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Revision 1.14 by root, Sat Jun 23 23:50:03 2007 UTC

1NAME 1NAME
2 JSON::XS - JSON serialising/deserialising, done correctly and fast 2 JSON::XS - JSON serialising/deserialising, done correctly and fast
3 3
4SYNOPSIS 4SYNOPSIS
5 use JSON::XS; 5 use JSON::XS;
6
7 # exported functions, they croak on error
8 # and expect/generate UTF-8
9
10 $utf8_encoded_json_text = to_json $perl_hash_or_arrayref;
11 $perl_hash_or_arrayref = from_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
17 # OO-interface
18
19 $coder = JSON::XS->new->ascii->pretty->allow_nonref;
20 $pretty_printed_unencoded = $coder->encode ($perl_scalar);
21 $perl_scalar = $coder->decode ($unicode_json_text);
6 22
7DESCRIPTION 23DESCRIPTION
8 This module converts Perl data structures to JSON and vice versa. Its 24 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*. 25 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. 26 To reach the latter goal it was written in C.
… …
15 cases their maintainers are unresponsive, gone missing, or not listening 31 cases their maintainers are unresponsive, gone missing, or not listening
16 to bug reports for other reasons. 32 to bug reports for other reasons.
17 33
18 See COMPARISON, below, for a comparison to some other JSON modules. 34 See COMPARISON, below, for a comparison to some other JSON modules.
19 35
36 See MAPPING, below, on how JSON::XS maps perl values to JSON values and
37 vice versa.
38
20 FEATURES 39 FEATURES
21 * correct handling of unicode issues 40 * correct unicode handling
22 This module knows how to handle Unicode, and even documents how it 41 This module knows how to handle Unicode, and even documents how and
23 does so. 42 when it does so.
24 43
25 * round-trip integrity 44 * round-trip integrity
26 When you serialise a perl data structure using only datatypes 45 When you serialise a perl data structure using only datatypes
27 supported by JSON, the deserialised data structure is identical on 46 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"). 47 the Perl level. (e.g. the string "2.0" doesn't suddenly become "2"
48 just because it looks like a number).
29 49
30 * strict checking of JSON correctness 50 * strict checking of JSON correctness
31 There is no guessing, no generating of illegal JSON strings by 51 There is no guessing, no generating of illegal JSON texts by
32 default, and only JSON is accepted as input (the latter is a 52 default, and only JSON is accepted as input by default (the latter
33 security feature). 53 is a security feature).
34 54
35 * fast 55 * fast
36 compared to other JSON modules, this module compares favourably. 56 Compared to other JSON modules, this module compares favourably in
57 terms of speed, too.
37 58
38 * simple to use 59 * simple to use
39 This module has both a simple functional interface as well as an OO 60 This module has both a simple functional interface as well as an OO
40 interface. 61 interface.
41 62
42 * reasonably versatile output formats 63 * reasonably versatile output formats
43 You can choose between the most compact format possible, a 64 You can choose between the most compact guarenteed single-line
44 pure-ascii format, or a pretty-printed format. Or you can combine 65 format possible (nice for simple line-based protocols), a pure-ascii
66 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
68 want to read that stuff). Or you can combine those features in
45 those features in whatever way you like. 69 whatever way you like.
46 70
47FUNCTIONAL INTERFACE 71FUNCTIONAL INTERFACE
48 The following convinience methods are provided by this module. They are 72 The following convinience methods are provided by this module. They are
49 exported by default: 73 exported by default:
50 74
51 $json_string = to_json $perl_scalar 75 $json_text = to_json $perl_scalar
52 Converts the given Perl data structure (a simple scalar or a 76 Converts the given Perl data structure (a simple scalar or a
53 reference to a hash or array) to a UTF-8 encoded, binary string 77 reference to a hash or array) to a UTF-8 encoded, binary string
54 (that is, the string contains octets only). Croaks on error. 78 (that is, the string contains octets only). Croaks on error.
55 79
56 This function call is functionally identical to "JSON::XS->new->utf8 80 This function call is functionally identical to:
57 (1)->encode ($perl_scalar)".
58 81
82 $json_text = JSON::XS->new->utf8->encode ($perl_scalar)
83
84 except being faster.
85
59 $perl_scalar = from_json $json_string 86 $perl_scalar = from_json $json_text
60 The opposite of "to_json": expects an UTF-8 (binary) string and 87 The opposite of "to_json": expects an UTF-8 (binary) string and
61 tries to parse that as an UTF-8 encoded JSON string, returning the 88 tries to parse that as an UTF-8 encoded JSON text, returning the
62 resulting simple scalar or reference. Croaks on error. 89 resulting simple scalar or reference. Croaks on error.
63 90
64 This function call is functionally identical to "JSON::XS->new->utf8 91 This function call is functionally identical to:
65 (1)->decode ($json_string)". 92
93 $perl_scalar = JSON::XS->new->utf8->decode ($json_text)
94
95 except being faster.
96
97 $is_boolean = JSON::XS::is_bool $scalar
98 Returns true if the passed scalar represents either JSON::XS::true
99 or JSON::XS::false, two constants that act like 1 and 0,
100 respectively and are used to represent JSON "true" and "false"
101 values in Perl.
102
103 See MAPPING, below, for more information on how JSON values are
104 mapped to Perl.
66 105
67OBJECT-ORIENTED INTERFACE 106OBJECT-ORIENTED INTERFACE
68 The object oriented interface lets you configure your own encoding or 107 The object oriented interface lets you configure your own encoding or
69 decoding style, within the limits of supported formats. 108 decoding style, within the limits of supported formats.
70 109
… …
74 *disabled*. 113 *disabled*.
75 114
76 The mutators for flags all return the JSON object again and thus 115 The mutators for flags all return the JSON object again and thus
77 calls can be chained: 116 calls can be chained:
78 117
79 my $json = JSON::XS->new->utf8(1)->space_after(1)->encode ({a => [1,2]}) 118 my $json = JSON::XS->new->utf8->space_after->encode ({a => [1,2]})
80 => {"a": [1, 2]} 119 => {"a": [1, 2]}
81 120
82 $json = $json->ascii ($enable) 121 $json = $json->ascii ([$enable])
83 If $enable is true, then the "encode" method will not generate 122 If $enable is true (or missing), then the "encode" method will not
84 characters outside the code range 0..127. Any unicode characters 123 generate characters outside the code range 0..127 (which is ASCII).
85 outside that range will be escaped using either a single \uXXXX (BMP 124 Any unicode characters outside that range will be escaped using
86 characters) or a double \uHHHH\uLLLLL escape sequence, as per 125 either a single \uXXXX (BMP characters) or a double \uHHHH\uLLLLL
87 RFC4627. 126 escape sequence, as per RFC4627. The resulting encoded JSON text can
127 be treated as a native unicode string, an ascii-encoded,
128 latin1-encoded or UTF-8 encoded string, or any other superset of
129 ASCII.
88 130
89 If $enable is false, then the "encode" method will not escape 131 If $enable is false, then the "encode" method will not escape
90 Unicode characters unless necessary. 132 Unicode characters unless required by the JSON syntax or other
133 flags. This results in a faster and more compact format.
91 134
135 The main use for this flag is to produce JSON texts that can be
136 transmitted over a 7-bit channel, as the encoded JSON texts will not
137 contain any 8 bit characters.
138
92 JSON::XS->new->ascii (1)->encode (chr 0x10401) 139 JSON::XS->new->ascii (1)->encode ([chr 0x10401])
93 => \ud801\udc01 140 => ["\ud801\udc01"]
94 141
142 $json = $json->latin1 ([$enable])
143 If $enable is true (or missing), then the "encode" method will
144 encode the resulting JSON text as latin1 (or iso-8859-1), escaping
145 any characters outside the code range 0..255. The resulting string
146 can be treated as a latin1-encoded JSON text or a native unicode
147 string. The "decode" method will not be affected in any way by this
148 flag, as "decode" by default expects unicode, which is a strict
149 superset of latin1.
150
151 If $enable is false, then the "encode" method will not escape
152 Unicode characters unless required by the JSON syntax or other
153 flags.
154
155 The main use for this flag is efficiently encoding binary data as
156 JSON text, as most octets will not be escaped, resulting in a
157 smaller encoded size. The disadvantage is that the resulting JSON
158 text is encoded in latin1 (and must correctly be treated as such
159 when storing and transfering), a rare encoding for JSON. It is
160 therefore most useful when you want to store data structures known
161 to contain binary data efficiently in files or databases, not when
162 talking to other JSON encoders/decoders.
163
164 JSON::XS->new->latin1->encode (["\x{89}\x{abc}"]
165 => ["\x{89}\\u0abc"] # (perl syntax, U+abc escaped, U+89 not)
166
95 $json = $json->utf8 ($enable) 167 $json = $json->utf8 ([$enable])
96 If $enable is true, then the "encode" method will encode the JSON 168 If $enable is true (or missing), then the "encode" method will
97 string into UTF-8, as required by many protocols, while the "decode" 169 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 170 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 171 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. 172 characters outside the range 0..255, they are thus useful for
173 bytewise/binary I/O. In future versions, enabling this option might
174 enable autodetection of the UTF-16 and UTF-32 encoding families, as
175 described in RFC4627.
101 176
102 If $enable is false, then the "encode" method will return the JSON 177 If $enable is false, then the "encode" method will return the JSON
103 string as a (non-encoded) unicode string, while "decode" expects 178 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 179 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. 180 UTF-16) needs to be done yourself, e.g. using the Encode module.
106 181
182 Example, output UTF-16BE-encoded JSON:
183
184 use Encode;
185 $jsontext = encode "UTF-16BE", JSON::XS->new->encode ($object);
186
187 Example, decode UTF-32LE-encoded JSON:
188
189 use Encode;
190 $object = JSON::XS->new->decode (decode "UTF-32LE", $jsontext);
191
107 $json = $json->pretty ($enable) 192 $json = $json->pretty ([$enable])
108 This enables (or disables) all of the "indent", "space_before" and 193 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 194 "space_after" (and in the future possibly more) flags in one call to
110 generate the most readable (or most compact) form possible. 195 generate the most readable (or most compact) form possible.
196
197 Example, pretty-print some simple structure:
111 198
112 my $json = JSON::XS->new->pretty(1)->encode ({a => [1,2]}) 199 my $json = JSON::XS->new->pretty(1)->encode ({a => [1,2]})
113 => 200 =>
114 { 201 {
115 "a" : [ 202 "a" : [
116 1, 203 1,
117 2 204 2
118 ] 205 ]
119 } 206 }
120 207
121 $json = $json->indent ($enable) 208 $json = $json->indent ([$enable])
122 If $enable is true, then the "encode" method will use a multiline 209 If $enable is true (or missing), then the "encode" method will use a
123 format as output, putting every array member or object/hash 210 multiline format as output, putting every array member or
124 key-value pair into its own line, identing them properly. 211 object/hash key-value pair into its own line, identing them
212 properly.
125 213
126 If $enable is false, no newlines or indenting will be produced, and 214 If $enable is false, no newlines or indenting will be produced, and
127 the resulting JSON strings is guarenteed not to contain any 215 the resulting JSON text is guarenteed not to contain any "newlines".
128 "newlines".
129 216
130 This setting has no effect when decoding JSON strings. 217 This setting has no effect when decoding JSON texts.
131 218
132 $json = $json->space_before ($enable) 219 $json = $json->space_before ([$enable])
133 If $enable is true, then the "encode" method will add an extra 220 If $enable is true (or missing), then the "encode" method will add
134 optional space before the ":" separating keys from values in JSON 221 an extra optional space before the ":" separating keys from values
135 objects. 222 in JSON objects.
136 223
137 If $enable is false, then the "encode" method will not add any extra 224 If $enable is false, then the "encode" method will not add any extra
138 space at those places. 225 space at those places.
139 226
140 This setting has no effect when decoding JSON strings. You will also 227 This setting has no effect when decoding JSON texts. You will also
141 most likely combine this setting with "space_after". 228 most likely combine this setting with "space_after".
142 229
230 Example, space_before enabled, space_after and indent disabled:
231
232 {"key" :"value"}
233
143 $json = $json->space_after ($enable) 234 $json = $json->space_after ([$enable])
144 If $enable is true, then the "encode" method will add an extra 235 If $enable is true (or missing), then the "encode" method will add
145 optional space after the ":" separating keys from values in JSON 236 an extra optional space after the ":" separating keys from values in
146 objects and extra whitespace after the "," separating key-value 237 JSON objects and extra whitespace after the "," separating key-value
147 pairs and array members. 238 pairs and array members.
148 239
149 If $enable is false, then the "encode" method will not add any extra 240 If $enable is false, then the "encode" method will not add any extra
150 space at those places. 241 space at those places.
151 242
152 This setting has no effect when decoding JSON strings. 243 This setting has no effect when decoding JSON texts.
153 244
245 Example, space_before and indent disabled, space_after enabled:
246
247 {"key": "value"}
248
154 $json = $json->canonical ($enable) 249 $json = $json->canonical ([$enable])
155 If $enable is true, then the "encode" method will output JSON 250 If $enable is true (or missing), then the "encode" method will
156 objects by sorting their keys. This is adding a comparatively high 251 output JSON objects by sorting their keys. This is adding a
157 overhead. 252 comparatively high overhead.
158 253
159 If $enable is false, then the "encode" method will output key-value 254 If $enable is false, then the "encode" method will output key-value
160 pairs in the order Perl stores them (which will likely change 255 pairs in the order Perl stores them (which will likely change
161 between runs of the same script). 256 between runs of the same script).
162 257
163 This option is useful if you want the same data structure to be 258 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). 259 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 260 it is disabled, the same hash migh be encoded differently even if
166 contains the same data, as key-value pairs have no inherent ordering 261 contains the same data, as key-value pairs have no inherent ordering
167 in Perl. 262 in Perl.
168 263
169 This setting has no effect when decoding JSON strings. 264 This setting has no effect when decoding JSON texts.
170 265
171 $json = $json->allow_nonref ($enable) 266 $json = $json->allow_nonref ([$enable])
172 If $enable is true, then the "encode" method can convert a 267 If $enable is true (or missing), then the "encode" method can
173 non-reference into its corresponding string, number or null JSON 268 convert a non-reference into its corresponding string, number or
174 value, which is an extension to RFC4627. Likewise, "decode" will 269 null JSON value, which is an extension to RFC4627. Likewise,
175 accept those JSON values instead of croaking. 270 "decode" will accept those JSON values instead of croaking.
176 271
177 If $enable is false, then the "encode" method will croak if it isn't 272 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 273 passed an arrayref or hashref, as JSON texts must either be an
179 object or array. Likewise, "decode" will croak if given something 274 object or array. Likewise, "decode" will croak if given something
180 that is not a JSON object or array. 275 that is not a JSON object or array.
181 276
277 Example, encode a Perl scalar as JSON value with enabled
278 "allow_nonref", resulting in an invalid JSON text:
279
280 JSON::XS->new->allow_nonref->encode ("Hello, World!")
281 => "Hello, World!"
282
283 $json = $json->shrink ([$enable])
284 Perl usually over-allocates memory a bit when allocating space for
285 strings. This flag optionally resizes strings generated by either
286 "encode" or "decode" to their minimum size possible. This can save
287 memory when your JSON texts are either very very long or you have
288 many short strings. It will also try to downgrade any strings to
289 octet-form if possible: perl stores strings internally either in an
290 encoding called UTF-X or in octet-form. The latter cannot store
291 everything but uses less space in general (and some buggy Perl or C
292 code might even rely on that internal representation being used).
293
294 The actual definition of what shrink does might change in future
295 versions, but it will always try to save space at the expense of
296 time.
297
298 If $enable is true (or missing), the string returned by "encode"
299 will be shrunk-to-fit, while all strings generated by "decode" will
300 also be shrunk-to-fit.
301
302 If $enable is false, then the normal perl allocation algorithms are
303 used. If you work with your data, then this is likely to be faster.
304
305 In the future, this setting might control other things, such as
306 converting strings that look like integers or floats into integers
307 or floats internally (there is no difference on the Perl level),
308 saving space.
309
310 $json = $json->max_depth ([$maximum_nesting_depth])
311 Sets the maximum nesting level (default 512) accepted while encoding
312 or decoding. If the JSON text or Perl data structure has an equal or
313 higher nesting level then this limit, then the encoder and decoder
314 will stop and croak at that point.
315
316 Nesting level is defined by number of hash- or arrayrefs that the
317 encoder needs to traverse to reach a given point or the number of
318 "{" or "[" characters without their matching closing parenthesis
319 crossed to reach a given character in a string.
320
321 Setting the maximum depth to one disallows any nesting, so that
322 ensures that the object is only a single hash/object or array.
323
324 The argument to "max_depth" will be rounded up to the next nearest
325 power of two.
326
327 See SECURITY CONSIDERATIONS, below, for more info on why this is
328 useful.
329
182 $json_string = $json->encode ($perl_scalar) 330 $json_text = $json->encode ($perl_scalar)
183 Converts the given Perl data structure (a simple scalar or a 331 Converts the given Perl data structure (a simple scalar or a
184 reference to a hash or array) to its JSON representation. Simple 332 reference to a hash or array) to its JSON representation. Simple
185 scalars will be converted into JSON string or number sequences, 333 scalars will be converted into JSON string or number sequences,
186 while references to arrays become JSON arrays and references to 334 while references to arrays become JSON arrays and references to
187 hashes become JSON objects. Undefined Perl values (e.g. "undef") 335 hashes become JSON objects. Undefined Perl values (e.g. "undef")
188 become JSON "null" values. Neither "true" nor "false" values will be 336 become JSON "null" values. Neither "true" nor "false" values will be
189 generated. 337 generated.
190 338
191 $perl_scalar = $json->decode ($json_string) 339 $perl_scalar = $json->decode ($json_text)
192 The opposite of "encode": expects a JSON string and tries to parse 340 The opposite of "encode": expects a JSON text and tries to parse it,
193 it, returning the resulting simple scalar or reference. Croaks on 341 returning the resulting simple scalar or reference. Croaks on error.
194 error.
195 342
196 JSON numbers and strings become simple Perl scalars. JSON arrays 343 JSON numbers and strings become simple Perl scalars. JSON arrays
197 become Perl arrayrefs and JSON objects become Perl hashrefs. "true" 344 become Perl arrayrefs and JSON objects become Perl hashrefs. "true"
198 becomes 1, "false" becomes 0 and "null" becomes "undef". 345 becomes 1, "false" becomes 0 and "null" becomes "undef".
346
347 ($perl_scalar, $characters) = $json->decode_prefix ($json_text)
348 This works like the "decode" method, but instead of raising an
349 exception when there is trailing garbage after the first JSON
350 object, it will silently stop parsing there and return the number of
351 characters consumed so far.
352
353 This is useful if your JSON texts are not delimited by an outer
354 protocol (which is not the brightest thing to do in the first place)
355 and you need to know where the JSON text ends.
356
357 JSON::XS->new->decode_prefix ("[1] the tail")
358 => ([], 3)
359
360MAPPING
361 This section describes how JSON::XS maps Perl values to JSON values and
362 vice versa. These mappings are designed to "do the right thing" in most
363 circumstances automatically, preserving round-tripping characteristics
364 (what you put in comes out as something equivalent).
365
366 For the more enlightened: note that in the following descriptions,
367 lowercase *perl* refers to the Perl interpreter, while uppcercase *Perl*
368 refers to the abstract Perl language itself.
369
370 JSON -> PERL
371 object
372 A JSON object becomes a reference to a hash in Perl. No ordering of
373 object keys is preserved (JSON does not preserver object key
374 ordering itself).
375
376 array
377 A JSON array becomes a reference to an array in Perl.
378
379 string
380 A JSON string becomes a string scalar in Perl - Unicode codepoints
381 in JSON are represented by the same codepoints in the Perl string,
382 so no manual decoding is necessary.
383
384 number
385 A JSON number becomes either an integer or numeric (floating point)
386 scalar in perl, depending on its range and any fractional parts. On
387 the Perl level, there is no difference between those as Perl handles
388 all the conversion details, but an integer may take slightly less
389 memory and might represent more values exactly than (floating point)
390 numbers.
391
392 true, false
393 These JSON atoms become "JSON::XS::true" and "JSON::XS::false",
394 respectively. They are overloaded to act almost exactly like the
395 numbers 1 and 0. You can check wether a scalar is a JSON boolean by
396 using the "JSON::XS::is_bool" function.
397
398 null
399 A JSON null atom becomes "undef" in Perl.
400
401 PERL -> JSON
402 The mapping from Perl to JSON is slightly more difficult, as Perl is a
403 truly typeless language, so we can only guess which JSON type is meant
404 by a Perl value.
405
406 hash references
407 Perl hash references become JSON objects. As there is no inherent
408 ordering in hash keys (or JSON objects), they will usually be
409 encoded in a pseudo-random order that can change between runs of the
410 same program but stays generally the same within a single run of a
411 program. JSON::XS can optionally sort the hash keys (determined by
412 the *canonical* flag), so the same datastructure will serialise to
413 the same JSON text (given same settings and version of JSON::XS),
414 but this incurs a runtime overhead and is only rarely useful, e.g.
415 when you want to compare some JSON text against another for
416 equality.
417
418 array references
419 Perl array references become JSON arrays.
420
421 other references
422 Other unblessed references are generally not allowed and will cause
423 an exception to be thrown, except for references to the integers 0
424 and 1, which get turned into "false" and "true" atoms in JSON. You
425 can also use "JSON::XS::false" and "JSON::XS::true" to improve
426 readability.
427
428 to_json [\0,JSON::XS::true] # yields [false,true]
429
430 JSON::XS::true, JSON::XS::false
431 These special values become JSON true and JSON false values,
432 respectively. You cna alos use "\1" and "\0" directly if you want.
433
434 blessed objects
435 Blessed objects are not allowed. JSON::XS currently tries to encode
436 their underlying representation (hash- or arrayref), but this
437 behaviour might change in future versions.
438
439 simple scalars
440 Simple Perl scalars (any scalar that is not a reference) are the
441 most difficult objects to encode: JSON::XS will encode undefined
442 scalars as JSON null value, scalars that have last been used in a
443 string context before encoding as JSON strings and anything else as
444 number value:
445
446 # dump as number
447 to_json [2] # yields [2]
448 to_json [-3.0e17] # yields [-3e+17]
449 my $value = 5; to_json [$value] # yields [5]
450
451 # used as string, so dump as string
452 print $value;
453 to_json [$value] # yields ["5"]
454
455 # undef becomes null
456 to_json [undef] # yields [null]
457
458 You can force the type to be a string by stringifying it:
459
460 my $x = 3.1; # some variable containing a number
461 "$x"; # stringified
462 $x .= ""; # another, more awkward way to stringify
463 print $x; # perl does it for you, too, quite often
464
465 You can force the type to be a number by numifying it:
466
467 my $x = "3"; # some variable containing a string
468 $x += 0; # numify it, ensuring it will be dumped as a number
469 $x *= 1; # same thing, the choise is yours.
470
471 You can not currently output JSON booleans or force the type in
472 other, less obscure, ways. Tell me if you need this capability.
199 473
200COMPARISON 474COMPARISON
201 As already mentioned, this module was created because none of the 475 As already mentioned, this module was created because none of the
202 existing JSON modules could be made to work correctly. First I will 476 existing JSON modules could be made to work correctly. First I will
203 describe the problems (or pleasures) I encountered with various existing 477 describe the problems (or pleasures) I encountered with various existing
… …
225 499
226 Has problems handling many Perl values (e.g. regex results and other 500 Has problems handling many Perl values (e.g. regex results and other
227 magic values will make it croak). 501 magic values will make it croak).
228 502
229 Does not even generate valid JSON ("{1,2}" gets converted to "{1:2}" 503 Does not even generate valid JSON ("{1,2}" gets converted to "{1:2}"
230 which is not a valid JSON string. 504 which is not a valid JSON text.
231 505
232 Unmaintained (maintainer unresponsive for many months, bugs are not 506 Unmaintained (maintainer unresponsive for many months, bugs are not
233 getting fixed). 507 getting fixed).
234 508
235 JSON::Syck 0.21 509 JSON::Syck 0.21
236 Very buggy (often crashes). 510 Very buggy (often crashes).
237 511
238 Very inflexible (no human-readable format supported, format pretty 512 Very inflexible (no human-readable format supported, format pretty
239 much undocumented. I need at least a format for easy reading by 513 much undocumented. I need at least a format for easy reading by
240 humans and a single-line compact format for use in a protocol, and 514 humans and a single-line compact format for use in a protocol, and
241 preferably a way to generate ASCII-only JSON strings). 515 preferably a way to generate ASCII-only JSON texts).
242 516
243 Completely broken (and confusingly documented) Unicode handling 517 Completely broken (and confusingly documented) Unicode handling
244 (unicode escapes are not working properly, you need to set 518 (unicode escapes are not working properly, you need to set
245 ImplicitUnicode to *different* values on en- and decoding to get 519 ImplicitUnicode to *different* values on en- and decoding to get
246 symmetric behaviour). 520 symmetric behaviour).
… …
269 543
270 Very inflexible. 544 Very inflexible.
271 545
272 No roundtripping. 546 No roundtripping.
273 547
274 Does not generate valid JSON (key strings are often unquoted, empty 548 Does not generate valid JSON texts (key strings are often unquoted,
275 keys result in nothing being output) 549 empty keys result in nothing being output)
276 550
277 Does not check input for validity. 551 Does not check input for validity.
552
553 JSON and YAML
554 You often hear that JSON is a subset (or a close subset) of YAML. This
555 is, however, a mass hysteria and very far from the truth. In general,
556 there is no way to configure JSON::XS to output a data structure as
557 valid YAML.
558
559 If you really must use JSON::XS to generate YAML, you should use this
560 algorithm (subject to change in future versions):
561
562 my $to_yaml = JSON::XS->new->utf8->space_after (1);
563 my $yaml = $to_yaml->encode ($ref) . "\n";
564
565 This will usually generate JSON texts that also parse as valid YAML.
566 Please note that YAML has hardcoded limits on (simple) object key
567 lengths that JSON doesn't have, so you should make sure that your hash
568 keys are noticably shorter than the 1024 characters YAML allows.
569
570 There might be other incompatibilities that I am not aware of. In
571 general you should not try to generate YAML with a JSON generator or
572 vice versa, or try to parse JSON with a YAML parser or vice versa:
573 chances are high that you will run into severe interoperability
574 problems.
278 575
279 SPEED 576 SPEED
280 It seems that JSON::XS is surprisingly fast, as shown in the following 577 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 578 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 579 in the JSON::XS distribution, to make it easy to compare on your own
283 system. 580 system.
284 581
285 First is a comparison between various modules using a very simple JSON 582 First comes a comparison between various modules using a very short
583 single-line JSON string:
584
585 {"method": "handleMessage", "params": ["user1", "we were just talking"], \
586 "id": null, "array":[1,11,234,-5,1e5,1e7, true, false]}
587
286 string, showing the number of encodes/decodes per second (JSON::XS is 588 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 589 functional interface, while JSON::XS/2 uses the OO interface with
288 pretty-printing and hashkey sorting enabled). 590 pretty-printing and hashkey sorting enabled, JSON::XS/3 enables shrink).
591 Higher is better:
289 592
290 module | encode | decode | 593 module | encode | decode |
291 -----------|------------|------------| 594 -----------|------------|------------|
292 JSON | 14006 | 6820 | 595 JSON | 7645.468 | 4208.613 |
293 JSON::DWIW | 200937 | 120386 | 596 JSON::DWIW | 40721.398 | 77101.176 |
294 JSON::PC | 85065 | 129366 | 597 JSON::PC | 65948.176 | 78251.940 |
295 JSON::Syck | 59898 | 44232 | 598 JSON::Syck | 22844.793 | 26479.192 |
296 JSON::XS | 1171478 | 342435 | 599 JSON::XS | 388361.481 | 199728.762 |
297 JSON::XS/2 | 730760 | 328714 | 600 JSON::XS/2 | 218453.333 | 192399.266 |
601 JSON::XS/3 | 338250.323 | 192399.266 |
602 Storable | 15779.925 | 14169.946 |
298 -----------+------------+------------+ 603 -----------+------------+------------+
299 604
300 That is, JSON::XS is 6 times faster than than JSON::DWIW and about 80 605 That is, JSON::XS is about five times faster than JSON::DWIW on
606 encoding, about three times faster on decoding, and over fourty times
301 times faster than JSON, even with pretty-printing and key sorting. 607 faster than JSON, even with pretty-printing and key sorting. It also
608 compares favourably to Storable for small amounts of data.
302 609
303 Using a longer test string (roughly 8KB, generated from Yahoo! Locals 610 Using a longer test string (roughly 18KB, generated from Yahoo! Locals
304 search API (http://nanoref.com/yahooapis/mgPdGg): 611 search API (http://nanoref.com/yahooapis/mgPdGg):
305 612
306 module | encode | decode | 613 module | encode | decode |
307 -----------|------------|------------| 614 -----------|------------|------------|
308 JSON | 673 | 38 | 615 JSON | 254.685 | 37.665 |
309 JSON::DWIW | 5271 | 770 | 616 JSON::DWIW | 843.343 | 1049.731 |
310 JSON::PC | 9901 | 2491 | 617 JSON::PC | 3602.116 | 2307.352 |
311 JSON::Syck | 2360 | 786 | 618 JSON::Syck | 505.107 | 787.899 |
312 JSON::XS | 37398 | 3202 | 619 JSON::XS | 5747.196 | 3690.220 |
313 JSON::XS/2 | 13765 | 3153 | 620 JSON::XS/2 | 3968.121 | 3676.634 |
621 JSON::XS/3 | 6105.246 | 3662.508 |
622 Storable | 4417.337 | 5285.161 |
314 -----------+------------+------------+ 623 -----------+------------+------------+
315 624
316 Again, JSON::XS leads by far in the encoding case, while still beating 625 Again, JSON::XS leads by far (except for Storable which non-surprisingly
317 every other module in the decoding case. 626 decodes faster).
318 627
319 Last example is an almost 8MB large hash with many large binary values 628 On large strings containing lots of high unicode characters, some
320 (PNG files), resulting in a lot of escaping: 629 modules (such as JSON::PC) seem to decode faster than JSON::XS, but the
630 result will be broken due to missing (or wrong) unicode handling. Others
631 refuse to decode or encode properly, so it was impossible to prepare a
632 fair comparison table for that case.
633
634SECURITY CONSIDERATIONS
635 When you are using JSON in a protocol, talking to untrusted potentially
636 hostile creatures requires relatively few measures.
637
638 First of all, your JSON decoder should be secure, that is, should not
639 have any buffer overflows. Obviously, this module should ensure that and
640 I am trying hard on making that true, but you never know.
641
642 Second, you need to avoid resource-starving attacks. That means you
643 should limit the size of JSON texts you accept, or make sure then when
644 your resources run out, thats just fine (e.g. by using a separate
645 process that can crash safely). The size of a JSON text in octets or
646 characters is usually a good indication of the size of the resources
647 required to decode it into a Perl structure.
648
649 Third, JSON::XS recurses using the C stack when decoding objects and
650 arrays. The C stack is a limited resource: for instance, on my amd64
651 machine with 8MB of stack size I can decode around 180k nested arrays
652 but only 14k nested JSON objects (due to perl itself recursing deeply on
653 croak to free the temporary). If that is exceeded, the program crashes.
654 to be conservative, the default nesting limit is set to 512. If your
655 process has a smaller stack, you should adjust this setting accordingly
656 with the "max_depth" method.
657
658 And last but least, something else could bomb you that I forgot to think
659 of. In that case, you get to keep the pieces. I am always open for
660 hints, though...
661
662 If you are using JSON::XS to return packets to consumption by javascript
663 scripts in a browser you should have a look at
664 <http://jpsykes.com/47/practical-csrf-and-json-security> to see wether
665 you are vulnerable to some common attack vectors (which really are
666 browser design bugs, but it is still you who will have to deal with it,
667 as major browser developers care only for features, not about doing
668 security right).
321 669
322BUGS 670BUGS
323 While the goal of this module is to be correct, that unfortunately does 671 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 672 not mean its 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 673 still relatively early in its development. If you keep reporting bugs
326 will be fixed swiftly, though. 674 they will be fixed swiftly, though.
327 675
328AUTHOR 676AUTHOR
329 Marc Lehmann <schmorp@schmorp.de> 677 Marc Lehmann <schmorp@schmorp.de>
330 http://home.schmorp.de/ 678 http://home.schmorp.de/
331 679

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