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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;
6 9
7 # exported functions, they croak on error 10 # exported functions, they croak on error
8 # and expect/generate UTF-8 11 # and expect/generate UTF-8
9 12
10 $utf8_encoded_json_text = to_json $perl_hash_or_arrayref; 13 $utf8_encoded_json_text = encode_json $perl_hash_or_arrayref;
11 $perl_hash_or_arrayref = from_json $utf8_encoded_json_text; 14 $perl_hash_or_arrayref = decode_json $utf8_encoded_json_text;
12
13 # objToJson and jsonToObj aliases to to_json and from_json
14 # are exported for compatibility to the JSON module,
15 # but should not be used in new code.
16 15
17 # OO-interface 16 # OO-interface
18 17
19 $coder = JSON::XS->new->ascii->pretty->allow_nonref; 18 $coder = JSON::XS->new->ascii->pretty->allow_nonref;
20 $pretty_printed_unencoded = $coder->encode ($perl_scalar); 19 $pretty_printed_unencoded = $coder->encode ($perl_scalar);
21 $perl_scalar = $coder->decode ($unicode_json_text); 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.
22 29
23DESCRIPTION 30DESCRIPTION
24 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
25 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*.
26 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 overridden) with no overhead due to emulation (by inheriting
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.
27 42
28 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
29 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
30 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
31 cases their maintainers are unresponsive, gone missing, or not listening 46 cases their maintainers are unresponsive, gone missing, or not listening
32 to bug reports for other reasons. 47 to bug reports for other reasons.
33 48
34 See COMPARISON, below, for a comparison to some other JSON modules.
35
36 See MAPPING, below, on how JSON::XS maps perl values to JSON values and 49 See MAPPING, below, on how JSON::XS maps perl values to JSON values and
37 vice versa. 50 vice versa.
38 51
39 FEATURES 52 FEATURES
40 * correct unicode handling 53 * correct Unicode handling
54
41 This module knows how to handle Unicode, and even documents how and 55 This module knows how to handle Unicode, documents how and when it
42 when it does so. 56 does so, and even documents what "correct" means.
43 57
44 * round-trip integrity 58 * round-trip integrity
59
45 When you serialise a perl data structure using only datatypes 60 When you serialise a perl data structure using only data types
46 supported by JSON, the deserialised data structure is identical on 61 supported by JSON and Perl, the deserialised data structure is
47 the Perl level. (e.g. the string "2.0" doesn't suddenly become "2" 62 identical on the Perl level. (e.g. the string "2.0" doesn't suddenly
48 just because it looks like a number). 63 become "2" just because it looks like a number). There *are* minor
64 exceptions to this, read the MAPPING section below to learn about
65 those.
49 66
50 * strict checking of JSON correctness 67 * strict checking of JSON correctness
68
51 There is no guessing, no generating of illegal JSON texts by 69 There is no guessing, no generating of illegal JSON texts by
52 default, and only JSON is accepted as input by default (the latter 70 default, and only JSON is accepted as input by default (the latter
53 is a security feature). 71 is a security feature).
54 72
55 * fast 73 * fast
56 Compared to other JSON modules, this module compares favourably in
57 terms of speed, too.
58 74
75 Compared to other JSON modules and other serialisers such as
76 Storable, this module usually compares favourably in terms of speed,
77 too.
78
59 * simple to use 79 * simple to use
80
60 This module has both a simple functional interface as well as an OO 81 This module has both a simple functional interface as well as an
61 interface. 82 object oriented interface interface.
62 83
63 * reasonably versatile output formats 84 * reasonably versatile output formats
85
64 You can choose between the most compact guarenteed single-line 86 You can choose between the most compact guaranteed-single-line
65 format possible (nice for simple line-based protocols), a pure-ascii 87 format possible (nice for simple line-based protocols), a pure-ASCII
66 format (for when your transport is not 8-bit clean, still supports 88 format (for when your transport is not 8-bit clean, still supports
67 the whole unicode range), or a pretty-printed format (for when you 89 the whole Unicode range), or a pretty-printed format (for when you
68 want to read that stuff). Or you can combine those features in 90 want to read that stuff). Or you can combine those features in
69 whatever way you like. 91 whatever way you like.
70 92
71FUNCTIONAL INTERFACE 93FUNCTIONAL INTERFACE
72 The following convinience methods are provided by this module. They are 94 The following convenience methods are provided by this module. They are
73 exported by default: 95 exported by default:
74 96
75 $json_text = to_json $perl_scalar 97 $json_text = encode_json $perl_scalar
76 Converts the given Perl data structure (a simple scalar or a 98 Converts the given Perl data structure to a UTF-8 encoded, binary
77 reference to a hash or array) to a UTF-8 encoded, binary string
78 (that is, the string contains octets only). Croaks on error. 99 string (that is, the string contains octets only). Croaks on error.
79 100
80 This function call is functionally identical to: 101 This function call is functionally identical to:
81 102
82 $json_text = JSON::XS->new->utf8->encode ($perl_scalar) 103 $json_text = JSON::XS->new->utf8->encode ($perl_scalar)
83 104
84 except being faster. 105 Except being faster.
85 106
86 $perl_scalar = from_json $json_text 107 $perl_scalar = decode_json $json_text
87 The opposite of "to_json": expects an UTF-8 (binary) string and 108 The opposite of "encode_json": expects an UTF-8 (binary) string and
88 tries to parse that as an UTF-8 encoded JSON text, returning the 109 tries to parse that as an UTF-8 encoded JSON text, returning the
89 resulting simple scalar or reference. Croaks on error. 110 resulting reference. Croaks on error.
90 111
91 This function call is functionally identical to: 112 This function call is functionally identical to:
92 113
93 $perl_scalar = JSON::XS->new->utf8->decode ($json_text) 114 $perl_scalar = JSON::XS->new->utf8->decode ($json_text)
94 115
95 except being faster. 116 Except being faster.
117
118 $is_boolean = JSON::XS::is_bool $scalar
119 Returns true if the passed scalar represents either JSON::XS::true
120 or JSON::XS::false, two constants that act like 1 and 0,
121 respectively and are used to represent JSON "true" and "false"
122 values in Perl.
123
124 See MAPPING, below, for more information on how JSON values are
125 mapped to Perl.
126
127A FEW NOTES ON UNICODE AND PERL
128 Since this often leads to confusion, here are a few very clear words on
129 how Unicode works in Perl, modulo bugs.
130
131 1. Perl strings can store characters with ordinal values > 255.
132 This enables you to store Unicode characters as single characters in
133 a Perl string - very natural.
134
135 2. Perl does *not* associate an encoding with your strings.
136 ... until you force it to, e.g. when matching it against a regex, or
137 printing the scalar to a file, in which case Perl either interprets
138 your string as locale-encoded text, octets/binary, or as Unicode,
139 depending on various settings. In no case is an encoding stored
140 together with your data, it is *use* that decides encoding, not any
141 magical meta data.
142
143 3. The internal utf-8 flag has no meaning with regards to the encoding
144 of your string.
145 Just ignore that flag unless you debug a Perl bug, a module written
146 in XS or want to dive into the internals of perl. Otherwise it will
147 only confuse you, as, despite the name, it says nothing about how
148 your string is encoded. You can have Unicode strings with that flag
149 set, with that flag clear, and you can have binary data with that
150 flag set and that flag clear. Other possibilities exist, too.
151
152 If you didn't know about that flag, just the better, pretend it
153 doesn't exist.
154
155 4. A "Unicode String" is simply a string where each character can be
156 validly interpreted as a Unicode code point.
157 If you have UTF-8 encoded data, it is no longer a Unicode string,
158 but a Unicode string encoded in UTF-8, giving you a binary string.
159
160 5. A string containing "high" (> 255) character values is *not* a UTF-8
161 string.
162 It's a fact. Learn to live with it.
163
164 I hope this helps :)
96 165
97OBJECT-ORIENTED INTERFACE 166OBJECT-ORIENTED INTERFACE
98 The object oriented interface lets you configure your own encoding or 167 The object oriented interface lets you configure your own encoding or
99 decoding style, within the limits of supported formats. 168 decoding style, within the limits of supported formats.
100 169
… …
108 177
109 my $json = JSON::XS->new->utf8->space_after->encode ({a => [1,2]}) 178 my $json = JSON::XS->new->utf8->space_after->encode ({a => [1,2]})
110 => {"a": [1, 2]} 179 => {"a": [1, 2]}
111 180
112 $json = $json->ascii ([$enable]) 181 $json = $json->ascii ([$enable])
182 $enabled = $json->get_ascii
113 If $enable is true (or missing), then the "encode" method will not 183 If $enable is true (or missing), then the "encode" method will not
114 generate characters outside the code range 0..127 (which is ASCII). 184 generate characters outside the code range 0..127 (which is ASCII).
115 Any unicode characters outside that range will be escaped using 185 Any Unicode characters outside that range will be escaped using
116 either a single \uXXXX (BMP characters) or a double \uHHHH\uLLLLL 186 either a single \uXXXX (BMP characters) or a double \uHHHH\uLLLLL
117 escape sequence, as per RFC4627. 187 escape sequence, as per RFC4627. The resulting encoded JSON text can
188 be treated as a native Unicode string, an ascii-encoded,
189 latin1-encoded or UTF-8 encoded string, or any other superset of
190 ASCII.
118 191
119 If $enable is false, then the "encode" method will not escape 192 If $enable is false, then the "encode" method will not escape
120 Unicode characters unless required by the JSON syntax. This results 193 Unicode characters unless required by the JSON syntax or other
121 in a faster and more compact format. 194 flags. This results in a faster and more compact format.
195
196 See also the section *ENCODING/CODESET FLAG NOTES* later in this
197 document.
198
199 The main use for this flag is to produce JSON texts that can be
200 transmitted over a 7-bit channel, as the encoded JSON texts will not
201 contain any 8 bit characters.
122 202
123 JSON::XS->new->ascii (1)->encode ([chr 0x10401]) 203 JSON::XS->new->ascii (1)->encode ([chr 0x10401])
124 => ["\ud801\udc01"] 204 => ["\ud801\udc01"]
125 205
206 $json = $json->latin1 ([$enable])
207 $enabled = $json->get_latin1
208 If $enable is true (or missing), then the "encode" method will
209 encode the resulting JSON text as latin1 (or iso-8859-1), escaping
210 any characters outside the code range 0..255. The resulting string
211 can be treated as a latin1-encoded JSON text or a native Unicode
212 string. The "decode" method will not be affected in any way by this
213 flag, as "decode" by default expects Unicode, which is a strict
214 superset of latin1.
215
216 If $enable is false, then the "encode" method will not escape
217 Unicode characters unless required by the JSON syntax or other
218 flags.
219
220 See also the section *ENCODING/CODESET FLAG NOTES* later in this
221 document.
222
223 The main use for this flag is efficiently encoding binary data as
224 JSON text, as most octets will not be escaped, resulting in a
225 smaller encoded size. The disadvantage is that the resulting JSON
226 text is encoded in latin1 (and must correctly be treated as such
227 when storing and transferring), a rare encoding for JSON. It is
228 therefore most useful when you want to store data structures known
229 to contain binary data efficiently in files or databases, not when
230 talking to other JSON encoders/decoders.
231
232 JSON::XS->new->latin1->encode (["\x{89}\x{abc}"]
233 => ["\x{89}\\u0abc"] # (perl syntax, U+abc escaped, U+89 not)
234
126 $json = $json->utf8 ([$enable]) 235 $json = $json->utf8 ([$enable])
236 $enabled = $json->get_utf8
127 If $enable is true (or missing), then the "encode" method will 237 If $enable is true (or missing), then the "encode" method will
128 encode the JSON result into UTF-8, as required by many protocols, 238 encode the JSON result into UTF-8, as required by many protocols,
129 while the "decode" method expects to be handled an UTF-8-encoded 239 while the "decode" method expects to be handled an UTF-8-encoded
130 string. Please note that UTF-8-encoded strings do not contain any 240 string. Please note that UTF-8-encoded strings do not contain any
131 characters outside the range 0..255, they are thus useful for 241 characters outside the range 0..255, they are thus useful for
132 bytewise/binary I/O. In future versions, enabling this option might 242 bytewise/binary I/O. In future versions, enabling this option might
133 enable autodetection of the UTF-16 and UTF-32 encoding families, as 243 enable autodetection of the UTF-16 and UTF-32 encoding families, as
134 described in RFC4627. 244 described in RFC4627.
135 245
136 If $enable is false, then the "encode" method will return the JSON 246 If $enable is false, then the "encode" method will return the JSON
137 string as a (non-encoded) unicode string, while "decode" expects 247 string as a (non-encoded) Unicode string, while "decode" expects
138 thus a unicode string. Any decoding or encoding (e.g. to UTF-8 or 248 thus a Unicode string. Any decoding or encoding (e.g. to UTF-8 or
139 UTF-16) needs to be done yourself, e.g. using the Encode module. 249 UTF-16) needs to be done yourself, e.g. using the Encode module.
250
251 See also the section *ENCODING/CODESET FLAG NOTES* later in this
252 document.
140 253
141 Example, output UTF-16BE-encoded JSON: 254 Example, output UTF-16BE-encoded JSON:
142 255
143 use Encode; 256 use Encode;
144 $jsontext = encode "UTF-16BE", JSON::XS->new->encode ($object); 257 $jsontext = encode "UTF-16BE", JSON::XS->new->encode ($object);
… …
163 2 276 2
164 ] 277 ]
165 } 278 }
166 279
167 $json = $json->indent ([$enable]) 280 $json = $json->indent ([$enable])
281 $enabled = $json->get_indent
168 If $enable is true (or missing), then the "encode" method will use a 282 If $enable is true (or missing), then the "encode" method will use a
169 multiline format as output, putting every array member or 283 multiline format as output, putting every array member or
170 object/hash key-value pair into its own line, identing them 284 object/hash key-value pair into its own line, indenting them
171 properly. 285 properly.
172 286
173 If $enable is false, no newlines or indenting will be produced, and 287 If $enable is false, no newlines or indenting will be produced, and
174 the resulting JSON text is guarenteed not to contain any "newlines". 288 the resulting JSON text is guaranteed not to contain any "newlines".
175 289
176 This setting has no effect when decoding JSON texts. 290 This setting has no effect when decoding JSON texts.
177 291
178 $json = $json->space_before ([$enable]) 292 $json = $json->space_before ([$enable])
293 $enabled = $json->get_space_before
179 If $enable is true (or missing), then the "encode" method will add 294 If $enable is true (or missing), then the "encode" method will add
180 an extra optional space before the ":" separating keys from values 295 an extra optional space before the ":" separating keys from values
181 in JSON objects. 296 in JSON objects.
182 297
183 If $enable is false, then the "encode" method will not add any extra 298 If $enable is false, then the "encode" method will not add any extra
… …
189 Example, space_before enabled, space_after and indent disabled: 304 Example, space_before enabled, space_after and indent disabled:
190 305
191 {"key" :"value"} 306 {"key" :"value"}
192 307
193 $json = $json->space_after ([$enable]) 308 $json = $json->space_after ([$enable])
309 $enabled = $json->get_space_after
194 If $enable is true (or missing), then the "encode" method will add 310 If $enable is true (or missing), then the "encode" method will add
195 an extra optional space after the ":" separating keys from values in 311 an extra optional space after the ":" separating keys from values in
196 JSON objects and extra whitespace after the "," separating key-value 312 JSON objects and extra whitespace after the "," separating key-value
197 pairs and array members. 313 pairs and array members.
198 314
… …
203 319
204 Example, space_before and indent disabled, space_after enabled: 320 Example, space_before and indent disabled, space_after enabled:
205 321
206 {"key": "value"} 322 {"key": "value"}
207 323
324 $json = $json->relaxed ([$enable])
325 $enabled = $json->get_relaxed
326 If $enable is true (or missing), then "decode" will accept some
327 extensions to normal JSON syntax (see below). "encode" will not be
328 affected in anyway. *Be aware that this option makes you accept
329 invalid JSON texts as if they were valid!*. I suggest only to use
330 this option to parse application-specific files written by humans
331 (configuration files, resource files etc.)
332
333 If $enable is false (the default), then "decode" will only accept
334 valid JSON texts.
335
336 Currently accepted extensions are:
337
338 * list items can have an end-comma
339
340 JSON *separates* array elements and key-value pairs with commas.
341 This can be annoying if you write JSON texts manually and want
342 to be able to quickly append elements, so this extension accepts
343 comma at the end of such items not just between them:
344
345 [
346 1,
347 2, <- this comma not normally allowed
348 ]
349 {
350 "k1": "v1",
351 "k2": "v2", <- this comma not normally allowed
352 }
353
354 * shell-style '#'-comments
355
356 Whenever JSON allows whitespace, shell-style comments are
357 additionally allowed. They are terminated by the first
358 carriage-return or line-feed character, after which more
359 white-space and comments are allowed.
360
361 [
362 1, # this comment not allowed in JSON
363 # neither this one...
364 ]
365
208 $json = $json->canonical ([$enable]) 366 $json = $json->canonical ([$enable])
367 $enabled = $json->get_canonical
209 If $enable is true (or missing), then the "encode" method will 368 If $enable is true (or missing), then the "encode" method will
210 output JSON objects by sorting their keys. This is adding a 369 output JSON objects by sorting their keys. This is adding a
211 comparatively high overhead. 370 comparatively high overhead.
212 371
213 If $enable is false, then the "encode" method will output key-value 372 If $enable is false, then the "encode" method will output key-value
214 pairs in the order Perl stores them (which will likely change 373 pairs in the order Perl stores them (which will likely change
215 between runs of the same script). 374 between runs of the same script, and can change even within the same
375 run from 5.18 onwards).
216 376
217 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
218 encoded as the same JSON text (given the same overall settings). If 378 encoded as the same JSON text (given the same overall settings). If
219 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
220 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
221 in Perl. 381 in Perl.
222 382
223 This setting has no effect when decoding JSON texts. 383 This setting has no effect when decoding JSON texts.
224 384
385 This setting has currently no effect on tied hashes.
386
225 $json = $json->allow_nonref ([$enable]) 387 $json = $json->allow_nonref ([$enable])
388 $enabled = $json->get_allow_nonref
226 If $enable is true (or missing), then the "encode" method can 389 If $enable is true (or missing), then the "encode" method can
227 convert a non-reference into its corresponding string, number or 390 convert a non-reference into its corresponding string, number or
228 null JSON value, which is an extension to RFC4627. Likewise, 391 null JSON value, which is an extension to RFC4627. Likewise,
229 "decode" will accept those JSON values instead of croaking. 392 "decode" will accept those JSON values instead of croaking.
230 393
… …
237 "allow_nonref", resulting in an invalid JSON text: 400 "allow_nonref", resulting in an invalid JSON text:
238 401
239 JSON::XS->new->allow_nonref->encode ("Hello, World!") 402 JSON::XS->new->allow_nonref->encode ("Hello, World!")
240 => "Hello, World!" 403 => "Hello, World!"
241 404
405 $json = $json->allow_unknown ([$enable])
406 $enabled = $json->get_allow_unknown
407 If $enable is true (or missing), then "encode" will *not* throw an
408 exception when it encounters values it cannot represent in JSON (for
409 example, filehandles) but instead will encode a JSON "null" value.
410 Note that blessed objects are not included here and are handled
411 separately by c<allow_nonref>.
412
413 If $enable is false (the default), then "encode" will throw an
414 exception when it encounters anything it cannot encode as JSON.
415
416 This option does not affect "decode" in any way, and it is
417 recommended to leave it off unless you know your communications
418 partner.
419
420 $json = $json->allow_blessed ([$enable])
421 $enabled = $json->get_allow_blessed
422 If $enable is true (or missing), then the "encode" method will not
423 barf when it encounters a blessed reference. Instead, the value of
424 the convert_blessed option will decide whether "null"
425 ("convert_blessed" disabled or no "TO_JSON" method found) or a
426 representation of the object ("convert_blessed" enabled and
427 "TO_JSON" method found) is being encoded. Has no effect on "decode".
428
429 If $enable is false (the default), then "encode" will throw an
430 exception when it encounters a blessed object.
431
432 $json = $json->convert_blessed ([$enable])
433 $enabled = $json->get_convert_blessed
434 If $enable is true (or missing), then "encode", upon encountering a
435 blessed object, will check for the availability of the "TO_JSON"
436 method on the object's class. If found, it will be called in scalar
437 context and the resulting scalar will be encoded instead of the
438 object. If no "TO_JSON" method is found, the value of
439 "allow_blessed" will decide what to do.
440
441 The "TO_JSON" method may safely call die if it wants. If "TO_JSON"
442 returns other blessed objects, those will be handled in the same
443 way. "TO_JSON" must take care of not causing an endless recursion
444 cycle (== crash) in this case. The name of "TO_JSON" was chosen
445 because other methods called by the Perl core (== not by the user of
446 the object) are usually in upper case letters and to avoid
447 collisions with any "to_json" function or method.
448
449 This setting does not yet influence "decode" in any way, but in the
450 future, global hooks might get installed that influence "decode" and
451 are enabled by this setting.
452
453 If $enable is false, then the "allow_blessed" setting will decide
454 what to do when a blessed object is found.
455
456 $json = $json->filter_json_object ([$coderef->($hashref)])
457 When $coderef is specified, it will be called from "decode" each
458 time it decodes a JSON object. The only argument is a reference to
459 the newly-created hash. If the code references returns a single
460 scalar (which need not be a reference), this value (i.e. a copy of
461 that scalar to avoid aliasing) is inserted into the deserialised
462 data structure. If it returns an empty list (NOTE: *not* "undef",
463 which is a valid scalar), the original deserialised hash will be
464 inserted. This setting can slow down decoding considerably.
465
466 When $coderef is omitted or undefined, any existing callback will be
467 removed and "decode" will not change the deserialised hash in any
468 way.
469
470 Example, convert all JSON objects into the integer 5:
471
472 my $js = JSON::XS->new->filter_json_object (sub { 5 });
473 # returns [5]
474 $js->decode ('[{}]')
475 # throw an exception because allow_nonref is not enabled
476 # so a lone 5 is not allowed.
477 $js->decode ('{"a":1, "b":2}');
478
479 $json = $json->filter_json_single_key_object ($key [=>
480 $coderef->($value)])
481 Works remotely similar to "filter_json_object", but is only called
482 for JSON objects having a single key named $key.
483
484 This $coderef is called before the one specified via
485 "filter_json_object", if any. It gets passed the single value in the
486 JSON object. If it returns a single value, it will be inserted into
487 the data structure. If it returns nothing (not even "undef" but the
488 empty list), the callback from "filter_json_object" will be called
489 next, as if no single-key callback were specified.
490
491 If $coderef is omitted or undefined, the corresponding callback will
492 be disabled. There can only ever be one callback for a given key.
493
494 As this callback gets called less often then the
495 "filter_json_object" one, decoding speed will not usually suffer as
496 much. Therefore, single-key objects make excellent targets to
497 serialise Perl objects into, especially as single-key JSON objects
498 are as close to the type-tagged value concept as JSON gets (it's
499 basically an ID/VALUE tuple). Of course, JSON does not support this
500 in any way, so you need to make sure your data never looks like a
501 serialised Perl hash.
502
503 Typical names for the single object key are "__class_whatever__", or
504 "$__dollars_are_rarely_used__$" or "}ugly_brace_placement", or even
505 things like "__class_md5sum(classname)__", to reduce the risk of
506 clashing with real hashes.
507
508 Example, decode JSON objects of the form "{ "__widget__" => <id> }"
509 into the corresponding $WIDGET{<id>} object:
510
511 # return whatever is in $WIDGET{5}:
512 JSON::XS
513 ->new
514 ->filter_json_single_key_object (__widget__ => sub {
515 $WIDGET{ $_[0] }
516 })
517 ->decode ('{"__widget__": 5')
518
519 # this can be used with a TO_JSON method in some "widget" class
520 # for serialisation to json:
521 sub WidgetBase::TO_JSON {
522 my ($self) = @_;
523
524 unless ($self->{id}) {
525 $self->{id} = ..get..some..id..;
526 $WIDGET{$self->{id}} = $self;
527 }
528
529 { __widget__ => $self->{id} }
530 }
531
242 $json = $json->shrink ([$enable]) 532 $json = $json->shrink ([$enable])
533 $enabled = $json->get_shrink
243 Perl usually over-allocates memory a bit when allocating space for 534 Perl usually over-allocates memory a bit when allocating space for
244 strings. This flag optionally resizes strings generated by either 535 strings. This flag optionally resizes strings generated by either
245 "encode" or "decode" to their minimum size possible. This can save 536 "encode" or "decode" to their minimum size possible. This can save
246 memory when your JSON texts are either very very long or you have 537 memory when your JSON texts are either very very long or you have
247 many short strings. It will also try to downgrade any strings to 538 many short strings. It will also try to downgrade any strings to
248 octet-form if possible: perl stores strings internally either in an 539 octet-form if possible: perl stores strings internally either in an
249 encoding called UTF-X or in octet-form. The latter cannot store 540 encoding called UTF-X or in octet-form. The latter cannot store
250 everything but uses less space in general. 541 everything but uses less space in general (and some buggy Perl or C
542 code might even rely on that internal representation being used).
543
544 The actual definition of what shrink does might change in future
545 versions, but it will always try to save space at the expense of
546 time.
251 547
252 If $enable is true (or missing), the string returned by "encode" 548 If $enable is true (or missing), the string returned by "encode"
253 will be shrunk-to-fit, while all strings generated by "decode" will 549 will be shrunk-to-fit, while all strings generated by "decode" will
254 also be shrunk-to-fit. 550 also be shrunk-to-fit.
255 551
… …
260 converting strings that look like integers or floats into integers 556 converting strings that look like integers or floats into integers
261 or floats internally (there is no difference on the Perl level), 557 or floats internally (there is no difference on the Perl level),
262 saving space. 558 saving space.
263 559
264 $json = $json->max_depth ([$maximum_nesting_depth]) 560 $json = $json->max_depth ([$maximum_nesting_depth])
561 $max_depth = $json->get_max_depth
265 Sets the maximum nesting level (default 8192) accepted while 562 Sets the maximum nesting level (default 512) accepted while encoding
266 encoding or decoding. If the JSON text or Perl data structure has an 563 or decoding. If a higher nesting level is detected in JSON text or a
267 equal or higher nesting level then this limit, then the encoder and 564 Perl data structure, then the encoder and decoder will stop and
268 decoder will stop and croak at that point. 565 croak at that point.
269 566
270 Nesting level is defined by number of hash- or arrayrefs that the 567 Nesting level is defined by number of hash- or arrayrefs that the
271 encoder needs to traverse to reach a given point or the number of 568 encoder needs to traverse to reach a given point or the number of
272 "{" or "[" characters without their matching closing parenthesis 569 "{" or "[" characters without their matching closing parenthesis
273 crossed to reach a given character in a string. 570 crossed to reach a given character in a string.
274 571
275 Setting the maximum depth to one disallows any nesting, so that 572 Setting the maximum depth to one disallows any nesting, so that
276 ensures that the object is only a single hash/object or array. 573 ensures that the object is only a single hash/object or array.
277 574
278 The argument to "max_depth" will be rounded up to the next nearest 575 If no argument is given, the highest possible setting will be used,
279 power of two. 576 which is rarely useful.
577
578 Note that nesting is implemented by recursion in C. The default
579 value has been chosen to be as large as typical operating systems
580 allow without crashing.
581
582 See SECURITY CONSIDERATIONS, below, for more info on why this is
583 useful.
584
585 $json = $json->max_size ([$maximum_string_size])
586 $max_size = $json->get_max_size
587 Set the maximum length a JSON text may have (in bytes) where
588 decoding is being attempted. The default is 0, meaning no limit.
589 When "decode" is called on a string that is longer then this many
590 bytes, it will not attempt to decode the string but throw an
591 exception. This setting has no effect on "encode" (yet).
592
593 If no argument is given, the limit check will be deactivated (same
594 as when 0 is specified).
280 595
281 See SECURITY CONSIDERATIONS, below, for more info on why this is 596 See SECURITY CONSIDERATIONS, below, for more info on why this is
282 useful. 597 useful.
283 598
284 $json_text = $json->encode ($perl_scalar) 599 $json_text = $json->encode ($perl_scalar)
… …
296 611
297 JSON numbers and strings become simple Perl scalars. JSON arrays 612 JSON numbers and strings become simple Perl scalars. JSON arrays
298 become Perl arrayrefs and JSON objects become Perl hashrefs. "true" 613 become Perl arrayrefs and JSON objects become Perl hashrefs. "true"
299 becomes 1, "false" becomes 0 and "null" becomes "undef". 614 becomes 1, "false" becomes 0 and "null" becomes "undef".
300 615
616 ($perl_scalar, $characters) = $json->decode_prefix ($json_text)
617 This works like the "decode" method, but instead of raising an
618 exception when there is trailing garbage after the first JSON
619 object, it will silently stop parsing there and return the number of
620 characters consumed so far.
621
622 This is useful if your JSON texts are not delimited by an outer
623 protocol (which is not the brightest thing to do in the first place)
624 and you need to know where the JSON text ends.
625
626 JSON::XS->new->decode_prefix ("[1] the tail")
627 => ([], 3)
628
629INCREMENTAL PARSING
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 (and can be implemented with a minimum of method
637 calls).
638
639 JSON::XS will only attempt to parse the JSON text once it is sure it has
640 enough text to get a decisive result, using a very simple but truly
641 incremental parser. This means that it sometimes won't stop as early as
642 the full parser, for example, it doesn't detect mismatched parentheses.
643 The only thing it guarantees is that it starts decoding as soon as a
644 syntactically valid JSON text has been seen. This means you need to set
645 resource limits (e.g. "max_size") to ensure the parser will stop parsing
646 in the presence if syntax errors.
647
648 The following methods implement this incremental parser.
649
650 [void, scalar or list context] = $json->incr_parse ([$string])
651 This is the central parsing function. It can both append new text
652 and extract objects from the stream accumulated so far (both of
653 these functions are optional).
654
655 If $string is given, then this string is appended to the already
656 existing JSON fragment stored in the $json object.
657
658 After that, if the function is called in void context, it will
659 simply return without doing anything further. This can be used to
660 add more text in as many chunks as you want.
661
662 If the method is called in scalar context, then it will try to
663 extract exactly *one* JSON object. If that is successful, it will
664 return this object, otherwise it will return "undef". If there is a
665 parse error, this method will croak just as "decode" would do (one
666 can then use "incr_skip" to skip the errornous part). This is the
667 most common way of using the method.
668
669 And finally, in list context, it will try to extract as many objects
670 from the stream as it can find and return them, or the empty list
671 otherwise. For this to work, there must be no separators between the
672 JSON objects or arrays, instead they must be concatenated
673 back-to-back. If an error occurs, an exception will be raised as in
674 the scalar context case. Note that in this case, any
675 previously-parsed JSON texts will be lost.
676
677 Example: Parse some JSON arrays/objects in a given string and return
678 them.
679
680 my @objs = JSON::XS->new->incr_parse ("[5][7][1,2]");
681
682 $lvalue_string = $json->incr_text
683 This method returns the currently stored JSON fragment as an lvalue,
684 that is, you can manipulate it. This *only* works when a preceding
685 call to "incr_parse" in *scalar context* successfully returned an
686 object. Under all other circumstances you must not call this
687 function (I mean it. although in simple tests it might actually
688 work, it *will* fail under real world conditions). As a special
689 exception, you can also call this method before having parsed
690 anything.
691
692 This function is useful in two cases: a) finding the trailing text
693 after a JSON object or b) parsing multiple JSON objects separated by
694 non-JSON text (such as commas).
695
696 $json->incr_skip
697 This will reset the state of the incremental parser and will remove
698 the parsed text from the input buffer so far. This is useful after
699 "incr_parse" died, in which case the input buffer and incremental
700 parser state is left unchanged, to skip the text parsed so far and
701 to reset the parse state.
702
703 The difference to "incr_reset" is that only text until the parse
704 error occured is removed.
705
706 $json->incr_reset
707 This completely resets the incremental parser, that is, after this
708 call, it will be as if the parser had never parsed anything.
709
710 This is useful if you want to repeatedly parse JSON objects and want
711 to ignore any trailing data, which means you have to reset the
712 parser after each successful decode.
713
714 LIMITATIONS
715 All options that affect decoding are supported, except "allow_nonref".
716 The reason for this is that it cannot be made to work sensibly: JSON
717 objects and arrays are self-delimited, i.e. you can concatenate them
718 back to back and still decode them perfectly. This does not hold true
719 for JSON numbers, however.
720
721 For example, is the string 1 a single JSON number, or is it simply the
722 start of 12? Or is 12 a single JSON number, or the concatenation of 1
723 and 2? In neither case you can tell, and this is why JSON::XS takes the
724 conservative route and disallows this case.
725
726 EXAMPLES
727 Some examples will make all this clearer. First, a simple example that
728 works similarly to "decode_prefix": We want to decode the JSON object at
729 the start of a string and identify the portion after the JSON object:
730
731 my $text = "[1,2,3] hello";
732
733 my $json = new JSON::XS;
734
735 my $obj = $json->incr_parse ($text)
736 or die "expected JSON object or array at beginning of string";
737
738 my $tail = $json->incr_text;
739 # $tail now contains " hello"
740
741 Easy, isn't it?
742
743 Now for a more complicated example: Imagine a hypothetical protocol
744 where you read some requests from a TCP stream, and each request is a
745 JSON array, without any separation between them (in fact, it is often
746 useful to use newlines as "separators", as these get interpreted as
747 whitespace at the start of the JSON text, which makes it possible to
748 test said protocol with "telnet"...).
749
750 Here is how you'd do it (it is trivial to write this in an event-based
751 manner):
752
753 my $json = new JSON::XS;
754
755 # read some data from the socket
756 while (sysread $socket, my $buf, 4096) {
757
758 # split and decode as many requests as possible
759 for my $request ($json->incr_parse ($buf)) {
760 # act on the $request
761 }
762 }
763
764 Another complicated example: Assume you have a string with JSON objects
765 or arrays, all separated by (optional) comma characters (e.g. "[1],[2],
766 [3]"). To parse them, we have to skip the commas between the JSON texts,
767 and here is where the lvalue-ness of "incr_text" comes in useful:
768
769 my $text = "[1],[2], [3]";
770 my $json = new JSON::XS;
771
772 # void context, so no parsing done
773 $json->incr_parse ($text);
774
775 # now extract as many objects as possible. note the
776 # use of scalar context so incr_text can be called.
777 while (my $obj = $json->incr_parse) {
778 # do something with $obj
779
780 # now skip the optional comma
781 $json->incr_text =~ s/^ \s* , //x;
782 }
783
784 Now lets go for a very complex example: Assume that you have a gigantic
785 JSON array-of-objects, many gigabytes in size, and you want to parse it,
786 but you cannot load it into memory fully (this has actually happened in
787 the real world :).
788
789 Well, you lost, you have to implement your own JSON parser. But JSON::XS
790 can still help you: You implement a (very simple) array parser and let
791 JSON decode the array elements, which are all full JSON objects on their
792 own (this wouldn't work if the array elements could be JSON numbers, for
793 example):
794
795 my $json = new JSON::XS;
796
797 # open the monster
798 open my $fh, "<bigfile.json"
799 or die "bigfile: $!";
800
801 # first parse the initial "["
802 for (;;) {
803 sysread $fh, my $buf, 65536
804 or die "read error: $!";
805 $json->incr_parse ($buf); # void context, so no parsing
806
807 # Exit the loop once we found and removed(!) the initial "[".
808 # In essence, we are (ab-)using the $json object as a simple scalar
809 # we append data to.
810 last if $json->incr_text =~ s/^ \s* \[ //x;
811 }
812
813 # now we have the skipped the initial "[", so continue
814 # parsing all the elements.
815 for (;;) {
816 # in this loop we read data until we got a single JSON object
817 for (;;) {
818 if (my $obj = $json->incr_parse) {
819 # do something with $obj
820 last;
821 }
822
823 # add more data
824 sysread $fh, my $buf, 65536
825 or die "read error: $!";
826 $json->incr_parse ($buf); # void context, so no parsing
827 }
828
829 # in this loop we read data until we either found and parsed the
830 # separating "," between elements, or the final "]"
831 for (;;) {
832 # first skip whitespace
833 $json->incr_text =~ s/^\s*//;
834
835 # if we find "]", we are done
836 if ($json->incr_text =~ s/^\]//) {
837 print "finished.\n";
838 exit;
839 }
840
841 # if we find ",", we can continue with the next element
842 if ($json->incr_text =~ s/^,//) {
843 last;
844 }
845
846 # if we find anything else, we have a parse error!
847 if (length $json->incr_text) {
848 die "parse error near ", $json->incr_text;
849 }
850
851 # else add more data
852 sysread $fh, my $buf, 65536
853 or die "read error: $!";
854 $json->incr_parse ($buf); # void context, so no parsing
855 }
856
857 This is a complex example, but most of the complexity comes from the
858 fact that we are trying to be correct (bear with me if I am wrong, I
859 never ran the above example :).
860
301MAPPING 861MAPPING
302 This section describes how JSON::XS maps Perl values to JSON values and 862 This section describes how JSON::XS maps Perl values to JSON values and
303 vice versa. These mappings are designed to "do the right thing" in most 863 vice versa. These mappings are designed to "do the right thing" in most
304 circumstances automatically, preserving round-tripping characteristics 864 circumstances automatically, preserving round-tripping characteristics
305 (what you put in comes out as something equivalent). 865 (what you put in comes out as something equivalent).
306 866
307 For the more enlightened: note that in the following descriptions, 867 For the more enlightened: note that in the following descriptions,
308 lowercase *perl* refers to the Perl interpreter, while uppcercase *Perl* 868 lowercase *perl* refers to the Perl interpreter, while uppercase *Perl*
309 refers to the abstract Perl language itself. 869 refers to the abstract Perl language itself.
310 870
311 JSON -> PERL 871 JSON -> PERL
312 object 872 object
313 A JSON object becomes a reference to a hash in Perl. No ordering of 873 A JSON object becomes a reference to a hash in Perl. No ordering of
314 object keys is preserved (JSON does not preserver object key 874 object keys is preserved (JSON does not preserve object key ordering
315 ordering itself). 875 itself).
316 876
317 array 877 array
318 A JSON array becomes a reference to an array in Perl. 878 A JSON array becomes a reference to an array in Perl.
319 879
320 string 880 string
321 A JSON string becomes a string scalar in Perl - Unicode codepoints 881 A JSON string becomes a string scalar in Perl - Unicode codepoints
322 in JSON are represented by the same codepoints in the Perl string, 882 in JSON are represented by the same codepoints in the Perl string,
323 so no manual decoding is necessary. 883 so no manual decoding is necessary.
324 884
325 number 885 number
326 A JSON number becomes either an integer or numeric (floating point) 886 A JSON number becomes either an integer, numeric (floating point) or
327 scalar in perl, depending on its range and any fractional parts. On 887 string scalar in perl, depending on its range and any fractional
328 the Perl level, there is no difference between those as Perl handles 888 parts. On the Perl level, there is no difference between those as
329 all the conversion details, but an integer may take slightly less 889 Perl handles all the conversion details, but an integer may take
330 memory and might represent more values exactly than (floating point) 890 slightly less memory and might represent more values exactly than
891 floating point numbers.
892
893 If the number consists of digits only, JSON::XS will try to
894 represent it as an integer value. If that fails, it will try to
895 represent it as a numeric (floating point) value if that is possible
896 without loss of precision. Otherwise it will preserve the number as
897 a string value (in which case you lose roundtripping ability, as the
898 JSON number will be re-encoded toa JSON string).
899
900 Numbers containing a fractional or exponential part will always be
901 represented as numeric (floating point) values, possibly at a loss
902 of precision (in which case you might lose perfect roundtripping
903 ability, but the JSON number will still be re-encoded as a JSON
331 numbers. 904 number).
905
906 Note that precision is not accuracy - binary floating point values
907 cannot represent most decimal fractions exactly, and when converting
908 from and to floating point, JSON::XS only guarantees precision up to
909 but not including the leats significant bit.
332 910
333 true, false 911 true, false
334 These JSON atoms become 0, 1, respectively. Information is lost in 912 These JSON atoms become "JSON::XS::true" and "JSON::XS::false",
335 this process. Future versions might represent those values 913 respectively. They are overloaded to act almost exactly like the
336 differently, but they will be guarenteed to act like these integers 914 numbers 1 and 0. You can check whether a scalar is a JSON boolean by
337 would normally in Perl. 915 using the "JSON::XS::is_bool" function.
338 916
339 null 917 null
340 A JSON null atom becomes "undef" in Perl. 918 A JSON null atom becomes "undef" in Perl.
341 919
342 PERL -> JSON 920 PERL -> JSON
… …
344 truly typeless language, so we can only guess which JSON type is meant 922 truly typeless language, so we can only guess which JSON type is meant
345 by a Perl value. 923 by a Perl value.
346 924
347 hash references 925 hash references
348 Perl hash references become JSON objects. As there is no inherent 926 Perl hash references become JSON objects. As there is no inherent
349 ordering in hash keys, they will usually be encoded in a 927 ordering in hash keys (or JSON objects), they will usually be
350 pseudo-random order that can change between runs of the same program 928 encoded in a pseudo-random order that can change between runs of the
351 but stays generally the same within a single run of a program. 929 same program but stays generally the same within a single run of a
352 JSON::XS can optionally sort the hash keys (determined by the 930 program. JSON::XS can optionally sort the hash keys (determined by
353 *canonical* flag), so the same datastructure will serialise to the 931 the *canonical* flag), so the same datastructure will serialise to
354 same JSON text (given same settings and version of JSON::XS), but 932 the same JSON text (given same settings and version of JSON::XS),
355 this incurs a runtime overhead. 933 but this incurs a runtime overhead and is only rarely useful, e.g.
934 when you want to compare some JSON text against another for
935 equality.
356 936
357 array references 937 array references
358 Perl array references become JSON arrays. 938 Perl array references become JSON arrays.
359 939
940 other references
941 Other unblessed references are generally not allowed and will cause
942 an exception to be thrown, except for references to the integers 0
943 and 1, which get turned into "false" and "true" atoms in JSON. You
944 can also use "JSON::XS::false" and "JSON::XS::true" to improve
945 readability.
946
947 encode_json [\0, JSON::XS::true] # yields [false,true]
948
949 JSON::XS::true, JSON::XS::false
950 These special values become JSON true and JSON false values,
951 respectively. You can also use "\1" and "\0" directly if you want.
952
360 blessed objects 953 blessed objects
361 Blessed objects are not allowed. JSON::XS currently tries to encode 954 Blessed objects are not directly representable in JSON. See the
362 their underlying representation (hash- or arrayref), but this 955 "allow_blessed" and "convert_blessed" methods on various options on
363 behaviour might change in future versions. 956 how to deal with this: basically, you can choose between throwing an
957 exception, encoding the reference as if it weren't blessed, or
958 provide your own serialiser method.
364 959
365 simple scalars 960 simple scalars
366 Simple Perl scalars (any scalar that is not a reference) are the 961 Simple Perl scalars (any scalar that is not a reference) are the
367 most difficult objects to encode: JSON::XS will encode undefined 962 most difficult objects to encode: JSON::XS will encode undefined
368 scalars as JSON null value, scalars that have last been used in a 963 scalars as JSON "null" values, scalars that have last been used in a
369 string context before encoding as JSON strings and anything else as 964 string context before encoding as JSON strings, and anything else as
370 number value: 965 number value:
371 966
372 # dump as number 967 # dump as number
373 to_json [2] # yields [2] 968 encode_json [2] # yields [2]
374 to_json [-3.0e17] # yields [-3e+17] 969 encode_json [-3.0e17] # yields [-3e+17]
375 my $value = 5; to_json [$value] # yields [5] 970 my $value = 5; encode_json [$value] # yields [5]
376 971
377 # used as string, so dump as string 972 # used as string, so dump as string
378 print $value; 973 print $value;
379 to_json [$value] # yields ["5"] 974 encode_json [$value] # yields ["5"]
380 975
381 # undef becomes null 976 # undef becomes null
382 to_json [undef] # yields [null] 977 encode_json [undef] # yields [null]
383 978
384 You can force the type to be a string by stringifying it: 979 You can force the type to be a JSON string by stringifying it:
385 980
386 my $x = 3.1; # some variable containing a number 981 my $x = 3.1; # some variable containing a number
387 "$x"; # stringified 982 "$x"; # stringified
388 $x .= ""; # another, more awkward way to stringify 983 $x .= ""; # another, more awkward way to stringify
389 print $x; # perl does it for you, too, quite often 984 print $x; # perl does it for you, too, quite often
390 985
391 You can force the type to be a number by numifying it: 986 You can force the type to be a JSON number by numifying it:
392 987
393 my $x = "3"; # some variable containing a string 988 my $x = "3"; # some variable containing a string
394 $x += 0; # numify it, ensuring it will be dumped as a number 989 $x += 0; # numify it, ensuring it will be dumped as a number
395 $x *= 1; # same thing, the choise is yours. 990 $x *= 1; # same thing, the choice is yours.
396 991
397 You can not currently output JSON booleans or force the type in 992 You can not currently force the type in other, less obscure, ways.
398 other, less obscure, ways. Tell me if you need this capability. 993 Tell me if you need this capability (but don't forget to explain why
994 it's needed :).
399 995
400 circular data structures 996 Note that numerical precision has the same meaning as under Perl (so
401 Those will be encoded until memory or stackspace runs out. 997 binary to decimal conversion follows the same rules as in Perl,
998 which can differ to other languages). Also, your perl interpreter
999 might expose extensions to the floating point numbers of your
1000 platform, such as infinities or NaN's - these cannot be represented
1001 in JSON, and it is an error to pass those in.
402 1002
403COMPARISON 1003ENCODING/CODESET FLAG NOTES
404 As already mentioned, this module was created because none of the 1004 The interested reader might have seen a number of flags that signify
405 existing JSON modules could be made to work correctly. First I will 1005 encodings or codesets - "utf8", "latin1" and "ascii". There seems to be
406 describe the problems (or pleasures) I encountered with various existing 1006 some confusion on what these do, so here is a short comparison:
407 JSON modules, followed by some benchmark values. JSON::XS was designed
408 not to suffer from any of these problems or limitations.
409 1007
410 JSON 1.07 1008 "utf8" controls whether the JSON text created by "encode" (and expected
411 Slow (but very portable, as it is written in pure Perl). 1009 by "decode") is UTF-8 encoded or not, while "latin1" and "ascii" only
1010 control whether "encode" escapes character values outside their
1011 respective codeset range. Neither of these flags conflict with each
1012 other, although some combinations make less sense than others.
412 1013
413 Undocumented/buggy Unicode handling (how JSON handles unicode values 1014 Care has been taken to make all flags symmetrical with respect to
414 is undocumented. One can get far by feeding it unicode strings and 1015 "encode" and "decode", that is, texts encoded with any combination of
415 doing en-/decoding oneself, but unicode escapes are not working 1016 these flag values will be correctly decoded when the same flags are used
1017 - in general, if you use different flag settings while encoding vs. when
1018 decoding you likely have a bug somewhere.
1019
1020 Below comes a verbose discussion of these flags. Note that a "codeset"
1021 is simply an abstract set of character-codepoint pairs, while an
1022 encoding takes those codepoint numbers and *encodes* them, in our case
1023 into octets. Unicode is (among other things) a codeset, UTF-8 is an
1024 encoding, and ISO-8859-1 (= latin 1) and ASCII are both codesets *and*
1025 encodings at the same time, which can be confusing.
1026
1027 "utf8" flag disabled
1028 When "utf8" is disabled (the default), then "encode"/"decode"
1029 generate and expect Unicode strings, that is, characters with high
1030 ordinal Unicode values (> 255) will be encoded as such characters,
1031 and likewise such characters are decoded as-is, no canges to them
1032 will be done, except "(re-)interpreting" them as Unicode codepoints
1033 or Unicode characters, respectively (to Perl, these are the same
1034 thing in strings unless you do funny/weird/dumb stuff).
1035
1036 This is useful when you want to do the encoding yourself (e.g. when
1037 you want to have UTF-16 encoded JSON texts) or when some other layer
1038 does the encoding for you (for example, when printing to a terminal
1039 using a filehandle that transparently encodes to UTF-8 you certainly
1040 do NOT want to UTF-8 encode your data first and have Perl encode it
1041 another time).
1042
1043 "utf8" flag enabled
1044 If the "utf8"-flag is enabled, "encode"/"decode" will encode all
1045 characters using the corresponding UTF-8 multi-byte sequence, and
1046 will expect your input strings to be encoded as UTF-8, that is, no
1047 "character" of the input string must have any value > 255, as UTF-8
1048 does not allow that.
1049
1050 The "utf8" flag therefore switches between two modes: disabled means
1051 you will get a Unicode string in Perl, enabled means you get an
1052 UTF-8 encoded octet/binary string in Perl.
1053
1054 "latin1" or "ascii" flags enabled
1055 With "latin1" (or "ascii") enabled, "encode" will escape characters
1056 with ordinal values > 255 (> 127 with "ascii") and encode the
1057 remaining characters as specified by the "utf8" flag.
1058
1059 If "utf8" is disabled, then the result is also correctly encoded in
1060 those character sets (as both are proper subsets of Unicode, meaning
1061 that a Unicode string with all character values < 256 is the same
1062 thing as a ISO-8859-1 string, and a Unicode string with all
1063 character values < 128 is the same thing as an ASCII string in
416 properly). 1064 Perl).
417 1065
418 No roundtripping (strings get clobbered if they look like numbers, 1066 If "utf8" is enabled, you still get a correct UTF-8-encoded string,
419 e.g. the string 2.0 will encode to 2.0 instead of "2.0", and that 1067 regardless of these flags, just some more characters will be escaped
420 will decode into the number 2. 1068 using "\uXXXX" then before.
421 1069
422 JSON::PC 0.01 1070 Note that ISO-8859-1-*encoded* strings are not compatible with UTF-8
423 Very fast. 1071 encoding, while ASCII-encoded strings are. That is because the
1072 ISO-8859-1 encoding is NOT a subset of UTF-8 (despite the ISO-8859-1
1073 *codeset* being a subset of Unicode), while ASCII is.
424 1074
425 Undocumented/buggy Unicode handling. 1075 Surprisingly, "decode" will ignore these flags and so treat all
1076 input values as governed by the "utf8" flag. If it is disabled, this
1077 allows you to decode ISO-8859-1- and ASCII-encoded strings, as both
1078 strict subsets of Unicode. If it is enabled, you can correctly
1079 decode UTF-8 encoded strings.
426 1080
427 No roundtripping. 1081 So neither "latin1" nor "ascii" are incompatible with the "utf8"
1082 flag - they only govern when the JSON output engine escapes a
1083 character or not.
428 1084
429 Has problems handling many Perl values (e.g. regex results and other 1085 The main use for "latin1" is to relatively efficiently store binary
430 magic values will make it croak). 1086 data as JSON, at the expense of breaking compatibility with most
1087 JSON decoders.
431 1088
432 Does not even generate valid JSON ("{1,2}" gets converted to "{1:2}" 1089 The main use for "ascii" is to force the output to not contain
433 which is not a valid JSON text. 1090 characters with values > 127, which means you can interpret the
1091 resulting string as UTF-8, ISO-8859-1, ASCII, KOI8-R or most about
1092 any character set and 8-bit-encoding, and still get the same data
1093 structure back. This is useful when your channel for JSON transfer
1094 is not 8-bit clean or the encoding might be mangled in between (e.g.
1095 in mail), and works because ASCII is a proper subset of most 8-bit
1096 and multibyte encodings in use in the world.
434 1097
435 Unmaintained (maintainer unresponsive for many months, bugs are not 1098 JSON and ECMAscript
436 getting fixed). 1099 JSON syntax is based on how literals are represented in javascript (the
1100 not-standardised predecessor of ECMAscript) which is presumably why it
1101 is called "JavaScript Object Notation".
437 1102
438 JSON::Syck 0.21 1103 However, JSON is not a subset (and also not a superset of course) of
439 Very buggy (often crashes). 1104 ECMAscript (the standard) or javascript (whatever browsers actually
1105 implement).
440 1106
441 Very inflexible (no human-readable format supported, format pretty 1107 If you want to use javascript's "eval" function to "parse" JSON, you
442 much undocumented. I need at least a format for easy reading by 1108 might run into parse errors for valid JSON texts, or the resulting data
443 humans and a single-line compact format for use in a protocol, and 1109 structure might not be queryable:
444 preferably a way to generate ASCII-only JSON texts).
445 1110
446 Completely broken (and confusingly documented) Unicode handling 1111 One of the problems is that U+2028 and U+2029 are valid characters
447 (unicode escapes are not working properly, you need to set 1112 inside JSON strings, but are not allowed in ECMAscript string literals,
448 ImplicitUnicode to *different* values on en- and decoding to get 1113 so the following Perl fragment will not output something that can be
449 symmetric behaviour). 1114 guaranteed to be parsable by javascript's "eval":
450 1115
451 No roundtripping (simple cases work, but this depends on wether the 1116 use JSON::XS;
452 scalar value was used in a numeric context or not).
453 1117
454 Dumping hashes may skip hash values depending on iterator state. 1118 print encode_json [chr 0x2028];
455 1119
456 Unmaintained (maintainer unresponsive for many months, bugs are not 1120 The right fix for this is to use a proper JSON parser in your javascript
457 getting fixed). 1121 programs, and not rely on "eval" (see for example Douglas Crockford's
1122 json2.js parser).
458 1123
459 Does not check input for validity (i.e. will accept non-JSON input 1124 If this is not an option, you can, as a stop-gap measure, simply encode
460 and return "something" instead of raising an exception. This is a 1125 to ASCII-only JSON:
461 security issue: imagine two banks transfering money between each
462 other using JSON. One bank might parse a given non-JSON request and
463 deduct money, while the other might reject the transaction with a
464 syntax error. While a good protocol will at least recover, that is
465 extra unnecessary work and the transaction will still not succeed).
466 1126
467 JSON::DWIW 0.04 1127 use JSON::XS;
468 Very fast. Very natural. Very nice.
469 1128
470 Undocumented unicode handling (but the best of the pack. Unicode 1129 print JSON::XS->new->ascii->encode ([chr 0x2028]);
471 escapes still don't get parsed properly).
472 1130
473 Very inflexible. 1131 Note that this will enlarge the resulting JSON text quite a bit if you
1132 have many non-ASCII characters. You might be tempted to run some regexes
1133 to only escape U+2028 and U+2029, e.g.:
474 1134
475 No roundtripping. 1135 # DO NOT USE THIS!
1136 my $json = JSON::XS->new->utf8->encode ([chr 0x2028]);
1137 $json =~ s/\xe2\x80\xa8/\\u2028/g; # escape U+2028
1138 $json =~ s/\xe2\x80\xa9/\\u2029/g; # escape U+2029
1139 print $json;
476 1140
477 Does not generate valid JSON texts (key strings are often unquoted, 1141 Note that *this is a bad idea*: the above only works for U+2028 and
478 empty keys result in nothing being output) 1142 U+2029 and thus only for fully ECMAscript-compliant parsers. Many
1143 existing javascript implementations, however, have issues with other
1144 characters as well - using "eval" naively simply *will* cause problems.
479 1145
480 Does not check input for validity. 1146 Another problem is that some javascript implementations reserve some
1147 property names for their own purposes (which probably makes them
1148 non-ECMAscript-compliant). For example, Iceweasel reserves the
1149 "__proto__" property name for its own purposes.
1150
1151 If that is a problem, you could parse try to filter the resulting JSON
1152 output for these property strings, e.g.:
1153
1154 $json =~ s/"__proto__"\s*:/"__proto__renamed":/g;
1155
1156 This works because "__proto__" is not valid outside of strings, so every
1157 occurence of ""__proto__"\s*:" must be a string used as property name.
1158
1159 If you know of other incompatibilities, please let me know.
1160
1161 JSON and YAML
1162 You often hear that JSON is a subset of YAML. This is, however, a mass
1163 hysteria(*) and very far from the truth (as of the time of this
1164 writing), so let me state it clearly: *in general, there is no way to
1165 configure JSON::XS to output a data structure as valid YAML* that works
1166 in all cases.
1167
1168 If you really must use JSON::XS to generate YAML, you should use this
1169 algorithm (subject to change in future versions):
1170
1171 my $to_yaml = JSON::XS->new->utf8->space_after (1);
1172 my $yaml = $to_yaml->encode ($ref) . "\n";
1173
1174 This will *usually* generate JSON texts that also parse as valid YAML.
1175 Please note that YAML has hardcoded limits on (simple) object key
1176 lengths that JSON doesn't have and also has different and incompatible
1177 unicode character escape syntax, so you should make sure that your hash
1178 keys are noticeably shorter than the 1024 "stream characters" YAML
1179 allows and that you do not have characters with codepoint values outside
1180 the Unicode BMP (basic multilingual page). YAML also does not allow "\/"
1181 sequences in strings (which JSON::XS does not *currently* generate, but
1182 other JSON generators might).
1183
1184 There might be other incompatibilities that I am not aware of (or the
1185 YAML specification has been changed yet again - it does so quite often).
1186 In general you should not try to generate YAML with a JSON generator or
1187 vice versa, or try to parse JSON with a YAML parser or vice versa:
1188 chances are high that you will run into severe interoperability problems
1189 when you least expect it.
1190
1191 (*) I have been pressured multiple times by Brian Ingerson (one of the
1192 authors of the YAML specification) to remove this paragraph, despite
1193 him acknowledging that the actual incompatibilities exist. As I was
1194 personally bitten by this "JSON is YAML" lie, I refused and said I
1195 will continue to educate people about these issues, so others do not
1196 run into the same problem again and again. After this, Brian called
1197 me a (quote)*complete and worthless idiot*(unquote).
1198
1199 In my opinion, instead of pressuring and insulting people who
1200 actually clarify issues with YAML and the wrong statements of some
1201 of its proponents, I would kindly suggest reading the JSON spec
1202 (which is not that difficult or long) and finally make YAML
1203 compatible to it, and educating users about the changes, instead of
1204 spreading lies about the real compatibility for many *years* and
1205 trying to silence people who point out that it isn't true.
1206
1207 Addendum/2009: the YAML 1.2 spec is still incompatible with JSON,
1208 even though the incompatibilities have been documented (and are
1209 known to Brian) for many years and the spec makes explicit claims
1210 that YAML is a superset of JSON. It would be so easy to fix, but
1211 apparently, bullying people and corrupting userdata is so much
1212 easier.
481 1213
482 SPEED 1214 SPEED
483 It seems that JSON::XS is surprisingly fast, as shown in the following 1215 It seems that JSON::XS is surprisingly fast, as shown in the following
484 tables. They have been generated with the help of the "eg/bench" program 1216 tables. They have been generated with the help of the "eg/bench" program
485 in the JSON::XS distribution, to make it easy to compare on your own 1217 in the JSON::XS distribution, to make it easy to compare on your own
486 system. 1218 system.
487 1219
488 First comes a comparison between various modules using a very short JSON 1220 First comes a comparison between various modules using a very short
489 string: 1221 single-line JSON string (also available at
1222 <http://dist.schmorp.de/misc/json/short.json>).
490 1223
491 {"method": "handleMessage", "params": ["user1", "we were just talking"], "id": null} 1224 {"method": "handleMessage", "params": ["user1",
1225 "we were just talking"], "id": null, "array":[1,11,234,-5,1e5,1e7,
1226 1, 0]}
492 1227
493 It shows the number of encodes/decodes per second (JSON::XS uses the 1228 It shows the number of encodes/decodes per second (JSON::XS uses the
494 functional interface, while JSON::XS/2 uses the OO interface with 1229 functional interface, while JSON::XS/2 uses the OO interface with
495 pretty-printing and hashkey sorting enabled). Higher is better: 1230 pretty-printing and hashkey sorting enabled, JSON::XS/3 enables shrink.
1231 JSON::DWIW/DS uses the deserialise function, while JSON::DWIW::FJ uses
1232 the from_json method). Higher is better:
496 1233
497 module | encode | decode | 1234 module | encode | decode |
498 -----------|------------|------------| 1235 --------------|------------|------------|
499 JSON | 11488.516 | 7823.035 | 1236 JSON::DWIW/DS | 86302.551 | 102300.098 |
500 JSON::DWIW | 94708.054 | 129094.260 | 1237 JSON::DWIW/FJ | 86302.551 | 75983.768 |
501 JSON::PC | 63884.157 | 128528.212 | 1238 JSON::PP | 15827.562 | 6638.658 |
502 JSON::Syck | 34898.677 | 42096.911 | 1239 JSON::Syck | 63358.066 | 47662.545 |
503 JSON::XS | 654027.064 | 396423.669 | 1240 JSON::XS | 511500.488 | 511500.488 |
504 JSON::XS/2 | 371564.190 | 371725.613 | 1241 JSON::XS/2 | 291271.111 | 388361.481 |
1242 JSON::XS/3 | 361577.931 | 361577.931 |
1243 Storable | 66788.280 | 265462.278 |
505 -----------+------------+------------+ 1244 --------------+------------+------------+
506 1245
507 That is, JSON::XS is more than six times faster than JSON::DWIW on 1246 That is, JSON::XS is almost six times faster than JSON::DWIW on
508 encoding, more than three times faster on decoding, and about thirty 1247 encoding, about five times faster on decoding, and over thirty to
509 times faster than JSON, even with pretty-printing and key sorting. 1248 seventy times faster than JSON's pure perl implementation. It also
1249 compares favourably to Storable for small amounts of data.
510 1250
511 Using a longer test string (roughly 18KB, generated from Yahoo! Locals 1251 Using a longer test string (roughly 18KB, generated from Yahoo! Locals
512 search API (http://nanoref.com/yahooapis/mgPdGg): 1252 search API (<http://dist.schmorp.de/misc/json/long.json>).
513 1253
514 module | encode | decode | 1254 module | encode | decode |
515 -----------|------------|------------| 1255 --------------|------------|------------|
516 JSON | 273.023 | 44.674 | 1256 JSON::DWIW/DS | 1647.927 | 2673.916 |
517 JSON::DWIW | 1089.383 | 1145.704 | 1257 JSON::DWIW/FJ | 1630.249 | 2596.128 |
518 JSON::PC | 3097.419 | 2393.921 | 1258 JSON::PP | 400.640 | 62.311 |
519 JSON::Syck | 514.060 | 843.053 | 1259 JSON::Syck | 1481.040 | 1524.869 |
520 JSON::XS | 6479.668 | 3636.364 | 1260 JSON::XS | 20661.596 | 9541.183 |
521 JSON::XS/2 | 3774.221 | 3599.124 | 1261 JSON::XS/2 | 10683.403 | 9416.938 |
1262 JSON::XS/3 | 20661.596 | 9400.054 |
1263 Storable | 19765.806 | 10000.725 |
522 -----------+------------+------------+ 1264 --------------+------------+------------+
523 1265
524 Again, JSON::XS leads by far. 1266 Again, JSON::XS leads by far (except for Storable which non-surprisingly
1267 decodes a bit faster).
525 1268
526 On large strings containing lots of high unicode characters, some 1269 On large strings containing lots of high Unicode characters, some
527 modules (such as JSON::PC) seem to decode faster than JSON::XS, but the 1270 modules (such as JSON::PC) seem to decode faster than JSON::XS, but the
528 result will be broken due to missing (or wrong) unicode handling. Others 1271 result will be broken due to missing (or wrong) Unicode handling. Others
529 refuse to decode or encode properly, so it was impossible to prepare a 1272 refuse to decode or encode properly, so it was impossible to prepare a
530 fair comparison table for that case. 1273 fair comparison table for that case.
531 1274
532SECURITY CONSIDERATIONS 1275SECURITY CONSIDERATIONS
533 When you are using JSON in a protocol, talking to untrusted potentially 1276 When you are using JSON in a protocol, talking to untrusted potentially
… …
537 have any buffer overflows. Obviously, this module should ensure that and 1280 have any buffer overflows. Obviously, this module should ensure that and
538 I am trying hard on making that true, but you never know. 1281 I am trying hard on making that true, but you never know.
539 1282
540 Second, you need to avoid resource-starving attacks. That means you 1283 Second, you need to avoid resource-starving attacks. That means you
541 should limit the size of JSON texts you accept, or make sure then when 1284 should limit the size of JSON texts you accept, or make sure then when
542 your resources run out, thats just fine (e.g. by using a separate 1285 your resources run out, that's just fine (e.g. by using a separate
543 process that can crash safely). The size of a JSON text in octets or 1286 process that can crash safely). The size of a JSON text in octets or
544 characters is usually a good indication of the size of the resources 1287 characters is usually a good indication of the size of the resources
545 required to decode it into a Perl structure. 1288 required to decode it into a Perl structure. While JSON::XS can check
1289 the size of the JSON text, it might be too late when you already have it
1290 in memory, so you might want to check the size before you accept the
1291 string.
546 1292
547 Third, JSON::XS recurses using the C stack when decoding objects and 1293 Third, JSON::XS recurses using the C stack when decoding objects and
548 arrays. The C stack is a limited resource: for instance, on my amd64 1294 arrays. The C stack is a limited resource: for instance, on my amd64
549 machine with 8MB of stack size I can decode around 180k nested arrays 1295 machine with 8MB of stack size I can decode around 180k nested arrays
550 but only 14k nested JSON objects. If that is exceeded, the program 1296 but only 14k nested JSON objects (due to perl itself recursing deeply on
1297 croak to free the temporary). If that is exceeded, the program crashes.
551 crashes. Thats why the default nesting limit is set to 8192. If your 1298 To be conservative, the default nesting limit is set to 512. If your
552 process has a smaller stack, you should adjust this setting accordingly 1299 process has a smaller stack, you should adjust this setting accordingly
553 with the "max_depth" method. 1300 with the "max_depth" method.
554 1301
555 And last but least, something else could bomb you that I forgot to think 1302 Something else could bomb you, too, that I forgot to think of. In that
556 of. In that case, you get to keep the pieces. I am alway sopen for 1303 case, you get to keep the pieces. I am always open for hints, though...
557 hints, though... 1304
1305 Also keep in mind that JSON::XS might leak contents of your Perl data
1306 structures in its error messages, so when you serialise sensitive
1307 information you might want to make sure that exceptions thrown by
1308 JSON::XS will not end up in front of untrusted eyes.
1309
1310 If you are using JSON::XS to return packets to consumption by JavaScript
1311 scripts in a browser you should have a look at
1312 <http://blog.archive.jpsykes.com/47/practical-csrf-and-json-security/>
1313 to see whether you are vulnerable to some common attack vectors (which
1314 really are browser design bugs, but it is still you who will have to
1315 deal with it, as major browser developers care only for features, not
1316 about getting security right).
1317
1318THREADS
1319 This module is *not* guaranteed to be thread safe and there are no plans
1320 to change this until Perl gets thread support (as opposed to the
1321 horribly slow so-called "threads" which are simply slow and bloated
1322 process simulations - use fork, it's *much* faster, cheaper, better).
1323
1324 (It might actually work, but you have been warned).
1325
1326THE PERILS OF SETLOCALE
1327 Sometimes people avoid the Perl locale support and directly call the
1328 system's setlocale function with "LC_ALL".
1329
1330 This breaks both perl and modules such as JSON::XS, as stringification
1331 of numbers no longer works correcly (e.g. "$x = 0.1; print "$x"+1" might
1332 print 1, and JSON::XS might output illegal JSON as JSON::XS relies on
1333 perl to stringify numbers).
1334
1335 The solution is simple: don't call "setlocale", or use it for only those
1336 categories you need, such as "LC_MESSAGES" or "LC_CTYPE".
1337
1338 If you need "LC_NUMERIC", you should enable it only around the code that
1339 actually needs it (avoiding stringification of numbers), and restore it
1340 afterwards.
558 1341
559BUGS 1342BUGS
560 While the goal of this module is to be correct, that unfortunately does 1343 While the goal of this module is to be correct, that unfortunately does
561 not mean its bug-free, only that I think its design is bug-free. It is 1344 not mean it's bug-free, only that I think its design is bug-free. If you
562 still relatively early in its development. If you keep reporting bugs
563 they will be fixed swiftly, though. 1345 keep reporting bugs they will be fixed swiftly, though.
1346
1347 Please refrain from using rt.cpan.org or any other bug reporting
1348 service. I put the contact address into my modules for a reason.
1349
1350SEE ALSO
1351 The json_xs command line utility for quick experiments.
564 1352
565AUTHOR 1353AUTHOR
566 Marc Lehmann <schmorp@schmorp.de> 1354 Marc Lehmann <schmorp@schmorp.de>
567 http://home.schmorp.de/ 1355 http://home.schmorp.de/
568 1356

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