1 |
NAME |
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JSON::XS - JSON serialising/deserialising, done correctly and fast |
3 |
|
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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 = to_json $perl_hash_or_arrayref; |
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$perl_hash_or_arrayref = from_json $utf8_encoded_json_text; |
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|
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# objToJson and jsonToObj aliases to to_json and from_json |
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# are exported for compatibility to the JSON module, |
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# but should not be used in new code. |
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|
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# OO-interface |
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|
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$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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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. |
27 |
|
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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 |
31 |
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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This module knows how to handle Unicode, and even documents how and |
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when it does so. |
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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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$json_text = to_json $perl_scalar |
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Converts the given Perl data structure (a simple scalar or a |
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reference to a hash or array) to a UTF-8 encoded, binary string |
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(that is, the string contains octets only). Croaks on error. |
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|
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This function call is functionally identical to: |
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|
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$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 = from_json $json_text |
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The opposite of "to_json": expects an UTF-8 (binary) string and |
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tries to parse that as an UTF-8 encoded JSON text, returning the |
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resulting simple scalar or reference. Croaks on error. |
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|
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This function call is functionally identical to: |
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|
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$perl_scalar = JSON::XS->new->utf8->decode ($json_text) |
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|
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except being faster. |
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|
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OBJECT-ORIENTED INTERFACE |
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The object oriented interface lets you configure your own encoding or |
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decoding style, within the limits of supported formats. |
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|
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$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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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. |
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|
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If $enable is false, then the "encode" method will not escape |
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Unicode characters unless required by the JSON syntax. This results |
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in a faster and more compact format. |
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|
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JSON::XS->new->ascii (1)->encode ([chr 0x10401]) |
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=> ["\ud801\udc01"] |
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|
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$json = $json->utf8 ([$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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|
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If $enable is false, then the "encode" method will return the JSON |
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string as a (non-encoded) unicode string, while "decode" expects |
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thus a unicode string. Any decoding or encoding (e.g. to UTF-8 or |
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UTF-16) needs to be done yourself, e.g. using the Encode module. |
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|
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Example, output UTF-16BE-encoded JSON: |
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|
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use Encode; |
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$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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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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|
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If $enable is false, no newlines or indenting will be produced, and |
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the resulting JSON text is guarenteed not to contain any "newlines". |
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|
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This setting has no effect when decoding JSON texts. |
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|
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$json = $json->space_before ([$enable]) |
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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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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 = $json->canonical ([$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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|
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If $enable is false, then the "encode" method will output key-value |
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pairs in the order Perl stores them (which will likely change |
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between runs of the same script). |
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|
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This option is useful if you want the same data structure to be |
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encoded as the same JSON text (given the same overall settings). If |
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it is disabled, the same hash migh be encoded differently even if |
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contains the same data, as key-value pairs have no inherent ordering |
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in Perl. |
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|
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This setting has no effect when decoding JSON texts. |
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|
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$json = $json->allow_nonref ([$enable]) |
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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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|
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If $enable is false, then the "encode" method will croak if it isn't |
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passed an arrayref or hashref, as JSON texts must either be an |
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object or array. Likewise, "decode" will croak if given something |
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that is not a JSON object or array. |
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|
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Example, encode a Perl scalar as JSON value with enabled |
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"allow_nonref", resulting in an invalid JSON text: |
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|
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JSON::XS->new->allow_nonref->encode ("Hello, World!") |
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=> "Hello, World!" |
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|
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$json = $json->shrink ([$enable]) |
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Perl usually over-allocates memory a bit when allocating space for |
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strings. This flag optionally resizes strings generated by either |
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"encode" or "decode" to their minimum size possible. This can save |
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memory when your JSON texts are either very very long or you have |
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many short strings. It will also try to downgrade any strings to |
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octet-form if possible: perl stores strings internally either in an |
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encoding called UTF-X or in octet-form. The latter cannot store |
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everything but uses less space in general (and some buggy Perl or C |
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code might even rely on that internal representation being used). |
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|
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The actual definition of what shrink does might change in future |
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versions, but it will always try to save space at the expense of |
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time. |
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|
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If $enable is true (or missing), the string returned by "encode" |
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will be shrunk-to-fit, while all strings generated by "decode" will |
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also be shrunk-to-fit. |
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|
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If $enable is false, then the normal perl allocation algorithms are |
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used. If you work with your data, then this is likely to be faster. |
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|
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In the future, this setting might control other things, such as |
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converting strings that look like integers or floats into integers |
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or floats internally (there is no difference on the Perl level), |
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saving space. |
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|
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$json = $json->max_depth ([$maximum_nesting_depth]) |
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Sets the maximum nesting level (default 4096) accepted while |
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encoding or decoding. If the JSON text or Perl data structure has an |
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equal or higher nesting level then this limit, then the encoder and |
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decoder will stop and croak at that point. |
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|
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Nesting level is defined by number of hash- or arrayrefs that the |
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encoder needs to traverse to reach a given point or the number of |
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"{" or "[" characters without their matching closing parenthesis |
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crossed to reach a given character in a string. |
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|
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Setting the maximum depth to one disallows any nesting, so that |
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ensures that the object is only a single hash/object or array. |
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|
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The argument to "max_depth" will be rounded up to the next nearest |
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power of two. |
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|
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See SECURITY CONSIDERATIONS, below, for more info on why this is |
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useful. |
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|
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$json_text = $json->encode ($perl_scalar) |
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Converts the given Perl data structure (a simple scalar or a |
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reference to a hash or array) to its JSON representation. Simple |
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scalars will be converted into JSON string or number sequences, |
293 |
while references to arrays become JSON arrays and references to |
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hashes become JSON objects. Undefined Perl values (e.g. "undef") |
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become JSON "null" values. Neither "true" nor "false" values will be |
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generated. |
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|
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$perl_scalar = $json->decode ($json_text) |
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The opposite of "encode": expects a JSON text and tries to parse it, |
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returning the resulting simple scalar or reference. Croaks on error. |
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|
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JSON numbers and strings become simple Perl scalars. JSON arrays |
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become Perl arrayrefs and JSON objects become Perl hashrefs. "true" |
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becomes 1, "false" becomes 0 and "null" becomes "undef". |
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|
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MAPPING |
307 |
This section describes how JSON::XS maps Perl values to JSON values and |
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vice versa. These mappings are designed to "do the right thing" in most |
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circumstances automatically, preserving round-tripping characteristics |
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(what you put in comes out as something equivalent). |
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|
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For the more enlightened: note that in the following descriptions, |
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lowercase *perl* refers to the Perl interpreter, while uppcercase *Perl* |
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refers to the abstract Perl language itself. |
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|
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JSON -> PERL |
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object |
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A JSON object becomes a reference to a hash in Perl. No ordering of |
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object keys is preserved (JSON does not preserver object key |
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ordering itself). |
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|
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array |
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A JSON array becomes a reference to an array in Perl. |
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|
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string |
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A JSON string becomes a string scalar in Perl - Unicode codepoints |
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in JSON are represented by the same codepoints in the Perl string, |
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so no manual decoding is necessary. |
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|
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number |
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A JSON number becomes either an integer or numeric (floating point) |
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scalar in perl, depending on its range and any fractional parts. On |
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the Perl level, there is no difference between those as Perl handles |
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all the conversion details, but an integer may take slightly less |
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memory and might represent more values exactly than (floating point) |
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numbers. |
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|
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true, false |
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These JSON atoms become 0, 1, respectively. Information is lost in |
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this process. Future versions might represent those values |
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differently, but they will be guarenteed to act like these integers |
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would normally in Perl. |
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|
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null |
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A JSON null atom becomes "undef" in Perl. |
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|
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PERL -> JSON |
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The mapping from Perl to JSON is slightly more difficult, as Perl is a |
349 |
truly typeless language, so we can only guess which JSON type is meant |
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by a Perl value. |
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|
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hash references |
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Perl hash references become JSON objects. As there is no inherent |
354 |
ordering in hash keys (or JSON objects), they will usually be |
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encoded in a pseudo-random order that can change between runs of the |
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same program but stays generally the same within a single run of a |
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program. JSON::XS can optionally sort the hash keys (determined by |
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the *canonical* flag), so the same datastructure will serialise to |
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the same JSON text (given same settings and version of JSON::XS), |
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but this incurs a runtime overhead and is only rarely useful, e.g. |
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when you want to compare some JSON text against another for |
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equality. |
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|
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array references |
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Perl array references become JSON arrays. |
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|
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other references |
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Other unblessed references are generally not allowed and will cause |
369 |
an exception to be thrown, except for references to the integers 0 |
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and 1, which get turned into "false" and "true" atoms in JSON. You |
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can also use "JSON::XS::false" and "JSON::XS::true" to improve |
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readability. |
373 |
|
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to_json [\0,JSON::XS::true] # yields [false,true] |
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|
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blessed objects |
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Blessed objects are not allowed. JSON::XS currently tries to encode |
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their underlying representation (hash- or arrayref), but this |
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behaviour might change in future versions. |
380 |
|
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simple scalars |
382 |
Simple Perl scalars (any scalar that is not a reference) are the |
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most difficult objects to encode: JSON::XS will encode undefined |
384 |
scalars as JSON null value, scalars that have last been used in a |
385 |
string context before encoding as JSON strings and anything else as |
386 |
number value: |
387 |
|
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# dump as number |
389 |
to_json [2] # yields [2] |
390 |
to_json [-3.0e17] # yields [-3e+17] |
391 |
my $value = 5; to_json [$value] # yields [5] |
392 |
|
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# used as string, so dump as string |
394 |
print $value; |
395 |
to_json [$value] # yields ["5"] |
396 |
|
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# undef becomes null |
398 |
to_json [undef] # yields [null] |
399 |
|
400 |
You can force the type to be a string by stringifying it: |
401 |
|
402 |
my $x = 3.1; # some variable containing a number |
403 |
"$x"; # stringified |
404 |
$x .= ""; # another, more awkward way to stringify |
405 |
print $x; # perl does it for you, too, quite often |
406 |
|
407 |
You can force the type to be a number by numifying it: |
408 |
|
409 |
my $x = "3"; # some variable containing a string |
410 |
$x += 0; # numify it, ensuring it will be dumped as a number |
411 |
$x *= 1; # same thing, the choise is yours. |
412 |
|
413 |
You can not currently output JSON booleans or force the type in |
414 |
other, less obscure, ways. Tell me if you need this capability. |
415 |
|
416 |
circular data structures |
417 |
Those will be encoded until memory or stackspace runs out. |
418 |
|
419 |
COMPARISON |
420 |
As already mentioned, this module was created because none of the |
421 |
existing JSON modules could be made to work correctly. First I will |
422 |
describe the problems (or pleasures) I encountered with various existing |
423 |
JSON modules, followed by some benchmark values. JSON::XS was designed |
424 |
not to suffer from any of these problems or limitations. |
425 |
|
426 |
JSON 1.07 |
427 |
Slow (but very portable, as it is written in pure Perl). |
428 |
|
429 |
Undocumented/buggy Unicode handling (how JSON handles unicode values |
430 |
is undocumented. One can get far by feeding it unicode strings and |
431 |
doing en-/decoding oneself, but unicode escapes are not working |
432 |
properly). |
433 |
|
434 |
No roundtripping (strings get clobbered if they look like numbers, |
435 |
e.g. the string 2.0 will encode to 2.0 instead of "2.0", and that |
436 |
will decode into the number 2. |
437 |
|
438 |
JSON::PC 0.01 |
439 |
Very fast. |
440 |
|
441 |
Undocumented/buggy Unicode handling. |
442 |
|
443 |
No roundtripping. |
444 |
|
445 |
Has problems handling many Perl values (e.g. regex results and other |
446 |
magic values will make it croak). |
447 |
|
448 |
Does not even generate valid JSON ("{1,2}" gets converted to "{1:2}" |
449 |
which is not a valid JSON text. |
450 |
|
451 |
Unmaintained (maintainer unresponsive for many months, bugs are not |
452 |
getting fixed). |
453 |
|
454 |
JSON::Syck 0.21 |
455 |
Very buggy (often crashes). |
456 |
|
457 |
Very inflexible (no human-readable format supported, format pretty |
458 |
much undocumented. I need at least a format for easy reading by |
459 |
humans and a single-line compact format for use in a protocol, and |
460 |
preferably a way to generate ASCII-only JSON texts). |
461 |
|
462 |
Completely broken (and confusingly documented) Unicode handling |
463 |
(unicode escapes are not working properly, you need to set |
464 |
ImplicitUnicode to *different* values on en- and decoding to get |
465 |
symmetric behaviour). |
466 |
|
467 |
No roundtripping (simple cases work, but this depends on wether the |
468 |
scalar value was used in a numeric context or not). |
469 |
|
470 |
Dumping hashes may skip hash values depending on iterator state. |
471 |
|
472 |
Unmaintained (maintainer unresponsive for many months, bugs are not |
473 |
getting fixed). |
474 |
|
475 |
Does not check input for validity (i.e. will accept non-JSON input |
476 |
and return "something" instead of raising an exception. This is a |
477 |
security issue: imagine two banks transfering money between each |
478 |
other using JSON. One bank might parse a given non-JSON request and |
479 |
deduct money, while the other might reject the transaction with a |
480 |
syntax error. While a good protocol will at least recover, that is |
481 |
extra unnecessary work and the transaction will still not succeed). |
482 |
|
483 |
JSON::DWIW 0.04 |
484 |
Very fast. Very natural. Very nice. |
485 |
|
486 |
Undocumented unicode handling (but the best of the pack. Unicode |
487 |
escapes still don't get parsed properly). |
488 |
|
489 |
Very inflexible. |
490 |
|
491 |
No roundtripping. |
492 |
|
493 |
Does not generate valid JSON texts (key strings are often unquoted, |
494 |
empty keys result in nothing being output) |
495 |
|
496 |
Does not check input for validity. |
497 |
|
498 |
SPEED |
499 |
It seems that JSON::XS is surprisingly fast, as shown in the following |
500 |
tables. They have been generated with the help of the "eg/bench" program |
501 |
in the JSON::XS distribution, to make it easy to compare on your own |
502 |
system. |
503 |
|
504 |
First comes a comparison between various modules using a very short JSON |
505 |
string: |
506 |
|
507 |
{"method": "handleMessage", "params": ["user1", "we were just talking"], "id": null} |
508 |
|
509 |
It shows the number of encodes/decodes per second (JSON::XS uses the |
510 |
functional interface, while JSON::XS/2 uses the OO interface with |
511 |
pretty-printing and hashkey sorting enabled). Higher is better: |
512 |
|
513 |
module | encode | decode | |
514 |
-----------|------------|------------| |
515 |
JSON | 11488.516 | 7823.035 | |
516 |
JSON::DWIW | 94708.054 | 129094.260 | |
517 |
JSON::PC | 63884.157 | 128528.212 | |
518 |
JSON::Syck | 34898.677 | 42096.911 | |
519 |
JSON::XS | 654027.064 | 396423.669 | |
520 |
JSON::XS/2 | 371564.190 | 371725.613 | |
521 |
-----------+------------+------------+ |
522 |
|
523 |
That is, JSON::XS is more than six times faster than JSON::DWIW on |
524 |
encoding, more than three times faster on decoding, and about thirty |
525 |
times faster than JSON, even with pretty-printing and key sorting. |
526 |
|
527 |
Using a longer test string (roughly 18KB, generated from Yahoo! Locals |
528 |
search API (http://nanoref.com/yahooapis/mgPdGg): |
529 |
|
530 |
module | encode | decode | |
531 |
-----------|------------|------------| |
532 |
JSON | 273.023 | 44.674 | |
533 |
JSON::DWIW | 1089.383 | 1145.704 | |
534 |
JSON::PC | 3097.419 | 2393.921 | |
535 |
JSON::Syck | 514.060 | 843.053 | |
536 |
JSON::XS | 6479.668 | 3636.364 | |
537 |
JSON::XS/2 | 3774.221 | 3599.124 | |
538 |
-----------+------------+------------+ |
539 |
|
540 |
Again, JSON::XS leads by far. |
541 |
|
542 |
On large strings containing lots of high unicode characters, some |
543 |
modules (such as JSON::PC) seem to decode faster than JSON::XS, but the |
544 |
result will be broken due to missing (or wrong) unicode handling. Others |
545 |
refuse to decode or encode properly, so it was impossible to prepare a |
546 |
fair comparison table for that case. |
547 |
|
548 |
SECURITY CONSIDERATIONS |
549 |
When you are using JSON in a protocol, talking to untrusted potentially |
550 |
hostile creatures requires relatively few measures. |
551 |
|
552 |
First of all, your JSON decoder should be secure, that is, should not |
553 |
have any buffer overflows. Obviously, this module should ensure that and |
554 |
I am trying hard on making that true, but you never know. |
555 |
|
556 |
Second, you need to avoid resource-starving attacks. That means you |
557 |
should limit the size of JSON texts you accept, or make sure then when |
558 |
your resources run out, thats just fine (e.g. by using a separate |
559 |
process that can crash safely). The size of a JSON text in octets or |
560 |
characters is usually a good indication of the size of the resources |
561 |
required to decode it into a Perl structure. |
562 |
|
563 |
Third, JSON::XS recurses using the C stack when decoding objects and |
564 |
arrays. The C stack is a limited resource: for instance, on my amd64 |
565 |
machine with 8MB of stack size I can decode around 180k nested arrays |
566 |
but only 14k nested JSON objects. If that is exceeded, the program |
567 |
crashes. Thats why the default nesting limit is set to 4096. If your |
568 |
process has a smaller stack, you should adjust this setting accordingly |
569 |
with the "max_depth" method. |
570 |
|
571 |
And last but least, something else could bomb you that I forgot to think |
572 |
of. In that case, you get to keep the pieces. I am alway sopen for |
573 |
hints, though... |
574 |
|
575 |
BUGS |
576 |
While the goal of this module is to be correct, that unfortunately does |
577 |
not mean its bug-free, only that I think its design is bug-free. It is |
578 |
still relatively early in its development. If you keep reporting bugs |
579 |
they will be fixed swiftly, though. |
580 |
|
581 |
AUTHOR |
582 |
Marc Lehmann <schmorp@schmorp.de> |
583 |
http://home.schmorp.de/ |
584 |
|