… | |
… | |
606 | and you need to know where the JSON text ends. |
606 | and you need to know where the JSON text ends. |
607 | |
607 | |
608 | JSON::XS->new->decode_prefix ("[1] the tail") |
608 | JSON::XS->new->decode_prefix ("[1] the tail") |
609 | => ([], 3) |
609 | => ([], 3) |
610 | |
610 | |
|
|
611 | INCREMENTAL PARSING |
|
|
612 | [This section and the API it details is still EXPERIMENTAL] |
|
|
613 | |
|
|
614 | In some cases, there is the need for incremental parsing of JSON texts. |
|
|
615 | While this module always has to keep both JSON text and resulting Perl |
|
|
616 | data structure in memory at one time, it does allow you to parse a JSON |
|
|
617 | stream incrementally. It does so by accumulating text until it has a |
|
|
618 | full JSON object, which it then can decode. This process is similar to |
|
|
619 | using "decode_prefix" to see if a full JSON object is available, but is |
|
|
620 | much more efficient (JSON::XS will only attempt to parse the JSON text |
|
|
621 | once it is sure it has enough text to get a decisive result, using a |
|
|
622 | very simple but truly incremental parser). |
|
|
623 | |
|
|
624 | The following two methods deal with this. |
|
|
625 | |
|
|
626 | [void, scalar or list context] = $json->incr_parse ([$string]) |
|
|
627 | This is the central parsing function. It can both append new text |
|
|
628 | and extract objects from the stream accumulated so far (both of |
|
|
629 | these functions are optional). |
|
|
630 | |
|
|
631 | If $string is given, then this string is appended to the already |
|
|
632 | existing JSON fragment stored in the $json object. |
|
|
633 | |
|
|
634 | After that, if the function is called in void context, it will |
|
|
635 | simply return without doing anything further. This can be used to |
|
|
636 | add more text in as many chunks as you want. |
|
|
637 | |
|
|
638 | If the method is called in scalar context, then it will try to |
|
|
639 | extract exactly *one* JSON object. If that is successful, it will |
|
|
640 | return this object, otherwise it will return "undef". If there is a |
|
|
641 | parse error, this method will croak just as "decode" would do (one |
|
|
642 | can then use "incr_skip" to skip the errornous part). This is the |
|
|
643 | most common way of using the method. |
|
|
644 | |
|
|
645 | And finally, in list context, it will try to extract as many objects |
|
|
646 | from the stream as it can find and return them, or the empty list |
|
|
647 | otherwise. For this to work, there must be no separators between the |
|
|
648 | JSON objects or arrays, instead they must be concatenated |
|
|
649 | back-to-back. If an error occurs, an exception will be raised as in |
|
|
650 | the scalar context case. Note that in this case, any |
|
|
651 | previously-parsed JSON texts will be lost. |
|
|
652 | |
|
|
653 | $lvalue_string = $json->incr_text |
|
|
654 | This method returns the currently stored JSON fragment as an lvalue, |
|
|
655 | that is, you can manipulate it. This *only* works when a preceding |
|
|
656 | call to "incr_parse" in *scalar context* successfully returned an |
|
|
657 | object. Under all other circumstances you must not call this |
|
|
658 | function (I mean it. although in simple tests it might actually |
|
|
659 | work, it *will* fail under real world conditions). As a special |
|
|
660 | exception, you can also call this method before having parsed |
|
|
661 | anything. |
|
|
662 | |
|
|
663 | This function is useful in two cases: a) finding the trailing text |
|
|
664 | after a JSON object or b) parsing multiple JSON objects separated by |
|
|
665 | non-JSON text (such as commas). |
|
|
666 | |
|
|
667 | $json->incr_skip |
|
|
668 | This will reset the state of the incremental parser and will remove |
|
|
669 | the parsed text from the input buffer. This is useful after |
|
|
670 | "incr_parse" died, in which case the input buffer and incremental |
|
|
671 | parser state is left unchanged, to skip the text parsed so far and |
|
|
672 | to reset the parse state. |
|
|
673 | |
|
|
674 | LIMITATIONS |
|
|
675 | All options that affect decoding are supported, except "allow_nonref". |
|
|
676 | The reason for this is that it cannot be made to work sensibly: JSON |
|
|
677 | objects and arrays are self-delimited, i.e. you can concatenate them |
|
|
678 | back to back and still decode them perfectly. This does not hold true |
|
|
679 | for JSON numbers, however. |
|
|
680 | |
|
|
681 | For example, is the string 1 a single JSON number, or is it simply the |
|
|
682 | start of 12? Or is 12 a single JSON number, or the concatenation of 1 |
|
|
683 | and 2? In neither case you can tell, and this is why JSON::XS takes the |
|
|
684 | conservative route and disallows this case. |
|
|
685 | |
|
|
686 | EXAMPLES |
|
|
687 | Some examples will make all this clearer. First, a simple example that |
|
|
688 | works similarly to "decode_prefix": We want to decode the JSON object at |
|
|
689 | the start of a string and identify the portion after the JSON object: |
|
|
690 | |
|
|
691 | my $text = "[1,2,3] hello"; |
|
|
692 | |
|
|
693 | my $json = new JSON::XS; |
|
|
694 | |
|
|
695 | my $obj = $json->incr_parse ($text) |
|
|
696 | or die "expected JSON object or array at beginning of string"; |
|
|
697 | |
|
|
698 | my $tail = $json->incr_text; |
|
|
699 | # $tail now contains " hello" |
|
|
700 | |
|
|
701 | Easy, isn't it? |
|
|
702 | |
|
|
703 | Now for a more complicated example: Imagine a hypothetical protocol |
|
|
704 | where you read some requests from a TCP stream, and each request is a |
|
|
705 | JSON array, without any separation between them (in fact, it is often |
|
|
706 | useful to use newlines as "separators", as these get interpreted as |
|
|
707 | whitespace at the start of the JSON text, which makes it possible to |
|
|
708 | test said protocol with "telnet"...). |
|
|
709 | |
|
|
710 | Here is how you'd do it (it is trivial to write this in an event-based |
|
|
711 | manner): |
|
|
712 | |
|
|
713 | my $json = new JSON::XS; |
|
|
714 | |
|
|
715 | # read some data from the socket |
|
|
716 | while (sysread $socket, my $buf, 4096) { |
|
|
717 | |
|
|
718 | # split and decode as many requests as possible |
|
|
719 | for my $request ($json->incr_parse ($buf)) { |
|
|
720 | # act on the $request |
|
|
721 | } |
|
|
722 | } |
|
|
723 | |
|
|
724 | Another complicated example: Assume you have a string with JSON objects |
|
|
725 | or arrays, all separated by (optional) comma characters (e.g. "[1],[2], |
|
|
726 | [3]"). To parse them, we have to skip the commas between the JSON texts, |
|
|
727 | and here is where the lvalue-ness of "incr_text" comes in useful: |
|
|
728 | |
|
|
729 | my $text = "[1],[2], [3]"; |
|
|
730 | my $json = new JSON::XS; |
|
|
731 | |
|
|
732 | # void context, so no parsing done |
|
|
733 | $json->incr_parse ($text); |
|
|
734 | |
|
|
735 | # now extract as many objects as possible. note the |
|
|
736 | # use of scalar context so incr_text can be called. |
|
|
737 | while (my $obj = $json->incr_parse) { |
|
|
738 | # do something with $obj |
|
|
739 | |
|
|
740 | # now skip the optional comma |
|
|
741 | $json->incr_text =~ s/^ \s* , //x; |
|
|
742 | } |
|
|
743 | |
|
|
744 | Now lets go for a very complex example: Assume that you have a gigantic |
|
|
745 | JSON array-of-objects, many gigabytes in size, and you want to parse it, |
|
|
746 | but you cannot load it into memory fully (this has actually happened in |
|
|
747 | the real world :). |
|
|
748 | |
|
|
749 | Well, you lost, you have to implement your own JSON parser. But JSON::XS |
|
|
750 | can still help you: You implement a (very simple) array parser and let |
|
|
751 | JSON decode the array elements, which are all full JSON objects on their |
|
|
752 | own (this wouldn't work if the array elements could be JSON numbers, for |
|
|
753 | example): |
|
|
754 | |
|
|
755 | my $json = new JSON::XS; |
|
|
756 | |
|
|
757 | # open the monster |
|
|
758 | open my $fh, "<bigfile.json" |
|
|
759 | or die "bigfile: $!"; |
|
|
760 | |
|
|
761 | # first parse the initial "[" |
|
|
762 | for (;;) { |
|
|
763 | sysread $fh, my $buf, 65536 |
|
|
764 | or die "read error: $!"; |
|
|
765 | $json->incr_parse ($buf); # void context, so no parsing |
|
|
766 | |
|
|
767 | # Exit the loop once we found and removed(!) the initial "[". |
|
|
768 | # In essence, we are (ab-)using the $json object as a simple scalar |
|
|
769 | # we append data to. |
|
|
770 | last if $json->incr_text =~ s/^ \s* \[ //x; |
|
|
771 | } |
|
|
772 | |
|
|
773 | # now we have the skipped the initial "[", so continue |
|
|
774 | # parsing all the elements. |
|
|
775 | for (;;) { |
|
|
776 | # in this loop we read data until we got a single JSON object |
|
|
777 | for (;;) { |
|
|
778 | if (my $obj = $json->incr_parse) { |
|
|
779 | # do something with $obj |
|
|
780 | last; |
|
|
781 | } |
|
|
782 | |
|
|
783 | # add more data |
|
|
784 | sysread $fh, my $buf, 65536 |
|
|
785 | or die "read error: $!"; |
|
|
786 | $json->incr_parse ($buf); # void context, so no parsing |
|
|
787 | } |
|
|
788 | |
|
|
789 | # in this loop we read data until we either found and parsed the |
|
|
790 | # separating "," between elements, or the final "]" |
|
|
791 | for (;;) { |
|
|
792 | # first skip whitespace |
|
|
793 | $json->incr_text =~ s/^\s*//; |
|
|
794 | |
|
|
795 | # if we find "]", we are done |
|
|
796 | if ($json->incr_text =~ s/^\]//) { |
|
|
797 | print "finished.\n"; |
|
|
798 | exit; |
|
|
799 | } |
|
|
800 | |
|
|
801 | # if we find ",", we can continue with the next element |
|
|
802 | if ($json->incr_text =~ s/^,//) { |
|
|
803 | last; |
|
|
804 | } |
|
|
805 | |
|
|
806 | # if we find anything else, we have a parse error! |
|
|
807 | if (length $json->incr_text) { |
|
|
808 | die "parse error near ", $json->incr_text; |
|
|
809 | } |
|
|
810 | |
|
|
811 | # else add more data |
|
|
812 | sysread $fh, my $buf, 65536 |
|
|
813 | or die "read error: $!"; |
|
|
814 | $json->incr_parse ($buf); # void context, so no parsing |
|
|
815 | } |
|
|
816 | |
|
|
817 | This is a complex example, but most of the complexity comes from the |
|
|
818 | fact that we are trying to be correct (bear with me if I am wrong, I |
|
|
819 | never ran the above example :). |
|
|
820 | |
611 | MAPPING |
821 | MAPPING |
612 | This section describes how JSON::XS maps Perl values to JSON values and |
822 | This section describes how JSON::XS maps Perl values to JSON values and |
613 | vice versa. These mappings are designed to "do the right thing" in most |
823 | vice versa. These mappings are designed to "do the right thing" in most |
614 | circumstances automatically, preserving round-tripping characteristics |
824 | circumstances automatically, preserving round-tripping characteristics |
615 | (what you put in comes out as something equivalent). |
825 | (what you put in comes out as something equivalent). |
… | |
… | |
734 | $x += 0; # numify it, ensuring it will be dumped as a number |
944 | $x += 0; # numify it, ensuring it will be dumped as a number |
735 | $x *= 1; # same thing, the choice is yours. |
945 | $x *= 1; # same thing, the choice is yours. |
736 | |
946 | |
737 | You can not currently force the type in other, less obscure, ways. |
947 | You can not currently force the type in other, less obscure, ways. |
738 | Tell me if you need this capability (but don't forget to explain why |
948 | Tell me if you need this capability (but don't forget to explain why |
739 | its needed :). |
949 | it's needed :). |
740 | |
950 | |
741 | ENCODING/CODESET FLAG NOTES |
951 | ENCODING/CODESET FLAG NOTES |
742 | The interested reader might have seen a number of flags that signify |
952 | The interested reader might have seen a number of flags that signify |
743 | encodings or codesets - "utf8", "latin1" and "ascii". There seems to be |
953 | encodings or codesets - "utf8", "latin1" and "ascii". There seems to be |
744 | some confusion on what these do, so here is a short comparison: |
954 | some confusion on what these do, so here is a short comparison: |
745 | |
955 | |
746 | "utf8" controls wether the JSON text created by "encode" (and expected |
956 | "utf8" controls whether the JSON text created by "encode" (and expected |
747 | by "decode") is UTF-8 encoded or not, while "latin1" and "ascii" only |
957 | by "decode") is UTF-8 encoded or not, while "latin1" and "ascii" only |
748 | control wether "encode" escapes character values outside their |
958 | control whether "encode" escapes character values outside their |
749 | respective codeset range. Neither of these flags conflict with each |
959 | respective codeset range. Neither of these flags conflict with each |
750 | other, although some combinations make less sense than others. |
960 | other, although some combinations make less sense than others. |
751 | |
961 | |
752 | Care has been taken to make all flags symmetrical with respect to |
962 | Care has been taken to make all flags symmetrical with respect to |
753 | "encode" and "decode", that is, texts encoded with any combination of |
963 | "encode" and "decode", that is, texts encoded with any combination of |
… | |
… | |
830 | any character set and 8-bit-encoding, and still get the same data |
1040 | any character set and 8-bit-encoding, and still get the same data |
831 | structure back. This is useful when your channel for JSON transfer |
1041 | structure back. This is useful when your channel for JSON transfer |
832 | is not 8-bit clean or the encoding might be mangled in between (e.g. |
1042 | is not 8-bit clean or the encoding might be mangled in between (e.g. |
833 | in mail), and works because ASCII is a proper subset of most 8-bit |
1043 | in mail), and works because ASCII is a proper subset of most 8-bit |
834 | and multibyte encodings in use in the world. |
1044 | and multibyte encodings in use in the world. |
835 | |
|
|
836 | COMPARISON |
|
|
837 | As already mentioned, this module was created because none of the |
|
|
838 | existing JSON modules could be made to work correctly. First I will |
|
|
839 | describe the problems (or pleasures) I encountered with various existing |
|
|
840 | JSON modules, followed by some benchmark values. JSON::XS was designed |
|
|
841 | not to suffer from any of these problems or limitations. |
|
|
842 | |
|
|
843 | JSON 2.xx |
|
|
844 | A marvellous piece of engineering, this module either uses JSON::XS |
|
|
845 | directly when available (so will be 100% compatible with it, |
|
|
846 | including speed), or it uses JSON::PP, which is basically JSON::XS |
|
|
847 | translated to Pure Perl, which should be 100% compatible with |
|
|
848 | JSON::XS, just a bit slower. |
|
|
849 | |
|
|
850 | You cannot really lose by using this module, especially as it tries |
|
|
851 | very hard to work even with ancient Perl versions, while JSON::XS |
|
|
852 | does not. |
|
|
853 | |
|
|
854 | JSON 1.07 |
|
|
855 | Slow (but very portable, as it is written in pure Perl). |
|
|
856 | |
|
|
857 | Undocumented/buggy Unicode handling (how JSON handles Unicode values |
|
|
858 | is undocumented. One can get far by feeding it Unicode strings and |
|
|
859 | doing en-/decoding oneself, but Unicode escapes are not working |
|
|
860 | properly). |
|
|
861 | |
|
|
862 | No round-tripping (strings get clobbered if they look like numbers, |
|
|
863 | e.g. the string 2.0 will encode to 2.0 instead of "2.0", and that |
|
|
864 | will decode into the number 2. |
|
|
865 | |
|
|
866 | JSON::PC 0.01 |
|
|
867 | Very fast. |
|
|
868 | |
|
|
869 | Undocumented/buggy Unicode handling. |
|
|
870 | |
|
|
871 | No round-tripping. |
|
|
872 | |
|
|
873 | Has problems handling many Perl values (e.g. regex results and other |
|
|
874 | magic values will make it croak). |
|
|
875 | |
|
|
876 | Does not even generate valid JSON ("{1,2}" gets converted to "{1:2}" |
|
|
877 | which is not a valid JSON text. |
|
|
878 | |
|
|
879 | Unmaintained (maintainer unresponsive for many months, bugs are not |
|
|
880 | getting fixed). |
|
|
881 | |
|
|
882 | JSON::Syck 0.21 |
|
|
883 | Very buggy (often crashes). |
|
|
884 | |
|
|
885 | Very inflexible (no human-readable format supported, format pretty |
|
|
886 | much undocumented. I need at least a format for easy reading by |
|
|
887 | humans and a single-line compact format for use in a protocol, and |
|
|
888 | preferably a way to generate ASCII-only JSON texts). |
|
|
889 | |
|
|
890 | Completely broken (and confusingly documented) Unicode handling |
|
|
891 | (Unicode escapes are not working properly, you need to set |
|
|
892 | ImplicitUnicode to *different* values on en- and decoding to get |
|
|
893 | symmetric behaviour). |
|
|
894 | |
|
|
895 | No round-tripping (simple cases work, but this depends on whether |
|
|
896 | the scalar value was used in a numeric context or not). |
|
|
897 | |
|
|
898 | Dumping hashes may skip hash values depending on iterator state. |
|
|
899 | |
|
|
900 | Unmaintained (maintainer unresponsive for many months, bugs are not |
|
|
901 | getting fixed). |
|
|
902 | |
|
|
903 | Does not check input for validity (i.e. will accept non-JSON input |
|
|
904 | and return "something" instead of raising an exception. This is a |
|
|
905 | security issue: imagine two banks transferring money between each |
|
|
906 | other using JSON. One bank might parse a given non-JSON request and |
|
|
907 | deduct money, while the other might reject the transaction with a |
|
|
908 | syntax error. While a good protocol will at least recover, that is |
|
|
909 | extra unnecessary work and the transaction will still not succeed). |
|
|
910 | |
|
|
911 | JSON::DWIW 0.04 |
|
|
912 | Very fast. Very natural. Very nice. |
|
|
913 | |
|
|
914 | Undocumented Unicode handling (but the best of the pack. Unicode |
|
|
915 | escapes still don't get parsed properly). |
|
|
916 | |
|
|
917 | Very inflexible. |
|
|
918 | |
|
|
919 | No round-tripping. |
|
|
920 | |
|
|
921 | Does not generate valid JSON texts (key strings are often unquoted, |
|
|
922 | empty keys result in nothing being output) |
|
|
923 | |
|
|
924 | Does not check input for validity. |
|
|
925 | |
1045 | |
926 | JSON and YAML |
1046 | JSON and YAML |
927 | You often hear that JSON is a subset of YAML. This is, however, a mass |
1047 | You often hear that JSON is a subset of YAML. This is, however, a mass |
928 | hysteria(*) and very far from the truth (as of the time of this |
1048 | hysteria(*) and very far from the truth (as of the time of this |
929 | writing), so let me state it clearly: *in general, there is no way to |
1049 | writing), so let me state it clearly: *in general, there is no way to |
… | |
… | |
1075 | |
1195 | |
1076 | THREADS |
1196 | THREADS |
1077 | This module is *not* guaranteed to be thread safe and there are no plans |
1197 | This module is *not* guaranteed to be thread safe and there are no plans |
1078 | to change this until Perl gets thread support (as opposed to the |
1198 | to change this until Perl gets thread support (as opposed to the |
1079 | horribly slow so-called "threads" which are simply slow and bloated |
1199 | horribly slow so-called "threads" which are simply slow and bloated |
1080 | process simulations - use fork, its *much* faster, cheaper, better). |
1200 | process simulations - use fork, it's *much* faster, cheaper, better). |
1081 | |
1201 | |
1082 | (It might actually work, but you have been warned). |
1202 | (It might actually work, but you have been warned). |
1083 | |
1203 | |
1084 | BUGS |
1204 | BUGS |
1085 | While the goal of this module is to be correct, that unfortunately does |
1205 | While the goal of this module is to be correct, that unfortunately does |
1086 | not mean its bug-free, only that I think its design is bug-free. It is |
1206 | not mean it's bug-free, only that I think its design is bug-free. It is |
1087 | still relatively early in its development. If you keep reporting bugs |
1207 | still relatively early in its development. If you keep reporting bugs |
1088 | they will be fixed swiftly, though. |
1208 | they will be fixed swiftly, though. |
1089 | |
1209 | |
1090 | Please refrain from using rt.cpan.org or any other bug reporting |
1210 | Please refrain from using rt.cpan.org or any other bug reporting |
1091 | service. I put the contact address into my modules for a reason. |
1211 | service. I put the contact address into my modules for a reason. |
|
|
1212 | |
|
|
1213 | SEE ALSO |
|
|
1214 | The json_xs command line utility for quick experiments. |
1092 | |
1215 | |
1093 | AUTHOR |
1216 | AUTHOR |
1094 | Marc Lehmann <schmorp@schmorp.de> |
1217 | Marc Lehmann <schmorp@schmorp.de> |
1095 | http://home.schmorp.de/ |
1218 | http://home.schmorp.de/ |
1096 | |
1219 | |