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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.
34
35 Beginning with version 2.0 of the JSON module, when both JSON and
36 JSON::XS are installed, then JSON will fall back on JSON::XS (this can
37 be overriden) with no overhead due to emulation (by inheritign
38 constructor and methods). If JSON::XS is not available, it will fall
39 back to the compatible JSON::PP module as backend, so using JSON instead
40 of JSON::XS gives you a portable JSON API that can be fast when you need
41 and doesn't require a C compiler when that is a problem.
11 42
12 As this is the n-th-something JSON module on CPAN, what was the reason 43 As this is the n-th-something JSON module on CPAN, what was the reason
13 to write yet another JSON module? While it seems there are many JSON 44 to write yet another JSON module? While it seems there are many JSON
14 modules, none of them correctly handle all corner cases, and in most 45 modules, none of them correctly handle all corner cases, and in most
15 cases their maintainers are unresponsive, gone missing, or not listening 46 cases their maintainers are unresponsive, gone missing, or not listening
16 to bug reports for other reasons. 47 to bug reports for other reasons.
17 48
18 See COMPARISON, below, for a comparison to some other JSON modules. 49 See COMPARISON, below, for a comparison to some other JSON modules.
19 50
51 See MAPPING, below, on how JSON::XS maps perl values to JSON values and
52 vice versa.
53
20 FEATURES 54 FEATURES
21 * correct handling of unicode issues 55 * correct Unicode handling
56
22 This module knows how to handle Unicode, and even documents how it 57 This module knows how to handle Unicode, documents how and when it
23 does so. 58 does so, and even documents what "correct" means.
24 59
25 * round-trip integrity 60 * round-trip integrity
61
26 When you serialise a perl data structure using only datatypes 62 When you serialise a perl data structure using only datatypes
27 supported by JSON, the deserialised data structure is identical on 63 supported by JSON, the deserialised data structure is identical on
28 the Perl level. (e.g. the string "2.0" doesn't suddenly become "2"). 64 the Perl level. (e.g. the string "2.0" doesn't suddenly become "2"
65 just because it looks like a number). There minor *are* exceptions
66 to this, read the MAPPING section below to learn about those.
29 67
30 * strict checking of JSON correctness 68 * strict checking of JSON correctness
69
31 There is no guessing, no generating of illegal JSON strings by 70 There is no guessing, no generating of illegal JSON texts by
32 default, and only JSON is accepted as input (the latter is a 71 default, and only JSON is accepted as input by default (the latter
33 security feature). 72 is a security feature).
34 73
35 * fast 74 * fast
36 compared to other JSON modules, this module compares favourably.
37 75
76 Compared to other JSON modules and other serialisers such as
77 Storable, this module usually compares favourably in terms of speed,
78 too.
79
38 * simple to use 80 * simple to use
81
39 This module has both a simple functional interface as well as an OO 82 This module has both a simple functional interface as well as an
40 interface. 83 objetc oriented interface interface.
41 84
42 * reasonably versatile output formats 85 * reasonably versatile output formats
86
43 You can choose between the most compact format possible, a 87 You can choose between the most compact guaranteed-single-line
44 pure-ascii format, or a pretty-printed format. Or you can combine 88 format possible (nice for simple line-based protocols), a pure-ascii
89 format (for when your transport is not 8-bit clean, still supports
90 the whole Unicode range), or a pretty-printed format (for when you
91 want to read that stuff). Or you can combine those features in
45 those features in whatever way you like. 92 whatever way you like.
46 93
47FUNCTIONAL INTERFACE 94FUNCTIONAL INTERFACE
48 The following convinience methods are provided by this module. They are 95 The following convenience methods are provided by this module. They are
49 exported by default: 96 exported by default:
50 97
51 $json_string = to_json $perl_scalar 98 $json_text = encode_json $perl_scalar
52 Converts the given Perl data structure (a simple scalar or a 99 Converts the given Perl data structure to a UTF-8 encoded, binary
53 reference to a hash or array) to a UTF-8 encoded, binary string
54 (that is, the string contains octets only). Croaks on error. 100 string (that is, the string contains octets only). Croaks on error.
55 101
56 This function call is functionally identical to "JSON::XS->new->utf8 102 This function call is functionally identical to:
57 (1)->encode ($perl_scalar)".
58 103
104 $json_text = JSON::XS->new->utf8->encode ($perl_scalar)
105
106 except being faster.
107
59 $perl_scalar = from_json $json_string 108 $perl_scalar = decode_json $json_text
60 The opposite of "to_json": expects an UTF-8 (binary) string and 109 The opposite of "encode_json": expects an UTF-8 (binary) string and
61 tries to parse that as an UTF-8 encoded JSON string, returning the 110 tries to parse that as an UTF-8 encoded JSON text, returning the
62 resulting simple scalar or reference. Croaks on error. 111 resulting reference. Croaks on error.
63 112
64 This function call is functionally identical to "JSON::XS->new->utf8 113 This function call is functionally identical to:
65 (1)->decode ($json_string)". 114
115 $perl_scalar = JSON::XS->new->utf8->decode ($json_text)
116
117 except being faster.
118
119 $is_boolean = JSON::XS::is_bool $scalar
120 Returns true if the passed scalar represents either JSON::XS::true
121 or JSON::XS::false, two constants that act like 1 and 0,
122 respectively and are used to represent JSON "true" and "false"
123 values in Perl.
124
125 See MAPPING, below, for more information on how JSON values are
126 mapped to Perl.
127
128A FEW NOTES ON UNICODE AND PERL
129 Since this often leads to confusion, here are a few very clear words on
130 how Unicode works in Perl, modulo bugs.
131
132 1. Perl strings can store characters with ordinal values > 255.
133 This enables you to store Unicode characters as single characters in
134 a Perl string - very natural.
135
136 2. Perl does *not* associate an encoding with your strings.
137 ... until you force it to, e.g. when matching it against a regex, or
138 printing the scalar to a file, in which case Perl either interprets
139 your string as locale-encoded text, octets/binary, or as Unicode,
140 depending on various settings. In no case is an encoding stored
141 together with your data, it is *use* that decides encoding, not any
142 magical meta data.
143
144 3. The internal utf-8 flag has no meaning with regards to the encoding
145 of your string.
146 Just ignore that flag unless you debug a Perl bug, a module written
147 in XS or want to dive into the internals of perl. Otherwise it will
148 only confuse you, as, despite the name, it says nothing about how
149 your string is encoded. You can have Unicode strings with that flag
150 set, with that flag clear, and you can have binary data with that
151 flag set and that flag clear. Other possibilities exist, too.
152
153 If you didn't know about that flag, just the better, pretend it
154 doesn't exist.
155
156 4. A "Unicode String" is simply a string where each character can be
157 validly interpreted as a Unicode codepoint.
158 If you have UTF-8 encoded data, it is no longer a Unicode string,
159 but a Unicode string encoded in UTF-8, giving you a binary string.
160
161 5. A string containing "high" (> 255) character values is *not* a UTF-8
162 string.
163 It's a fact. Learn to live with it.
164
165 I hope this helps :)
66 166
67OBJECT-ORIENTED INTERFACE 167OBJECT-ORIENTED INTERFACE
68 The object oriented interface lets you configure your own encoding or 168 The object oriented interface lets you configure your own encoding or
69 decoding style, within the limits of supported formats. 169 decoding style, within the limits of supported formats.
70 170
… …
74 *disabled*. 174 *disabled*.
75 175
76 The mutators for flags all return the JSON object again and thus 176 The mutators for flags all return the JSON object again and thus
77 calls can be chained: 177 calls can be chained:
78 178
79 my $json = JSON::XS->new->utf8(1)->space_after(1)->encode ({a => [1,2]}) 179 my $json = JSON::XS->new->utf8->space_after->encode ({a => [1,2]})
80 => {"a": [1, 2]} 180 => {"a": [1, 2]}
81 181
82 $json = $json->ascii ($enable) 182 $json = $json->ascii ([$enable])
183 $enabled = $json->get_ascii
83 If $enable is true, then the "encode" method will not generate 184 If $enable is true (or missing), then the "encode" method will not
84 characters outside the code range 0..127. Any unicode characters 185 generate characters outside the code range 0..127 (which is ASCII).
85 outside that range will be escaped using either a single \uXXXX (BMP 186 Any Unicode characters outside that range will be escaped using
86 characters) or a double \uHHHH\uLLLLL escape sequence, as per 187 either a single \uXXXX (BMP characters) or a double \uHHHH\uLLLLL
87 RFC4627. 188 escape sequence, as per RFC4627. The resulting encoded JSON text can
189 be treated as a native Unicode string, an ascii-encoded,
190 latin1-encoded or UTF-8 encoded string, or any other superset of
191 ASCII.
88 192
89 If $enable is false, then the "encode" method will not escape 193 If $enable is false, then the "encode" method will not escape
90 Unicode characters unless necessary. 194 Unicode characters unless required by the JSON syntax or other
195 flags. This results in a faster and more compact format.
91 196
197 See also the section *ENCODING/CODESET FLAG NOTES* later in this
198 document.
199
200 The main use for this flag is to produce JSON texts that can be
201 transmitted over a 7-bit channel, as the encoded JSON texts will not
202 contain any 8 bit characters.
203
92 JSON::XS->new->ascii (1)->encode (chr 0x10401) 204 JSON::XS->new->ascii (1)->encode ([chr 0x10401])
93 => \ud801\udc01 205 => ["\ud801\udc01"]
94 206
207 $json = $json->latin1 ([$enable])
208 $enabled = $json->get_latin1
209 If $enable is true (or missing), then the "encode" method will
210 encode the resulting JSON text as latin1 (or iso-8859-1), escaping
211 any characters outside the code range 0..255. The resulting string
212 can be treated as a latin1-encoded JSON text or a native Unicode
213 string. The "decode" method will not be affected in any way by this
214 flag, as "decode" by default expects Unicode, which is a strict
215 superset of latin1.
216
217 If $enable is false, then the "encode" method will not escape
218 Unicode characters unless required by the JSON syntax or other
219 flags.
220
221 See also the section *ENCODING/CODESET FLAG NOTES* later in this
222 document.
223
224 The main use for this flag is efficiently encoding binary data as
225 JSON text, as most octets will not be escaped, resulting in a
226 smaller encoded size. The disadvantage is that the resulting JSON
227 text is encoded in latin1 (and must correctly be treated as such
228 when storing and transferring), a rare encoding for JSON. It is
229 therefore most useful when you want to store data structures known
230 to contain binary data efficiently in files or databases, not when
231 talking to other JSON encoders/decoders.
232
233 JSON::XS->new->latin1->encode (["\x{89}\x{abc}"]
234 => ["\x{89}\\u0abc"] # (perl syntax, U+abc escaped, U+89 not)
235
95 $json = $json->utf8 ($enable) 236 $json = $json->utf8 ([$enable])
237 $enabled = $json->get_utf8
96 If $enable is true, then the "encode" method will encode the JSON 238 If $enable is true (or missing), then the "encode" method will
97 string into UTF-8, as required by many protocols, while the "decode" 239 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 240 while the "decode" method expects to be handled an UTF-8-encoded
99 that UTF-8-encoded strings do not contain any characters outside the 241 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. 242 characters outside the range 0..255, they are thus useful for
243 bytewise/binary I/O. In future versions, enabling this option might
244 enable autodetection of the UTF-16 and UTF-32 encoding families, as
245 described in RFC4627.
101 246
102 If $enable is false, then the "encode" method will return the JSON 247 If $enable is false, then the "encode" method will return the JSON
103 string as a (non-encoded) unicode string, while "decode" expects 248 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 249 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. 250 UTF-16) needs to be done yourself, e.g. using the Encode module.
106 251
252 See also the section *ENCODING/CODESET FLAG NOTES* later in this
253 document.
254
255 Example, output UTF-16BE-encoded JSON:
256
257 use Encode;
258 $jsontext = encode "UTF-16BE", JSON::XS->new->encode ($object);
259
260 Example, decode UTF-32LE-encoded JSON:
261
262 use Encode;
263 $object = JSON::XS->new->decode (decode "UTF-32LE", $jsontext);
264
107 $json = $json->pretty ($enable) 265 $json = $json->pretty ([$enable])
108 This enables (or disables) all of the "indent", "space_before" and 266 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 267 "space_after" (and in the future possibly more) flags in one call to
110 generate the most readable (or most compact) form possible. 268 generate the most readable (or most compact) form possible.
269
270 Example, pretty-print some simple structure:
111 271
112 my $json = JSON::XS->new->pretty(1)->encode ({a => [1,2]}) 272 my $json = JSON::XS->new->pretty(1)->encode ({a => [1,2]})
113 => 273 =>
114 { 274 {
115 "a" : [ 275 "a" : [
116 1, 276 1,
117 2 277 2
118 ] 278 ]
119 } 279 }
120 280
121 $json = $json->indent ($enable) 281 $json = $json->indent ([$enable])
282 $enabled = $json->get_indent
122 If $enable is true, then the "encode" method will use a multiline 283 If $enable is true (or missing), then the "encode" method will use a
123 format as output, putting every array member or object/hash 284 multiline format as output, putting every array member or
124 key-value pair into its own line, identing them properly. 285 object/hash key-value pair into its own line, indenting them
286 properly.
125 287
126 If $enable is false, no newlines or indenting will be produced, and 288 If $enable is false, no newlines or indenting will be produced, and
127 the resulting JSON strings is guarenteed not to contain any 289 the resulting JSON text is guaranteed not to contain any "newlines".
128 "newlines".
129 290
130 This setting has no effect when decoding JSON strings. 291 This setting has no effect when decoding JSON texts.
131 292
132 $json = $json->space_before ($enable) 293 $json = $json->space_before ([$enable])
294 $enabled = $json->get_space_before
133 If $enable is true, then the "encode" method will add an extra 295 If $enable is true (or missing), then the "encode" method will add
134 optional space before the ":" separating keys from values in JSON 296 an extra optional space before the ":" separating keys from values
135 objects. 297 in JSON objects.
136 298
137 If $enable is false, then the "encode" method will not add any extra 299 If $enable is false, then the "encode" method will not add any extra
138 space at those places. 300 space at those places.
139 301
140 This setting has no effect when decoding JSON strings. You will also 302 This setting has no effect when decoding JSON texts. You will also
141 most likely combine this setting with "space_after". 303 most likely combine this setting with "space_after".
142 304
305 Example, space_before enabled, space_after and indent disabled:
306
307 {"key" :"value"}
308
143 $json = $json->space_after ($enable) 309 $json = $json->space_after ([$enable])
310 $enabled = $json->get_space_after
144 If $enable is true, then the "encode" method will add an extra 311 If $enable is true (or missing), then the "encode" method will add
145 optional space after the ":" separating keys from values in JSON 312 an extra optional space after the ":" separating keys from values in
146 objects and extra whitespace after the "," separating key-value 313 JSON objects and extra whitespace after the "," separating key-value
147 pairs and array members. 314 pairs and array members.
148 315
149 If $enable is false, then the "encode" method will not add any extra 316 If $enable is false, then the "encode" method will not add any extra
150 space at those places. 317 space at those places.
151 318
152 This setting has no effect when decoding JSON strings. 319 This setting has no effect when decoding JSON texts.
153 320
321 Example, space_before and indent disabled, space_after enabled:
322
323 {"key": "value"}
324
325 $json = $json->relaxed ([$enable])
326 $enabled = $json->get_relaxed
327 If $enable is true (or missing), then "decode" will accept some
328 extensions to normal JSON syntax (see below). "encode" will not be
329 affected in anyway. *Be aware that this option makes you accept
330 invalid JSON texts as if they were valid!*. I suggest only to use
331 this option to parse application-specific files written by humans
332 (configuration files, resource files etc.)
333
334 If $enable is false (the default), then "decode" will only accept
335 valid JSON texts.
336
337 Currently accepted extensions are:
338
339 * list items can have an end-comma
340
341 JSON *separates* array elements and key-value pairs with commas.
342 This can be annoying if you write JSON texts manually and want
343 to be able to quickly append elements, so this extension accepts
344 comma at the end of such items not just between them:
345
346 [
347 1,
348 2, <- this comma not normally allowed
349 ]
350 {
351 "k1": "v1",
352 "k2": "v2", <- this comma not normally allowed
353 }
354
355 * shell-style '#'-comments
356
357 Whenever JSON allows whitespace, shell-style comments are
358 additionally allowed. They are terminated by the first
359 carriage-return or line-feed character, after which more
360 white-space and comments are allowed.
361
362 [
363 1, # this comment not allowed in JSON
364 # neither this one...
365 ]
366
154 $json = $json->canonical ($enable) 367 $json = $json->canonical ([$enable])
368 $enabled = $json->get_canonical
155 If $enable is true, then the "encode" method will output JSON 369 If $enable is true (or missing), then the "encode" method will
156 objects by sorting their keys. This is adding a comparatively high 370 output JSON objects by sorting their keys. This is adding a
157 overhead. 371 comparatively high overhead.
158 372
159 If $enable is false, then the "encode" method will output key-value 373 If $enable is false, then the "encode" method will output key-value
160 pairs in the order Perl stores them (which will likely change 374 pairs in the order Perl stores them (which will likely change
161 between runs of the same script). 375 between runs of the same script).
162 376
163 This option is useful if you want the same data structure to be 377 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). 378 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 379 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 380 contains the same data, as key-value pairs have no inherent ordering
167 in Perl. 381 in Perl.
168 382
169 This setting has no effect when decoding JSON strings. 383 This setting has no effect when decoding JSON texts.
170 384
171 $json = $json->allow_nonref ($enable) 385 $json = $json->allow_nonref ([$enable])
386 $enabled = $json->get_allow_nonref
172 If $enable is true, then the "encode" method can convert a 387 If $enable is true (or missing), then the "encode" method can
173 non-reference into its corresponding string, number or null JSON 388 convert a non-reference into its corresponding string, number or
174 value, which is an extension to RFC4627. Likewise, "decode" will 389 null JSON value, which is an extension to RFC4627. Likewise,
175 accept those JSON values instead of croaking. 390 "decode" will accept those JSON values instead of croaking.
176 391
177 If $enable is false, then the "encode" method will croak if it isn't 392 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 393 passed an arrayref or hashref, as JSON texts must either be an
179 object or array. Likewise, "decode" will croak if given something 394 object or array. Likewise, "decode" will croak if given something
180 that is not a JSON object or array. 395 that is not a JSON object or array.
181 396
397 Example, encode a Perl scalar as JSON value with enabled
398 "allow_nonref", resulting in an invalid JSON text:
399
400 JSON::XS->new->allow_nonref->encode ("Hello, World!")
401 => "Hello, World!"
402
403 $json = $json->allow_unknown ([$enable])
404 $enabled = $json->get_allow_unknown
405 If $enable is true (or missing), then "encode" will *not* throw an
406 exception when it encounters values it cannot represent in JSON (for
407 example, filehandles) but instead will encode a JSON "null" value.
408 Note that blessed objects are not included here and are handled
409 separately by c<allow_nonref>.
410
411 If $enable is false (the default), then "encode" will throw an
412 exception when it encounters anything it cannot encode as JSON.
413
414 This option does not affect "decode" in any way, and it is
415 recommended to leave it off unless you know your communications
416 partner.
417
418 $json = $json->allow_blessed ([$enable])
419 $enabled = $json->get_allow_blessed
420 If $enable is true (or missing), then the "encode" method will not
421 barf when it encounters a blessed reference. Instead, the value of
422 the convert_blessed option will decide whether "null"
423 ("convert_blessed" disabled or no "TO_JSON" method found) or a
424 representation of the object ("convert_blessed" enabled and
425 "TO_JSON" method found) is being encoded. Has no effect on "decode".
426
427 If $enable is false (the default), then "encode" will throw an
428 exception when it encounters a blessed object.
429
430 $json = $json->convert_blessed ([$enable])
431 $enabled = $json->get_convert_blessed
432 If $enable is true (or missing), then "encode", upon encountering a
433 blessed object, will check for the availability of the "TO_JSON"
434 method on the object's class. If found, it will be called in scalar
435 context and the resulting scalar will be encoded instead of the
436 object. If no "TO_JSON" method is found, the value of
437 "allow_blessed" will decide what to do.
438
439 The "TO_JSON" method may safely call die if it wants. If "TO_JSON"
440 returns other blessed objects, those will be handled in the same
441 way. "TO_JSON" must take care of not causing an endless recursion
442 cycle (== crash) in this case. The name of "TO_JSON" was chosen
443 because other methods called by the Perl core (== not by the user of
444 the object) are usually in upper case letters and to avoid
445 collisions with any "to_json" function or method.
446
447 This setting does not yet influence "decode" in any way, but in the
448 future, global hooks might get installed that influence "decode" and
449 are enabled by this setting.
450
451 If $enable is false, then the "allow_blessed" setting will decide
452 what to do when a blessed object is found.
453
454 $json = $json->filter_json_object ([$coderef->($hashref)])
455 When $coderef is specified, it will be called from "decode" each
456 time it decodes a JSON object. The only argument is a reference to
457 the newly-created hash. If the code references returns a single
458 scalar (which need not be a reference), this value (i.e. a copy of
459 that scalar to avoid aliasing) is inserted into the deserialised
460 data structure. If it returns an empty list (NOTE: *not* "undef",
461 which is a valid scalar), the original deserialised hash will be
462 inserted. This setting can slow down decoding considerably.
463
464 When $coderef is omitted or undefined, any existing callback will be
465 removed and "decode" will not change the deserialised hash in any
466 way.
467
468 Example, convert all JSON objects into the integer 5:
469
470 my $js = JSON::XS->new->filter_json_object (sub { 5 });
471 # returns [5]
472 $js->decode ('[{}]')
473 # throw an exception because allow_nonref is not enabled
474 # so a lone 5 is not allowed.
475 $js->decode ('{"a":1, "b":2}');
476
477 $json = $json->filter_json_single_key_object ($key [=>
478 $coderef->($value)])
479 Works remotely similar to "filter_json_object", but is only called
480 for JSON objects having a single key named $key.
481
482 This $coderef is called before the one specified via
483 "filter_json_object", if any. It gets passed the single value in the
484 JSON object. If it returns a single value, it will be inserted into
485 the data structure. If it returns nothing (not even "undef" but the
486 empty list), the callback from "filter_json_object" will be called
487 next, as if no single-key callback were specified.
488
489 If $coderef is omitted or undefined, the corresponding callback will
490 be disabled. There can only ever be one callback for a given key.
491
492 As this callback gets called less often then the
493 "filter_json_object" one, decoding speed will not usually suffer as
494 much. Therefore, single-key objects make excellent targets to
495 serialise Perl objects into, especially as single-key JSON objects
496 are as close to the type-tagged value concept as JSON gets (it's
497 basically an ID/VALUE tuple). Of course, JSON does not support this
498 in any way, so you need to make sure your data never looks like a
499 serialised Perl hash.
500
501 Typical names for the single object key are "__class_whatever__", or
502 "$__dollars_are_rarely_used__$" or "}ugly_brace_placement", or even
503 things like "__class_md5sum(classname)__", to reduce the risk of
504 clashing with real hashes.
505
506 Example, decode JSON objects of the form "{ "__widget__" => <id> }"
507 into the corresponding $WIDGET{<id>} object:
508
509 # return whatever is in $WIDGET{5}:
510 JSON::XS
511 ->new
512 ->filter_json_single_key_object (__widget__ => sub {
513 $WIDGET{ $_[0] }
514 })
515 ->decode ('{"__widget__": 5')
516
517 # this can be used with a TO_JSON method in some "widget" class
518 # for serialisation to json:
519 sub WidgetBase::TO_JSON {
520 my ($self) = @_;
521
522 unless ($self->{id}) {
523 $self->{id} = ..get..some..id..;
524 $WIDGET{$self->{id}} = $self;
525 }
526
527 { __widget__ => $self->{id} }
528 }
529
530 $json = $json->shrink ([$enable])
531 $enabled = $json->get_shrink
532 Perl usually over-allocates memory a bit when allocating space for
533 strings. This flag optionally resizes strings generated by either
534 "encode" or "decode" to their minimum size possible. This can save
535 memory when your JSON texts are either very very long or you have
536 many short strings. It will also try to downgrade any strings to
537 octet-form if possible: perl stores strings internally either in an
538 encoding called UTF-X or in octet-form. The latter cannot store
539 everything but uses less space in general (and some buggy Perl or C
540 code might even rely on that internal representation being used).
541
542 The actual definition of what shrink does might change in future
543 versions, but it will always try to save space at the expense of
544 time.
545
546 If $enable is true (or missing), the string returned by "encode"
547 will be shrunk-to-fit, while all strings generated by "decode" will
548 also be shrunk-to-fit.
549
550 If $enable is false, then the normal perl allocation algorithms are
551 used. If you work with your data, then this is likely to be faster.
552
553 In the future, this setting might control other things, such as
554 converting strings that look like integers or floats into integers
555 or floats internally (there is no difference on the Perl level),
556 saving space.
557
558 $json = $json->max_depth ([$maximum_nesting_depth])
559 $max_depth = $json->get_max_depth
560 Sets the maximum nesting level (default 512) accepted while encoding
561 or decoding. If a higher nesting level is detected in JSON text or a
562 Perl data structure, then the encoder and decoder will stop and
563 croak at that point.
564
565 Nesting level is defined by number of hash- or arrayrefs that the
566 encoder needs to traverse to reach a given point or the number of
567 "{" or "[" characters without their matching closing parenthesis
568 crossed to reach a given character in a string.
569
570 Setting the maximum depth to one disallows any nesting, so that
571 ensures that the object is only a single hash/object or array.
572
573 If no argument is given, the highest possible setting will be used,
574 which is rarely useful.
575
576 Note that nesting is implemented by recursion in C. The default
577 value has been chosen to be as large as typical operating systems
578 allow without crashing.
579
580 See SECURITY CONSIDERATIONS, below, for more info on why this is
581 useful.
582
583 $json = $json->max_size ([$maximum_string_size])
584 $max_size = $json->get_max_size
585 Set the maximum length a JSON text may have (in bytes) where
586 decoding is being attempted. The default is 0, meaning no limit.
587 When "decode" is called on a string that is longer then this many
588 bytes, it will not attempt to decode the string but throw an
589 exception. This setting has no effect on "encode" (yet).
590
591 If no argument is given, the limit check will be deactivated (same
592 as when 0 is specified).
593
594 See SECURITY CONSIDERATIONS, below, for more info on why this is
595 useful.
596
182 $json_string = $json->encode ($perl_scalar) 597 $json_text = $json->encode ($perl_scalar)
183 Converts the given Perl data structure (a simple scalar or a 598 Converts the given Perl data structure (a simple scalar or a
184 reference to a hash or array) to its JSON representation. Simple 599 reference to a hash or array) to its JSON representation. Simple
185 scalars will be converted into JSON string or number sequences, 600 scalars will be converted into JSON string or number sequences,
186 while references to arrays become JSON arrays and references to 601 while references to arrays become JSON arrays and references to
187 hashes become JSON objects. Undefined Perl values (e.g. "undef") 602 hashes become JSON objects. Undefined Perl values (e.g. "undef")
188 become JSON "null" values. Neither "true" nor "false" values will be 603 become JSON "null" values. Neither "true" nor "false" values will be
189 generated. 604 generated.
190 605
191 $perl_scalar = $json->decode ($json_string) 606 $perl_scalar = $json->decode ($json_text)
192 The opposite of "encode": expects a JSON string and tries to parse 607 The opposite of "encode": expects a JSON text and tries to parse it,
193 it, returning the resulting simple scalar or reference. Croaks on 608 returning the resulting simple scalar or reference. Croaks on error.
194 error.
195 609
196 JSON numbers and strings become simple Perl scalars. JSON arrays 610 JSON numbers and strings become simple Perl scalars. JSON arrays
197 become Perl arrayrefs and JSON objects become Perl hashrefs. "true" 611 become Perl arrayrefs and JSON objects become Perl hashrefs. "true"
198 becomes 1, "false" becomes 0 and "null" becomes "undef". 612 becomes 1, "false" becomes 0 and "null" becomes "undef".
199 613
200COMPARISON 614 ($perl_scalar, $characters) = $json->decode_prefix ($json_text)
201 As already mentioned, this module was created because none of the 615 This works like the "decode" method, but instead of raising an
202 existing JSON modules could be made to work correctly. First I will 616 exception when there is trailing garbage after the first JSON
203 describe the problems (or pleasures) I encountered with various existing 617 object, it will silently stop parsing there and return the number of
204 JSON modules, followed by some benchmark values. JSON::XS was designed 618 characters consumed so far.
205 not to suffer from any of these problems or limitations.
206 619
207 JSON 1.07 620 This is useful if your JSON texts are not delimited by an outer
208 Slow (but very portable, as it is written in pure Perl). 621 protocol (which is not the brightest thing to do in the first place)
622 and you need to know where the JSON text ends.
209 623
210 Undocumented/buggy Unicode handling (how JSON handles unicode values 624 JSON::XS->new->decode_prefix ("[1] the tail")
211 is undocumented. One can get far by feeding it unicode strings and 625 => ([], 3)
212 doing en-/decoding oneself, but unicode escapes are not working 626
627INCREMENTAL PARSING
628 [This section and the API it details is still EXPERIMENTAL]
629
630 In some cases, there is the need for incremental parsing of JSON texts.
631 While this module always has to keep both JSON text and resulting Perl
632 data structure in memory at one time, it does allow you to parse a JSON
633 stream incrementally. It does so by accumulating text until it has a
634 full JSON object, which it then can decode. This process is similar to
635 using "decode_prefix" to see if a full JSON object is available, but is
636 much more efficient (JSON::XS will only attempt to parse the JSON text
637 once it is sure it has enough text to get a decisive result, using a
638 very simple but truly incremental parser).
639
640 The following two methods deal with this.
641
642 [void, scalar or list context] = $json->incr_parse ([$string])
643 This is the central parsing function. It can both append new text
644 and extract objects from the stream accumulated so far (both of
645 these functions are optional).
646
647 If $string is given, then this string is appended to the already
648 existing JSON fragment stored in the $json object.
649
650 After that, if the function is called in void context, it will
651 simply return without doing anything further. This can be used to
652 add more text in as many chunks as you want.
653
654 If the method is called in scalar context, then it will try to
655 extract exactly *one* JSON object. If that is successful, it will
656 return this object, otherwise it will return "undef". If there is a
657 parse error, this method will croak just as "decode" would do (one
658 can then use "incr_skip" to skip the errornous part). This is the
659 most common way of using the method.
660
661 And finally, in list context, it will try to extract as many objects
662 from the stream as it can find and return them, or the empty list
663 otherwise. For this to work, there must be no separators between the
664 JSON objects or arrays, instead they must be concatenated
665 back-to-back. If an error occurs, an exception will be raised as in
666 the scalar context case. Note that in this case, any
667 previously-parsed JSON texts will be lost.
668
669 $lvalue_string = $json->incr_text
670 This method returns the currently stored JSON fragment as an lvalue,
671 that is, you can manipulate it. This *only* works when a preceding
672 call to "incr_parse" in *scalar context* successfully returned an
673 object. Under all other circumstances you must not call this
674 function (I mean it. although in simple tests it might actually
675 work, it *will* fail under real world conditions). As a special
676 exception, you can also call this method before having parsed
677 anything.
678
679 This function is useful in two cases: a) finding the trailing text
680 after a JSON object or b) parsing multiple JSON objects separated by
681 non-JSON text (such as commas).
682
683 $json->incr_skip
684 This will reset the state of the incremental parser and will remove
685 the parsed text from the input buffer. This is useful after
686 "incr_parse" died, in which case the input buffer and incremental
687 parser state is left unchanged, to skip the text parsed so far and
688 to reset the parse state.
689
690 LIMITATIONS
691 All options that affect decoding are supported, except "allow_nonref".
692 The reason for this is that it cannot be made to work sensibly: JSON
693 objects and arrays are self-delimited, i.e. you can concatenate them
694 back to back and still decode them perfectly. This does not hold true
695 for JSON numbers, however.
696
697 For example, is the string 1 a single JSON number, or is it simply the
698 start of 12? Or is 12 a single JSON number, or the concatenation of 1
699 and 2? In neither case you can tell, and this is why JSON::XS takes the
700 conservative route and disallows this case.
701
702 EXAMPLES
703 Some examples will make all this clearer. First, a simple example that
704 works similarly to "decode_prefix": We want to decode the JSON object at
705 the start of a string and identify the portion after the JSON object:
706
707 my $text = "[1,2,3] hello";
708
709 my $json = new JSON::XS;
710
711 my $obj = $json->incr_parse ($text)
712 or die "expected JSON object or array at beginning of string";
713
714 my $tail = $json->incr_text;
715 # $tail now contains " hello"
716
717 Easy, isn't it?
718
719 Now for a more complicated example: Imagine a hypothetical protocol
720 where you read some requests from a TCP stream, and each request is a
721 JSON array, without any separation between them (in fact, it is often
722 useful to use newlines as "separators", as these get interpreted as
723 whitespace at the start of the JSON text, which makes it possible to
724 test said protocol with "telnet"...).
725
726 Here is how you'd do it (it is trivial to write this in an event-based
727 manner):
728
729 my $json = new JSON::XS;
730
731 # read some data from the socket
732 while (sysread $socket, my $buf, 4096) {
733
734 # split and decode as many requests as possible
735 for my $request ($json->incr_parse ($buf)) {
736 # act on the $request
737 }
738 }
739
740 Another complicated example: Assume you have a string with JSON objects
741 or arrays, all separated by (optional) comma characters (e.g. "[1],[2],
742 [3]"). To parse them, we have to skip the commas between the JSON texts,
743 and here is where the lvalue-ness of "incr_text" comes in useful:
744
745 my $text = "[1],[2], [3]";
746 my $json = new JSON::XS;
747
748 # void context, so no parsing done
749 $json->incr_parse ($text);
750
751 # now extract as many objects as possible. note the
752 # use of scalar context so incr_text can be called.
753 while (my $obj = $json->incr_parse) {
754 # do something with $obj
755
756 # now skip the optional comma
757 $json->incr_text =~ s/^ \s* , //x;
758 }
759
760 Now lets go for a very complex example: Assume that you have a gigantic
761 JSON array-of-objects, many gigabytes in size, and you want to parse it,
762 but you cannot load it into memory fully (this has actually happened in
763 the real world :).
764
765 Well, you lost, you have to implement your own JSON parser. But JSON::XS
766 can still help you: You implement a (very simple) array parser and let
767 JSON decode the array elements, which are all full JSON objects on their
768 own (this wouldn't work if the array elements could be JSON numbers, for
769 example):
770
771 my $json = new JSON::XS;
772
773 # open the monster
774 open my $fh, "<bigfile.json"
775 or die "bigfile: $!";
776
777 # first parse the initial "["
778 for (;;) {
779 sysread $fh, my $buf, 65536
780 or die "read error: $!";
781 $json->incr_parse ($buf); # void context, so no parsing
782
783 # Exit the loop once we found and removed(!) the initial "[".
784 # In essence, we are (ab-)using the $json object as a simple scalar
785 # we append data to.
786 last if $json->incr_text =~ s/^ \s* \[ //x;
787 }
788
789 # now we have the skipped the initial "[", so continue
790 # parsing all the elements.
791 for (;;) {
792 # in this loop we read data until we got a single JSON object
793 for (;;) {
794 if (my $obj = $json->incr_parse) {
795 # do something with $obj
796 last;
797 }
798
799 # add more data
800 sysread $fh, my $buf, 65536
801 or die "read error: $!";
802 $json->incr_parse ($buf); # void context, so no parsing
803 }
804
805 # in this loop we read data until we either found and parsed the
806 # separating "," between elements, or the final "]"
807 for (;;) {
808 # first skip whitespace
809 $json->incr_text =~ s/^\s*//;
810
811 # if we find "]", we are done
812 if ($json->incr_text =~ s/^\]//) {
813 print "finished.\n";
814 exit;
815 }
816
817 # if we find ",", we can continue with the next element
818 if ($json->incr_text =~ s/^,//) {
819 last;
820 }
821
822 # if we find anything else, we have a parse error!
823 if (length $json->incr_text) {
824 die "parse error near ", $json->incr_text;
825 }
826
827 # else add more data
828 sysread $fh, my $buf, 65536
829 or die "read error: $!";
830 $json->incr_parse ($buf); # void context, so no parsing
831 }
832
833 This is a complex example, but most of the complexity comes from the
834 fact that we are trying to be correct (bear with me if I am wrong, I
835 never ran the above example :).
836
837MAPPING
838 This section describes how JSON::XS maps Perl values to JSON values and
839 vice versa. These mappings are designed to "do the right thing" in most
840 circumstances automatically, preserving round-tripping characteristics
841 (what you put in comes out as something equivalent).
842
843 For the more enlightened: note that in the following descriptions,
844 lowercase *perl* refers to the Perl interpreter, while uppercase *Perl*
845 refers to the abstract Perl language itself.
846
847 JSON -> PERL
848 object
849 A JSON object becomes a reference to a hash in Perl. No ordering of
850 object keys is preserved (JSON does not preserve object key ordering
851 itself).
852
853 array
854 A JSON array becomes a reference to an array in Perl.
855
856 string
857 A JSON string becomes a string scalar in Perl - Unicode codepoints
858 in JSON are represented by the same codepoints in the Perl string,
859 so no manual decoding is necessary.
860
861 number
862 A JSON number becomes either an integer, numeric (floating point) or
863 string scalar in perl, depending on its range and any fractional
864 parts. On the Perl level, there is no difference between those as
865 Perl handles all the conversion details, but an integer may take
866 slightly less memory and might represent more values exactly than
867 floating point numbers.
868
869 If the number consists of digits only, JSON::XS will try to
870 represent it as an integer value. If that fails, it will try to
871 represent it as a numeric (floating point) value if that is possible
872 without loss of precision. Otherwise it will preserve the number as
873 a string value (in which case you lose roundtripping ability, as the
874 JSON number will be re-encoded toa JSON string).
875
876 Numbers containing a fractional or exponential part will always be
877 represented as numeric (floating point) values, possibly at a loss
878 of precision (in which case you might lose perfect roundtripping
879 ability, but the JSON number will still be re-encoded as a JSON
880 number).
881
882 true, false
883 These JSON atoms become "JSON::XS::true" and "JSON::XS::false",
884 respectively. They are overloaded to act almost exactly like the
885 numbers 1 and 0. You can check whether a scalar is a JSON boolean by
886 using the "JSON::XS::is_bool" function.
887
888 null
889 A JSON null atom becomes "undef" in Perl.
890
891 PERL -> JSON
892 The mapping from Perl to JSON is slightly more difficult, as Perl is a
893 truly typeless language, so we can only guess which JSON type is meant
894 by a Perl value.
895
896 hash references
897 Perl hash references become JSON objects. As there is no inherent
898 ordering in hash keys (or JSON objects), they will usually be
899 encoded in a pseudo-random order that can change between runs of the
900 same program but stays generally the same within a single run of a
901 program. JSON::XS can optionally sort the hash keys (determined by
902 the *canonical* flag), so the same datastructure will serialise to
903 the same JSON text (given same settings and version of JSON::XS),
904 but this incurs a runtime overhead and is only rarely useful, e.g.
905 when you want to compare some JSON text against another for
906 equality.
907
908 array references
909 Perl array references become JSON arrays.
910
911 other references
912 Other unblessed references are generally not allowed and will cause
913 an exception to be thrown, except for references to the integers 0
914 and 1, which get turned into "false" and "true" atoms in JSON. You
915 can also use "JSON::XS::false" and "JSON::XS::true" to improve
916 readability.
917
918 encode_json [\0,JSON::XS::true] # yields [false,true]
919
920 JSON::XS::true, JSON::XS::false
921 These special values become JSON true and JSON false values,
922 respectively. You can also use "\1" and "\0" directly if you want.
923
924 blessed objects
925 Blessed objects are not directly representable in JSON. See the
926 "allow_blessed" and "convert_blessed" methods on various options on
927 how to deal with this: basically, you can choose between throwing an
928 exception, encoding the reference as if it weren't blessed, or
929 provide your own serialiser method.
930
931 simple scalars
932 Simple Perl scalars (any scalar that is not a reference) are the
933 most difficult objects to encode: JSON::XS will encode undefined
934 scalars as JSON "null" values, scalars that have last been used in a
935 string context before encoding as JSON strings, and anything else as
936 number value:
937
938 # dump as number
939 encode_json [2] # yields [2]
940 encode_json [-3.0e17] # yields [-3e+17]
941 my $value = 5; encode_json [$value] # yields [5]
942
943 # used as string, so dump as string
944 print $value;
945 encode_json [$value] # yields ["5"]
946
947 # undef becomes null
948 encode_json [undef] # yields [null]
949
950 You can force the type to be a JSON string by stringifying it:
951
952 my $x = 3.1; # some variable containing a number
953 "$x"; # stringified
954 $x .= ""; # another, more awkward way to stringify
955 print $x; # perl does it for you, too, quite often
956
957 You can force the type to be a JSON number by numifying it:
958
959 my $x = "3"; # some variable containing a string
960 $x += 0; # numify it, ensuring it will be dumped as a number
961 $x *= 1; # same thing, the choice is yours.
962
963 You can not currently force the type in other, less obscure, ways.
964 Tell me if you need this capability (but don't forget to explain why
965 it's needed :).
966
967ENCODING/CODESET FLAG NOTES
968 The interested reader might have seen a number of flags that signify
969 encodings or codesets - "utf8", "latin1" and "ascii". There seems to be
970 some confusion on what these do, so here is a short comparison:
971
972 "utf8" controls whether the JSON text created by "encode" (and expected
973 by "decode") is UTF-8 encoded or not, while "latin1" and "ascii" only
974 control whether "encode" escapes character values outside their
975 respective codeset range. Neither of these flags conflict with each
976 other, although some combinations make less sense than others.
977
978 Care has been taken to make all flags symmetrical with respect to
979 "encode" and "decode", that is, texts encoded with any combination of
980 these flag values will be correctly decoded when the same flags are used
981 - in general, if you use different flag settings while encoding vs. when
982 decoding you likely have a bug somewhere.
983
984 Below comes a verbose discussion of these flags. Note that a "codeset"
985 is simply an abstract set of character-codepoint pairs, while an
986 encoding takes those codepoint numbers and *encodes* them, in our case
987 into octets. Unicode is (among other things) a codeset, UTF-8 is an
988 encoding, and ISO-8859-1 (= latin 1) and ASCII are both codesets *and*
989 encodings at the same time, which can be confusing.
990
991 "utf8" flag disabled
992 When "utf8" is disabled (the default), then "encode"/"decode"
993 generate and expect Unicode strings, that is, characters with high
994 ordinal Unicode values (> 255) will be encoded as such characters,
995 and likewise such characters are decoded as-is, no canges to them
996 will be done, except "(re-)interpreting" them as Unicode codepoints
997 or Unicode characters, respectively (to Perl, these are the same
998 thing in strings unless you do funny/weird/dumb stuff).
999
1000 This is useful when you want to do the encoding yourself (e.g. when
1001 you want to have UTF-16 encoded JSON texts) or when some other layer
1002 does the encoding for you (for example, when printing to a terminal
1003 using a filehandle that transparently encodes to UTF-8 you certainly
1004 do NOT want to UTF-8 encode your data first and have Perl encode it
1005 another time).
1006
1007 "utf8" flag enabled
1008 If the "utf8"-flag is enabled, "encode"/"decode" will encode all
1009 characters using the corresponding UTF-8 multi-byte sequence, and
1010 will expect your input strings to be encoded as UTF-8, that is, no
1011 "character" of the input string must have any value > 255, as UTF-8
1012 does not allow that.
1013
1014 The "utf8" flag therefore switches between two modes: disabled means
1015 you will get a Unicode string in Perl, enabled means you get an
1016 UTF-8 encoded octet/binary string in Perl.
1017
1018 "latin1" or "ascii" flags enabled
1019 With "latin1" (or "ascii") enabled, "encode" will escape characters
1020 with ordinal values > 255 (> 127 with "ascii") and encode the
1021 remaining characters as specified by the "utf8" flag.
1022
1023 If "utf8" is disabled, then the result is also correctly encoded in
1024 those character sets (as both are proper subsets of Unicode, meaning
1025 that a Unicode string with all character values < 256 is the same
1026 thing as a ISO-8859-1 string, and a Unicode string with all
1027 character values < 128 is the same thing as an ASCII string in
213 properly). 1028 Perl).
214 1029
215 No roundtripping (strings get clobbered if they look like numbers, 1030 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 1031 regardless of these flags, just some more characters will be escaped
217 will decode into the number 2. 1032 using "\uXXXX" then before.
218 1033
219 JSON::PC 0.01 1034 Note that ISO-8859-1-*encoded* strings are not compatible with UTF-8
220 Very fast. 1035 encoding, while ASCII-encoded strings are. That is because the
1036 ISO-8859-1 encoding is NOT a subset of UTF-8 (despite the ISO-8859-1
1037 *codeset* being a subset of Unicode), while ASCII is.
221 1038
222 Undocumented/buggy Unicode handling. 1039 Surprisingly, "decode" will ignore these flags and so treat all
1040 input values as governed by the "utf8" flag. If it is disabled, this
1041 allows you to decode ISO-8859-1- and ASCII-encoded strings, as both
1042 strict subsets of Unicode. If it is enabled, you can correctly
1043 decode UTF-8 encoded strings.
223 1044
224 No roundtripping. 1045 So neither "latin1" nor "ascii" are incompatible with the "utf8"
1046 flag - they only govern when the JSON output engine escapes a
1047 character or not.
225 1048
226 Has problems handling many Perl values (e.g. regex results and other 1049 The main use for "latin1" is to relatively efficiently store binary
227 magic values will make it croak). 1050 data as JSON, at the expense of breaking compatibility with most
1051 JSON decoders.
228 1052
229 Does not even generate valid JSON ("{1,2}" gets converted to "{1:2}" 1053 The main use for "ascii" is to force the output to not contain
230 which is not a valid JSON string. 1054 characters with values > 127, which means you can interpret the
1055 resulting string as UTF-8, ISO-8859-1, ASCII, KOI8-R or most about
1056 any character set and 8-bit-encoding, and still get the same data
1057 structure back. This is useful when your channel for JSON transfer
1058 is not 8-bit clean or the encoding might be mangled in between (e.g.
1059 in mail), and works because ASCII is a proper subset of most 8-bit
1060 and multibyte encodings in use in the world.
231 1061
232 Unmaintained (maintainer unresponsive for many months, bugs are not 1062 JSON and YAML
233 getting fixed). 1063 You often hear that JSON is a subset of YAML. This is, however, a mass
1064 hysteria(*) and very far from the truth (as of the time of this
1065 writing), so let me state it clearly: *in general, there is no way to
1066 configure JSON::XS to output a data structure as valid YAML* that works
1067 in all cases.
234 1068
235 JSON::Syck 0.21 1069 If you really must use JSON::XS to generate YAML, you should use this
236 Very buggy (often crashes). 1070 algorithm (subject to change in future versions):
237 1071
238 Very inflexible (no human-readable format supported, format pretty 1072 my $to_yaml = JSON::XS->new->utf8->space_after (1);
239 much undocumented. I need at least a format for easy reading by 1073 my $yaml = $to_yaml->encode ($ref) . "\n";
240 humans and a single-line compact format for use in a protocol, and
241 preferably a way to generate ASCII-only JSON strings).
242 1074
243 Completely broken (and confusingly documented) Unicode handling 1075 This will *usually* generate JSON texts that also parse as valid YAML.
244 (unicode escapes are not working properly, you need to set 1076 Please note that YAML has hardcoded limits on (simple) object key
245 ImplicitUnicode to *different* values on en- and decoding to get 1077 lengths that JSON doesn't have and also has different and incompatible
246 symmetric behaviour). 1078 unicode handling, so you should make sure that your hash keys are
1079 noticeably shorter than the 1024 "stream characters" YAML allows and
1080 that you do not have characters with codepoint values outside the
1081 Unicode BMP (basic multilingual page). YAML also does not allow "\/"
1082 sequences in strings (which JSON::XS does not *currently* generate, but
1083 other JSON generators might).
247 1084
248 No roundtripping (simple cases work, but this depends on wether the 1085 There might be other incompatibilities that I am not aware of (or the
249 scalar value was used in a numeric context or not). 1086 YAML specification has been changed yet again - it does so quite often).
1087 In general you should not try to generate YAML with a JSON generator or
1088 vice versa, or try to parse JSON with a YAML parser or vice versa:
1089 chances are high that you will run into severe interoperability problems
1090 when you least expect it.
250 1091
251 Dumping hashes may skip hash values depending on iterator state. 1092 (*) I have been pressured multiple times by Brian Ingerson (one of the
1093 authors of the YAML specification) to remove this paragraph, despite
1094 him acknowledging that the actual incompatibilities exist. As I was
1095 personally bitten by this "JSON is YAML" lie, I refused and said I
1096 will continue to educate people about these issues, so others do not
1097 run into the same problem again and again. After this, Brian called
1098 me a (quote)*complete and worthless idiot*(unquote).
252 1099
253 Unmaintained (maintainer unresponsive for many months, bugs are not 1100 In my opinion, instead of pressuring and insulting people who
254 getting fixed). 1101 actually clarify issues with YAML and the wrong statements of some
255 1102 of its proponents, I would kindly suggest reading the JSON spec
256 Does not check input for validity (i.e. will accept non-JSON input 1103 (which is not that difficult or long) and finally make YAML
257 and return "something" instead of raising an exception. This is a 1104 compatible to it, and educating users about the changes, instead of
258 security issue: imagine two banks transfering money between each 1105 spreading lies about the real compatibility for many *years* and
259 other using JSON. One bank might parse a given non-JSON request and 1106 trying to silence people who point out that it isn't true.
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
264 JSON::DWIW 0.04
265 Very fast. Very natural. Very nice.
266
267 Undocumented unicode handling (but the best of the pack. Unicode
268 escapes still don't get parsed properly).
269
270 Very inflexible.
271
272 No roundtripping.
273
274 Does not generate valid JSON (key strings are often unquoted, empty
275 keys result in nothing being output)
276
277 Does not check input for validity.
278 1107
279 SPEED 1108 SPEED
280 It seems that JSON::XS is surprisingly fast, as shown in the following 1109 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 1110 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 1111 in the JSON::XS distribution, to make it easy to compare on your own
283 system. 1112 system.
284 1113
285 First is a comparison between various modules using a very simple JSON 1114 First comes a comparison between various modules using a very short
1115 single-line JSON string (also available at
1116 <http://dist.schmorp.de/misc/json/short.json>).
1117
1118 {"method": "handleMessage", "params": ["user1",
1119 "we were just talking"], "id": null, "array":[1,11,234,-5,1e5,1e7,
1120 true, false]}
1121
286 string, showing the number of encodes/decodes per second (JSON::XS is 1122 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 1123 functional interface, while JSON::XS/2 uses the OO interface with
288 pretty-printing and hashkey sorting enabled). 1124 pretty-printing and hashkey sorting enabled, JSON::XS/3 enables shrink).
1125 Higher is better:
289 1126
290 module | encode | decode | 1127 module | encode | decode |
291 -----------|------------|------------| 1128 -----------|------------|------------|
292 JSON | 14006 | 6820 | 1129 JSON 1.x | 4990.842 | 4088.813 |
293 JSON::DWIW | 200937 | 120386 | 1130 JSON::DWIW | 51653.990 | 71575.154 |
294 JSON::PC | 85065 | 129366 | 1131 JSON::PC | 65948.176 | 74631.744 |
295 JSON::Syck | 59898 | 44232 | 1132 JSON::PP | 8931.652 | 3817.168 |
296 JSON::XS | 1171478 | 342435 | 1133 JSON::Syck | 24877.248 | 27776.848 |
297 JSON::XS/2 | 730760 | 328714 | 1134 JSON::XS | 388361.481 | 227951.304 |
1135 JSON::XS/2 | 227951.304 | 218453.333 |
1136 JSON::XS/3 | 338250.323 | 218453.333 |
1137 Storable | 16500.016 | 135300.129 |
298 -----------+------------+------------+ 1138 -----------+------------+------------+
299 1139
300 That is, JSON::XS is 6 times faster than than JSON::DWIW and about 80 1140 That is, JSON::XS is about five times faster than JSON::DWIW on
1141 encoding, about three times faster on decoding, and over forty times
301 times faster than JSON, even with pretty-printing and key sorting. 1142 faster than JSON, even with pretty-printing and key sorting. It also
1143 compares favourably to Storable for small amounts of data.
302 1144
303 Using a longer test string (roughly 8KB, generated from Yahoo! Locals 1145 Using a longer test string (roughly 18KB, generated from Yahoo! Locals
304 search API (http://nanoref.com/yahooapis/mgPdGg): 1146 search API (<http://dist.schmorp.de/misc/json/long.json>).
305 1147
306 module | encode | decode | 1148 module | encode | decode |
307 -----------|------------|------------| 1149 -----------|------------|------------|
308 JSON | 673 | 38 | 1150 JSON 1.x | 55.260 | 34.971 |
309 JSON::DWIW | 5271 | 770 | 1151 JSON::DWIW | 825.228 | 1082.513 |
1152 JSON::PC | 3571.444 | 2394.829 |
310 JSON::PC | 9901 | 2491 | 1153 JSON::PP | 210.987 | 32.574 |
311 JSON::Syck | 2360 | 786 | 1154 JSON::Syck | 552.551 | 787.544 |
312 JSON::XS | 37398 | 3202 | 1155 JSON::XS | 5780.463 | 4854.519 |
313 JSON::XS/2 | 13765 | 3153 | 1156 JSON::XS/2 | 3869.998 | 4798.975 |
1157 JSON::XS/3 | 5862.880 | 4798.975 |
1158 Storable | 4445.002 | 5235.027 |
314 -----------+------------+------------+ 1159 -----------+------------+------------+
315 1160
316 Again, JSON::XS leads by far in the encoding case, while still beating 1161 Again, JSON::XS leads by far (except for Storable which non-surprisingly
317 every other module in the decoding case. 1162 decodes faster).
318 1163
319 Last example is an almost 8MB large hash with many large binary values 1164 On large strings containing lots of high Unicode characters, some
320 (PNG files), resulting in a lot of escaping: 1165 modules (such as JSON::PC) seem to decode faster than JSON::XS, but the
1166 result will be broken due to missing (or wrong) Unicode handling. Others
1167 refuse to decode or encode properly, so it was impossible to prepare a
1168 fair comparison table for that case.
1169
1170SECURITY CONSIDERATIONS
1171 When you are using JSON in a protocol, talking to untrusted potentially
1172 hostile creatures requires relatively few measures.
1173
1174 First of all, your JSON decoder should be secure, that is, should not
1175 have any buffer overflows. Obviously, this module should ensure that and
1176 I am trying hard on making that true, but you never know.
1177
1178 Second, you need to avoid resource-starving attacks. That means you
1179 should limit the size of JSON texts you accept, or make sure then when
1180 your resources run out, that's just fine (e.g. by using a separate
1181 process that can crash safely). The size of a JSON text in octets or
1182 characters is usually a good indication of the size of the resources
1183 required to decode it into a Perl structure. While JSON::XS can check
1184 the size of the JSON text, it might be too late when you already have it
1185 in memory, so you might want to check the size before you accept the
1186 string.
1187
1188 Third, JSON::XS recurses using the C stack when decoding objects and
1189 arrays. The C stack is a limited resource: for instance, on my amd64
1190 machine with 8MB of stack size I can decode around 180k nested arrays
1191 but only 14k nested JSON objects (due to perl itself recursing deeply on
1192 croak to free the temporary). If that is exceeded, the program crashes.
1193 To be conservative, the default nesting limit is set to 512. If your
1194 process has a smaller stack, you should adjust this setting accordingly
1195 with the "max_depth" method.
1196
1197 Something else could bomb you, too, that I forgot to think of. In that
1198 case, you get to keep the pieces. I am always open for hints, though...
1199
1200 Also keep in mind that JSON::XS might leak contents of your Perl data
1201 structures in its error messages, so when you serialise sensitive
1202 information you might want to make sure that exceptions thrown by
1203 JSON::XS will not end up in front of untrusted eyes.
1204
1205 If you are using JSON::XS to return packets to consumption by JavaScript
1206 scripts in a browser you should have a look at
1207 <http://jpsykes.com/47/practical-csrf-and-json-security> to see whether
1208 you are vulnerable to some common attack vectors (which really are
1209 browser design bugs, but it is still you who will have to deal with it,
1210 as major browser developers care only for features, not about getting
1211 security right).
1212
1213THREADS
1214 This module is *not* guaranteed to be thread safe and there are no plans
1215 to change this until Perl gets thread support (as opposed to the
1216 horribly slow so-called "threads" which are simply slow and bloated
1217 process simulations - use fork, it's *much* faster, cheaper, better).
1218
1219 (It might actually work, but you have been warned).
321 1220
322BUGS 1221BUGS
323 While the goal of this module is to be correct, that unfortunately does 1222 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 1223 not mean it's bug-free, only that I think its design is bug-free. It is
325 still very young and not well-tested. If you keep reporting bugs they 1224 still relatively early in its development. If you keep reporting bugs
326 will be fixed swiftly, though. 1225 they will be fixed swiftly, though.
1226
1227 Please refrain from using rt.cpan.org or any other bug reporting
1228 service. I put the contact address into my modules for a reason.
1229
1230SEE ALSO
1231 The json_xs command line utility for quick experiments.
327 1232
328AUTHOR 1233AUTHOR
329 Marc Lehmann <schmorp@schmorp.de> 1234 Marc Lehmann <schmorp@schmorp.de>
330 http://home.schmorp.de/ 1235 http://home.schmorp.de/
331 1236

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