
252 1260
253 Unmaintained (maintainer unresponsive for many months, bugs are not 1261 The right fix for this is to use a proper JSON parser in your javascript
254 getting fixed). 1262 programs, and not rely on "eval" (see for example Douglas Crockford's
1263 json2.js parser).
255 1264
256 Does not check input for validity (i.e. will accept non-JSON input 1265 If this is not an option, you can, as a stop-gap measure, simply encode
257 and return "something" instead of raising an exception. This is a 1266 to ASCII-only JSON:
258 security issue: imagine two banks transfering money between each
259 other using JSON. One bank might parse a given non-JSON request and
260 deduct money, while the other might reject the transaction with a
261 syntax error. While a good protocol will at least recover, that is
262 extra unnecessary work and the transaction will still not succeed).
263 1267
264 JSON::DWIW 1268 use JSON::XS;
265 Very fast. Very natural. Very nice.
266 1269
267 Undocumented unicode handling (but the best of the pack. Unicode 1270 print JSON::XS->new->ascii->encode ([chr 0x2028]);
268 escapes still don't get parsed properly).
269 1271
270 Very inflexible. 1272 Note that this will enlarge the resulting JSON text quite a bit if you
1273 have many non-ASCII characters. You might be tempted to run some regexes
1274 to only escape U+2028 and U+2029, e.g.:
271 1275
272 No roundtripping. 1276 # DO NOT USE THIS!
1277 my $json = JSON::XS->new->utf8->encode ([chr 0x2028]);
1278 $json =~ s/\xe2\x80\xa8/\\u2028/g; # escape U+2028
1279 $json =~ s/\xe2\x80\xa9/\\u2029/g; # escape U+2029
1280 print $json;
273 1281
274 Does not generate valid JSON (key strings are often unquoted, empty 1282 Note that *this is a bad idea*: the above only works for U+2028 and
275 keys result in nothing being output) 1283 U+2029 and thus only for fully ECMAscript-compliant parsers. Many
1284 existing javascript implementations, however, have issues with other
1285 characters as well - using "eval" naively simply *will* cause problems.
276 1286
277 Does not check input for validity. 1287 Another problem is that some javascript implementations reserve some
1288 property names for their own purposes (which probably makes them
1289 non-ECMAscript-compliant). For example, Iceweasel reserves the
1290 "__proto__" property name for its own purposes.
1291
1292 If that is a problem, you could parse try to filter the resulting JSON
1293 output for these property strings, e.g.:
1294
1295 $json =~ s/"__proto__"\s*:/"__proto__renamed":/g;
1296
1297 This works because "__proto__" is not valid outside of strings, so every
1298 occurrence of ""__proto__"\s*:" must be a string used as property name.
1299
1300 If you know of other incompatibilities, please let me know.
1301
1302 JSON and YAML
1303 You often hear that JSON is a subset of YAML. This is, however, a mass
1304 hysteria(*) and very far from the truth (as of the time of this
1305 writing), so let me state it clearly: *in general, there is no way to
1306 configure JSON::XS to output a data structure as valid YAML* that works
1307 in all cases.
1308
1309 If you really must use JSON::XS to generate YAML, you should use this
1310 algorithm (subject to change in future versions):
1311
1312 my $to_yaml = JSON::XS->new->utf8->space_after (1);
1313 my $yaml = $to_yaml->encode ($ref) . "\n";
1314
1315 This will *usually* generate JSON texts that also parse as valid YAML.
1316 Please note that YAML has hardcoded limits on (simple) object key
1317 lengths that JSON doesn't have and also has different and incompatible
1318 unicode character escape syntax, so you should make sure that your hash
1319 keys are noticeably shorter than the 1024 "stream characters" YAML
1320 allows and that you do not have characters with codepoint values outside
1321 the Unicode BMP (basic multilingual page). YAML also does not allow "\/"
1322 sequences in strings (which JSON::XS does not *currently* generate, but
1323 other JSON generators might).
1324
1325 There might be other incompatibilities that I am not aware of (or the
1326 YAML specification has been changed yet again - it does so quite often).
1327 In general you should not try to generate YAML with a JSON generator or
1328 vice versa, or try to parse JSON with a YAML parser or vice versa:
1329 chances are high that you will run into severe interoperability problems
1330 when you least expect it.
1331
1332 (*) I have been pressured multiple times by Brian Ingerson (one of the
1333 authors of the YAML specification) to remove this paragraph, despite
1334 him acknowledging that the actual incompatibilities exist. As I was
1335 personally bitten by this "JSON is YAML" lie, I refused and said I
1336 will continue to educate people about these issues, so others do not
1337 run into the same problem again and again. After this, Brian called
1338 me a (quote)*complete and worthless idiot*(unquote).
1339
1340 In my opinion, instead of pressuring and insulting people who
1341 actually clarify issues with YAML and the wrong statements of some
1342 of its proponents, I would kindly suggest reading the JSON spec
1343 (which is not that difficult or long) and finally make YAML
1344 compatible to it, and educating users about the changes, instead of
1345 spreading lies about the real compatibility for many *years* and
1346 trying to silence people who point out that it isn't true.
1347
1348 Addendum/2009: the YAML 1.2 spec is still incompatible with JSON,
1349 even though the incompatibilities have been documented (and are
1350 known to Brian) for many years and the spec makes explicit claims
1351 that YAML is a superset of JSON. It would be so easy to fix, but
1352 apparently, bullying people and corrupting userdata is so much
1353 easier.
278 1354
279 SPEED 1355 SPEED
280 It seems that JSON::XS is surprisingly fast, as shown in the following 1356 It seems that JSON::XS is surprisingly fast, as shown in the following
281 tables. They have been generated with the help of the "eg/bench" program 1357 tables. They have been generated with the help of the "eg/bench" program
282 in the JSON::XS distribution, to make it easy to compare on your own 1358 in the JSON::XS distribution, to make it easy to compare on your own
283 system. 1359 system.
284 1360
285 First is a comparison between various modules using a very simple JSON 1361 First comes a comparison between various modules using a very short
1362 single-line JSON string (also available at
1363 <http://dist.schmorp.de/misc/json/short.json>).
1364
1365 {"method": "handleMessage", "params": ["user1",
1366 "we were just talking"], "id": null, "array":[1,11,234,-5,1e5,1e7,
1367 1, 0]}
1368
286 string, showing the number of encodes/decodes per second (JSON::XS is 1369 It shows the number of encodes/decodes per second (JSON::XS uses the
287 the functional interface, while JSON::XS/2 is the OO interface with 1370 functional interface, while JSON::XS/2 uses the OO interface with
288 pretty-printing and hashkey sorting enabled). 1371 pretty-printing and hashkey sorting enabled, JSON::XS/3 enables shrink.
1372 JSON::DWIW/DS uses the deserialise function, while JSON::DWIW::FJ uses
1373 the from_json method). Higher is better:
289 1374
290 module | encode | decode | 1375 module | encode | decode |
291 -----------|------------|------------| 1376 --------------|------------|------------|
292 JSON | 14006 | 6820 | 1377 JSON::DWIW/DS | 86302.551 | 102300.098 |
293 JSON::DWIW | 200937 | 120386 | 1378 JSON::DWIW/FJ | 86302.551 | 75983.768 |
294 JSON::PC | 85065 | 129366 | 1379 JSON::PP | 15827.562 | 6638.658 |
295 JSON::Syck | 59898 | 44232 | 1380 JSON::Syck | 63358.066 | 47662.545 |
296 JSON::XS | 1171478 | 342435 | 1381 JSON::XS | 511500.488 | 511500.488 |
297 JSON::XS/2 | 730760 | 328714 | 1382 JSON::XS/2 | 291271.111 | 388361.481 |
1383 JSON::XS/3 | 361577.931 | 361577.931 |
1384 Storable | 66788.280 | 265462.278 |
298 -----------+------------+------------+ 1385 --------------+------------+------------+
299 1386
300 That is, JSON::XS is 6 times faster than than JSON::DWIW and about 80 1387 That is, JSON::XS is almost six times faster than JSON::DWIW on
301 times faster than JSON, even with pretty-printing and key sorting. 1388 encoding, about five times faster on decoding, and over thirty to
1389 seventy times faster than JSON's pure perl implementation. It also
1390 compares favourably to Storable for small amounts of data.
302 1391
303 Using a longer test string (roughly 8KB, generated from Yahoo! Locals 1392 Using a longer test string (roughly 18KB, generated from Yahoo! Locals
304 search API (http://nanoref.com/yahooapis/mgPdGg): 1393 search API (<http://dist.schmorp.de/misc/json/long.json>).
305 1394
306 module | encode | decode | 1395 module | encode | decode |
307 -----------|------------|------------| 1396 --------------|------------|------------|
308 JSON | 673 | 38 | 1397 JSON::DWIW/DS | 1647.927 | 2673.916 |
309 JSON::DWIW | 5271 | 770 | 1398 JSON::DWIW/FJ | 1630.249 | 2596.128 |
310 JSON::PC | 9901 | 2491 | 1399 JSON::PP | 400.640 | 62.311 |
311 JSON::Syck | 2360 | 786 | 1400 JSON::Syck | 1481.040 | 1524.869 |
312 JSON::XS | 37398 | 3202 | 1401 JSON::XS | 20661.596 | 9541.183 |
313 JSON::XS/2 | 13765 | 3153 | 1402 JSON::XS/2 | 10683.403 | 9416.938 |
1403 JSON::XS/3 | 20661.596 | 9400.054 |
1404 Storable | 19765.806 | 10000.725 |
314 -----------+------------+------------+ 1405 --------------+------------+------------+
315 1406
316 Again, JSON::XS leads by far in the encoding case, while still beating 1407 Again, JSON::XS leads by far (except for Storable which non-surprisingly
317 every other module in the decoding case. 1408 decodes a bit faster).
318 1409
319 Last example is an almost 8MB large hash with many large binary values 1410 On large strings containing lots of high Unicode characters, some
320 (PNG files), resulting in a lot of escaping: 1411 modules (such as JSON::PC) seem to decode faster than JSON::XS, but the
1412 result will be broken due to missing (or wrong) Unicode handling. Others
1413 refuse to decode or encode properly, so it was impossible to prepare a
1414 fair comparison table for that case.
1415
1416SECURITY CONSIDERATIONS
1417 When you are using JSON in a protocol, talking to untrusted potentially
1418 hostile creatures requires relatively few measures.
1419
1420 First of all, your JSON decoder should be secure, that is, should not
1421 have any buffer overflows. Obviously, this module should ensure that and
1422 I am trying hard on making that true, but you never know.
1423
1424 Second, you need to avoid resource-starving attacks. That means you
1425 should limit the size of JSON texts you accept, or make sure then when
1426 your resources run out, that's just fine (e.g. by using a separate
1427 process that can crash safely). The size of a JSON text in octets or
1428 characters is usually a good indication of the size of the resources
1429 required to decode it into a Perl structure. While JSON::XS can check
1430 the size of the JSON text, it might be too late when you already have it
1431 in memory, so you might want to check the size before you accept the
1432 string.
1433
1434 Third, JSON::XS recurses using the C stack when decoding objects and
1435 arrays. The C stack is a limited resource: for instance, on my amd64
1436 machine with 8MB of stack size I can decode around 180k nested arrays
1437 but only 14k nested JSON objects (due to perl itself recursing deeply on
1438 croak to free the temporary). If that is exceeded, the program crashes.
1439 To be conservative, the default nesting limit is set to 512. If your
1440 process has a smaller stack, you should adjust this setting accordingly
1441 with the "max_depth" method.
1442
1443 Something else could bomb you, too, that I forgot to think of. In that
1444 case, you get to keep the pieces. I am always open for hints, though...
1445
1446 Also keep in mind that JSON::XS might leak contents of your Perl data
1447 structures in its error messages, so when you serialise sensitive
1448 information you might want to make sure that exceptions thrown by
1449 JSON::XS will not end up in front of untrusted eyes.
1450
1451 If you are using JSON::XS to return packets to consumption by JavaScript
1452 scripts in a browser you should have a look at
1453 <http://blog.archive.jpsykes.com/47/practical-csrf-and-json-security/>
1454 to see whether you are vulnerable to some common attack vectors (which
1455 really are browser design bugs, but it is still you who will have to
1456 deal with it, as major browser developers care only for features, not
1457 about getting security right).
1458
1459 "OLD" VS. "NEW" JSON (RFC4627 VS. RFC7159)
1460 JSON originally required JSON texts to represent an array or object -
1461 scalar values were explicitly not allowed. This has changed, and
1462 versions of JSON::XS beginning with 4.0 reflect this by allowing scalar
1463 values by default.
1464
1465 One reason why one might not want this is that this removes a
1466 fundamental property of JSON texts, namely that they are self-delimited
1467 and self-contained, or in other words, you could take any number of
1468 "old" JSON texts and paste them together, and the result would be
1469 unambiguously parseable:
1470
1471 [1,3]{"k":5}[][null] # four JSON texts, without doubt
1472
1473 By allowing scalars, this property is lost: in the following example, is
1474 this one JSON text (the number 12) or two JSON texts (the numbers 1 and
1475 2):
1476
1477 12 # could be 12, or 1 and 2
1478
1479 Another lost property of "old" JSON is that no lookahead is required to
1480 know the end of a JSON text, i.e. the JSON text definitely ended at the
1481 last "]" or "}" character, there was no need to read extra characters.
1482
1483 For example, a viable network protocol with "old" JSON was to simply
1484 exchange JSON texts without delimiter. For "new" JSON, you have to use a
1485 suitable delimiter (such as a newline) after every JSON text or ensure
1486 you never encode/decode scalar values.
1487
1488 Most protocols do work by only transferring arrays or objects, and the
1489 easiest way to avoid problems with the "new" JSON definition is to
1490 explicitly disallow scalar values in your encoder and decoder:
1491
1492 $json_coder = JSON::XS->new->allow_nonref (0)
1493
1494 This is a somewhat unhappy situation, and the blame can fully be put on
1495 JSON's inmventor, Douglas Crockford, who unilaterally changed the format
1496 in 2006 without consulting the IETF, forcing the IETF to either fork the
1497 format or go with it (as I was told, the IETF wasn't amused).
1498
1499RELATIONSHIP WITH I-JSON
1500 JSON is a somewhat sloppily-defined format - it carries around obvious
1501 Javascript baggage, such as not really defining number range, probably
1502 because Javascript only has one type of numbers: IEEE 64 bit floats
1503 ("binary64").
1504
1505 For this reaosn, RFC7493 defines "Internet JSON", which is a restricted
1506 subset of JSON that is supposedly more interoperable on the internet.
1507
1508 While "JSON::XS" does not offer specific support for I-JSON, it of
1509 course accepts valid I-JSON and by default implements some of the
1510 limitations of I-JSON, such as parsing numbers as perl numbers, which
1511 are usually a superset of binary64 numbers.
1512
1513 To generate I-JSON, follow these rules:
1514
1515 * always generate UTF-8
1516
1517 I-JSON must be encoded in UTF-8, the default for "encode_json".
1518
1519 * numbers should be within IEEE 754 binary64 range
1520
1521 Basically all existing perl installations use binary64 to represent
1522 floating point numbers, so all you need to do is to avoid large
1523 integers.
1524
1525 * objects must not have duplicate keys
1526
1527 This is trivially done, as "JSON::XS" does not allow duplicate keys.
1528
1529 * do not generate scalar JSON texts, use "->allow_nonref (0)"
1530
1531 I-JSON strongly requests you to only encode arrays and objects into
1532 JSON.
1533
1534 * times should be strings in ISO 8601 format
1535
1536 There are a myriad of modules on CPAN dealing with ISO 8601 - search
1537 for "ISO8601" on CPAN and use one.
1538
1539 * encode binary data as base64
1540
1541 While it's tempting to just dump binary data as a string (and let
1542 "JSON::XS" do the escaping), for I-JSON, it's *recommended* to
1543 encode binary data as base64.
1544
1545 There are some other considerations - read RFC7493 for the details if
1546 interested.
1547
1548INTEROPERABILITY WITH OTHER MODULES
1549 "JSON::XS" uses the Types::Serialiser module to provide boolean
1550 constants. That means that the JSON true and false values will be
1551 comaptible to true and false values of other modules that do the same,
1552 such as JSON::PP and CBOR::XS.
1553
1554INTEROPERABILITY WITH OTHER JSON DECODERS
1555 As long as you only serialise data that can be directly expressed in
1556 JSON, "JSON::XS" is incapable of generating invalid JSON output (modulo
1557 bugs, but "JSON::XS" has found more bugs in the official JSON testsuite
1558 (1) than the official JSON testsuite has found in "JSON::XS" (0)).
1559
1560 When you have trouble decoding JSON generated by this module using other
1561 decoders, then it is very likely that you have an encoding mismatch or
1562 the other decoder is broken.
1563
1564 When decoding, "JSON::XS" is strict by default and will likely catch all
1565 errors. There are currently two settings that change this: "relaxed"
1566 makes "JSON::XS" accept (but not generate) some non-standard extensions,
1567 and "allow_tags" will allow you to encode and decode Perl objects, at
1568 the cost of not outputting valid JSON anymore.
1569
1570 TAGGED VALUE SYNTAX AND STANDARD JSON EN/DECODERS
1571 When you use "allow_tags" to use the extended (and also nonstandard and
1572 invalid) JSON syntax for serialised objects, and you still want to
1573 decode the generated When you want to serialise objects, you can run a
1574 regex to replace the tagged syntax by standard JSON arrays (it only
1575 works for "normal" package names without comma, newlines or single
1576 colons). First, the readable Perl version:
1577
1578 # if your FREEZE methods return no values, you need this replace first:
1579 $json =~ s/\( \s* (" (?: [^\\":,]+|\\.|::)* ") \s* \) \s* \[\s*\]/[$1]/gx;
1580
1581 # this works for non-empty constructor arg lists:
1582 $json =~ s/\( \s* (" (?: [^\\":,]+|\\.|::)* ") \s* \) \s* \[/[$1,/gx;
1583
1584 And here is a less readable version that is easy to adapt to other
1585 languages:
1586
1587 $json =~ s/\(\s*("([^\\":,]+|\\.|::)*")\s*\)\s*\[/[$1,/g;
1588
1589 Here is an ECMAScript version (same regex):
1590
1591 json = json.replace (/\(\s*("([^\\":,]+|\\.|::)*")\s*\)\s*\[/g, "[$1,");
1592
1593 Since this syntax converts to standard JSON arrays, it might be hard to
1594 distinguish serialised objects from normal arrays. You can prepend a
1595 "magic number" as first array element to reduce chances of a collision:
1596
1597 $json =~ s/\(\s*("([^\\":,]+|\\.|::)*")\s*\)\s*\[/["XU1peReLzT4ggEllLanBYq4G9VzliwKF",$1,/g;
1598
1599 And after decoding the JSON text, you could walk the data structure
1600 looking for arrays with a first element of
1601 "XU1peReLzT4ggEllLanBYq4G9VzliwKF".
1602
1603 The same approach can be used to create the tagged format with another
1604 encoder. First, you create an array with the magic string as first
1605 member, the classname as second, and constructor arguments last, encode
1606 it as part of your JSON structure, and then:
1607
1608 $json =~ s/\[\s*"XU1peReLzT4ggEllLanBYq4G9VzliwKF"\s*,\s*("([^\\":,]+|\\.|::)*")\s*,/($1)[/g;
1609
1610 Again, this has some limitations - the magic string must not be encoded
1611 with character escapes, and the constructor arguments must be non-empty.
1612
1613(I-)THREADS
1614 This module is *not* guaranteed to be ithread (or MULTIPLICITY-) safe
1615 and there are no plans to change this. Note that perl's builtin
1616 so-called threads/ithreads are officially deprecated and should not be
1617 used.
1618
1619THE PERILS OF SETLOCALE
1620 Sometimes people avoid the Perl locale support and directly call the
1621 system's setlocale function with "LC_ALL".
1622
1623 This breaks both perl and modules such as JSON::XS, as stringification
1624 of numbers no longer works correctly (e.g. "$x = 0.1; print "$x"+1"
1625 might print 1, and JSON::XS might output illegal JSON as JSON::XS relies
1626 on perl to stringify numbers).
1627
1628 The solution is simple: don't call "setlocale", or use it for only those
1629 categories you need, such as "LC_MESSAGES" or "LC_CTYPE".
1630
1631 If you need "LC_NUMERIC", you should enable it only around the code that
1632 actually needs it (avoiding stringification of numbers), and restore it
1633 afterwards.
1634
1635SOME HISTORY
1636 At the time this module was created there already were a number of JSON
1637 modules available on CPAN, so what was the reason to write yet another
1638 JSON module? While it seems there are many JSON modules, none of them
1639 correctly handled all corner cases, and in most cases their maintainers
1640 are unresponsive, gone missing, or not listening to bug reports for
1641 other reasons.
1642
1643 Beginning with version 2.0 of the JSON module, when both JSON and
1644 JSON::XS are installed, then JSON will fall back on JSON::XS (this can
1645 be overridden) with no overhead due to emulation (by inheriting
1646 constructor and methods). If JSON::XS is not available, it will fall
1647 back to the compatible JSON::PP module as backend, so using JSON instead
1648 of JSON::XS gives you a portable JSON API that can be fast when you need
1649 it and doesn't require a C compiler when that is a problem.
1650
1651 Somewhere around version 3, this module was forked into
1652 "Cpanel::JSON::XS", because its maintainer had serious trouble
1653 understanding JSON and insisted on a fork with many bugs "fixed" that
1654 weren't actually bugs, while spreading FUD about this module without
1655 actually giving any details on his accusations. You be the judge, but in
1656 my personal opinion, if you want quality, you will stay away from
1657 dangerous forks like that.
321 1658
322BUGS 1659BUGS
323 While the goal of this module is to be correct, that unfortunately does 1660 While the goal of this module is to be correct, that unfortunately does
324 not mean its bug-free, only that I think its design is bug-free. It is 1661 not mean it's bug-free, only that I think its design is bug-free. If you
325 still very young and not well-tested. If you keep reporting bugs they 1662 keep reporting bugs they will be fixed swiftly, though.
326 will be fixed swiftly, though. 1663
1664 Please refrain from using rt.cpan.org or any other bug reporting
1665 service. I put the contact address into my modules for a reason.
1666
1667SEE ALSO
1668 The json_xs command line utility for quick experiments.
327 1669
328AUTHOR 1670AUTHOR
329 Marc Lehmann <schmorp@schmorp.de> 1671 Marc Lehmann <schmorp@schmorp.de>
330 http://home.schmorp.de/ 1672 http://home.schmorp.de/
331 1673

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