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NAME |
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JSON::XS - JSON serialising/deserialising, done correctly and fast |
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|
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JSON::XS - 正しくて高速な JSON 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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|
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SYNOPSIS |
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use JSON::XS; |
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|
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# exported functions, they croak on error |
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# and expect/generate UTF-8 |
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|
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$utf8_encoded_json_text = encode_json $perl_hash_or_arrayref; |
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$perl_hash_or_arrayref = decode_json $utf8_encoded_json_text; |
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|
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# OO-interface |
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|
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$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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|
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# Note that JSON version 2.0 and above will automatically use JSON::XS |
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# if available, at virtually no speed overhead either, so you should |
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# be able to just: |
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|
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use JSON; |
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|
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# and do the same things, except that you have a pure-perl fallback now. |
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|
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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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|
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Beginning with version 2.0 of the JSON module, when both JSON and |
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JSON::XS are installed, then JSON will fall back on JSON::XS (this can |
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be overriden) with no overhead due to emulation (by inheritign |
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constructor and methods). If JSON::XS is not available, it will fall |
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back to the compatible JSON::PP module as backend, so using JSON instead |
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of JSON::XS gives you a portable JSON API that can be fast when you need |
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and doesn't require a C compiler when that is a problem. |
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|
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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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|
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See COMPARISON, below, for a comparison to some other JSON modules. |
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|
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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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|
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FEATURES |
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* correct Unicode handling |
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|
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This module knows how to handle Unicode, documents how and when it |
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does so, and even documents what "correct" means. |
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|
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* round-trip integrity |
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|
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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). There minor *are* exceptions |
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to this, read the MAPPING section below to learn about those. |
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|
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* strict checking of JSON correctness |
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|
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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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|
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* fast |
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|
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Compared to other JSON modules and other serialisers such as |
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Storable, this module usually compares favourably in terms of speed, |
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too. |
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|
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* simple to use |
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|
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This module has both a simple functional interface as well as an |
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objetc oriented interface interface. |
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|
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* reasonably versatile output formats |
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|
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You can choose between the most compact guaranteed-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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|
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FUNCTIONAL INTERFACE |
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The following convenience methods are provided by this module. They are |
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exported by default: |
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|
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$json_text = encode_json $perl_scalar |
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Converts the given Perl data structure to a UTF-8 encoded, binary |
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string (that is, the string contains octets only). Croaks on error. |
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|
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This function call is functionally identical to: |
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|
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$json_text = JSON::XS->new->utf8->encode ($perl_scalar) |
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|
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except being faster. |
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|
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$perl_scalar = decode_json $json_text |
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The opposite of "encode_json": expects an UTF-8 (binary) string and |
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tries to parse that as an UTF-8 encoded JSON text, returning the |
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resulting reference. Croaks on error. |
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|
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This function call is functionally identical to: |
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|
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$perl_scalar = JSON::XS->new->utf8->decode ($json_text) |
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|
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except being faster. |
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|
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$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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|
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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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|
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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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|
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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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|
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2. Perl does *not* associate an encoding with your strings. |
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... until 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 meta data. |
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|
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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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|
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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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|
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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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|
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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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It's a fact. Learn to live with it. |
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|
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I hope this helps :) |
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|
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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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|
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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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|
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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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|
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my $json = JSON::XS->new->utf8->space_after->encode ({a => [1,2]}) |
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=> {"a": [1, 2]} |
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|
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$json = $json->ascii ([$enable]) |
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$enabled = $json->get_ascii |
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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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|
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If $enable is false, then the "encode" method will not escape |
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Unicode characters unless required by the JSON syntax or other |
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flags. This results in a faster and more compact format. |
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|
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See also the section *ENCODING/CODESET FLAG NOTES* later in this |
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document. |
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|
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The main use for this flag is to produce JSON texts that can be |
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transmitted over a 7-bit channel, as the encoded JSON texts will not |
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contain any 8 bit characters. |
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|
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JSON::XS->new->ascii (1)->encode ([chr 0x10401]) |
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=> ["\ud801\udc01"] |
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|
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$json = $json->latin1 ([$enable]) |
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$enabled = $json->get_latin1 |
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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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|
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If $enable is false, then the "encode" method will not escape |
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Unicode characters unless required by the JSON syntax or other |
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flags. |
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|
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See also the section *ENCODING/CODESET FLAG NOTES* later in this |
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document. |
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|
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The main use for this flag is efficiently encoding binary data as |
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JSON text, as most octets will not be escaped, resulting in a |
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smaller encoded size. The disadvantage is that the resulting JSON |
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text is encoded in latin1 (and must correctly be treated as such |
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when storing and transferring), a rare encoding for JSON. It is |
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therefore most useful when you want to store data structures known |
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to contain binary data efficiently in files or databases, not when |
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talking to other JSON encoders/decoders. |
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|
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JSON::XS->new->latin1->encode (["\x{89}\x{abc}"] |
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=> ["\x{89}\\u0abc"] # (perl syntax, U+abc escaped, U+89 not) |
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|
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$json = $json->utf8 ([$enable]) |
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$enabled = $json->get_utf8 |
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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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|
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If $enable is false, then the "encode" method will return the JSON |
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string as a (non-encoded) Unicode string, while "decode" expects |
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thus a Unicode string. Any decoding or encoding (e.g. to UTF-8 or |
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UTF-16) needs to be done yourself, e.g. using the Encode module. |
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|
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See also the section *ENCODING/CODESET FLAG NOTES* later in this |
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document. |
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|
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Example, output UTF-16BE-encoded JSON: |
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|
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use Encode; |
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$jsontext = encode "UTF-16BE", JSON::XS->new->encode ($object); |
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|
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Example, decode UTF-32LE-encoded JSON: |
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|
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use Encode; |
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$object = JSON::XS->new->decode (decode "UTF-32LE", $jsontext); |
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|
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$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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|
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Example, pretty-print some simple structure: |
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|
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my $json = JSON::XS->new->pretty(1)->encode ({a => [1,2]}) |
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=> |
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{ |
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"a" : [ |
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1, |
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2 |
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] |
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} |
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|
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$json = $json->indent ([$enable]) |
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$enabled = $json->get_indent |
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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, indenting them |
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properly. |
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|
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If $enable is false, no newlines or indenting will be produced, and |
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the resulting JSON text is guaranteed not to contain any "newlines". |
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|
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This setting has no effect when decoding JSON texts. |
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|
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$json = $json->space_before ([$enable]) |
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$enabled = $json->get_space_before |
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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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|
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If $enable is false, then the "encode" method will not add any extra |
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space at those places. |
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|
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This setting has no effect when decoding JSON texts. You will also |
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most likely combine this setting with "space_after". |
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|
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Example, space_before enabled, space_after and indent disabled: |
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|
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{"key" :"value"} |
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|
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$json = $json->space_after ([$enable]) |
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$enabled = $json->get_space_after |
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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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|
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If $enable is false, then the "encode" method will not add any extra |
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space at those places. |
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|
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This setting has no effect when decoding JSON texts. |
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|
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Example, space_before and indent disabled, space_after enabled: |
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|
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{"key": "value"} |
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|
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$json = $json->relaxed ([$enable]) |
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$enabled = $json->get_relaxed |
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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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|
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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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|
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Currently accepted extensions are: |
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|
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* list items can have an end-comma |
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|
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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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[ |
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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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|
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* shell-style '#'-comments |
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|
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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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[ |
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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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|
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$json = $json->canonical ([$enable]) |
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$enabled = $json->get_canonical |
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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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|
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If $enable is false, then the "encode" method will output key-value |
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pairs in the order Perl stores them (which will likely change |
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between runs of the same script). |
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|
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This option is useful if you want the same data structure to be |
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encoded as the same JSON text (given the same overall settings). If |
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it is disabled, the same hash might be encoded differently even if |
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contains the same data, as key-value pairs have no inherent ordering |
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in Perl. |
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|
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This setting has no effect when decoding JSON texts. |
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|
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$json = $json->allow_nonref ([$enable]) |
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$enabled = $json->get_allow_nonref |
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If $enable is true (or missing), then the "encode" method can |
388 |
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. |
391 |
|
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If $enable is false, then the "encode" method will croak if it isn't |
393 |
passed an arrayref or hashref, as JSON texts must either be an |
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object or array. Likewise, "decode" will croak if given something |
395 |
that is not a JSON object or array. |
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|
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Example, encode a Perl scalar as JSON value with enabled |
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"allow_nonref", resulting in an invalid JSON text: |
399 |
|
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JSON::XS->new->allow_nonref->encode ("Hello, World!") |
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=> "Hello, World!" |
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|
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$json = $json->allow_blessed ([$enable]) |
404 |
$enabled = $json->get_allow_blessed |
405 |
If $enable is true (or missing), then the "encode" method will not |
406 |
barf when it encounters a blessed reference. Instead, the value of |
407 |
the convert_blessed option will decide whether "null" |
408 |
("convert_blessed" disabled or no "TO_JSON" method found) or a |
409 |
representation of the object ("convert_blessed" enabled and |
410 |
"TO_JSON" method found) is being encoded. Has no effect on "decode". |
411 |
|
412 |
If $enable is false (the default), then "encode" will throw an |
413 |
exception when it encounters a blessed object. |
414 |
|
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$json = $json->convert_blessed ([$enable]) |
416 |
$enabled = $json->get_convert_blessed |
417 |
If $enable is true (or missing), then "encode", upon encountering a |
418 |
blessed object, will check for the availability of the "TO_JSON" |
419 |
method on the object's class. If found, it will be called in scalar |
420 |
context and the resulting scalar will be encoded instead of the |
421 |
object. If no "TO_JSON" method is found, the value of |
422 |
"allow_blessed" will decide what to do. |
423 |
|
424 |
The "TO_JSON" method may safely call die if it wants. If "TO_JSON" |
425 |
returns other blessed objects, those will be handled in the same |
426 |
way. "TO_JSON" must take care of not causing an endless recursion |
427 |
cycle (== crash) in this case. The name of "TO_JSON" was chosen |
428 |
because other methods called by the Perl core (== not by the user of |
429 |
the object) are usually in upper case letters and to avoid |
430 |
collisions with any "to_json" function or method. |
431 |
|
432 |
This setting does not yet influence "decode" in any way, but in the |
433 |
future, global hooks might get installed that influence "decode" and |
434 |
are enabled by this setting. |
435 |
|
436 |
If $enable is false, then the "allow_blessed" setting will decide |
437 |
what to do when a blessed object is found. |
438 |
|
439 |
$json = $json->filter_json_object ([$coderef->($hashref)]) |
440 |
When $coderef is specified, it will be called from "decode" each |
441 |
time it decodes a JSON object. The only argument is a reference to |
442 |
the newly-created hash. If the code references returns a single |
443 |
scalar (which need not be a reference), this value (i.e. a copy of |
444 |
that scalar to avoid aliasing) is inserted into the deserialised |
445 |
data structure. If it returns an empty list (NOTE: *not* "undef", |
446 |
which is a valid scalar), the original deserialised hash will be |
447 |
inserted. This setting can slow down decoding considerably. |
448 |
|
449 |
When $coderef is omitted or undefined, any existing callback will be |
450 |
removed and "decode" will not change the deserialised hash in any |
451 |
way. |
452 |
|
453 |
Example, convert all JSON objects into the integer 5: |
454 |
|
455 |
my $js = JSON::XS->new->filter_json_object (sub { 5 }); |
456 |
# returns [5] |
457 |
$js->decode ('[{}]') |
458 |
# throw an exception because allow_nonref is not enabled |
459 |
# so a lone 5 is not allowed. |
460 |
$js->decode ('{"a":1, "b":2}'); |
461 |
|
462 |
$json = $json->filter_json_single_key_object ($key [=> |
463 |
$coderef->($value)]) |
464 |
Works remotely similar to "filter_json_object", but is only called |
465 |
for JSON objects having a single key named $key. |
466 |
|
467 |
This $coderef is called before the one specified via |
468 |
"filter_json_object", if any. It gets passed the single value in the |
469 |
JSON object. If it returns a single value, it will be inserted into |
470 |
the data structure. If it returns nothing (not even "undef" but the |
471 |
empty list), the callback from "filter_json_object" will be called |
472 |
next, as if no single-key callback were specified. |
473 |
|
474 |
If $coderef is omitted or undefined, the corresponding callback will |
475 |
be disabled. There can only ever be one callback for a given key. |
476 |
|
477 |
As this callback gets called less often then the |
478 |
"filter_json_object" one, decoding speed will not usually suffer as |
479 |
much. Therefore, single-key objects make excellent targets to |
480 |
serialise Perl objects into, especially as single-key JSON objects |
481 |
are as close to the type-tagged value concept as JSON gets (it's |
482 |
basically an ID/VALUE tuple). Of course, JSON does not support this |
483 |
in any way, so you need to make sure your data never looks like a |
484 |
serialised Perl hash. |
485 |
|
486 |
Typical names for the single object key are "__class_whatever__", or |
487 |
"$__dollars_are_rarely_used__$" or "}ugly_brace_placement", or even |
488 |
things like "__class_md5sum(classname)__", to reduce the risk of |
489 |
clashing with real hashes. |
490 |
|
491 |
Example, decode JSON objects of the form "{ "__widget__" => <id> }" |
492 |
into the corresponding $WIDGET{<id>} object: |
493 |
|
494 |
# return whatever is in $WIDGET{5}: |
495 |
JSON::XS |
496 |
->new |
497 |
->filter_json_single_key_object (__widget__ => sub { |
498 |
$WIDGET{ $_[0] } |
499 |
}) |
500 |
->decode ('{"__widget__": 5') |
501 |
|
502 |
# this can be used with a TO_JSON method in some "widget" class |
503 |
# for serialisation to json: |
504 |
sub WidgetBase::TO_JSON { |
505 |
my ($self) = @_; |
506 |
|
507 |
unless ($self->{id}) { |
508 |
$self->{id} = ..get..some..id..; |
509 |
$WIDGET{$self->{id}} = $self; |
510 |
} |
511 |
|
512 |
{ __widget__ => $self->{id} } |
513 |
} |
514 |
|
515 |
$json = $json->shrink ([$enable]) |
516 |
$enabled = $json->get_shrink |
517 |
Perl usually over-allocates memory a bit when allocating space for |
518 |
strings. This flag optionally resizes strings generated by either |
519 |
"encode" or "decode" to their minimum size possible. This can save |
520 |
memory when your JSON texts are either very very long or you have |
521 |
many short strings. It will also try to downgrade any strings to |
522 |
octet-form if possible: perl stores strings internally either in an |
523 |
encoding called UTF-X or in octet-form. The latter cannot store |
524 |
everything but uses less space in general (and some buggy Perl or C |
525 |
code might even rely on that internal representation being used). |
526 |
|
527 |
The actual definition of what shrink does might change in future |
528 |
versions, but it will always try to save space at the expense of |
529 |
time. |
530 |
|
531 |
If $enable is true (or missing), the string returned by "encode" |
532 |
will be shrunk-to-fit, while all strings generated by "decode" will |
533 |
also be shrunk-to-fit. |
534 |
|
535 |
If $enable is false, then the normal perl allocation algorithms are |
536 |
used. If you work with your data, then this is likely to be faster. |
537 |
|
538 |
In the future, this setting might control other things, such as |
539 |
converting strings that look like integers or floats into integers |
540 |
or floats internally (there is no difference on the Perl level), |
541 |
saving space. |
542 |
|
543 |
$json = $json->max_depth ([$maximum_nesting_depth]) |
544 |
$max_depth = $json->get_max_depth |
545 |
Sets the maximum nesting level (default 512) accepted while encoding |
546 |
or decoding. If the JSON text or Perl data structure has an equal or |
547 |
higher nesting level then this limit, then the encoder and decoder |
548 |
will stop and croak at that point. |
549 |
|
550 |
Nesting level is defined by number of hash- or arrayrefs that the |
551 |
encoder needs to traverse to reach a given point or the number of |
552 |
"{" or "[" characters without their matching closing parenthesis |
553 |
crossed to reach a given character in a string. |
554 |
|
555 |
Setting the maximum depth to one disallows any nesting, so that |
556 |
ensures that the object is only a single hash/object or array. |
557 |
|
558 |
The argument to "max_depth" will be rounded up to the next highest |
559 |
power of two. If no argument is given, the highest possible setting |
560 |
will be used, which is rarely useful. |
561 |
|
562 |
See SECURITY CONSIDERATIONS, below, for more info on why this is |
563 |
useful. |
564 |
|
565 |
$json = $json->max_size ([$maximum_string_size]) |
566 |
$max_size = $json->get_max_size |
567 |
Set the maximum length a JSON text may have (in bytes) where |
568 |
decoding is being attempted. The default is 0, meaning no limit. |
569 |
When "decode" is called on a string longer then this number of |
570 |
characters it will not attempt to decode the string but throw an |
571 |
exception. This setting has no effect on "encode" (yet). |
572 |
|
573 |
The argument to "max_size" will be rounded up to the next highest |
574 |
power of two (so may be more than requested). If no argument is |
575 |
given, the limit check will be deactivated (same as when 0 is |
576 |
specified). |
577 |
|
578 |
See SECURITY CONSIDERATIONS, below, for more info on why this is |
579 |
useful. |
580 |
|
581 |
$json_text = $json->encode ($perl_scalar) |
582 |
Converts the given Perl data structure (a simple scalar or a |
583 |
reference to a hash or array) to its JSON representation. Simple |
584 |
scalars will be converted into JSON string or number sequences, |
585 |
while references to arrays become JSON arrays and references to |
586 |
hashes become JSON objects. Undefined Perl values (e.g. "undef") |
587 |
become JSON "null" values. Neither "true" nor "false" values will be |
588 |
generated. |
589 |
|
590 |
$perl_scalar = $json->decode ($json_text) |
591 |
The opposite of "encode": expects a JSON text and tries to parse it, |
592 |
returning the resulting simple scalar or reference. Croaks on error. |
593 |
|
594 |
JSON numbers and strings become simple Perl scalars. JSON arrays |
595 |
become Perl arrayrefs and JSON objects become Perl hashrefs. "true" |
596 |
becomes 1, "false" becomes 0 and "null" becomes "undef". |
597 |
|
598 |
($perl_scalar, $characters) = $json->decode_prefix ($json_text) |
599 |
This works like the "decode" method, but instead of raising an |
600 |
exception when there is trailing garbage after the first JSON |
601 |
object, it will silently stop parsing there and return the number of |
602 |
characters consumed so far. |
603 |
|
604 |
This is useful if your JSON texts are not delimited by an outer |
605 |
protocol (which is not the brightest thing to do in the first place) |
606 |
and you need to know where the JSON text ends. |
607 |
|
608 |
JSON::XS->new->decode_prefix ("[1] the tail") |
609 |
=> ([], 3) |
610 |
|
611 |
MAPPING |
612 |
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 |
614 |
circumstances automatically, preserving round-tripping characteristics |
615 |
(what you put in comes out as something equivalent). |
616 |
|
617 |
For the more enlightened: note that in the following descriptions, |
618 |
lowercase *perl* refers to the Perl interpreter, while uppercase *Perl* |
619 |
refers to the abstract Perl language itself. |
620 |
|
621 |
JSON -> PERL |
622 |
object |
623 |
A JSON object becomes a reference to a hash in Perl. No ordering of |
624 |
object keys is preserved (JSON does not preserve object key ordering |
625 |
itself). |
626 |
|
627 |
array |
628 |
A JSON array becomes a reference to an array in Perl. |
629 |
|
630 |
string |
631 |
A JSON string becomes a string scalar in Perl - Unicode codepoints |
632 |
in JSON are represented by the same codepoints in the Perl string, |
633 |
so no manual decoding is necessary. |
634 |
|
635 |
number |
636 |
A JSON number becomes either an integer, numeric (floating point) or |
637 |
string scalar in perl, depending on its range and any fractional |
638 |
parts. On the Perl level, there is no difference between those as |
639 |
Perl handles all the conversion details, but an integer may take |
640 |
slightly less memory and might represent more values exactly than |
641 |
floating point numbers. |
642 |
|
643 |
If the number consists of digits only, JSON::XS will try to |
644 |
represent it as an integer value. If that fails, it will try to |
645 |
represent it as a numeric (floating point) value if that is possible |
646 |
without loss of precision. Otherwise it will preserve the number as |
647 |
a string value (in which case you lose roundtripping ability, as the |
648 |
JSON number will be re-encoded toa JSON string). |
649 |
|
650 |
Numbers containing a fractional or exponential part will always be |
651 |
represented as numeric (floating point) values, possibly at a loss |
652 |
of precision (in which case you might lose perfect roundtripping |
653 |
ability, but the JSON number will still be re-encoded as a JSON |
654 |
number). |
655 |
|
656 |
true, false |
657 |
These JSON atoms become "JSON::XS::true" and "JSON::XS::false", |
658 |
respectively. They are overloaded to act almost exactly like the |
659 |
numbers 1 and 0. You can check whether a scalar is a JSON boolean by |
660 |
using the "JSON::XS::is_bool" function. |
661 |
|
662 |
null |
663 |
A JSON null atom becomes "undef" in Perl. |
664 |
|
665 |
PERL -> JSON |
666 |
The mapping from Perl to JSON is slightly more difficult, as Perl is a |
667 |
truly typeless language, so we can only guess which JSON type is meant |
668 |
by a Perl value. |
669 |
|
670 |
hash references |
671 |
Perl hash references become JSON objects. As there is no inherent |
672 |
ordering in hash keys (or JSON objects), they will usually be |
673 |
encoded in a pseudo-random order that can change between runs of the |
674 |
same program but stays generally the same within a single run of a |
675 |
program. JSON::XS can optionally sort the hash keys (determined by |
676 |
the *canonical* flag), so the same datastructure will serialise to |
677 |
the same JSON text (given same settings and version of JSON::XS), |
678 |
but this incurs a runtime overhead and is only rarely useful, e.g. |
679 |
when you want to compare some JSON text against another for |
680 |
equality. |
681 |
|
682 |
array references |
683 |
Perl array references become JSON arrays. |
684 |
|
685 |
other references |
686 |
Other unblessed references are generally not allowed and will cause |
687 |
an exception to be thrown, except for references to the integers 0 |
688 |
and 1, which get turned into "false" and "true" atoms in JSON. You |
689 |
can also use "JSON::XS::false" and "JSON::XS::true" to improve |
690 |
readability. |
691 |
|
692 |
encode_json [\0,JSON::XS::true] # yields [false,true] |
693 |
|
694 |
JSON::XS::true, JSON::XS::false |
695 |
These special values become JSON true and JSON false values, |
696 |
respectively. You can also use "\1" and "\0" directly if you want. |
697 |
|
698 |
blessed objects |
699 |
Blessed objects are not directly representable in JSON. See the |
700 |
"allow_blessed" and "convert_blessed" methods on various options on |
701 |
how to deal with this: basically, you can choose between throwing an |
702 |
exception, encoding the reference as if it weren't blessed, or |
703 |
provide your own serialiser method. |
704 |
|
705 |
simple scalars |
706 |
Simple Perl scalars (any scalar that is not a reference) are the |
707 |
most difficult objects to encode: JSON::XS will encode undefined |
708 |
scalars as JSON "null" values, scalars that have last been used in a |
709 |
string context before encoding as JSON strings, and anything else as |
710 |
number value: |
711 |
|
712 |
# dump as number |
713 |
encode_json [2] # yields [2] |
714 |
encode_json [-3.0e17] # yields [-3e+17] |
715 |
my $value = 5; encode_json [$value] # yields [5] |
716 |
|
717 |
# used as string, so dump as string |
718 |
print $value; |
719 |
encode_json [$value] # yields ["5"] |
720 |
|
721 |
# undef becomes null |
722 |
encode_json [undef] # yields [null] |
723 |
|
724 |
You can force the type to be a JSON string by stringifying it: |
725 |
|
726 |
my $x = 3.1; # some variable containing a number |
727 |
"$x"; # stringified |
728 |
$x .= ""; # another, more awkward way to stringify |
729 |
print $x; # perl does it for you, too, quite often |
730 |
|
731 |
You can force the type to be a JSON number by numifying it: |
732 |
|
733 |
my $x = "3"; # some variable containing a string |
734 |
$x += 0; # numify it, ensuring it will be dumped as a number |
735 |
$x *= 1; # same thing, the choice is yours. |
736 |
|
737 |
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 |
739 |
its needed :). |
740 |
|
741 |
ENCODING/CODESET FLAG NOTES |
742 |
The interested reader might have seen a number of flags that signify |
743 |
encodings or codesets - "utf8", "latin1" and "ascii". There seems to be |
744 |
some confusion on what these do, so here is a short comparison: |
745 |
|
746 |
"utf8" controls wether the JSON text created by "encode" (and expected |
747 |
by "decode") is UTF-8 encoded or not, while "latin1" and "ascii" only |
748 |
control wether "encode" escapes character values outside their |
749 |
respective codeset range. Neither of these flags conflict with each |
750 |
other, although some combinations make less sense than others. |
751 |
|
752 |
Care has been taken to make all flags symmetrical with respect to |
753 |
"encode" and "decode", that is, texts encoded with any combination of |
754 |
these flag values will be correctly decoded when the same flags are used |
755 |
- in general, if you use different flag settings while encoding vs. when |
756 |
decoding you likely have a bug somewhere. |
757 |
|
758 |
Below comes a verbose discussion of these flags. Note that a "codeset" |
759 |
is simply an abstract set of character-codepoint pairs, while an |
760 |
encoding takes those codepoint numbers and *encodes* them, in our case |
761 |
into octets. Unicode is (among other things) a codeset, UTF-8 is an |
762 |
encoding, and ISO-8859-1 (= latin 1) and ASCII are both codesets *and* |
763 |
encodings at the same time, which can be confusing. |
764 |
|
765 |
"utf8" flag disabled |
766 |
When "utf8" is disabled (the default), then "encode"/"decode" |
767 |
generate and expect Unicode strings, that is, characters with high |
768 |
ordinal Unicode values (> 255) will be encoded as such characters, |
769 |
and likewise such characters are decoded as-is, no canges to them |
770 |
will be done, except "(re-)interpreting" them as Unicode codepoints |
771 |
or Unicode characters, respectively (to Perl, these are the same |
772 |
thing in strings unless you do funny/weird/dumb stuff). |
773 |
|
774 |
This is useful when you want to do the encoding yourself (e.g. when |
775 |
you want to have UTF-16 encoded JSON texts) or when some other layer |
776 |
does the encoding for you (for example, when printing to a terminal |
777 |
using a filehandle that transparently encodes to UTF-8 you certainly |
778 |
do NOT want to UTF-8 encode your data first and have Perl encode it |
779 |
another time). |
780 |
|
781 |
"utf8" flag enabled |
782 |
If the "utf8"-flag is enabled, "encode"/"decode" will encode all |
783 |
characters using the corresponding UTF-8 multi-byte sequence, and |
784 |
will expect your input strings to be encoded as UTF-8, that is, no |
785 |
"character" of the input string must have any value > 255, as UTF-8 |
786 |
does not allow that. |
787 |
|
788 |
The "utf8" flag therefore switches between two modes: disabled means |
789 |
you will get a Unicode string in Perl, enabled means you get an |
790 |
UTF-8 encoded octet/binary string in Perl. |
791 |
|
792 |
"latin1" or "ascii" flags enabled |
793 |
With "latin1" (or "ascii") enabled, "encode" will escape characters |
794 |
with ordinal values > 255 (> 127 with "ascii") and encode the |
795 |
remaining characters as specified by the "utf8" flag. |
796 |
|
797 |
If "utf8" is disabled, then the result is also correctly encoded in |
798 |
those character sets (as both are proper subsets of Unicode, meaning |
799 |
that a Unicode string with all character values < 256 is the same |
800 |
thing as a ISO-8859-1 string, and a Unicode string with all |
801 |
character values < 128 is the same thing as an ASCII string in |
802 |
Perl). |
803 |
|
804 |
If "utf8" is enabled, you still get a correct UTF-8-encoded string, |
805 |
regardless of these flags, just some more characters will be escaped |
806 |
using "\uXXXX" then before. |
807 |
|
808 |
Note that ISO-8859-1-*encoded* strings are not compatible with UTF-8 |
809 |
encoding, while ASCII-encoded strings are. That is because the |
810 |
ISO-8859-1 encoding is NOT a subset of UTF-8 (despite the ISO-8859-1 |
811 |
*codeset* being a subset of Unicode), while ASCII is. |
812 |
|
813 |
Surprisingly, "decode" will ignore these flags and so treat all |
814 |
input values as governed by the "utf8" flag. If it is disabled, this |
815 |
allows you to decode ISO-8859-1- and ASCII-encoded strings, as both |
816 |
strict subsets of Unicode. If it is enabled, you can correctly |
817 |
decode UTF-8 encoded strings. |
818 |
|
819 |
So neither "latin1" nor "ascii" are incompatible with the "utf8" |
820 |
flag - they only govern when the JSON output engine escapes a |
821 |
character or not. |
822 |
|
823 |
The main use for "latin1" is to relatively efficiently store binary |
824 |
data as JSON, at the expense of breaking compatibility with most |
825 |
JSON decoders. |
826 |
|
827 |
The main use for "ascii" is to force the output to not contain |
828 |
characters with values > 127, which means you can interpret the |
829 |
resulting string as UTF-8, ISO-8859-1, ASCII, KOI8-R or most about |
830 |
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 |
832 |
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 |
834 |
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 |
|
926 |
JSON and YAML |
927 |
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 |
929 |
writing), so let me state it clearly: *in general, there is no way to |
930 |
configure JSON::XS to output a data structure as valid YAML* that works |
931 |
in all cases. |
932 |
|
933 |
If you really must use JSON::XS to generate YAML, you should use this |
934 |
algorithm (subject to change in future versions): |
935 |
|
936 |
my $to_yaml = JSON::XS->new->utf8->space_after (1); |
937 |
my $yaml = $to_yaml->encode ($ref) . "\n"; |
938 |
|
939 |
This will *usually* generate JSON texts that also parse as valid YAML. |
940 |
Please note that YAML has hardcoded limits on (simple) object key |
941 |
lengths that JSON doesn't have and also has different and incompatible |
942 |
unicode handling, so you should make sure that your hash keys are |
943 |
noticeably shorter than the 1024 "stream characters" YAML allows and |
944 |
that you do not have characters with codepoint values outside the |
945 |
Unicode BMP (basic multilingual page). YAML also does not allow "\/" |
946 |
sequences in strings (which JSON::XS does not *currently* generate, but |
947 |
other JSON generators might). |
948 |
|
949 |
There might be other incompatibilities that I am not aware of (or the |
950 |
YAML specification has been changed yet again - it does so quite often). |
951 |
In general you should not try to generate YAML with a JSON generator or |
952 |
vice versa, or try to parse JSON with a YAML parser or vice versa: |
953 |
chances are high that you will run into severe interoperability problems |
954 |
when you least expect it. |
955 |
|
956 |
(*) I have been pressured multiple times by Brian Ingerson (one of the |
957 |
authors of the YAML specification) to remove this paragraph, despite |
958 |
him acknowledging that the actual incompatibilities exist. As I was |
959 |
personally bitten by this "JSON is YAML" lie, I refused and said I |
960 |
will continue to educate people about these issues, so others do not |
961 |
run into the same problem again and again. After this, Brian called |
962 |
me a (quote)*complete and worthless idiot*(unquote). |
963 |
|
964 |
In my opinion, instead of pressuring and insulting people who |
965 |
actually clarify issues with YAML and the wrong statements of some |
966 |
of its proponents, I would kindly suggest reading the JSON spec |
967 |
(which is not that difficult or long) and finally make YAML |
968 |
compatible to it, and educating users about the changes, instead of |
969 |
spreading lies about the real compatibility for many *years* and |
970 |
trying to silence people who point out that it isn't true. |
971 |
|
972 |
SPEED |
973 |
It seems that JSON::XS is surprisingly fast, as shown in the following |
974 |
tables. They have been generated with the help of the "eg/bench" program |
975 |
in the JSON::XS distribution, to make it easy to compare on your own |
976 |
system. |
977 |
|
978 |
First comes a comparison between various modules using a very short |
979 |
single-line JSON string (also available at |
980 |
<http://dist.schmorp.de/misc/json/short.json>). |
981 |
|
982 |
{"method": "handleMessage", "params": ["user1", "we were just talking"], \ |
983 |
"id": null, "array":[1,11,234,-5,1e5,1e7, true, false]} |
984 |
|
985 |
It shows the number of encodes/decodes per second (JSON::XS uses the |
986 |
functional interface, while JSON::XS/2 uses the OO interface with |
987 |
pretty-printing and hashkey sorting enabled, JSON::XS/3 enables shrink). |
988 |
Higher is better: |
989 |
|
990 |
module | encode | decode | |
991 |
-----------|------------|------------| |
992 |
JSON 1.x | 4990.842 | 4088.813 | |
993 |
JSON::DWIW | 51653.990 | 71575.154 | |
994 |
JSON::PC | 65948.176 | 74631.744 | |
995 |
JSON::PP | 8931.652 | 3817.168 | |
996 |
JSON::Syck | 24877.248 | 27776.848 | |
997 |
JSON::XS | 388361.481 | 227951.304 | |
998 |
JSON::XS/2 | 227951.304 | 218453.333 | |
999 |
JSON::XS/3 | 338250.323 | 218453.333 | |
1000 |
Storable | 16500.016 | 135300.129 | |
1001 |
-----------+------------+------------+ |
1002 |
|
1003 |
That is, JSON::XS is about five times faster than JSON::DWIW on |
1004 |
encoding, about three times faster on decoding, and over forty times |
1005 |
faster than JSON, even with pretty-printing and key sorting. It also |
1006 |
compares favourably to Storable for small amounts of data. |
1007 |
|
1008 |
Using a longer test string (roughly 18KB, generated from Yahoo! Locals |
1009 |
search API (<http://dist.schmorp.de/misc/json/long.json>). |
1010 |
|
1011 |
module | encode | decode | |
1012 |
-----------|------------|------------| |
1013 |
JSON 1.x | 55.260 | 34.971 | |
1014 |
JSON::DWIW | 825.228 | 1082.513 | |
1015 |
JSON::PC | 3571.444 | 2394.829 | |
1016 |
JSON::PP | 210.987 | 32.574 | |
1017 |
JSON::Syck | 552.551 | 787.544 | |
1018 |
JSON::XS | 5780.463 | 4854.519 | |
1019 |
JSON::XS/2 | 3869.998 | 4798.975 | |
1020 |
JSON::XS/3 | 5862.880 | 4798.975 | |
1021 |
Storable | 4445.002 | 5235.027 | |
1022 |
-----------+------------+------------+ |
1023 |
|
1024 |
Again, JSON::XS leads by far (except for Storable which non-surprisingly |
1025 |
decodes faster). |
1026 |
|
1027 |
On large strings containing lots of high Unicode characters, some |
1028 |
modules (such as JSON::PC) seem to decode faster than JSON::XS, but the |
1029 |
result will be broken due to missing (or wrong) Unicode handling. Others |
1030 |
refuse to decode or encode properly, so it was impossible to prepare a |
1031 |
fair comparison table for that case. |
1032 |
|
1033 |
SECURITY CONSIDERATIONS |
1034 |
When you are using JSON in a protocol, talking to untrusted potentially |
1035 |
hostile creatures requires relatively few measures. |
1036 |
|
1037 |
First of all, your JSON decoder should be secure, that is, should not |
1038 |
have any buffer overflows. Obviously, this module should ensure that and |
1039 |
I am trying hard on making that true, but you never know. |
1040 |
|
1041 |
Second, you need to avoid resource-starving attacks. That means you |
1042 |
should limit the size of JSON texts you accept, or make sure then when |
1043 |
your resources run out, that's just fine (e.g. by using a separate |
1044 |
process that can crash safely). The size of a JSON text in octets or |
1045 |
characters is usually a good indication of the size of the resources |
1046 |
required to decode it into a Perl structure. While JSON::XS can check |
1047 |
the size of the JSON text, it might be too late when you already have it |
1048 |
in memory, so you might want to check the size before you accept the |
1049 |
string. |
1050 |
|
1051 |
Third, JSON::XS recurses using the C stack when decoding objects and |
1052 |
arrays. The C stack is a limited resource: for instance, on my amd64 |
1053 |
machine with 8MB of stack size I can decode around 180k nested arrays |
1054 |
but only 14k nested JSON objects (due to perl itself recursing deeply on |
1055 |
croak to free the temporary). If that is exceeded, the program crashes. |
1056 |
To be conservative, the default nesting limit is set to 512. If your |
1057 |
process has a smaller stack, you should adjust this setting accordingly |
1058 |
with the "max_depth" method. |
1059 |
|
1060 |
Something else could bomb you, too, that I forgot to think of. In that |
1061 |
case, you get to keep the pieces. I am always open for hints, though... |
1062 |
|
1063 |
Also keep in mind that JSON::XS might leak contents of your Perl data |
1064 |
structures in its error messages, so when you serialise sensitive |
1065 |
information you might want to make sure that exceptions thrown by |
1066 |
JSON::XS will not end up in front of untrusted eyes. |
1067 |
|
1068 |
If you are using JSON::XS to return packets to consumption by JavaScript |
1069 |
scripts in a browser you should have a look at |
1070 |
<http://jpsykes.com/47/practical-csrf-and-json-security> to see whether |
1071 |
you are vulnerable to some common attack vectors (which really are |
1072 |
browser design bugs, but it is still you who will have to deal with it, |
1073 |
as major browser developers care only for features, not about getting |
1074 |
security right). |
1075 |
|
1076 |
THREADS |
1077 |
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 |
1079 |
horribly slow so-called "threads" which are simply slow and bloated |
1080 |
process simulations - use fork, its *much* faster, cheaper, better). |
1081 |
|
1082 |
(It might actually work, but you have been warned). |
1083 |
|
1084 |
BUGS |
1085 |
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 |
1087 |
still relatively early in its development. If you keep reporting bugs |
1088 |
they will be fixed swiftly, though. |
1089 |
|
1090 |
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. |
1092 |
|
1093 |
AUTHOR |
1094 |
Marc Lehmann <schmorp@schmorp.de> |
1095 |
http://home.schmorp.de/ |
1096 |
|