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1=head1 NAME
2
3JSON::XS - JSON serialising/deserialising, done correctly and fast
4
1=encoding utf-8 5=encoding utf-8
2
3=head1 NAME
4
5JSON::XS - JSON serialising/deserialising, done correctly and fast
6 6
7JSON::XS - 正しくて高速な JSON シリアライザ/デシリアライザ 7JSON::XS - 正しくて高速な JSON シリアライザ/デシリアライザ
8 (http://fleur.hio.jp/perldoc/mix/lib/JSON/XS.html) 8 (http://fleur.hio.jp/perldoc/mix/lib/JSON/XS.html)
9 9
10=head1 SYNOPSIS 10=head1 SYNOPSIS
37primary goal is to be I<correct> and its secondary goal is to be 37primary goal is to be I<correct> and its secondary goal is to be
38I<fast>. To reach the latter goal it was written in C. 38I<fast>. To reach the latter goal it was written in C.
39 39
40Beginning with version 2.0 of the JSON module, when both JSON and 40Beginning with version 2.0 of the JSON module, when both JSON and
41JSON::XS are installed, then JSON will fall back on JSON::XS (this can be 41JSON::XS are installed, then JSON will fall back on JSON::XS (this can be
42overriden) with no overhead due to emulation (by inheritign constructor 42overridden) with no overhead due to emulation (by inheriting constructor
43and methods). If JSON::XS is not available, it will fall back to the 43and methods). If JSON::XS is not available, it will fall back to the
44compatible JSON::PP module as backend, so using JSON instead of JSON::XS 44compatible JSON::PP module as backend, so using JSON instead of JSON::XS
45gives you a portable JSON API that can be fast when you need and doesn't 45gives you a portable JSON API that can be fast when you need and doesn't
46require a C compiler when that is a problem. 46require a C compiler when that is a problem.
47 47
65This module knows how to handle Unicode, documents how and when it does 65This module knows how to handle Unicode, documents how and when it does
66so, and even documents what "correct" means. 66so, and even documents what "correct" means.
67 67
68=item * round-trip integrity 68=item * round-trip integrity
69 69
70When you serialise a perl data structure using only datatypes supported 70When you serialise a perl data structure using only data types supported
71by JSON, the deserialised data structure is identical on the Perl level. 71by JSON, the deserialised data structure is identical on the Perl level.
72(e.g. the string "2.0" doesn't suddenly become "2" just because it looks 72(e.g. the string "2.0" doesn't suddenly become "2" just because it looks
73like a number). There minor I<are> exceptions to this, read the MAPPING 73like a number). There minor I<are> exceptions to this, read the MAPPING
74section below to learn about those. 74section below to learn about those.
75 75
84Compared to other JSON modules and other serialisers such as Storable, 84Compared to other JSON modules and other serialisers such as Storable,
85this module usually compares favourably in terms of speed, too. 85this module usually compares favourably in terms of speed, too.
86 86
87=item * simple to use 87=item * simple to use
88 88
89This module has both a simple functional interface as well as an objetc 89This module has both a simple functional interface as well as an object
90oriented interface interface. 90oriented interface interface.
91 91
92=item * reasonably versatile output formats 92=item * reasonably versatile output formats
93 93
94You can choose between the most compact guaranteed-single-line format 94You can choose between the most compact guaranteed-single-line format
95possible (nice for simple line-based protocols), a pure-ascii format 95possible (nice for simple line-based protocols), a pure-ASCII format
96(for when your transport is not 8-bit clean, still supports the whole 96(for when your transport is not 8-bit clean, still supports the whole
97Unicode range), or a pretty-printed format (for when you want to read that 97Unicode range), or a pretty-printed format (for when you want to read that
98stuff). Or you can combine those features in whatever way you like. 98stuff). Or you can combine those features in whatever way you like.
99 99
100=back 100=back
103 103
104package JSON::XS; 104package JSON::XS;
105 105
106use strict; 106use strict;
107 107
108our $VERSION = '2.01'; 108our $VERSION = '2.21';
109our @ISA = qw(Exporter); 109our @ISA = qw(Exporter);
110 110
111our @EXPORT = qw(encode_json decode_json to_json from_json); 111our @EXPORT = qw(encode_json decode_json to_json from_json);
112 112
113sub to_json($) { 113sub to_json($) {
137 137
138This function call is functionally identical to: 138This function call is functionally identical to:
139 139
140 $json_text = JSON::XS->new->utf8->encode ($perl_scalar) 140 $json_text = JSON::XS->new->utf8->encode ($perl_scalar)
141 141
142except being faster. 142Except being faster.
143 143
144=item $perl_scalar = decode_json $json_text 144=item $perl_scalar = decode_json $json_text
145 145
146The opposite of C<encode_json>: expects an UTF-8 (binary) string and tries 146The opposite of C<encode_json>: expects an UTF-8 (binary) string and tries
147to parse that as an UTF-8 encoded JSON text, returning the resulting 147to parse that as an UTF-8 encoded JSON text, returning the resulting
149 149
150This function call is functionally identical to: 150This function call is functionally identical to:
151 151
152 $perl_scalar = JSON::XS->new->utf8->decode ($json_text) 152 $perl_scalar = JSON::XS->new->utf8->decode ($json_text)
153 153
154except being faster. 154Except being faster.
155 155
156=item $is_boolean = JSON::XS::is_bool $scalar 156=item $is_boolean = JSON::XS::is_bool $scalar
157 157
158Returns true if the passed scalar represents either JSON::XS::true or 158Returns true if the passed scalar represents either JSON::XS::true or
159JSON::XS::false, two constants that act like C<1> and C<0>, respectively 159JSON::XS::false, two constants that act like C<1> and C<0>, respectively
197 197
198If you didn't know about that flag, just the better, pretend it doesn't 198If you didn't know about that flag, just the better, pretend it doesn't
199exist. 199exist.
200 200
201=item 4. A "Unicode String" is simply a string where each character can be 201=item 4. A "Unicode String" is simply a string where each character can be
202validly interpreted as a Unicode codepoint. 202validly interpreted as a Unicode code point.
203 203
204If you have UTF-8 encoded data, it is no longer a Unicode string, but a 204If you have UTF-8 encoded data, it is no longer a Unicode string, but a
205Unicode string encoded in UTF-8, giving you a binary string. 205Unicode string encoded in UTF-8, giving you a binary string.
206 206
207=item 5. A string containing "high" (> 255) character values is I<not> a UTF-8 string. 207=item 5. A string containing "high" (> 255) character values is I<not> a UTF-8 string.
245 245
246If C<$enable> is false, then the C<encode> method will not escape Unicode 246If C<$enable> is false, then the C<encode> method will not escape Unicode
247characters unless required by the JSON syntax or other flags. This results 247characters unless required by the JSON syntax or other flags. This results
248in a faster and more compact format. 248in a faster and more compact format.
249 249
250See also the section I<ENCODING/CODESET FLAG NOTES> later in this
251document.
252
250The main use for this flag is to produce JSON texts that can be 253The main use for this flag is to produce JSON texts that can be
251transmitted over a 7-bit channel, as the encoded JSON texts will not 254transmitted over a 7-bit channel, as the encoded JSON texts will not
252contain any 8 bit characters. 255contain any 8 bit characters.
253 256
254 JSON::XS->new->ascii (1)->encode ([chr 0x10401]) 257 JSON::XS->new->ascii (1)->encode ([chr 0x10401])
265will not be affected in any way by this flag, as C<decode> by default 268will not be affected in any way by this flag, as C<decode> by default
266expects Unicode, which is a strict superset of latin1. 269expects Unicode, which is a strict superset of latin1.
267 270
268If C<$enable> is false, then the C<encode> method will not escape Unicode 271If C<$enable> is false, then the C<encode> method will not escape Unicode
269characters unless required by the JSON syntax or other flags. 272characters unless required by the JSON syntax or other flags.
273
274See also the section I<ENCODING/CODESET FLAG NOTES> later in this
275document.
270 276
271The main use for this flag is efficiently encoding binary data as JSON 277The main use for this flag is efficiently encoding binary data as JSON
272text, as most octets will not be escaped, resulting in a smaller encoded 278text, as most octets will not be escaped, resulting in a smaller encoded
273size. The disadvantage is that the resulting JSON text is encoded 279size. The disadvantage is that the resulting JSON text is encoded
274in latin1 (and must correctly be treated as such when storing and 280in latin1 (and must correctly be treated as such when storing and
293 299
294If C<$enable> is false, then the C<encode> method will return the JSON 300If C<$enable> is false, then the C<encode> method will return the JSON
295string as a (non-encoded) Unicode string, while C<decode> expects thus a 301string as a (non-encoded) Unicode string, while C<decode> expects thus a
296Unicode string. Any decoding or encoding (e.g. to UTF-8 or UTF-16) needs 302Unicode string. Any decoding or encoding (e.g. to UTF-8 or UTF-16) needs
297to be done yourself, e.g. using the Encode module. 303to be done yourself, e.g. using the Encode module.
304
305See also the section I<ENCODING/CODESET FLAG NOTES> later in this
306document.
298 307
299Example, output UTF-16BE-encoded JSON: 308Example, output UTF-16BE-encoded JSON:
300 309
301 use Encode; 310 use Encode;
302 $jsontext = encode "UTF-16BE", JSON::XS->new->encode ($object); 311 $jsontext = encode "UTF-16BE", JSON::XS->new->encode ($object);
453Example, encode a Perl scalar as JSON value with enabled C<allow_nonref>, 462Example, encode a Perl scalar as JSON value with enabled C<allow_nonref>,
454resulting in an invalid JSON text: 463resulting in an invalid JSON text:
455 464
456 JSON::XS->new->allow_nonref->encode ("Hello, World!") 465 JSON::XS->new->allow_nonref->encode ("Hello, World!")
457 => "Hello, World!" 466 => "Hello, World!"
467
468=item $json = $json->allow_unknown ([$enable])
469
470=item $enabled = $json->get_allow_unknown
471
472If C<$enable> is true (or missing), then C<encode> will I<not> throw an
473exception when it encounters values it cannot represent in JSON (for
474example, filehandles) but instead will encode a JSON C<null> value. Note
475that blessed objects are not included here and are handled separately by
476c<allow_nonref>.
477
478If C<$enable> is false (the default), then C<encode> will throw an
479exception when it encounters anything it cannot encode as JSON.
480
481This option does not affect C<decode> in any way, and it is recommended to
482leave it off unless you know your communications partner.
458 483
459=item $json = $json->allow_blessed ([$enable]) 484=item $json = $json->allow_blessed ([$enable])
460 485
461=item $enabled = $json->get_allow_blessed 486=item $enabled = $json->get_allow_blessed
462 487
603=item $json = $json->max_depth ([$maximum_nesting_depth]) 628=item $json = $json->max_depth ([$maximum_nesting_depth])
604 629
605=item $max_depth = $json->get_max_depth 630=item $max_depth = $json->get_max_depth
606 631
607Sets the maximum nesting level (default C<512>) accepted while encoding 632Sets the maximum nesting level (default C<512>) accepted while encoding
608or decoding. If the JSON text or Perl data structure has an equal or 633or decoding. If a higher nesting level is detected in JSON text or a Perl
609higher nesting level then this limit, then the encoder and decoder will 634data structure, then the encoder and decoder will stop and croak at that
610stop and croak at that point. 635point.
611 636
612Nesting level is defined by number of hash- or arrayrefs that the encoder 637Nesting level is defined by number of hash- or arrayrefs that the encoder
613needs to traverse to reach a given point or the number of C<{> or C<[> 638needs to traverse to reach a given point or the number of C<{> or C<[>
614characters without their matching closing parenthesis crossed to reach a 639characters without their matching closing parenthesis crossed to reach a
615given character in a string. 640given character in a string.
616 641
617Setting the maximum depth to one disallows any nesting, so that ensures 642Setting the maximum depth to one disallows any nesting, so that ensures
618that the object is only a single hash/object or array. 643that the object is only a single hash/object or array.
619 644
620The argument to C<max_depth> will be rounded up to the next highest power
621of two. If no argument is given, the highest possible setting will be 645If no argument is given, the highest possible setting will be used, which
622used, which is rarely useful. 646is rarely useful.
647
648Note that nesting is implemented by recursion in C. The default value has
649been chosen to be as large as typical operating systems allow without
650crashing.
623 651
624See SECURITY CONSIDERATIONS, below, for more info on why this is useful. 652See SECURITY CONSIDERATIONS, below, for more info on why this is useful.
625 653
626=item $json = $json->max_size ([$maximum_string_size]) 654=item $json = $json->max_size ([$maximum_string_size])
627 655
628=item $max_size = $json->get_max_size 656=item $max_size = $json->get_max_size
629 657
630Set the maximum length a JSON text may have (in bytes) where decoding is 658Set the maximum length a JSON text may have (in bytes) where decoding is
631being attempted. The default is C<0>, meaning no limit. When C<decode> 659being attempted. The default is C<0>, meaning no limit. When C<decode>
632is called on a string longer then this number of characters it will not 660is called on a string that is longer then this many bytes, it will not
633attempt to decode the string but throw an exception. This setting has no 661attempt to decode the string but throw an exception. This setting has no
634effect on C<encode> (yet). 662effect on C<encode> (yet).
635 663
636The argument to C<max_size> will be rounded up to the next B<highest> 664If no argument is given, the limit check will be deactivated (same as when
637power of two (so may be more than requested). If no argument is given, the 665C<0> is specified).
638limit check will be deactivated (same as when C<0> is specified).
639 666
640See SECURITY CONSIDERATIONS, below, for more info on why this is useful. 667See SECURITY CONSIDERATIONS, below, for more info on why this is useful.
641 668
642=item $json_text = $json->encode ($perl_scalar) 669=item $json_text = $json->encode ($perl_scalar)
643 670
670 697
671 JSON::XS->new->decode_prefix ("[1] the tail") 698 JSON::XS->new->decode_prefix ("[1] the tail")
672 => ([], 3) 699 => ([], 3)
673 700
674=back 701=back
702
703
704=head1 INCREMENTAL PARSING
705
706In some cases, there is the need for incremental parsing of JSON
707texts. While this module always has to keep both JSON text and resulting
708Perl data structure in memory at one time, it does allow you to parse a
709JSON stream incrementally. It does so by accumulating text until it has
710a full JSON object, which it then can decode. This process is similar to
711using C<decode_prefix> to see if a full JSON object is available, but is
712much more efficient (JSON::XS will only attempt to parse the JSON text
713once it is sure it has enough text to get a decisive result, using a very
714simple but truly incremental parser).
715
716The following two methods deal with this.
717
718=over 4
719
720=item [void, scalar or list context] = $json->incr_parse ([$string])
721
722This is the central parsing function. It can both append new text and
723extract objects from the stream accumulated so far (both of these
724functions are optional).
725
726If C<$string> is given, then this string is appended to the already
727existing JSON fragment stored in the C<$json> object.
728
729After that, if the function is called in void context, it will simply
730return without doing anything further. This can be used to add more text
731in as many chunks as you want.
732
733If the method is called in scalar context, then it will try to extract
734exactly I<one> JSON object. If that is successful, it will return this
735object, otherwise it will return C<undef>. If there is a parse error,
736this method will croak just as C<decode> would do (one can then use
737C<incr_skip> to skip the errornous part). This is the most common way of
738using the method.
739
740And finally, in list context, it will try to extract as many objects
741from the stream as it can find and return them, or the empty list
742otherwise. For this to work, there must be no separators between the JSON
743objects or arrays, instead they must be concatenated back-to-back. If
744an error occurs, an exception will be raised as in the scalar context
745case. Note that in this case, any previously-parsed JSON texts will be
746lost.
747
748=item $lvalue_string = $json->incr_text
749
750This method returns the currently stored JSON fragment as an lvalue, that
751is, you can manipulate it. This I<only> works when a preceding call to
752C<incr_parse> in I<scalar context> successfully returned an object. Under
753all other circumstances you must not call this function (I mean it.
754although in simple tests it might actually work, it I<will> fail under
755real world conditions). As a special exception, you can also call this
756method before having parsed anything.
757
758This function is useful in two cases: a) finding the trailing text after a
759JSON object or b) parsing multiple JSON objects separated by non-JSON text
760(such as commas).
761
762=item $json->incr_skip
763
764This will reset the state of the incremental parser and will remove the
765parsed text from the input buffer. This is useful after C<incr_parse>
766died, in which case the input buffer and incremental parser state is left
767unchanged, to skip the text parsed so far and to reset the parse state.
768
769=item $json->incr_reset
770
771This completely resets the incremental parser, that is, after this call,
772it will be as if the parser had never parsed anything.
773
774This is useful if you want ot repeatedly parse JSON objects and want to
775ignore any trailing data, which means you have to reset the parser after
776each successful decode.
777
778=back
779
780=head2 LIMITATIONS
781
782All options that affect decoding are supported, except
783C<allow_nonref>. The reason for this is that it cannot be made to
784work sensibly: JSON objects and arrays are self-delimited, i.e. you can concatenate
785them back to back and still decode them perfectly. This does not hold true
786for JSON numbers, however.
787
788For example, is the string C<1> a single JSON number, or is it simply the
789start of C<12>? Or is C<12> a single JSON number, or the concatenation
790of C<1> and C<2>? In neither case you can tell, and this is why JSON::XS
791takes the conservative route and disallows this case.
792
793=head2 EXAMPLES
794
795Some examples will make all this clearer. First, a simple example that
796works similarly to C<decode_prefix>: We want to decode the JSON object at
797the start of a string and identify the portion after the JSON object:
798
799 my $text = "[1,2,3] hello";
800
801 my $json = new JSON::XS;
802
803 my $obj = $json->incr_parse ($text)
804 or die "expected JSON object or array at beginning of string";
805
806 my $tail = $json->incr_text;
807 # $tail now contains " hello"
808
809Easy, isn't it?
810
811Now for a more complicated example: Imagine a hypothetical protocol where
812you read some requests from a TCP stream, and each request is a JSON
813array, without any separation between them (in fact, it is often useful to
814use newlines as "separators", as these get interpreted as whitespace at
815the start of the JSON text, which makes it possible to test said protocol
816with C<telnet>...).
817
818Here is how you'd do it (it is trivial to write this in an event-based
819manner):
820
821 my $json = new JSON::XS;
822
823 # read some data from the socket
824 while (sysread $socket, my $buf, 4096) {
825
826 # split and decode as many requests as possible
827 for my $request ($json->incr_parse ($buf)) {
828 # act on the $request
829 }
830 }
831
832Another complicated example: Assume you have a string with JSON objects
833or arrays, all separated by (optional) comma characters (e.g. C<[1],[2],
834[3]>). To parse them, we have to skip the commas between the JSON texts,
835and here is where the lvalue-ness of C<incr_text> comes in useful:
836
837 my $text = "[1],[2], [3]";
838 my $json = new JSON::XS;
839
840 # void context, so no parsing done
841 $json->incr_parse ($text);
842
843 # now extract as many objects as possible. note the
844 # use of scalar context so incr_text can be called.
845 while (my $obj = $json->incr_parse) {
846 # do something with $obj
847
848 # now skip the optional comma
849 $json->incr_text =~ s/^ \s* , //x;
850 }
851
852Now lets go for a very complex example: Assume that you have a gigantic
853JSON array-of-objects, many gigabytes in size, and you want to parse it,
854but you cannot load it into memory fully (this has actually happened in
855the real world :).
856
857Well, you lost, you have to implement your own JSON parser. But JSON::XS
858can still help you: You implement a (very simple) array parser and let
859JSON decode the array elements, which are all full JSON objects on their
860own (this wouldn't work if the array elements could be JSON numbers, for
861example):
862
863 my $json = new JSON::XS;
864
865 # open the monster
866 open my $fh, "<bigfile.json"
867 or die "bigfile: $!";
868
869 # first parse the initial "["
870 for (;;) {
871 sysread $fh, my $buf, 65536
872 or die "read error: $!";
873 $json->incr_parse ($buf); # void context, so no parsing
874
875 # Exit the loop once we found and removed(!) the initial "[".
876 # In essence, we are (ab-)using the $json object as a simple scalar
877 # we append data to.
878 last if $json->incr_text =~ s/^ \s* \[ //x;
879 }
880
881 # now we have the skipped the initial "[", so continue
882 # parsing all the elements.
883 for (;;) {
884 # in this loop we read data until we got a single JSON object
885 for (;;) {
886 if (my $obj = $json->incr_parse) {
887 # do something with $obj
888 last;
889 }
890
891 # add more data
892 sysread $fh, my $buf, 65536
893 or die "read error: $!";
894 $json->incr_parse ($buf); # void context, so no parsing
895 }
896
897 # in this loop we read data until we either found and parsed the
898 # separating "," between elements, or the final "]"
899 for (;;) {
900 # first skip whitespace
901 $json->incr_text =~ s/^\s*//;
902
903 # if we find "]", we are done
904 if ($json->incr_text =~ s/^\]//) {
905 print "finished.\n";
906 exit;
907 }
908
909 # if we find ",", we can continue with the next element
910 if ($json->incr_text =~ s/^,//) {
911 last;
912 }
913
914 # if we find anything else, we have a parse error!
915 if (length $json->incr_text) {
916 die "parse error near ", $json->incr_text;
917 }
918
919 # else add more data
920 sysread $fh, my $buf, 65536
921 or die "read error: $!";
922 $json->incr_parse ($buf); # void context, so no parsing
923 }
924
925This is a complex example, but most of the complexity comes from the fact
926that we are trying to be correct (bear with me if I am wrong, I never ran
927the above example :).
928
675 929
676 930
677=head1 MAPPING 931=head1 MAPPING
678 932
679This section describes how JSON::XS maps Perl values to JSON values and 933This section describes how JSON::XS maps Perl values to JSON values and
768Other unblessed references are generally not allowed and will cause an 1022Other unblessed references are generally not allowed and will cause an
769exception to be thrown, except for references to the integers C<0> and 1023exception to be thrown, except for references to the integers C<0> and
770C<1>, which get turned into C<false> and C<true> atoms in JSON. You can 1024C<1>, which get turned into C<false> and C<true> atoms in JSON. You can
771also use C<JSON::XS::false> and C<JSON::XS::true> to improve readability. 1025also use C<JSON::XS::false> and C<JSON::XS::true> to improve readability.
772 1026
773 encode_json [\0,JSON::XS::true] # yields [false,true] 1027 encode_json [\0, JSON::XS::true] # yields [false,true]
774 1028
775=item JSON::XS::true, JSON::XS::false 1029=item JSON::XS::true, JSON::XS::false
776 1030
777These special values become JSON true and JSON false values, 1031These special values become JSON true and JSON false values,
778respectively. You can also use C<\1> and C<\0> directly if you want. 1032respectively. You can also use C<\1> and C<\0> directly if you want.
816 my $x = "3"; # some variable containing a string 1070 my $x = "3"; # some variable containing a string
817 $x += 0; # numify it, ensuring it will be dumped as a number 1071 $x += 0; # numify it, ensuring it will be dumped as a number
818 $x *= 1; # same thing, the choice is yours. 1072 $x *= 1; # same thing, the choice is yours.
819 1073
820You can not currently force the type in other, less obscure, ways. Tell me 1074You can not currently force the type in other, less obscure, ways. Tell me
821if you need this capability (but don't forget to explain why its needed 1075if you need this capability (but don't forget to explain why it's needed
822:). 1076:).
823 1077
824=back 1078=back
825 1079
826 1080
828 1082
829The interested reader might have seen a number of flags that signify 1083The interested reader might have seen a number of flags that signify
830encodings or codesets - C<utf8>, C<latin1> and C<ascii>. There seems to be 1084encodings or codesets - C<utf8>, C<latin1> and C<ascii>. There seems to be
831some confusion on what these do, so here is a short comparison: 1085some confusion on what these do, so here is a short comparison:
832 1086
833C<utf8> controls wether the JSON text created by C<encode> (and expected 1087C<utf8> controls whether the JSON text created by C<encode> (and expected
834by C<decode>) is UTF-8 encoded or not, while C<latin1> and C<ascii> only 1088by C<decode>) is UTF-8 encoded or not, while C<latin1> and C<ascii> only
835control wether C<encode> escapes character values outside their respective 1089control whether C<encode> escapes character values outside their respective
836codeset range. Neither of these flags conflict with each other, although 1090codeset range. Neither of these flags conflict with each other, although
837some combinations make less sense than others. 1091some combinations make less sense than others.
838 1092
839Care has been taken to make all flags symmetrical with respect to 1093Care has been taken to make all flags symmetrical with respect to
840C<encode> and C<decode>, that is, texts encoded with any combination of 1094C<encode> and C<decode>, that is, texts encoded with any combination of
920proper subset of most 8-bit and multibyte encodings in use in the world. 1174proper subset of most 8-bit and multibyte encodings in use in the world.
921 1175
922=back 1176=back
923 1177
924 1178
925=head1 COMPARISON
926
927As already mentioned, this module was created because none of the existing
928JSON modules could be made to work correctly. First I will describe the
929problems (or pleasures) I encountered with various existing JSON modules,
930followed by some benchmark values. JSON::XS was designed not to suffer
931from any of these problems or limitations.
932
933=over 4
934
935=item JSON 2.xx
936
937A marvellous piece of engineering, this module either uses JSON::XS
938directly when available (so will be 100% compatible with it, including
939speed), or it uses JSON::PP, which is basically JSON::XS translated to
940Pure Perl, which should be 100% compatible with JSON::XS, just a bit
941slower.
942
943You cannot really lose by using this module, especially as it tries very
944hard to work even with ancient Perl versions, while JSON::XS does not.
945
946=item JSON 1.07
947
948Slow (but very portable, as it is written in pure Perl).
949
950Undocumented/buggy Unicode handling (how JSON handles Unicode values is
951undocumented. One can get far by feeding it Unicode strings and doing
952en-/decoding oneself, but Unicode escapes are not working properly).
953
954No round-tripping (strings get clobbered if they look like numbers, e.g.
955the string C<2.0> will encode to C<2.0> instead of C<"2.0">, and that will
956decode into the number 2.
957
958=item JSON::PC 0.01
959
960Very fast.
961
962Undocumented/buggy Unicode handling.
963
964No round-tripping.
965
966Has problems handling many Perl values (e.g. regex results and other magic
967values will make it croak).
968
969Does not even generate valid JSON (C<{1,2}> gets converted to C<{1:2}>
970which is not a valid JSON text.
971
972Unmaintained (maintainer unresponsive for many months, bugs are not
973getting fixed).
974
975=item JSON::Syck 0.21
976
977Very buggy (often crashes).
978
979Very inflexible (no human-readable format supported, format pretty much
980undocumented. I need at least a format for easy reading by humans and a
981single-line compact format for use in a protocol, and preferably a way to
982generate ASCII-only JSON texts).
983
984Completely broken (and confusingly documented) Unicode handling (Unicode
985escapes are not working properly, you need to set ImplicitUnicode to
986I<different> values on en- and decoding to get symmetric behaviour).
987
988No round-tripping (simple cases work, but this depends on whether the scalar
989value was used in a numeric context or not).
990
991Dumping hashes may skip hash values depending on iterator state.
992
993Unmaintained (maintainer unresponsive for many months, bugs are not
994getting fixed).
995
996Does not check input for validity (i.e. will accept non-JSON input and
997return "something" instead of raising an exception. This is a security
998issue: imagine two banks transferring money between each other using
999JSON. One bank might parse a given non-JSON request and deduct money,
1000while the other might reject the transaction with a syntax error. While a
1001good protocol will at least recover, that is extra unnecessary work and
1002the transaction will still not succeed).
1003
1004=item JSON::DWIW 0.04
1005
1006Very fast. Very natural. Very nice.
1007
1008Undocumented Unicode handling (but the best of the pack. Unicode escapes
1009still don't get parsed properly).
1010
1011Very inflexible.
1012
1013No round-tripping.
1014
1015Does not generate valid JSON texts (key strings are often unquoted, empty keys
1016result in nothing being output)
1017
1018Does not check input for validity.
1019
1020=back
1021
1022
1023=head2 JSON and YAML 1179=head2 JSON and YAML
1024 1180
1025You often hear that JSON is a subset of YAML. This is, however, a mass 1181You often hear that JSON is a subset of YAML. This is, however, a mass
1026hysteria(*) and very far from the truth. In general, there is no way to 1182hysteria(*) and very far from the truth (as of the time of this writing),
1183so let me state it clearly: I<in general, there is no way to configure
1027configure JSON::XS to output a data structure as valid YAML that works for 1184JSON::XS to output a data structure as valid YAML> that works in all
1028all cases. 1185cases.
1029 1186
1030If you really must use JSON::XS to generate YAML, you should use this 1187If you really must use JSON::XS to generate YAML, you should use this
1031algorithm (subject to change in future versions): 1188algorithm (subject to change in future versions):
1032 1189
1033 my $to_yaml = JSON::XS->new->utf8->space_after (1); 1190 my $to_yaml = JSON::XS->new->utf8->space_after (1);
1036This will I<usually> generate JSON texts that also parse as valid 1193This will I<usually> generate JSON texts that also parse as valid
1037YAML. Please note that YAML has hardcoded limits on (simple) object key 1194YAML. Please note that YAML has hardcoded limits on (simple) object key
1038lengths that JSON doesn't have and also has different and incompatible 1195lengths that JSON doesn't have and also has different and incompatible
1039unicode handling, so you should make sure that your hash keys are 1196unicode handling, so you should make sure that your hash keys are
1040noticeably shorter than the 1024 "stream characters" YAML allows and that 1197noticeably shorter than the 1024 "stream characters" YAML allows and that
1041you do not have codepoints with values outside the Unicode BMP (basic 1198you do not have characters with codepoint values outside the Unicode BMP
1042multilingual page). YAML also does not allow C<\/> sequences in strings 1199(basic multilingual page). YAML also does not allow C<\/> sequences in
1043(which JSON::XS does not I<currently> generate). 1200strings (which JSON::XS does not I<currently> generate, but other JSON
1201generators might).
1044 1202
1045There might be other incompatibilities that I am not aware of (or the YAML 1203There might be other incompatibilities that I am not aware of (or the YAML
1046specification has been changed yet again - it does so quite often). In 1204specification has been changed yet again - it does so quite often). In
1047general you should not try to generate YAML with a JSON generator or vice 1205general you should not try to generate YAML with a JSON generator or vice
1048versa, or try to parse JSON with a YAML parser or vice versa: chances are 1206versa, or try to parse JSON with a YAML parser or vice versa: chances are
1051 1209
1052=over 4 1210=over 4
1053 1211
1054=item (*) 1212=item (*)
1055 1213
1056This is spread actively by the YAML team, however. For many years now they 1214I have been pressured multiple times by Brian Ingerson (one of the
1057claim YAML were a superset of JSON, even when proven otherwise. 1215authors of the YAML specification) to remove this paragraph, despite him
1216acknowledging that the actual incompatibilities exist. As I was personally
1217bitten by this "JSON is YAML" lie, I refused and said I will continue to
1218educate people about these issues, so others do not run into the same
1219problem again and again. After this, Brian called me a (quote)I<complete
1220and worthless idiot>(unquote).
1058 1221
1059Even the author of this manpage was at some point accused of providing 1222In my opinion, instead of pressuring and insulting people who actually
1060"incorrect" information, despite the evidence presented (claims ranged 1223clarify issues with YAML and the wrong statements of some of its
1061from "your documentation contains inaccurate and negative statements about 1224proponents, I would kindly suggest reading the JSON spec (which is not
1062YAML" (the only negative comment is this footnote, and it didn't exist 1225that difficult or long) and finally make YAML compatible to it, and
1063back then; the question on which claims were inaccurate was never answered 1226educating users about the changes, instead of spreading lies about the
1064etc.) to "the YAML spec is not up-to-date" (the *real* and supposedly 1227real compatibility for many I<years> and trying to silence people who
1065JSON-compatible spec is apparently not currently publicly available) 1228point out that it isn't true.
1066to actual requests to replace this section by *incorrect* information,
1067suppressing information about the real problem).
1068
1069So whenever you are told that YAML was a superset of JSON, first check
1070wether it is really true (it might be when you check it, but it certainly
1071was not true when this was written). I would much prefer if the YAML team
1072would spent their time on actually making JSON compatibility a truth
1073(JSON, after all, has a very small and simple specification) instead of
1074trying to lobby/force people into reporting untruths.
1075 1229
1076=back 1230=back
1077 1231
1078 1232
1079=head2 SPEED 1233=head2 SPEED
1085 1239
1086First comes a comparison between various modules using 1240First comes a comparison between various modules using
1087a very short single-line JSON string (also available at 1241a very short single-line JSON string (also available at
1088L<http://dist.schmorp.de/misc/json/short.json>). 1242L<http://dist.schmorp.de/misc/json/short.json>).
1089 1243
1090 {"method": "handleMessage", "params": ["user1", "we were just talking"], \ 1244 {"method": "handleMessage", "params": ["user1",
1091 "id": null, "array":[1,11,234,-5,1e5,1e7, true, false]} 1245 "we were just talking"], "id": null, "array":[1,11,234,-5,1e5,1e7,
1246 true, false]}
1092 1247
1093It shows the number of encodes/decodes per second (JSON::XS uses 1248It shows the number of encodes/decodes per second (JSON::XS uses
1094the functional interface, while JSON::XS/2 uses the OO interface 1249the functional interface, while JSON::XS/2 uses the OO interface
1095with pretty-printing and hashkey sorting enabled, JSON::XS/3 enables 1250with pretty-printing and hashkey sorting enabled, JSON::XS/3 enables
1096shrink). Higher is better: 1251shrink). Higher is better:
1186=head1 THREADS 1341=head1 THREADS
1187 1342
1188This module is I<not> guaranteed to be thread safe and there are no 1343This module is I<not> guaranteed to be thread safe and there are no
1189plans to change this until Perl gets thread support (as opposed to the 1344plans to change this until Perl gets thread support (as opposed to the
1190horribly slow so-called "threads" which are simply slow and bloated 1345horribly slow so-called "threads" which are simply slow and bloated
1191process simulations - use fork, its I<much> faster, cheaper, better). 1346process simulations - use fork, it's I<much> faster, cheaper, better).
1192 1347
1193(It might actually work, but you have been warned). 1348(It might actually work, but you have been warned).
1194 1349
1195 1350
1196=head1 BUGS 1351=head1 BUGS
1197 1352
1198While the goal of this module is to be correct, that unfortunately does 1353While the goal of this module is to be correct, that unfortunately does
1199not mean its bug-free, only that I think its design is bug-free. It is 1354not mean it's bug-free, only that I think its design is bug-free. If you
1200still relatively early in its development. If you keep reporting bugs they 1355keep reporting bugs they will be fixed swiftly, though.
1201will be fixed swiftly, though.
1202 1356
1203Please refrain from using rt.cpan.org or any other bug reporting 1357Please refrain from using rt.cpan.org or any other bug reporting
1204service. I put the contact address into my modules for a reason. 1358service. I put the contact address into my modules for a reason.
1205 1359
1206=cut 1360=cut
1226 "--" => sub { $_[0] = ${$_[0]} - 1 }, 1380 "--" => sub { $_[0] = ${$_[0]} - 1 },
1227 fallback => 1; 1381 fallback => 1;
1228 1382
12291; 13831;
1230 1384
1385=head1 SEE ALSO
1386
1387The F<json_xs> command line utility for quick experiments.
1388
1231=head1 AUTHOR 1389=head1 AUTHOR
1232 1390
1233 Marc Lehmann <schmorp@schmorp.de> 1391 Marc Lehmann <schmorp@schmorp.de>
1234 http://home.schmorp.de/ 1392 http://home.schmorp.de/
1235 1393

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