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Revision 1.18 by root, Tue Aug 28 02:06:06 2007 UTC

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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 6
7 # exported functions, croak on error 7 # exported functions, they croak on error
8 # and expect/generate UTF-8
8 9
9 $utf8_encoded_json_text = to_json $perl_hash_or_arrayref; 10 $utf8_encoded_json_text = to_json $perl_hash_or_arrayref;
10 $perl_hash_or_arrayref = from_json $utf8_encoded_json_text; 11 $perl_hash_or_arrayref = from_json $utf8_encoded_json_text;
11 12
12 # oo-interface 13 # OO-interface
13 14
14 $coder = JSON::XS->new->ascii->pretty->allow_nonref; 15 $coder = JSON::XS->new->ascii->pretty->allow_nonref;
15 $pretty_printed_unencoded = $coder->encode ($perl_scalar); 16 $pretty_printed_unencoded = $coder->encode ($perl_scalar);
16 $perl_scalar = $coder->decode ($unicode_json_text); 17 $perl_scalar = $coder->decode ($unicode_json_text);
17 18
… …
30 31
31 See MAPPING, below, on how JSON::XS maps perl values to JSON values and 32 See MAPPING, below, on how JSON::XS maps perl values to JSON values and
32 vice versa. 33 vice versa.
33 34
34 FEATURES 35 FEATURES
35 * correct handling of unicode issues 36 * correct unicode handling
36 This module knows how to handle Unicode, and even documents how and 37 This module knows how to handle Unicode, and even documents how and
37 when it does so. 38 when it does so.
38 39
39 * round-trip integrity 40 * round-trip integrity
40 When you serialise a perl data structure using only datatypes 41 When you serialise a perl data structure using only datatypes
41 supported by JSON, the deserialised data structure is identical on 42 supported by JSON, the deserialised data structure is identical on
42 the Perl level. (e.g. the string "2.0" doesn't suddenly become "2"). 43 the Perl level. (e.g. the string "2.0" doesn't suddenly become "2"
44 just because it looks like a number).
43 45
44 * strict checking of JSON correctness 46 * strict checking of JSON correctness
45 There is no guessing, no generating of illegal JSON texts by 47 There is no guessing, no generating of illegal JSON texts by
46 default, and only JSON is accepted as input by default (the latter 48 default, and only JSON is accepted as input by default (the latter
47 is a security feature). 49 is a security feature).
… …
55 interface. 57 interface.
56 58
57 * reasonably versatile output formats 59 * reasonably versatile output formats
58 You can choose between the most compact guarenteed single-line 60 You can choose between the most compact guarenteed single-line
59 format possible (nice for simple line-based protocols), a pure-ascii 61 format possible (nice for simple line-based protocols), a pure-ascii
60 format (for when your transport is not 8-bit clean), or a 62 format (for when your transport is not 8-bit clean, still supports
61 pretty-printed format (for when you want to read that stuff). Or you 63 the whole unicode range), or a pretty-printed format (for when you
62 can combine those features in whatever way you like. 64 want to read that stuff). Or you can combine those features in
65 whatever way you like.
63 66
64FUNCTIONAL INTERFACE 67FUNCTIONAL INTERFACE
65 The following convinience methods are provided by this module. They are 68 The following convinience methods are provided by this module. They are
66 exported by default: 69 exported by default:
67 70
… …
84 This function call is functionally identical to: 87 This function call is functionally identical to:
85 88
86 $perl_scalar = JSON::XS->new->utf8->decode ($json_text) 89 $perl_scalar = JSON::XS->new->utf8->decode ($json_text)
87 90
88 except being faster. 91 except being faster.
92
93 $is_boolean = JSON::XS::is_bool $scalar
94 Returns true if the passed scalar represents either JSON::XS::true
95 or JSON::XS::false, two constants that act like 1 and 0,
96 respectively and are used to represent JSON "true" and "false"
97 values in Perl.
98
99 See MAPPING, below, for more information on how JSON values are
100 mapped to Perl.
89 101
90OBJECT-ORIENTED INTERFACE 102OBJECT-ORIENTED INTERFACE
91 The object oriented interface lets you configure your own encoding or 103 The object oriented interface lets you configure your own encoding or
92 decoding style, within the limits of supported formats. 104 decoding style, within the limits of supported formats.
93 105
… …
105 $json = $json->ascii ([$enable]) 117 $json = $json->ascii ([$enable])
106 If $enable is true (or missing), then the "encode" method will not 118 If $enable is true (or missing), then the "encode" method will not
107 generate characters outside the code range 0..127 (which is ASCII). 119 generate characters outside the code range 0..127 (which is ASCII).
108 Any unicode characters outside that range will be escaped using 120 Any unicode characters outside that range will be escaped using
109 either a single \uXXXX (BMP characters) or a double \uHHHH\uLLLLL 121 either a single \uXXXX (BMP characters) or a double \uHHHH\uLLLLL
110 escape sequence, as per RFC4627. 122 escape sequence, as per RFC4627. The resulting encoded JSON text can
123 be treated as a native unicode string, an ascii-encoded,
124 latin1-encoded or UTF-8 encoded string, or any other superset of
125 ASCII.
111 126
112 If $enable is false, then the "encode" method will not escape 127 If $enable is false, then the "encode" method will not escape
113 Unicode characters unless required by the JSON syntax. This results 128 Unicode characters unless required by the JSON syntax or other
114 in a faster and more compact format. 129 flags. This results in a faster and more compact format.
130
131 The main use for this flag is to produce JSON texts that can be
132 transmitted over a 7-bit channel, as the encoded JSON texts will not
133 contain any 8 bit characters.
115 134
116 JSON::XS->new->ascii (1)->encode ([chr 0x10401]) 135 JSON::XS->new->ascii (1)->encode ([chr 0x10401])
117 => ["\ud801\udc01"] 136 => ["\ud801\udc01"]
137
138 $json = $json->latin1 ([$enable])
139 If $enable is true (or missing), then the "encode" method will
140 encode the resulting JSON text as latin1 (or iso-8859-1), escaping
141 any characters outside the code range 0..255. The resulting string
142 can be treated as a latin1-encoded JSON text or a native unicode
143 string. The "decode" method will not be affected in any way by this
144 flag, as "decode" by default expects unicode, which is a strict
145 superset of latin1.
146
147 If $enable is false, then the "encode" method will not escape
148 Unicode characters unless required by the JSON syntax or other
149 flags.
150
151 The main use for this flag is efficiently encoding binary data as
152 JSON text, as most octets will not be escaped, resulting in a
153 smaller encoded size. The disadvantage is that the resulting JSON
154 text is encoded in latin1 (and must correctly be treated as such
155 when storing and transfering), a rare encoding for JSON. It is
156 therefore most useful when you want to store data structures known
157 to contain binary data efficiently in files or databases, not when
158 talking to other JSON encoders/decoders.
159
160 JSON::XS->new->latin1->encode (["\x{89}\x{abc}"]
161 => ["\x{89}\\u0abc"] # (perl syntax, U+abc escaped, U+89 not)
118 162
119 $json = $json->utf8 ([$enable]) 163 $json = $json->utf8 ([$enable])
120 If $enable is true (or missing), then the "encode" method will 164 If $enable is true (or missing), then the "encode" method will
121 encode the JSON result into UTF-8, as required by many protocols, 165 encode the JSON result into UTF-8, as required by many protocols,
122 while the "decode" method expects to be handled an UTF-8-encoded 166 while the "decode" method expects to be handled an UTF-8-encoded
… …
196 240
197 Example, space_before and indent disabled, space_after enabled: 241 Example, space_before and indent disabled, space_after enabled:
198 242
199 {"key": "value"} 243 {"key": "value"}
200 244
245 $json = $json->relaxed ([$enable])
246 If $enable is true (or missing), then "decode" will accept some
247 extensions to normal JSON syntax (see below). "encode" will not be
248 affected in anyway. *Be aware that this option makes you accept
249 invalid JSON texts as if they were valid!*. I suggest only to use
250 this option to parse application-specific files written by humans
251 (configuration files, resource files etc.)
252
253 If $enable is false (the default), then "decode" will only accept
254 valid JSON texts.
255
256 Currently accepted extensions are:
257
258 * list items can have an end-comma
259 JSON *separates* array elements and key-value pairs with commas.
260 This can be annoying if you write JSON texts manually and want
261 to be able to quickly append elements, so this extension accepts
262 comma at the end of such items not just between them:
263
264 [
265 1,
266 2, <- this comma not normally allowed
267 ]
268 {
269 "k1": "v1",
270 "k2": "v2", <- this comma not normally allowed
271 }
272
273 * shell-style '#'-comments
274 Whenever JSON allows whitespace, shell-style comments are
275 additionally allowed. They are terminated by the first
276 carriage-return or line-feed character, after which more
277 white-space and comments are allowed.
278
279 [
280 1, # this comment not allowed in JSON
281 # neither this one...
282 ]
283
201 $json = $json->canonical ([$enable]) 284 $json = $json->canonical ([$enable])
202 If $enable is true (or missing), then the "encode" method will 285 If $enable is true (or missing), then the "encode" method will
203 output JSON objects by sorting their keys. This is adding a 286 output JSON objects by sorting their keys. This is adding a
204 comparatively high overhead. 287 comparatively high overhead.
205 288
… …
229 Example, encode a Perl scalar as JSON value with enabled 312 Example, encode a Perl scalar as JSON value with enabled
230 "allow_nonref", resulting in an invalid JSON text: 313 "allow_nonref", resulting in an invalid JSON text:
231 314
232 JSON::XS->new->allow_nonref->encode ("Hello, World!") 315 JSON::XS->new->allow_nonref->encode ("Hello, World!")
233 => "Hello, World!" 316 => "Hello, World!"
317
318 $json = $json->allow_blessed ([$enable])
319 If $enable is true (or missing), then the "encode" method will not
320 barf when it encounters a blessed reference. Instead, the value of
321 the convert_blessed option will decide wether "null"
322 ("convert_blessed" disabled or no "to_json" method found) or a
323 representation of the object ("convert_blessed" enabled and
324 "to_json" method found) is being encoded. Has no effect on "decode".
325
326 If $enable is false (the default), then "encode" will throw an
327 exception when it encounters a blessed object.
328
329 $json = $json->convert_blessed ([$enable])
330 If $enable is true (or missing), then "encode", upon encountering a
331 blessed object, will check for the availability of the "TO_JSON"
332 method on the object's class. If found, it will be called in scalar
333 context and the resulting scalar will be encoded instead of the
334 object. If no "TO_JSON" method is found, the value of
335 "allow_blessed" will decide what to do.
336
337 The "TO_JSON" method may safely call die if it wants. If "TO_JSON"
338 returns other blessed objects, those will be handled in the same
339 way. "TO_JSON" must take care of not causing an endless recursion
340 cycle (== crash) in this case. The name of "TO_JSON" was chosen
341 because other methods called by the Perl core (== not by the user of
342 the object) are usually in upper case letters and to avoid
343 collisions with the "to_json" function.
344
345 This setting does not yet influence "decode" in any way, but in the
346 future, global hooks might get installed that influence "decode" and
347 are enabled by this setting.
348
349 If $enable is false, then the "allow_blessed" setting will decide
350 what to do when a blessed object is found.
351
352 $json = $json->filter_json_object ([$coderef->($hashref)])
353 When $coderef is specified, it will be called from "decode" each
354 time it decodes a JSON object. The only argument is a reference to
355 the newly-created hash. If the code references returns a single
356 scalar (which need not be a reference), this value (i.e. a copy of
357 that scalar to avoid aliasing) is inserted into the deserialised
358 data structure. If it returns an empty list (NOTE: *not* "undef",
359 which is a valid scalar), the original deserialised hash will be
360 inserted. This setting can slow down decoding considerably.
361
362 When $coderef is omitted or undefined, any existing callback will be
363 removed and "decode" will not change the deserialised hash in any
364 way.
365
366 Example, convert all JSON objects into the integer 5:
367
368 my $js = JSON::XS->new->filter_json_object (sub { 5 });
369 # returns [5]
370 $js->decode ('[{}]')
371 # throw an exception because allow_nonref is not enabled
372 # so a lone 5 is not allowed.
373 $js->decode ('{"a":1, "b":2}');
374
375 $json = $json->filter_json_single_key_object ($key [=>
376 $coderef->($value)])
377 Works remotely similar to "filter_json_object", but is only called
378 for JSON objects having a single key named $key.
379
380 This $coderef is called before the one specified via
381 "filter_json_object", if any. It gets passed the single value in the
382 JSON object. If it returns a single value, it will be inserted into
383 the data structure. If it returns nothing (not even "undef" but the
384 empty list), the callback from "filter_json_object" will be called
385 next, as if no single-key callback were specified.
386
387 If $coderef is omitted or undefined, the corresponding callback will
388 be disabled. There can only ever be one callback for a given key.
389
390 As this callback gets called less often then the
391 "filter_json_object" one, decoding speed will not usually suffer as
392 much. Therefore, single-key objects make excellent targets to
393 serialise Perl objects into, especially as single-key JSON objects
394 are as close to the type-tagged value concept as JSON gets (its
395 basically an ID/VALUE tuple). Of course, JSON does not support this
396 in any way, so you need to make sure your data never looks like a
397 serialised Perl hash.
398
399 Typical names for the single object key are "__class_whatever__", or
400 "$__dollars_are_rarely_used__$" or "}ugly_brace_placement", or even
401 things like "__class_md5sum(classname)__", to reduce the risk of
402 clashing with real hashes.
403
404 Example, decode JSON objects of the form "{ "__widget__" => <id> }"
405 into the corresponding $WIDGET{<id>} object:
406
407 # return whatever is in $WIDGET{5}:
408 JSON::XS
409 ->new
410 ->filter_json_single_key_object (__widget__ => sub {
411 $WIDGET{ $_[0] }
412 })
413 ->decode ('{"__widget__": 5')
414
415 # this can be used with a TO_JSON method in some "widget" class
416 # for serialisation to json:
417 sub WidgetBase::TO_JSON {
418 my ($self) = @_;
419
420 unless ($self->{id}) {
421 $self->{id} = ..get..some..id..;
422 $WIDGET{$self->{id}} = $self;
423 }
424
425 { __widget__ => $self->{id} }
426 }
234 427
235 $json = $json->shrink ([$enable]) 428 $json = $json->shrink ([$enable])
236 Perl usually over-allocates memory a bit when allocating space for 429 Perl usually over-allocates memory a bit when allocating space for
237 strings. This flag optionally resizes strings generated by either 430 strings. This flag optionally resizes strings generated by either
238 "encode" or "decode" to their minimum size possible. This can save 431 "encode" or "decode" to their minimum size possible. This can save
239 memory when your JSON texts are either very very long or you have 432 memory when your JSON texts are either very very long or you have
240 many short strings. It will also try to downgrade any strings to 433 many short strings. It will also try to downgrade any strings to
241 octet-form if possible: perl stores strings internally either in an 434 octet-form if possible: perl stores strings internally either in an
242 encoding called UTF-X or in octet-form. The latter cannot store 435 encoding called UTF-X or in octet-form. The latter cannot store
243 everything but uses less space in general. 436 everything but uses less space in general (and some buggy Perl or C
437 code might even rely on that internal representation being used).
438
439 The actual definition of what shrink does might change in future
440 versions, but it will always try to save space at the expense of
441 time.
244 442
245 If $enable is true (or missing), the string returned by "encode" 443 If $enable is true (or missing), the string returned by "encode"
246 will be shrunk-to-fit, while all strings generated by "decode" will 444 will be shrunk-to-fit, while all strings generated by "decode" will
247 also be shrunk-to-fit. 445 also be shrunk-to-fit.
248 446
… …
251 449
252 In the future, this setting might control other things, such as 450 In the future, this setting might control other things, such as
253 converting strings that look like integers or floats into integers 451 converting strings that look like integers or floats into integers
254 or floats internally (there is no difference on the Perl level), 452 or floats internally (there is no difference on the Perl level),
255 saving space. 453 saving space.
454
455 $json = $json->max_depth ([$maximum_nesting_depth])
456 Sets the maximum nesting level (default 512) accepted while encoding
457 or decoding. If the JSON text or Perl data structure has an equal or
458 higher nesting level then this limit, then the encoder and decoder
459 will stop and croak at that point.
460
461 Nesting level is defined by number of hash- or arrayrefs that the
462 encoder needs to traverse to reach a given point or the number of
463 "{" or "[" characters without their matching closing parenthesis
464 crossed to reach a given character in a string.
465
466 Setting the maximum depth to one disallows any nesting, so that
467 ensures that the object is only a single hash/object or array.
468
469 The argument to "max_depth" will be rounded up to the next highest
470 power of two. If no argument is given, the highest possible setting
471 will be used, which is rarely useful.
472
473 See SECURITY CONSIDERATIONS, below, for more info on why this is
474 useful.
475
476 $json = $json->max_size ([$maximum_string_size])
477 Set the maximum length a JSON text may have (in bytes) where
478 decoding is being attempted. The default is 0, meaning no limit.
479 When "decode" is called on a string longer then this number of
480 characters it will not attempt to decode the string but throw an
481 exception. This setting has no effect on "encode" (yet).
482
483 The argument to "max_size" will be rounded up to the next highest
484 power of two (so may be more than requested). If no argument is
485 given, the limit check will be deactivated (same as when 0 is
486 specified).
487
488 See SECURITY CONSIDERATIONS, below, for more info on why this is
489 useful.
256 490
257 $json_text = $json->encode ($perl_scalar) 491 $json_text = $json->encode ($perl_scalar)
258 Converts the given Perl data structure (a simple scalar or a 492 Converts the given Perl data structure (a simple scalar or a
259 reference to a hash or array) to its JSON representation. Simple 493 reference to a hash or array) to its JSON representation. Simple
260 scalars will be converted into JSON string or number sequences, 494 scalars will be converted into JSON string or number sequences,
… …
269 503
270 JSON numbers and strings become simple Perl scalars. JSON arrays 504 JSON numbers and strings become simple Perl scalars. JSON arrays
271 become Perl arrayrefs and JSON objects become Perl hashrefs. "true" 505 become Perl arrayrefs and JSON objects become Perl hashrefs. "true"
272 becomes 1, "false" becomes 0 and "null" becomes "undef". 506 becomes 1, "false" becomes 0 and "null" becomes "undef".
273 507
508 ($perl_scalar, $characters) = $json->decode_prefix ($json_text)
509 This works like the "decode" method, but instead of raising an
510 exception when there is trailing garbage after the first JSON
511 object, it will silently stop parsing there and return the number of
512 characters consumed so far.
513
514 This is useful if your JSON texts are not delimited by an outer
515 protocol (which is not the brightest thing to do in the first place)
516 and you need to know where the JSON text ends.
517
518 JSON::XS->new->decode_prefix ("[1] the tail")
519 => ([], 3)
520
274MAPPING 521MAPPING
275 This section describes how JSON::XS maps Perl values to JSON values and 522 This section describes how JSON::XS maps Perl values to JSON values and
276 vice versa. These mappings are designed to "do the right thing" in most 523 vice versa. These mappings are designed to "do the right thing" in most
277 circumstances automatically, preserving round-tripping characteristics 524 circumstances automatically, preserving round-tripping characteristics
278 (what you put in comes out as something equivalent). 525 (what you put in comes out as something equivalent).
… …
294 A JSON string becomes a string scalar in Perl - Unicode codepoints 541 A JSON string becomes a string scalar in Perl - Unicode codepoints
295 in JSON are represented by the same codepoints in the Perl string, 542 in JSON are represented by the same codepoints in the Perl string,
296 so no manual decoding is necessary. 543 so no manual decoding is necessary.
297 544
298 number 545 number
299 A JSON number becomes either an integer or numeric (floating point) 546 A JSON number becomes either an integer, numeric (floating point) or
300 scalar in perl, depending on its range and any fractional parts. On 547 string scalar in perl, depending on its range and any fractional
301 the Perl level, there is no difference between those as Perl handles 548 parts. On the Perl level, there is no difference between those as
302 all the conversion details, but an integer may take slightly less 549 Perl handles all the conversion details, but an integer may take
303 memory and might represent more values exactly than (floating point) 550 slightly less memory and might represent more values exactly than
304 numbers. 551 (floating point) numbers.
552
553 If the number consists of digits only, JSON::XS will try to
554 represent it as an integer value. If that fails, it will try to
555 represent it as a numeric (floating point) value if that is possible
556 without loss of precision. Otherwise it will preserve the number as
557 a string value.
558
559 Numbers containing a fractional or exponential part will always be
560 represented as numeric (floating point) values, possibly at a loss
561 of precision.
562
563 This might create round-tripping problems as numbers might become
564 strings, but as Perl is typeless there is no other way to do it.
305 565
306 true, false 566 true, false
307 These JSON atoms become 0, 1, respectively. Information is lost in 567 These JSON atoms become "JSON::XS::true" and "JSON::XS::false",
308 this process. Future versions might represent those values 568 respectively. They are overloaded to act almost exactly like the
309 differently, but they will be guarenteed to act like these integers 569 numbers 1 and 0. You can check wether a scalar is a JSON boolean by
310 would normally in Perl. 570 using the "JSON::XS::is_bool" function.
311 571
312 null 572 null
313 A JSON null atom becomes "undef" in Perl. 573 A JSON null atom becomes "undef" in Perl.
314 574
315 PERL -> JSON 575 PERL -> JSON
… …
317 truly typeless language, so we can only guess which JSON type is meant 577 truly typeless language, so we can only guess which JSON type is meant
318 by a Perl value. 578 by a Perl value.
319 579
320 hash references 580 hash references
321 Perl hash references become JSON objects. As there is no inherent 581 Perl hash references become JSON objects. As there is no inherent
322 ordering in hash keys, they will usually be encoded in a 582 ordering in hash keys (or JSON objects), they will usually be
323 pseudo-random order that can change between runs of the same program 583 encoded in a pseudo-random order that can change between runs of the
324 but stays generally the same within a single run of a program. 584 same program but stays generally the same within a single run of a
325 JSON::XS can optionally sort the hash keys (determined by the 585 program. JSON::XS can optionally sort the hash keys (determined by
326 *canonical* flag), so the same datastructure will serialise to the 586 the *canonical* flag), so the same datastructure will serialise to
327 same JSON text (given same settings and version of JSON::XS), but 587 the same JSON text (given same settings and version of JSON::XS),
328 this incurs a runtime overhead. 588 but this incurs a runtime overhead and is only rarely useful, e.g.
589 when you want to compare some JSON text against another for
590 equality.
329 591
330 array references 592 array references
331 Perl array references become JSON arrays. 593 Perl array references become JSON arrays.
594
595 other references
596 Other unblessed references are generally not allowed and will cause
597 an exception to be thrown, except for references to the integers 0
598 and 1, which get turned into "false" and "true" atoms in JSON. You
599 can also use "JSON::XS::false" and "JSON::XS::true" to improve
600 readability.
601
602 to_json [\0,JSON::XS::true] # yields [false,true]
603
604 JSON::XS::true, JSON::XS::false
605 These special values become JSON true and JSON false values,
606 respectively. You cna alos use "\1" and "\0" directly if you want.
332 607
333 blessed objects 608 blessed objects
334 Blessed objects are not allowed. JSON::XS currently tries to encode 609 Blessed objects are not allowed. JSON::XS currently tries to encode
335 their underlying representation (hash- or arrayref), but this 610 their underlying representation (hash- or arrayref), but this
336 behaviour might change in future versions. 611 behaviour might change in future versions.
… …
367 $x += 0; # numify it, ensuring it will be dumped as a number 642 $x += 0; # numify it, ensuring it will be dumped as a number
368 $x *= 1; # same thing, the choise is yours. 643 $x *= 1; # same thing, the choise is yours.
369 644
370 You can not currently output JSON booleans or force the type in 645 You can not currently output JSON booleans or force the type in
371 other, less obscure, ways. Tell me if you need this capability. 646 other, less obscure, ways. Tell me if you need this capability.
372
373 circular data structures
374 Those will be encoded until memory or stackspace runs out.
375 647
376COMPARISON 648COMPARISON
377 As already mentioned, this module was created because none of the 649 As already mentioned, this module was created because none of the
378 existing JSON modules could be made to work correctly. First I will 650 existing JSON modules could be made to work correctly. First I will
379 describe the problems (or pleasures) I encountered with various existing 651 describe the problems (or pleasures) I encountered with various existing
… …
450 Does not generate valid JSON texts (key strings are often unquoted, 722 Does not generate valid JSON texts (key strings are often unquoted,
451 empty keys result in nothing being output) 723 empty keys result in nothing being output)
452 724
453 Does not check input for validity. 725 Does not check input for validity.
454 726
727 JSON and YAML
728 You often hear that JSON is a subset (or a close subset) of YAML. This
729 is, however, a mass hysteria and very far from the truth. In general,
730 there is no way to configure JSON::XS to output a data structure as
731 valid YAML.
732
733 If you really must use JSON::XS to generate YAML, you should use this
734 algorithm (subject to change in future versions):
735
736 my $to_yaml = JSON::XS->new->utf8->space_after (1);
737 my $yaml = $to_yaml->encode ($ref) . "\n";
738
739 This will usually generate JSON texts that also parse as valid YAML.
740 Please note that YAML has hardcoded limits on (simple) object key
741 lengths that JSON doesn't have, so you should make sure that your hash
742 keys are noticably shorter than the 1024 characters YAML allows.
743
744 There might be other incompatibilities that I am not aware of. In
745 general you should not try to generate YAML with a JSON generator or
746 vice versa, or try to parse JSON with a YAML parser or vice versa:
747 chances are high that you will run into severe interoperability
748 problems.
749
455 SPEED 750 SPEED
456 It seems that JSON::XS is surprisingly fast, as shown in the following 751 It seems that JSON::XS is surprisingly fast, as shown in the following
457 tables. They have been generated with the help of the "eg/bench" program 752 tables. They have been generated with the help of the "eg/bench" program
458 in the JSON::XS distribution, to make it easy to compare on your own 753 in the JSON::XS distribution, to make it easy to compare on your own
459 system. 754 system.
460 755
461 First comes a comparison between various modules using a very short JSON 756 First comes a comparison between various modules using a very short
462 string: 757 single-line JSON string:
463 758
464 {"method": "handleMessage", "params": ["user1", "we were just talking"], "id": null} 759 {"method": "handleMessage", "params": ["user1", "we were just talking"], \
760 "id": null, "array":[1,11,234,-5,1e5,1e7, true, false]}
465 761
466 It shows the number of encodes/decodes per second (JSON::XS uses the 762 It shows the number of encodes/decodes per second (JSON::XS uses the
467 functional interface, while JSON::XS/2 uses the OO interface with 763 functional interface, while JSON::XS/2 uses the OO interface with
468 pretty-printing and hashkey sorting enabled). Higher is better: 764 pretty-printing and hashkey sorting enabled, JSON::XS/3 enables shrink).
765 Higher is better:
469 766
767 Storable | 15779.925 | 14169.946 |
768 -----------+------------+------------+
470 module | encode | decode | 769 module | encode | decode |
471 -----------|------------|------------| 770 -----------|------------|------------|
472 JSON | 11488.516 | 7823.035 | 771 JSON | 4990.842 | 4088.813 |
473 JSON::DWIW | 94708.054 | 129094.260 | 772 JSON::DWIW | 51653.990 | 71575.154 |
474 JSON::PC | 63884.157 | 128528.212 | 773 JSON::PC | 65948.176 | 74631.744 |
774 JSON::PP | 8931.652 | 3817.168 |
475 JSON::Syck | 34898.677 | 42096.911 | 775 JSON::Syck | 24877.248 | 27776.848 |
476 JSON::XS | 654027.064 | 396423.669 | 776 JSON::XS | 388361.481 | 227951.304 |
477 JSON::XS/2 | 371564.190 | 371725.613 | 777 JSON::XS/2 | 227951.304 | 218453.333 |
778 JSON::XS/3 | 338250.323 | 218453.333 |
779 Storable | 16500.016 | 135300.129 |
478 -----------+------------+------------+ 780 -----------+------------+------------+
479 781
480 That is, JSON::XS is more than six times faster than JSON::DWIW on 782 That is, JSON::XS is about five times faster than JSON::DWIW on
481 encoding, more than three times faster on decoding, and about thirty 783 encoding, about three times faster on decoding, and over fourty times
482 times faster than JSON, even with pretty-printing and key sorting. 784 faster than JSON, even with pretty-printing and key sorting. It also
785 compares favourably to Storable for small amounts of data.
483 786
484 Using a longer test string (roughly 18KB, generated from Yahoo! Locals 787 Using a longer test string (roughly 18KB, generated from Yahoo! Locals
485 search API (http://nanoref.com/yahooapis/mgPdGg): 788 search API (http://nanoref.com/yahooapis/mgPdGg):
486 789
487 module | encode | decode | 790 module | encode | decode |
488 -----------|------------|------------| 791 -----------|------------|------------|
489 JSON | 273.023 | 44.674 | 792 JSON | 55.260 | 34.971 |
490 JSON::DWIW | 1089.383 | 1145.704 | 793 JSON::DWIW | 825.228 | 1082.513 |
491 JSON::PC | 3097.419 | 2393.921 | 794 JSON::PC | 3571.444 | 2394.829 |
492 JSON::Syck | 514.060 | 843.053 | 795 JSON::PP | 210.987 | 32.574 |
493 JSON::XS | 6479.668 | 3636.364 | 796 JSON::Syck | 552.551 | 787.544 |
494 JSON::XS/2 | 3774.221 | 3599.124 | 797 JSON::XS | 5780.463 | 4854.519 |
798 JSON::XS/2 | 3869.998 | 4798.975 |
799 JSON::XS/3 | 5862.880 | 4798.975 |
800 Storable | 4445.002 | 5235.027 |
495 -----------+------------+------------+ 801 -----------+------------+------------+
496 802
497 Again, JSON::XS leads by far. 803 Again, JSON::XS leads by far (except for Storable which non-surprisingly
804 decodes faster).
498 805
499 On large strings containing lots of high unicode characters, some 806 On large strings containing lots of high unicode characters, some
500 modules (such as JSON::PC) seem to decode faster than JSON::XS, but the 807 modules (such as JSON::PC) seem to decode faster than JSON::XS, but the
501 result will be broken due to missing (or wrong) unicode handling. Others 808 result will be broken due to missing (or wrong) unicode handling. Others
502 refuse to decode or encode properly, so it was impossible to prepare a 809 refuse to decode or encode properly, so it was impossible to prepare a
503 fair comparison table for that case. 810 fair comparison table for that case.
504 811
505RESOURCE LIMITS 812SECURITY CONSIDERATIONS
506 JSON::XS does not impose any limits on the size of JSON texts or Perl 813 When you are using JSON in a protocol, talking to untrusted potentially
507 values they represent - if your machine can handle it, JSON::XS will 814 hostile creatures requires relatively few measures.
508 encode or decode it. Future versions might optionally impose structure 815
509 depth and memory use resource limits. 816 First of all, your JSON decoder should be secure, that is, should not
817 have any buffer overflows. Obviously, this module should ensure that and
818 I am trying hard on making that true, but you never know.
819
820 Second, you need to avoid resource-starving attacks. That means you
821 should limit the size of JSON texts you accept, or make sure then when
822 your resources run out, thats just fine (e.g. by using a separate
823 process that can crash safely). The size of a JSON text in octets or
824 characters is usually a good indication of the size of the resources
825 required to decode it into a Perl structure. While JSON::XS can check
826 the size of the JSON text, it might be too late when you already have it
827 in memory, so you might want to check the size before you accept the
828 string.
829
830 Third, JSON::XS recurses using the C stack when decoding objects and
831 arrays. The C stack is a limited resource: for instance, on my amd64
832 machine with 8MB of stack size I can decode around 180k nested arrays
833 but only 14k nested JSON objects (due to perl itself recursing deeply on
834 croak to free the temporary). If that is exceeded, the program crashes.
835 to be conservative, the default nesting limit is set to 512. If your
836 process has a smaller stack, you should adjust this setting accordingly
837 with the "max_depth" method.
838
839 And last but least, something else could bomb you that I forgot to think
840 of. In that case, you get to keep the pieces. I am always open for
841 hints, though...
842
843 If you are using JSON::XS to return packets to consumption by javascript
844 scripts in a browser you should have a look at
845 <http://jpsykes.com/47/practical-csrf-and-json-security> to see wether
846 you are vulnerable to some common attack vectors (which really are
847 browser design bugs, but it is still you who will have to deal with it,
848 as major browser developers care only for features, not about doing
849 security right).
510 850
511BUGS 851BUGS
512 While the goal of this module is to be correct, that unfortunately does 852 While the goal of this module is to be correct, that unfortunately does
513 not mean its bug-free, only that I think its design is bug-free. It is 853 not mean its bug-free, only that I think its design is bug-free. It is
514 still very young and not well-tested. If you keep reporting bugs they 854 still relatively early in its development. If you keep reporting bugs
515 will be fixed swiftly, though. 855 they will be fixed swiftly, though.
516 856
517AUTHOR 857AUTHOR
518 Marc Lehmann <schmorp@schmorp.de> 858 Marc Lehmann <schmorp@schmorp.de>
519 http://home.schmorp.de/ 859 http://home.schmorp.de/
520 860

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