… | |
… | |
43 | As this is the n-th-something JSON module on CPAN, what was the reason |
43 | As this is the n-th-something JSON module on CPAN, what was the reason |
44 | to write yet another JSON module? While it seems there are many JSON |
44 | to write yet another JSON module? While it seems there are many JSON |
45 | modules, none of them correctly handle all corner cases, and in most |
45 | modules, none of them correctly handle all corner cases, and in most |
46 | cases their maintainers are unresponsive, gone missing, or not listening |
46 | cases their maintainers are unresponsive, gone missing, or not listening |
47 | to bug reports for other reasons. |
47 | to bug reports for other reasons. |
48 | |
|
|
49 | See COMPARISON, below, for a comparison to some other JSON modules. |
|
|
50 | |
48 | |
51 | See MAPPING, below, on how JSON::XS maps perl values to JSON values and |
49 | See MAPPING, below, on how JSON::XS maps perl values to JSON values and |
52 | vice versa. |
50 | vice versa. |
53 | |
51 | |
54 | FEATURES |
52 | FEATURES |
… | |
… | |
629 | While this module always has to keep both JSON text and resulting Perl |
627 | While this module always has to keep both JSON text and resulting Perl |
630 | data structure in memory at one time, it does allow you to parse a JSON |
628 | data structure in memory at one time, it does allow you to parse a JSON |
631 | stream incrementally. It does so by accumulating text until it has a |
629 | stream incrementally. It does so by accumulating text until it has a |
632 | full JSON object, which it then can decode. This process is similar to |
630 | full JSON object, which it then can decode. This process is similar to |
633 | using "decode_prefix" to see if a full JSON object is available, but is |
631 | using "decode_prefix" to see if a full JSON object is available, but is |
634 | much more efficient (JSON::XS will only attempt to parse the JSON text |
632 | much more efficient (and can be implemented with a minimum of method |
635 | once it is sure it has enough text to get a decisive result, using a |
633 | calls). |
636 | very simple but truly incremental parser). |
|
|
637 | |
634 | |
638 | The following two methods deal with this. |
635 | JSON::XS will only attempt to parse the JSON text once it is sure it has |
|
|
636 | enough text to get a decisive result, using a very simple but truly |
|
|
637 | incremental parser. This means that it sometimes won't stop as early as |
|
|
638 | the full parser, for example, it doesn't detect parenthese mismatches. |
|
|
639 | The only thing it guarantees is that it starts decoding as soon as a |
|
|
640 | syntactically valid JSON text has been seen. This means you need to set |
|
|
641 | resource limits (e.g. "max_size") to ensure the parser will stop parsing |
|
|
642 | in the presence if syntax errors. |
|
|
643 | |
|
|
644 | The following methods implement this incremental parser. |
639 | |
645 | |
640 | [void, scalar or list context] = $json->incr_parse ([$string]) |
646 | [void, scalar or list context] = $json->incr_parse ([$string]) |
641 | This is the central parsing function. It can both append new text |
647 | This is the central parsing function. It can both append new text |
642 | and extract objects from the stream accumulated so far (both of |
648 | and extract objects from the stream accumulated so far (both of |
643 | these functions are optional). |
649 | these functions are optional). |
… | |
… | |
678 | after a JSON object or b) parsing multiple JSON objects separated by |
684 | after a JSON object or b) parsing multiple JSON objects separated by |
679 | non-JSON text (such as commas). |
685 | non-JSON text (such as commas). |
680 | |
686 | |
681 | $json->incr_skip |
687 | $json->incr_skip |
682 | This will reset the state of the incremental parser and will remove |
688 | This will reset the state of the incremental parser and will remove |
683 | the parsed text from the input buffer. This is useful after |
689 | the parsed text from the input buffer so far. This is useful after |
684 | "incr_parse" died, in which case the input buffer and incremental |
690 | "incr_parse" died, in which case the input buffer and incremental |
685 | parser state is left unchanged, to skip the text parsed so far and |
691 | parser state is left unchanged, to skip the text parsed so far and |
686 | to reset the parse state. |
692 | to reset the parse state. |
687 | |
693 | |
|
|
694 | The difference to "incr_reset" is that only text until the parse |
|
|
695 | error occured is removed. |
|
|
696 | |
688 | $json->incr_reset |
697 | $json->incr_reset |
689 | This completely resets the incremental parser, that is, after this |
698 | This completely resets the incremental parser, that is, after this |
690 | call, it will be as if the parser had never parsed anything. |
699 | call, it will be as if the parser had never parsed anything. |
691 | |
700 | |
692 | This is useful if you want ot repeatedly parse JSON objects and want |
701 | This is useful if you want to repeatedly parse JSON objects and want |
693 | to ignore any trailing data, which means you have to reset the |
702 | to ignore any trailing data, which means you have to reset the |
694 | parser after each successful decode. |
703 | parser after each successful decode. |
695 | |
704 | |
696 | LIMITATIONS |
705 | LIMITATIONS |
697 | All options that affect decoding are supported, except "allow_nonref". |
706 | All options that affect decoding are supported, except "allow_nonref". |
… | |
… | |
1062 | any character set and 8-bit-encoding, and still get the same data |
1071 | any character set and 8-bit-encoding, and still get the same data |
1063 | structure back. This is useful when your channel for JSON transfer |
1072 | structure back. This is useful when your channel for JSON transfer |
1064 | is not 8-bit clean or the encoding might be mangled in between (e.g. |
1073 | is not 8-bit clean or the encoding might be mangled in between (e.g. |
1065 | in mail), and works because ASCII is a proper subset of most 8-bit |
1074 | in mail), and works because ASCII is a proper subset of most 8-bit |
1066 | and multibyte encodings in use in the world. |
1075 | and multibyte encodings in use in the world. |
|
|
1076 | |
|
|
1077 | JSON and ECMAscript |
|
|
1078 | JSON syntax is based on how literals are represented in javascript (the |
|
|
1079 | not-standardised predecessor of ECMAscript) which is presumably why it |
|
|
1080 | is called "JavaScript Object Notation". |
|
|
1081 | |
|
|
1082 | However, JSON is not a subset (and also not a superset of course) of |
|
|
1083 | ECMAscript (the standard) or javascript (whatever browsers actually |
|
|
1084 | implement). |
|
|
1085 | |
|
|
1086 | If you want to use javascript's "eval" function to "parse" JSON, you |
|
|
1087 | might run into parse errors for valid JSON texts, or the resulting data |
|
|
1088 | structure might not be queryable: |
|
|
1089 | |
|
|
1090 | One of the problems is that U+2028 and U+2029 are valid characters |
|
|
1091 | inside JSON strings, but are not allowed in ECMAscript string literals, |
|
|
1092 | so the following Perl fragment will not output something that can be |
|
|
1093 | guaranteed to be parsable by javascript's "eval": |
|
|
1094 | |
|
|
1095 | use JSON::XS; |
|
|
1096 | |
|
|
1097 | print encode_json [chr 0x2028]; |
|
|
1098 | |
|
|
1099 | The right fix for this is to use a proper JSON parser in your javascript |
|
|
1100 | programs, and not rely on "eval" (see for example Douglas Crockford's |
|
|
1101 | json2.js parser). |
|
|
1102 | |
|
|
1103 | If this is not an option, you can, as a stop-gap measure, simply encode |
|
|
1104 | to ASCII-only JSON: |
|
|
1105 | |
|
|
1106 | use JSON::XS; |
|
|
1107 | |
|
|
1108 | print JSON::XS->new->ascii->encode ([chr 0x2028]); |
|
|
1109 | |
|
|
1110 | Note that this will enlarge the resulting JSON text quite a bit if you |
|
|
1111 | have many non-ASCII characters. You might be tempted to run some regexes |
|
|
1112 | to only escape U+2028 and U+2029, e.g.: |
|
|
1113 | |
|
|
1114 | # DO NOT USE THIS! |
|
|
1115 | my $json = JSON::XS->new->utf8->encode ([chr 0x2028]); |
|
|
1116 | $json =~ s/\xe2\x80\xa8/\\u2028/g; # escape U+2028 |
|
|
1117 | $json =~ s/\xe2\x80\xa9/\\u2029/g; # escape U+2029 |
|
|
1118 | print $json; |
|
|
1119 | |
|
|
1120 | Note that *this is a bad idea*: the above only works for U+2028 and |
|
|
1121 | U+2029 and thus only for fully ECMAscript-compliant parsers. Many |
|
|
1122 | existing javascript implementations, however, have issues with other |
|
|
1123 | characters as well - using "eval" naively simply *will* cause problems. |
|
|
1124 | |
|
|
1125 | Another problem is that some javascript implementations reserve some |
|
|
1126 | property names for their own purposes (which probably makes them |
|
|
1127 | non-ECMAscript-compliant). For example, Iceweasel reserves the |
|
|
1128 | "__proto__" property name for it's own purposes. |
|
|
1129 | |
|
|
1130 | If that is a problem, you could parse try to filter the resulting JSON |
|
|
1131 | output for these property strings, e.g.: |
|
|
1132 | |
|
|
1133 | $json =~ s/"__proto__"\s*:/"__proto__renamed":/g; |
|
|
1134 | |
|
|
1135 | This works because "__proto__" is not valid outside of strings, so every |
|
|
1136 | occurence of ""__proto__"\s*:" must be a string used as property name. |
|
|
1137 | |
|
|
1138 | If you know of other incompatibilities, please let me know. |
1067 | |
1139 | |
1068 | JSON and YAML |
1140 | JSON and YAML |
1069 | You often hear that JSON is a subset of YAML. This is, however, a mass |
1141 | You often hear that JSON is a subset of YAML. This is, however, a mass |
1070 | hysteria(*) and very far from the truth (as of the time of this |
1142 | hysteria(*) and very far from the truth (as of the time of this |
1071 | writing), so let me state it clearly: *in general, there is no way to |
1143 | writing), so let me state it clearly: *in general, there is no way to |