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NAME |
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JSON::XS - JSON serialising/deserialising, done correctly and fast |
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JSON::XS - 正しくて高速な JSON |
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シリアライザ/デシリアライザ |
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(http://fleur.hio.jp/perldoc/mix/lib/JSON/XS.html) |
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SYNOPSIS |
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use JSON::XS; |
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# exported functions, they croak on error |
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# and expect/generate UTF-8 |
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$utf8_encoded_json_text = to_json $perl_hash_or_arrayref; |
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$perl_hash_or_arrayref = from_json $utf8_encoded_json_text; |
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# OO-interface |
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$coder = JSON::XS->new->ascii->pretty->allow_nonref; |
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$pretty_printed_unencoded = $coder->encode ($perl_scalar); |
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$perl_scalar = $coder->decode ($unicode_json_text); |
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DESCRIPTION |
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This module converts Perl data structures to JSON and vice versa. Its |
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primary goal is to be *correct* and its secondary goal is to be *fast*. |
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To reach the latter goal it was written in C. |
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As this is the n-th-something JSON module on CPAN, what was the reason |
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to write yet another JSON module? While it seems there are many JSON |
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modules, none of them correctly handle all corner cases, and in most |
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cases their maintainers are unresponsive, gone missing, or not listening |
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to bug reports for other reasons. |
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See COMPARISON, below, for a comparison to some other JSON modules. |
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See MAPPING, below, on how JSON::XS maps perl values to JSON values and |
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vice versa. |
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FEATURES |
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* correct unicode handling |
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This module knows how to handle Unicode, and even documents how and |
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when it does so. |
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* round-trip integrity |
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When you serialise a perl data structure using only datatypes |
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supported by JSON, the deserialised data structure is identical on |
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the Perl level. (e.g. the string "2.0" doesn't suddenly become "2" |
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just because it looks like a number). |
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* strict checking of JSON correctness |
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There is no guessing, no generating of illegal JSON texts by |
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default, and only JSON is accepted as input by default (the latter |
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is a security feature). |
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* fast |
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Compared to other JSON modules, this module compares favourably in |
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terms of speed, too. |
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* simple to use |
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This module has both a simple functional interface as well as an OO |
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interface. |
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* reasonably versatile output formats |
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You can choose between the most compact guarenteed single-line |
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format possible (nice for simple line-based protocols), a pure-ascii |
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format (for when your transport is not 8-bit clean, still supports |
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the whole unicode range), or a pretty-printed format (for when you |
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want to read that stuff). Or you can combine those features in |
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whatever way you like. |
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FUNCTIONAL INTERFACE |
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The following convinience methods are provided by this module. They are |
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exported by default: |
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$json_text = to_json $perl_scalar |
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Converts the given Perl data structure to a UTF-8 encoded, binary |
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string (that is, the string contains octets only). Croaks on error. |
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This function call is functionally identical to: |
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$json_text = JSON::XS->new->utf8->encode ($perl_scalar) |
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except being faster. |
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$perl_scalar = from_json $json_text |
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The opposite of "to_json": expects an UTF-8 (binary) string and |
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tries to parse that as an UTF-8 encoded JSON text, returning the |
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resulting reference. Croaks on error. |
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This function call is functionally identical to: |
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$perl_scalar = JSON::XS->new->utf8->decode ($json_text) |
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except being faster. |
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$is_boolean = JSON::XS::is_bool $scalar |
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Returns true if the passed scalar represents either JSON::XS::true |
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or JSON::XS::false, two constants that act like 1 and 0, |
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respectively and are used to represent JSON "true" and "false" |
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values in Perl. |
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See MAPPING, below, for more information on how JSON values are |
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mapped to Perl. |
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A FEW NOTES ON UNICODE AND PERL |
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Since this often leads to confusion, here are a few very clear words on |
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how Unicode works in Perl, modulo bugs. |
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1. Perl strings can store characters with ordinal values > 255. |
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This enables you to store unicode characters as single characters in |
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a Perl string - very natural. |
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2. Perl does *not* associate an encoding with your strings. |
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Unless you force it to, e.g. when matching it against a regex, or |
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printing the scalar to a file, in which case Perl either interprets |
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your string as locale-encoded text, octets/binary, or as Unicode, |
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depending on various settings. In no case is an encoding stored |
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together with your data, it is *use* that decides encoding, not any |
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magical metadata. |
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3. The internal utf-8 flag has no meaning with regards to the encoding |
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of your string. |
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Just ignore that flag unless you debug a Perl bug, a module written |
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in XS or want to dive into the internals of perl. Otherwise it will |
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only confuse you, as, despite the name, it says nothing about how |
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your string is encoded. You can have unicode strings with that flag |
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set, with that flag clear, and you can have binary data with that |
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flag set and that flag clear. Other possibilities exist, too. |
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If you didn't know about that flag, just the better, pretend it |
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doesn't exist. |
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4. A "Unicode String" is simply a string where each character can be |
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validly interpreted as a Unicode codepoint. |
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If you have UTF-8 encoded data, it is no longer a Unicode string, |
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but a Unicode string encoded in UTF-8, giving you a binary string. |
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5. A string containing "high" (> 255) character values is *not* a UTF-8 |
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string. |
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Its a fact. Learn to live with it. |
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I hope this helps :) |
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OBJECT-ORIENTED INTERFACE |
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The object oriented interface lets you configure your own encoding or |
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decoding style, within the limits of supported formats. |
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$json = new JSON::XS |
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Creates a new JSON::XS object that can be used to de/encode JSON |
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strings. All boolean flags described below are by default |
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*disabled*. |
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The mutators for flags all return the JSON object again and thus |
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calls can be chained: |
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my $json = JSON::XS->new->utf8->space_after->encode ({a => [1,2]}) |
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=> {"a": [1, 2]} |
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$json = $json->ascii ([$enable]) |
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If $enable is true (or missing), then the "encode" method will not |
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generate characters outside the code range 0..127 (which is ASCII). |
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Any unicode characters outside that range will be escaped using |
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either a single \uXXXX (BMP characters) or a double \uHHHH\uLLLLL |
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escape sequence, as per RFC4627. The resulting encoded JSON text can |
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be treated as a native unicode string, an ascii-encoded, |
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latin1-encoded or UTF-8 encoded string, or any other superset of |
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ASCII. |
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If $enable is false, then the "encode" method will not escape |
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Unicode characters unless required by the JSON syntax or other |
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flags. This results in a faster and more compact format. |
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The main use for this flag is to produce JSON texts that can be |
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transmitted over a 7-bit channel, as the encoded JSON texts will not |
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contain any 8 bit characters. |
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JSON::XS->new->ascii (1)->encode ([chr 0x10401]) |
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=> ["\ud801\udc01"] |
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$json = $json->latin1 ([$enable]) |
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If $enable is true (or missing), then the "encode" method will |
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encode the resulting JSON text as latin1 (or iso-8859-1), escaping |
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any characters outside the code range 0..255. The resulting string |
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can be treated as a latin1-encoded JSON text or a native unicode |
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string. The "decode" method will not be affected in any way by this |
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flag, as "decode" by default expects unicode, which is a strict |
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superset of latin1. |
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If $enable is false, then the "encode" method will not escape |
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Unicode characters unless required by the JSON syntax or other |
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flags. |
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The main use for this flag is efficiently encoding binary data as |
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JSON text, as most octets will not be escaped, resulting in a |
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smaller encoded size. The disadvantage is that the resulting JSON |
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text is encoded in latin1 (and must correctly be treated as such |
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when storing and transfering), a rare encoding for JSON. It is |
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therefore most useful when you want to store data structures known |
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to contain binary data efficiently in files or databases, not when |
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talking to other JSON encoders/decoders. |
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JSON::XS->new->latin1->encode (["\x{89}\x{abc}"] |
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=> ["\x{89}\\u0abc"] # (perl syntax, U+abc escaped, U+89 not) |
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$json = $json->utf8 ([$enable]) |
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If $enable is true (or missing), then the "encode" method will |
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encode the JSON result into UTF-8, as required by many protocols, |
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while the "decode" method expects to be handled an UTF-8-encoded |
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string. Please note that UTF-8-encoded strings do not contain any |
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characters outside the range 0..255, they are thus useful for |
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bytewise/binary I/O. In future versions, enabling this option might |
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enable autodetection of the UTF-16 and UTF-32 encoding families, as |
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described in RFC4627. |
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If $enable is false, then the "encode" method will return the JSON |
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string as a (non-encoded) unicode string, while "decode" expects |
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thus a unicode string. Any decoding or encoding (e.g. to UTF-8 or |
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UTF-16) needs to be done yourself, e.g. using the Encode module. |
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Example, output UTF-16BE-encoded JSON: |
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use Encode; |
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$jsontext = encode "UTF-16BE", JSON::XS->new->encode ($object); |
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Example, decode UTF-32LE-encoded JSON: |
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use Encode; |
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$object = JSON::XS->new->decode (decode "UTF-32LE", $jsontext); |
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$json = $json->pretty ([$enable]) |
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This enables (or disables) all of the "indent", "space_before" and |
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"space_after" (and in the future possibly more) flags in one call to |
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generate the most readable (or most compact) form possible. |
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Example, pretty-print some simple structure: |
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my $json = JSON::XS->new->pretty(1)->encode ({a => [1,2]}) |
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=> |
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{ |
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"a" : [ |
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1, |
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2 |
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] |
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} |
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$json = $json->indent ([$enable]) |
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If $enable is true (or missing), then the "encode" method will use a |
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multiline format as output, putting every array member or |
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object/hash key-value pair into its own line, identing them |
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properly. |
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If $enable is false, no newlines or indenting will be produced, and |
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the resulting JSON text is guarenteed not to contain any "newlines". |
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This setting has no effect when decoding JSON texts. |
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$json = $json->space_before ([$enable]) |
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If $enable is true (or missing), then the "encode" method will add |
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an extra optional space before the ":" separating keys from values |
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in JSON objects. |
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If $enable is false, then the "encode" method will not add any extra |
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space at those places. |
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This setting has no effect when decoding JSON texts. You will also |
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most likely combine this setting with "space_after". |
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Example, space_before enabled, space_after and indent disabled: |
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{"key" :"value"} |
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$json = $json->space_after ([$enable]) |
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If $enable is true (or missing), then the "encode" method will add |
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an extra optional space after the ":" separating keys from values in |
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JSON objects and extra whitespace after the "," separating key-value |
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pairs and array members. |
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If $enable is false, then the "encode" method will not add any extra |
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space at those places. |
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This setting has no effect when decoding JSON texts. |
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Example, space_before and indent disabled, space_after enabled: |
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{"key": "value"} |
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$json = $json->relaxed ([$enable]) |
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If $enable is true (or missing), then "decode" will accept some |
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extensions to normal JSON syntax (see below). "encode" will not be |
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affected in anyway. *Be aware that this option makes you accept |
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invalid JSON texts as if they were valid!*. I suggest only to use |
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this option to parse application-specific files written by humans |
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(configuration files, resource files etc.) |
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If $enable is false (the default), then "decode" will only accept |
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valid JSON texts. |
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Currently accepted extensions are: |
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* list items can have an end-comma |
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JSON *separates* array elements and key-value pairs with commas. |
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This can be annoying if you write JSON texts manually and want |
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to be able to quickly append elements, so this extension accepts |
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comma at the end of such items not just between them: |
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[ |
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1, |
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2, <- this comma not normally allowed |
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] |
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{ |
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"k1": "v1", |
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"k2": "v2", <- this comma not normally allowed |
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} |
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* shell-style '#'-comments |
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Whenever JSON allows whitespace, shell-style comments are |
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additionally allowed. They are terminated by the first |
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carriage-return or line-feed character, after which more |
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white-space and comments are allowed. |
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[ |
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1, # this comment not allowed in JSON |
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# neither this one... |
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] |
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$json = $json->canonical ([$enable]) |
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If $enable is true (or missing), then the "encode" method will |
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output JSON objects by sorting their keys. This is adding a |
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comparatively high overhead. |
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If $enable is false, then the "encode" method will output key-value |
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pairs in the order Perl stores them (which will likely change |
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between runs of the same script). |
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This option is useful if you want the same data structure to be |
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encoded as the same JSON text (given the same overall settings). If |
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it is disabled, the same hash migh be encoded differently even if |
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contains the same data, as key-value pairs have no inherent ordering |
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in Perl. |
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This setting has no effect when decoding JSON texts. |
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$json = $json->allow_nonref ([$enable]) |
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If $enable is true (or missing), then the "encode" method can |
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convert a non-reference into its corresponding string, number or |
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null JSON value, which is an extension to RFC4627. Likewise, |
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"decode" will accept those JSON values instead of croaking. |
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If $enable is false, then the "encode" method will croak if it isn't |
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passed an arrayref or hashref, as JSON texts must either be an |
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object or array. Likewise, "decode" will croak if given something |
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that is not a JSON object or array. |
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Example, encode a Perl scalar as JSON value with enabled |
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"allow_nonref", resulting in an invalid JSON text: |
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JSON::XS->new->allow_nonref->encode ("Hello, World!") |
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=> "Hello, World!" |
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$json = $json->allow_blessed ([$enable]) |
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If $enable is true (or missing), then the "encode" method will not |
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barf when it encounters a blessed reference. Instead, the value of |
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the convert_blessed option will decide wether "null" |
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("convert_blessed" disabled or no "to_json" method found) or a |
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representation of the object ("convert_blessed" enabled and |
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"to_json" method found) is being encoded. Has no effect on "decode". |
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If $enable is false (the default), then "encode" will throw an |
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exception when it encounters a blessed object. |
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|
$json = $json->convert_blessed ([$enable]) |
372 |
|
|
If $enable is true (or missing), then "encode", upon encountering a |
373 |
|
|
blessed object, will check for the availability of the "TO_JSON" |
374 |
|
|
method on the object's class. If found, it will be called in scalar |
375 |
|
|
context and the resulting scalar will be encoded instead of the |
376 |
|
|
object. If no "TO_JSON" method is found, the value of |
377 |
|
|
"allow_blessed" will decide what to do. |
378 |
|
|
|
379 |
|
|
The "TO_JSON" method may safely call die if it wants. If "TO_JSON" |
380 |
|
|
returns other blessed objects, those will be handled in the same |
381 |
|
|
way. "TO_JSON" must take care of not causing an endless recursion |
382 |
|
|
cycle (== crash) in this case. The name of "TO_JSON" was chosen |
383 |
|
|
because other methods called by the Perl core (== not by the user of |
384 |
|
|
the object) are usually in upper case letters and to avoid |
385 |
|
|
collisions with the "to_json" function. |
386 |
|
|
|
387 |
|
|
This setting does not yet influence "decode" in any way, but in the |
388 |
|
|
future, global hooks might get installed that influence "decode" and |
389 |
|
|
are enabled by this setting. |
390 |
|
|
|
391 |
|
|
If $enable is false, then the "allow_blessed" setting will decide |
392 |
|
|
what to do when a blessed object is found. |
393 |
|
|
|
394 |
|
|
$json = $json->filter_json_object ([$coderef->($hashref)]) |
395 |
|
|
When $coderef is specified, it will be called from "decode" each |
396 |
|
|
time it decodes a JSON object. The only argument is a reference to |
397 |
|
|
the newly-created hash. If the code references returns a single |
398 |
|
|
scalar (which need not be a reference), this value (i.e. a copy of |
399 |
|
|
that scalar to avoid aliasing) is inserted into the deserialised |
400 |
|
|
data structure. If it returns an empty list (NOTE: *not* "undef", |
401 |
|
|
which is a valid scalar), the original deserialised hash will be |
402 |
|
|
inserted. This setting can slow down decoding considerably. |
403 |
|
|
|
404 |
|
|
When $coderef is omitted or undefined, any existing callback will be |
405 |
|
|
removed and "decode" will not change the deserialised hash in any |
406 |
|
|
way. |
407 |
|
|
|
408 |
|
|
Example, convert all JSON objects into the integer 5: |
409 |
|
|
|
410 |
|
|
my $js = JSON::XS->new->filter_json_object (sub { 5 }); |
411 |
|
|
# returns [5] |
412 |
|
|
$js->decode ('[{}]') |
413 |
|
|
# throw an exception because allow_nonref is not enabled |
414 |
|
|
# so a lone 5 is not allowed. |
415 |
|
|
$js->decode ('{"a":1, "b":2}'); |
416 |
|
|
|
417 |
|
|
$json = $json->filter_json_single_key_object ($key [=> |
418 |
|
|
$coderef->($value)]) |
419 |
|
|
Works remotely similar to "filter_json_object", but is only called |
420 |
|
|
for JSON objects having a single key named $key. |
421 |
|
|
|
422 |
|
|
This $coderef is called before the one specified via |
423 |
|
|
"filter_json_object", if any. It gets passed the single value in the |
424 |
|
|
JSON object. If it returns a single value, it will be inserted into |
425 |
|
|
the data structure. If it returns nothing (not even "undef" but the |
426 |
|
|
empty list), the callback from "filter_json_object" will be called |
427 |
|
|
next, as if no single-key callback were specified. |
428 |
|
|
|
429 |
|
|
If $coderef is omitted or undefined, the corresponding callback will |
430 |
|
|
be disabled. There can only ever be one callback for a given key. |
431 |
|
|
|
432 |
|
|
As this callback gets called less often then the |
433 |
|
|
"filter_json_object" one, decoding speed will not usually suffer as |
434 |
|
|
much. Therefore, single-key objects make excellent targets to |
435 |
|
|
serialise Perl objects into, especially as single-key JSON objects |
436 |
|
|
are as close to the type-tagged value concept as JSON gets (its |
437 |
|
|
basically an ID/VALUE tuple). Of course, JSON does not support this |
438 |
|
|
in any way, so you need to make sure your data never looks like a |
439 |
|
|
serialised Perl hash. |
440 |
|
|
|
441 |
|
|
Typical names for the single object key are "__class_whatever__", or |
442 |
|
|
"$__dollars_are_rarely_used__$" or "}ugly_brace_placement", or even |
443 |
|
|
things like "__class_md5sum(classname)__", to reduce the risk of |
444 |
|
|
clashing with real hashes. |
445 |
|
|
|
446 |
|
|
Example, decode JSON objects of the form "{ "__widget__" => <id> }" |
447 |
|
|
into the corresponding $WIDGET{<id>} object: |
448 |
|
|
|
449 |
|
|
# return whatever is in $WIDGET{5}: |
450 |
|
|
JSON::XS |
451 |
|
|
->new |
452 |
|
|
->filter_json_single_key_object (__widget__ => sub { |
453 |
|
|
$WIDGET{ $_[0] } |
454 |
|
|
}) |
455 |
|
|
->decode ('{"__widget__": 5') |
456 |
|
|
|
457 |
|
|
# this can be used with a TO_JSON method in some "widget" class |
458 |
|
|
# for serialisation to json: |
459 |
|
|
sub WidgetBase::TO_JSON { |
460 |
|
|
my ($self) = @_; |
461 |
|
|
|
462 |
|
|
unless ($self->{id}) { |
463 |
|
|
$self->{id} = ..get..some..id..; |
464 |
|
|
$WIDGET{$self->{id}} = $self; |
465 |
|
|
} |
466 |
|
|
|
467 |
|
|
{ __widget__ => $self->{id} } |
468 |
|
|
} |
469 |
|
|
|
470 |
|
|
$json = $json->shrink ([$enable]) |
471 |
|
|
Perl usually over-allocates memory a bit when allocating space for |
472 |
|
|
strings. This flag optionally resizes strings generated by either |
473 |
|
|
"encode" or "decode" to their minimum size possible. This can save |
474 |
|
|
memory when your JSON texts are either very very long or you have |
475 |
|
|
many short strings. It will also try to downgrade any strings to |
476 |
|
|
octet-form if possible: perl stores strings internally either in an |
477 |
|
|
encoding called UTF-X or in octet-form. The latter cannot store |
478 |
|
|
everything but uses less space in general (and some buggy Perl or C |
479 |
|
|
code might even rely on that internal representation being used). |
480 |
|
|
|
481 |
|
|
The actual definition of what shrink does might change in future |
482 |
|
|
versions, but it will always try to save space at the expense of |
483 |
|
|
time. |
484 |
|
|
|
485 |
|
|
If $enable is true (or missing), the string returned by "encode" |
486 |
|
|
will be shrunk-to-fit, while all strings generated by "decode" will |
487 |
|
|
also be shrunk-to-fit. |
488 |
|
|
|
489 |
|
|
If $enable is false, then the normal perl allocation algorithms are |
490 |
|
|
used. If you work with your data, then this is likely to be faster. |
491 |
|
|
|
492 |
|
|
In the future, this setting might control other things, such as |
493 |
|
|
converting strings that look like integers or floats into integers |
494 |
|
|
or floats internally (there is no difference on the Perl level), |
495 |
|
|
saving space. |
496 |
|
|
|
497 |
|
|
$json = $json->max_depth ([$maximum_nesting_depth]) |
498 |
|
|
Sets the maximum nesting level (default 512) accepted while encoding |
499 |
|
|
or decoding. If the JSON text or Perl data structure has an equal or |
500 |
|
|
higher nesting level then this limit, then the encoder and decoder |
501 |
|
|
will stop and croak at that point. |
502 |
|
|
|
503 |
|
|
Nesting level is defined by number of hash- or arrayrefs that the |
504 |
|
|
encoder needs to traverse to reach a given point or the number of |
505 |
|
|
"{" or "[" characters without their matching closing parenthesis |
506 |
|
|
crossed to reach a given character in a string. |
507 |
|
|
|
508 |
|
|
Setting the maximum depth to one disallows any nesting, so that |
509 |
|
|
ensures that the object is only a single hash/object or array. |
510 |
|
|
|
511 |
|
|
The argument to "max_depth" will be rounded up to the next highest |
512 |
|
|
power of two. If no argument is given, the highest possible setting |
513 |
|
|
will be used, which is rarely useful. |
514 |
|
|
|
515 |
|
|
See SECURITY CONSIDERATIONS, below, for more info on why this is |
516 |
|
|
useful. |
517 |
|
|
|
518 |
|
|
$json = $json->max_size ([$maximum_string_size]) |
519 |
|
|
Set the maximum length a JSON text may have (in bytes) where |
520 |
|
|
decoding is being attempted. The default is 0, meaning no limit. |
521 |
|
|
When "decode" is called on a string longer then this number of |
522 |
|
|
characters it will not attempt to decode the string but throw an |
523 |
|
|
exception. This setting has no effect on "encode" (yet). |
524 |
|
|
|
525 |
|
|
The argument to "max_size" will be rounded up to the next highest |
526 |
|
|
power of two (so may be more than requested). If no argument is |
527 |
|
|
given, the limit check will be deactivated (same as when 0 is |
528 |
|
|
specified). |
529 |
|
|
|
530 |
|
|
See SECURITY CONSIDERATIONS, below, for more info on why this is |
531 |
|
|
useful. |
532 |
|
|
|
533 |
|
|
$json_text = $json->encode ($perl_scalar) |
534 |
|
|
Converts the given Perl data structure (a simple scalar or a |
535 |
|
|
reference to a hash or array) to its JSON representation. Simple |
536 |
|
|
scalars will be converted into JSON string or number sequences, |
537 |
|
|
while references to arrays become JSON arrays and references to |
538 |
|
|
hashes become JSON objects. Undefined Perl values (e.g. "undef") |
539 |
|
|
become JSON "null" values. Neither "true" nor "false" values will be |
540 |
|
|
generated. |
541 |
|
|
|
542 |
|
|
$perl_scalar = $json->decode ($json_text) |
543 |
|
|
The opposite of "encode": expects a JSON text and tries to parse it, |
544 |
|
|
returning the resulting simple scalar or reference. Croaks on error. |
545 |
|
|
|
546 |
|
|
JSON numbers and strings become simple Perl scalars. JSON arrays |
547 |
|
|
become Perl arrayrefs and JSON objects become Perl hashrefs. "true" |
548 |
|
|
becomes 1, "false" becomes 0 and "null" becomes "undef". |
549 |
|
|
|
550 |
|
|
($perl_scalar, $characters) = $json->decode_prefix ($json_text) |
551 |
|
|
This works like the "decode" method, but instead of raising an |
552 |
|
|
exception when there is trailing garbage after the first JSON |
553 |
|
|
object, it will silently stop parsing there and return the number of |
554 |
|
|
characters consumed so far. |
555 |
|
|
|
556 |
|
|
This is useful if your JSON texts are not delimited by an outer |
557 |
|
|
protocol (which is not the brightest thing to do in the first place) |
558 |
|
|
and you need to know where the JSON text ends. |
559 |
|
|
|
560 |
|
|
JSON::XS->new->decode_prefix ("[1] the tail") |
561 |
|
|
=> ([], 3) |
562 |
|
|
|
563 |
|
|
MAPPING |
564 |
|
|
This section describes how JSON::XS maps Perl values to JSON values and |
565 |
|
|
vice versa. These mappings are designed to "do the right thing" in most |
566 |
|
|
circumstances automatically, preserving round-tripping characteristics |
567 |
|
|
(what you put in comes out as something equivalent). |
568 |
|
|
|
569 |
|
|
For the more enlightened: note that in the following descriptions, |
570 |
|
|
lowercase *perl* refers to the Perl interpreter, while uppcercase *Perl* |
571 |
|
|
refers to the abstract Perl language itself. |
572 |
|
|
|
573 |
|
|
JSON -> PERL |
574 |
|
|
object |
575 |
|
|
A JSON object becomes a reference to a hash in Perl. No ordering of |
576 |
|
|
object keys is preserved (JSON does not preserver object key |
577 |
|
|
ordering itself). |
578 |
|
|
|
579 |
|
|
array |
580 |
|
|
A JSON array becomes a reference to an array in Perl. |
581 |
|
|
|
582 |
|
|
string |
583 |
|
|
A JSON string becomes a string scalar in Perl - Unicode codepoints |
584 |
|
|
in JSON are represented by the same codepoints in the Perl string, |
585 |
|
|
so no manual decoding is necessary. |
586 |
|
|
|
587 |
|
|
number |
588 |
|
|
A JSON number becomes either an integer, numeric (floating point) or |
589 |
|
|
string scalar in perl, depending on its range and any fractional |
590 |
|
|
parts. On the Perl level, there is no difference between those as |
591 |
|
|
Perl handles all the conversion details, but an integer may take |
592 |
|
|
slightly less memory and might represent more values exactly than |
593 |
|
|
(floating point) numbers. |
594 |
|
|
|
595 |
|
|
If the number consists of digits only, JSON::XS will try to |
596 |
|
|
represent it as an integer value. If that fails, it will try to |
597 |
|
|
represent it as a numeric (floating point) value if that is possible |
598 |
|
|
without loss of precision. Otherwise it will preserve the number as |
599 |
|
|
a string value. |
600 |
|
|
|
601 |
|
|
Numbers containing a fractional or exponential part will always be |
602 |
|
|
represented as numeric (floating point) values, possibly at a loss |
603 |
|
|
of precision. |
604 |
|
|
|
605 |
|
|
This might create round-tripping problems as numbers might become |
606 |
|
|
strings, but as Perl is typeless there is no other way to do it. |
607 |
|
|
|
608 |
|
|
true, false |
609 |
|
|
These JSON atoms become "JSON::XS::true" and "JSON::XS::false", |
610 |
|
|
respectively. They are overloaded to act almost exactly like the |
611 |
|
|
numbers 1 and 0. You can check wether a scalar is a JSON boolean by |
612 |
|
|
using the "JSON::XS::is_bool" function. |
613 |
|
|
|
614 |
|
|
null |
615 |
|
|
A JSON null atom becomes "undef" in Perl. |
616 |
|
|
|
617 |
|
|
PERL -> JSON |
618 |
|
|
The mapping from Perl to JSON is slightly more difficult, as Perl is a |
619 |
|
|
truly typeless language, so we can only guess which JSON type is meant |
620 |
|
|
by a Perl value. |
621 |
|
|
|
622 |
|
|
hash references |
623 |
|
|
Perl hash references become JSON objects. As there is no inherent |
624 |
|
|
ordering in hash keys (or JSON objects), they will usually be |
625 |
|
|
encoded in a pseudo-random order that can change between runs of the |
626 |
|
|
same program but stays generally the same within a single run of a |
627 |
|
|
program. JSON::XS can optionally sort the hash keys (determined by |
628 |
|
|
the *canonical* flag), so the same datastructure will serialise to |
629 |
|
|
the same JSON text (given same settings and version of JSON::XS), |
630 |
|
|
but this incurs a runtime overhead and is only rarely useful, e.g. |
631 |
|
|
when you want to compare some JSON text against another for |
632 |
|
|
equality. |
633 |
|
|
|
634 |
|
|
array references |
635 |
|
|
Perl array references become JSON arrays. |
636 |
|
|
|
637 |
|
|
other references |
638 |
|
|
Other unblessed references are generally not allowed and will cause |
639 |
|
|
an exception to be thrown, except for references to the integers 0 |
640 |
|
|
and 1, which get turned into "false" and "true" atoms in JSON. You |
641 |
|
|
can also use "JSON::XS::false" and "JSON::XS::true" to improve |
642 |
|
|
readability. |
643 |
|
|
|
644 |
|
|
to_json [\0,JSON::XS::true] # yields [false,true] |
645 |
|
|
|
646 |
|
|
JSON::XS::true, JSON::XS::false |
647 |
|
|
These special values become JSON true and JSON false values, |
648 |
|
|
respectively. You can also use "\1" and "\0" directly if you want. |
649 |
|
|
|
650 |
|
|
blessed objects |
651 |
|
|
Blessed objects are not allowed. JSON::XS currently tries to encode |
652 |
|
|
their underlying representation (hash- or arrayref), but this |
653 |
|
|
behaviour might change in future versions. |
654 |
|
|
|
655 |
|
|
simple scalars |
656 |
|
|
Simple Perl scalars (any scalar that is not a reference) are the |
657 |
|
|
most difficult objects to encode: JSON::XS will encode undefined |
658 |
|
|
scalars as JSON null value, scalars that have last been used in a |
659 |
|
|
string context before encoding as JSON strings and anything else as |
660 |
|
|
number value: |
661 |
|
|
|
662 |
|
|
# dump as number |
663 |
|
|
to_json [2] # yields [2] |
664 |
|
|
to_json [-3.0e17] # yields [-3e+17] |
665 |
|
|
my $value = 5; to_json [$value] # yields [5] |
666 |
|
|
|
667 |
|
|
# used as string, so dump as string |
668 |
|
|
print $value; |
669 |
|
|
to_json [$value] # yields ["5"] |
670 |
|
|
|
671 |
|
|
# undef becomes null |
672 |
|
|
to_json [undef] # yields [null] |
673 |
|
|
|
674 |
|
|
You can force the type to be a string by stringifying it: |
675 |
|
|
|
676 |
|
|
my $x = 3.1; # some variable containing a number |
677 |
|
|
"$x"; # stringified |
678 |
|
|
$x .= ""; # another, more awkward way to stringify |
679 |
|
|
print $x; # perl does it for you, too, quite often |
680 |
|
|
|
681 |
|
|
You can force the type to be a number by numifying it: |
682 |
|
|
|
683 |
|
|
my $x = "3"; # some variable containing a string |
684 |
|
|
$x += 0; # numify it, ensuring it will be dumped as a number |
685 |
|
|
$x *= 1; # same thing, the choise is yours. |
686 |
|
|
|
687 |
|
|
You can not currently output JSON booleans or force the type in |
688 |
|
|
other, less obscure, ways. Tell me if you need this capability. |
689 |
|
|
|
690 |
|
|
COMPARISON |
691 |
|
|
As already mentioned, this module was created because none of the |
692 |
|
|
existing JSON modules could be made to work correctly. First I will |
693 |
|
|
describe the problems (or pleasures) I encountered with various existing |
694 |
|
|
JSON modules, followed by some benchmark values. JSON::XS was designed |
695 |
|
|
not to suffer from any of these problems or limitations. |
696 |
|
|
|
697 |
|
|
JSON 1.07 |
698 |
|
|
Slow (but very portable, as it is written in pure Perl). |
699 |
|
|
|
700 |
|
|
Undocumented/buggy Unicode handling (how JSON handles unicode values |
701 |
|
|
is undocumented. One can get far by feeding it unicode strings and |
702 |
|
|
doing en-/decoding oneself, but unicode escapes are not working |
703 |
|
|
properly). |
704 |
|
|
|
705 |
|
|
No roundtripping (strings get clobbered if they look like numbers, |
706 |
|
|
e.g. the string 2.0 will encode to 2.0 instead of "2.0", and that |
707 |
|
|
will decode into the number 2. |
708 |
|
|
|
709 |
|
|
JSON::PC 0.01 |
710 |
|
|
Very fast. |
711 |
|
|
|
712 |
|
|
Undocumented/buggy Unicode handling. |
713 |
|
|
|
714 |
|
|
No roundtripping. |
715 |
|
|
|
716 |
|
|
Has problems handling many Perl values (e.g. regex results and other |
717 |
|
|
magic values will make it croak). |
718 |
|
|
|
719 |
|
|
Does not even generate valid JSON ("{1,2}" gets converted to "{1:2}" |
720 |
|
|
which is not a valid JSON text. |
721 |
|
|
|
722 |
|
|
Unmaintained (maintainer unresponsive for many months, bugs are not |
723 |
|
|
getting fixed). |
724 |
|
|
|
725 |
|
|
JSON::Syck 0.21 |
726 |
|
|
Very buggy (often crashes). |
727 |
|
|
|
728 |
|
|
Very inflexible (no human-readable format supported, format pretty |
729 |
|
|
much undocumented. I need at least a format for easy reading by |
730 |
|
|
humans and a single-line compact format for use in a protocol, and |
731 |
|
|
preferably a way to generate ASCII-only JSON texts). |
732 |
|
|
|
733 |
|
|
Completely broken (and confusingly documented) Unicode handling |
734 |
|
|
(unicode escapes are not working properly, you need to set |
735 |
|
|
ImplicitUnicode to *different* values on en- and decoding to get |
736 |
|
|
symmetric behaviour). |
737 |
|
|
|
738 |
|
|
No roundtripping (simple cases work, but this depends on wether the |
739 |
|
|
scalar value was used in a numeric context or not). |
740 |
|
|
|
741 |
|
|
Dumping hashes may skip hash values depending on iterator state. |
742 |
|
|
|
743 |
|
|
Unmaintained (maintainer unresponsive for many months, bugs are not |
744 |
|
|
getting fixed). |
745 |
|
|
|
746 |
|
|
Does not check input for validity (i.e. will accept non-JSON input |
747 |
|
|
and return "something" instead of raising an exception. This is a |
748 |
|
|
security issue: imagine two banks transfering money between each |
749 |
|
|
other using JSON. One bank might parse a given non-JSON request and |
750 |
|
|
deduct money, while the other might reject the transaction with a |
751 |
|
|
syntax error. While a good protocol will at least recover, that is |
752 |
|
|
extra unnecessary work and the transaction will still not succeed). |
753 |
|
|
|
754 |
|
|
JSON::DWIW 0.04 |
755 |
|
|
Very fast. Very natural. Very nice. |
756 |
|
|
|
757 |
|
|
Undocumented unicode handling (but the best of the pack. Unicode |
758 |
|
|
escapes still don't get parsed properly). |
759 |
|
|
|
760 |
|
|
Very inflexible. |
761 |
|
|
|
762 |
|
|
No roundtripping. |
763 |
|
|
|
764 |
|
|
Does not generate valid JSON texts (key strings are often unquoted, |
765 |
|
|
empty keys result in nothing being output) |
766 |
|
|
|
767 |
|
|
Does not check input for validity. |
768 |
|
|
|
769 |
|
|
JSON and YAML |
770 |
|
|
You often hear that JSON is a subset (or a close subset) of YAML. This |
771 |
|
|
is, however, a mass hysteria and very far from the truth. In general, |
772 |
|
|
there is no way to configure JSON::XS to output a data structure as |
773 |
|
|
valid YAML. |
774 |
|
|
|
775 |
|
|
If you really must use JSON::XS to generate YAML, you should use this |
776 |
|
|
algorithm (subject to change in future versions): |
777 |
|
|
|
778 |
|
|
my $to_yaml = JSON::XS->new->utf8->space_after (1); |
779 |
|
|
my $yaml = $to_yaml->encode ($ref) . "\n"; |
780 |
|
|
|
781 |
|
|
This will usually generate JSON texts that also parse as valid YAML. |
782 |
|
|
Please note that YAML has hardcoded limits on (simple) object key |
783 |
|
|
lengths that JSON doesn't have, so you should make sure that your hash |
784 |
|
|
keys are noticably shorter than the 1024 characters YAML allows. |
785 |
|
|
|
786 |
|
|
There might be other incompatibilities that I am not aware of. In |
787 |
|
|
general you should not try to generate YAML with a JSON generator or |
788 |
|
|
vice versa, or try to parse JSON with a YAML parser or vice versa: |
789 |
|
|
chances are high that you will run into severe interoperability |
790 |
|
|
problems. |
791 |
|
|
|
792 |
|
|
SPEED |
793 |
|
|
It seems that JSON::XS is surprisingly fast, as shown in the following |
794 |
|
|
tables. They have been generated with the help of the "eg/bench" program |
795 |
|
|
in the JSON::XS distribution, to make it easy to compare on your own |
796 |
|
|
system. |
797 |
|
|
|
798 |
|
|
First comes a comparison between various modules using a very short |
799 |
|
|
single-line JSON string: |
800 |
|
|
|
801 |
|
|
{"method": "handleMessage", "params": ["user1", "we were just talking"], \ |
802 |
|
|
"id": null, "array":[1,11,234,-5,1e5,1e7, true, false]} |
803 |
|
|
|
804 |
|
|
It shows the number of encodes/decodes per second (JSON::XS uses the |
805 |
|
|
functional interface, while JSON::XS/2 uses the OO interface with |
806 |
|
|
pretty-printing and hashkey sorting enabled, JSON::XS/3 enables shrink). |
807 |
|
|
Higher is better: |
808 |
|
|
|
809 |
|
|
Storable | 15779.925 | 14169.946 | |
810 |
|
|
-----------+------------+------------+ |
811 |
|
|
module | encode | decode | |
812 |
|
|
-----------|------------|------------| |
813 |
|
|
JSON | 4990.842 | 4088.813 | |
814 |
|
|
JSON::DWIW | 51653.990 | 71575.154 | |
815 |
|
|
JSON::PC | 65948.176 | 74631.744 | |
816 |
|
|
JSON::PP | 8931.652 | 3817.168 | |
817 |
|
|
JSON::Syck | 24877.248 | 27776.848 | |
818 |
|
|
JSON::XS | 388361.481 | 227951.304 | |
819 |
|
|
JSON::XS/2 | 227951.304 | 218453.333 | |
820 |
|
|
JSON::XS/3 | 338250.323 | 218453.333 | |
821 |
|
|
Storable | 16500.016 | 135300.129 | |
822 |
|
|
-----------+------------+------------+ |
823 |
|
|
|
824 |
|
|
That is, JSON::XS is about five times faster than JSON::DWIW on |
825 |
|
|
encoding, about three times faster on decoding, and over fourty times |
826 |
|
|
faster than JSON, even with pretty-printing and key sorting. It also |
827 |
|
|
compares favourably to Storable for small amounts of data. |
828 |
|
|
|
829 |
|
|
Using a longer test string (roughly 18KB, generated from Yahoo! Locals |
830 |
|
|
search API (http://nanoref.com/yahooapis/mgPdGg): |
831 |
|
|
|
832 |
|
|
module | encode | decode | |
833 |
|
|
-----------|------------|------------| |
834 |
|
|
JSON | 55.260 | 34.971 | |
835 |
|
|
JSON::DWIW | 825.228 | 1082.513 | |
836 |
|
|
JSON::PC | 3571.444 | 2394.829 | |
837 |
|
|
JSON::PP | 210.987 | 32.574 | |
838 |
|
|
JSON::Syck | 552.551 | 787.544 | |
839 |
|
|
JSON::XS | 5780.463 | 4854.519 | |
840 |
|
|
JSON::XS/2 | 3869.998 | 4798.975 | |
841 |
|
|
JSON::XS/3 | 5862.880 | 4798.975 | |
842 |
|
|
Storable | 4445.002 | 5235.027 | |
843 |
|
|
-----------+------------+------------+ |
844 |
|
|
|
845 |
|
|
Again, JSON::XS leads by far (except for Storable which non-surprisingly |
846 |
|
|
decodes faster). |
847 |
|
|
|
848 |
|
|
On large strings containing lots of high unicode characters, some |
849 |
|
|
modules (such as JSON::PC) seem to decode faster than JSON::XS, but the |
850 |
|
|
result will be broken due to missing (or wrong) unicode handling. Others |
851 |
|
|
refuse to decode or encode properly, so it was impossible to prepare a |
852 |
|
|
fair comparison table for that case. |
853 |
|
|
|
854 |
|
|
SECURITY CONSIDERATIONS |
855 |
|
|
When you are using JSON in a protocol, talking to untrusted potentially |
856 |
|
|
hostile creatures requires relatively few measures. |
857 |
|
|
|
858 |
|
|
First of all, your JSON decoder should be secure, that is, should not |
859 |
|
|
have any buffer overflows. Obviously, this module should ensure that and |
860 |
|
|
I am trying hard on making that true, but you never know. |
861 |
|
|
|
862 |
|
|
Second, you need to avoid resource-starving attacks. That means you |
863 |
|
|
should limit the size of JSON texts you accept, or make sure then when |
864 |
|
|
your resources run out, thats just fine (e.g. by using a separate |
865 |
|
|
process that can crash safely). The size of a JSON text in octets or |
866 |
|
|
characters is usually a good indication of the size of the resources |
867 |
|
|
required to decode it into a Perl structure. While JSON::XS can check |
868 |
|
|
the size of the JSON text, it might be too late when you already have it |
869 |
|
|
in memory, so you might want to check the size before you accept the |
870 |
|
|
string. |
871 |
|
|
|
872 |
|
|
Third, JSON::XS recurses using the C stack when decoding objects and |
873 |
|
|
arrays. The C stack is a limited resource: for instance, on my amd64 |
874 |
|
|
machine with 8MB of stack size I can decode around 180k nested arrays |
875 |
|
|
but only 14k nested JSON objects (due to perl itself recursing deeply on |
876 |
|
|
croak to free the temporary). If that is exceeded, the program crashes. |
877 |
|
|
to be conservative, the default nesting limit is set to 512. If your |
878 |
|
|
process has a smaller stack, you should adjust this setting accordingly |
879 |
|
|
with the "max_depth" method. |
880 |
|
|
|
881 |
|
|
And last but least, something else could bomb you that I forgot to think |
882 |
|
|
of. In that case, you get to keep the pieces. I am always open for |
883 |
|
|
hints, though... |
884 |
|
|
|
885 |
|
|
If you are using JSON::XS to return packets to consumption by javascript |
886 |
|
|
scripts in a browser you should have a look at |
887 |
|
|
<http://jpsykes.com/47/practical-csrf-and-json-security> to see wether |
888 |
|
|
you are vulnerable to some common attack vectors (which really are |
889 |
|
|
browser design bugs, but it is still you who will have to deal with it, |
890 |
|
|
as major browser developers care only for features, not about doing |
891 |
|
|
security right). |
892 |
|
|
|
893 |
|
|
THREADS |
894 |
|
|
This module is *not* guarenteed to be thread safe and there are no plans |
895 |
|
|
to change this until Perl gets thread support (as opposed to the |
896 |
|
|
horribly slow so-called "threads" which are simply slow and bloated |
897 |
|
|
process simulations - use fork, its *much* faster, cheaper, better). |
898 |
|
|
|
899 |
|
|
(It might actually work, but you ahve ben warned). |
900 |
|
|
|
901 |
|
|
BUGS |
902 |
|
|
While the goal of this module is to be correct, that unfortunately does |
903 |
|
|
not mean its bug-free, only that I think its design is bug-free. It is |
904 |
|
|
still relatively early in its development. If you keep reporting bugs |
905 |
|
|
they will be fixed swiftly, though. |
906 |
|
|
|
907 |
|
|
Please refrain from using rt.cpan.org or any other bug reporting |
908 |
|
|
service. I put the contact address into my modules for a reason. |
909 |
|
|
|
910 |
|
|
AUTHOR |
911 |
|
|
Marc Lehmann <schmorp@schmorp.de> |
912 |
|
|
http://home.schmorp.de/ |
913 |
|
|
|