| /usr/local/perl/lib/site_perl/5.8.5/sun4-solaris/JSON/XS.pm |
JSON::XS - JSON serialising/deserialising, done correctly and fast
JSON::XS - 正しくて高速な JSON シリアライザ/デシリアライザ (http://fleur.hio.jp/perldoc/mix/lib/JSON/XS.html)
use JSON::XS;
# exported functions, they croak on error # and expect/generate UTF-8
$utf8_encoded_json_text = encode_json $perl_hash_or_arrayref; $perl_hash_or_arrayref = decode_json $utf8_encoded_json_text;
# OO-interface
$coder = JSON::XS->new->ascii->pretty->allow_nonref; $pretty_printed_unencoded = $coder->encode ($perl_scalar); $perl_scalar = $coder->decode ($unicode_json_text);
# Note that JSON version 2.0 and above will automatically use JSON::XS # if available, at virtually no speed overhead either, so you should # be able to just: use JSON;
# and do the same things, except that you have a pure-perl fallback now.
This module converts Perl data structures to JSON and vice versa. Its primary goal is to be correct and its secondary goal is to be fast. To reach the latter goal it was written in C.
Beginning with version 2.0 of the JSON module, when both JSON and JSON::XS are installed, then JSON will fall back on JSON::XS (this can be overriden) with no overhead due to emulation (by inheritign constructor and methods). If JSON::XS is not available, it will fall back to the compatible JSON::PP module as backend, so using JSON instead of JSON::XS gives you a portable JSON API that can be fast when you need and doesn't require a C compiler when that is a problem.
As this is the n-th-something JSON module on CPAN, what was the reason to write yet another JSON module? While it seems there are many JSON modules, none of them correctly handle all corner cases, and in most cases their maintainers are unresponsive, gone missing, or not listening to bug reports for other reasons.
See COMPARISON, below, for a comparison to some other JSON modules.
See MAPPING, below, on how JSON::XS maps perl values to JSON values and vice versa.
The following convenience methods are provided by this module. They are exported by default:
This function call is functionally identical to:
$json_text = JSON::XS->new->utf8->encode ($perl_scalar)
except being faster.
encode_json: expects an UTF-8 (binary) string and tries
to parse that as an UTF-8 encoded JSON text, returning the resulting
reference. Croaks on error.
This function call is functionally identical to:
$perl_scalar = JSON::XS->new->utf8->decode ($json_text)
except being faster.
1 and 0, respectively
and are used to represent JSON true and false values in Perl.
See MAPPING, below, for more information on how JSON values are mapped to Perl.
Since this often leads to confusion, here are a few very clear words on how Unicode works in Perl, modulo bugs.
If you didn't know about that flag, just the better, pretend it doesn't exist.
I hope this helps :)
The object oriented interface lets you configure your own encoding or decoding style, within the limits of supported formats.
The mutators for flags all return the JSON object again and thus calls can be chained:
my $json = JSON::XS->new->utf8->space_after->encode ({a => [1,2]})
=> {"a": [1, 2]}
$enable is true (or missing), then the encode method will not
generate characters outside the code range 0..127 (which is ASCII). Any
Unicode characters outside that range will be escaped using either a
single \uXXXX (BMP characters) or a double \uHHHH\uLLLLL escape sequence,
as per RFC4627. The resulting encoded JSON text can be treated as a native
Unicode string, an ascii-encoded, latin1-encoded or UTF-8 encoded string,
or any other superset of ASCII.
If $enable is false, then the encode method will not escape Unicode
characters unless required by the JSON syntax or other flags. This results
in a faster and more compact format.
See also the section ENCODING/CODESET FLAG NOTES later in this document.
The main use for this flag is to produce JSON texts that can be transmitted over a 7-bit channel, as the encoded JSON texts will not contain any 8 bit characters.
JSON::XS->new->ascii (1)->encode ([chr 0x10401]) => ["\ud801\udc01"]
$enable is true (or missing), then the encode method will encode
the resulting JSON text as latin1 (or iso-8859-1), escaping any characters
outside the code range 0..255. The resulting string can be treated as a
latin1-encoded JSON text or a native Unicode string. The decode method
will not be affected in any way by this flag, as decode by default
expects Unicode, which is a strict superset of latin1.
If $enable is false, then the encode method will not escape Unicode
characters unless required by the JSON syntax or other flags.
See also the section ENCODING/CODESET FLAG NOTES later in this document.
The main use for this flag is efficiently encoding binary data as JSON text, as most octets will not be escaped, resulting in a smaller encoded size. The disadvantage is that the resulting JSON text is encoded in latin1 (and must correctly be treated as such when storing and transferring), a rare encoding for JSON. It is therefore most useful when you want to store data structures known to contain binary data efficiently in files or databases, not when talking to other JSON encoders/decoders.
JSON::XS->new->latin1->encode (["\x{89}\x{abc}"]
=> ["\x{89}\\u0abc"] # (perl syntax, U+abc escaped, U+89 not)
$enable is true (or missing), then the encode method will encode
the JSON result into UTF-8, as required by many protocols, while the
decode method expects to be handled an UTF-8-encoded string. Please
note that UTF-8-encoded strings do not contain any characters outside the
range 0..255, they are thus useful for bytewise/binary I/O. In future
versions, enabling this option might enable autodetection of the UTF-16
and UTF-32 encoding families, as described in RFC4627.
If $enable is false, then the encode method will return the JSON
string as a (non-encoded) Unicode string, while decode expects thus a
Unicode string. Any decoding or encoding (e.g. to UTF-8 or UTF-16) needs
to be done yourself, e.g. using the Encode module.
See also the section ENCODING/CODESET FLAG NOTES later in this document.
Example, output UTF-16BE-encoded JSON:
use Encode; $jsontext = encode "UTF-16BE", JSON::XS->new->encode ($object);
Example, decode UTF-32LE-encoded JSON:
use Encode; $object = JSON::XS->new->decode (decode "UTF-32LE", $jsontext);
indent, space_before and
space_after (and in the future possibly more) flags in one call to
generate the most readable (or most compact) form possible.
Example, pretty-print some simple structure:
my $json = JSON::XS->new->pretty(1)->encode ({a => [1,2]})
=>
{
"a" : [
1,
2
]
}
$enable is true (or missing), then the encode method will use a multiline
format as output, putting every array member or object/hash key-value pair
into its own line, indenting them properly.
If $enable is false, no newlines or indenting will be produced, and the
resulting JSON text is guaranteed not to contain any newlines.
This setting has no effect when decoding JSON texts.
$enable is true (or missing), then the encode method will add an extra
optional space before the : separating keys from values in JSON objects.
If $enable is false, then the encode method will not add any extra
space at those places.
This setting has no effect when decoding JSON texts. You will also
most likely combine this setting with space_after.
Example, space_before enabled, space_after and indent disabled:
{"key" :"value"}
$enable is true (or missing), then the encode method will add an extra
optional space after the : separating keys from values in JSON objects
and extra whitespace after the , separating key-value pairs and array
members.
If $enable is false, then the encode method will not add any extra
space at those places.
This setting has no effect when decoding JSON texts.
Example, space_before and indent disabled, space_after enabled:
{"key": "value"}
$enable is true (or missing), then decode will accept some
extensions to normal JSON syntax (see below). encode will not be
affected in anyway. Be aware that this option makes you accept invalid
JSON texts as if they were valid!. I suggest only to use this option to
parse application-specific files written by humans (configuration files,
resource files etc.)
If $enable is false (the default), then decode will only accept
valid JSON texts.
Currently accepted extensions are:
[
1,
2, <- this comma not normally allowed
]
{
"k1": "v1",
"k2": "v2", <- this comma not normally allowed
}
[
1, # this comment not allowed in JSON
# neither this one...
]
$enable is true (or missing), then the encode method will output JSON objects
by sorting their keys. This is adding a comparatively high overhead.
If $enable is false, then the encode method will output key-value
pairs in the order Perl stores them (which will likely change between runs
of the same script).
This option is useful if you want the same data structure to be encoded as the same JSON text (given the same overall settings). If it is disabled, the same hash might be encoded differently even if contains the same data, as key-value pairs have no inherent ordering in Perl.
This setting has no effect when decoding JSON texts.
$enable is true (or missing), then the encode method can convert a
non-reference into its corresponding string, number or null JSON value,
which is an extension to RFC4627. Likewise, decode will accept those JSON
values instead of croaking.
If $enable is false, then the encode method will croak if it isn't
passed an arrayref or hashref, as JSON texts must either be an object
or array. Likewise, decode will croak if given something that is not a
JSON object or array.
Example, encode a Perl scalar as JSON value with enabled allow_nonref,
resulting in an invalid JSON text:
JSON::XS->new->allow_nonref->encode ("Hello, World!")
=> "Hello, World!"
$enable is true (or missing), then encode will not throw an
exception when it encounters values it cannot represent in JSON (for
example, filehandles) but instead will encode a JSON null value. Note
that blessed objects are not included here and are handled separately by
c<allow_nonref>.
If $enable is false (the default), then encode will throw an
exception when it encounters anything it cannot encode as JSON.
This option does not affect decode in any way, and it is recommended to
leave it off unless you know your communications partner.
$enable is true (or missing), then the encode method will not
barf when it encounters a blessed reference. Instead, the value of the
convert_blessed option will decide whether null (convert_blessed
disabled or no TO_JSON method found) or a representation of the
object (convert_blessed enabled and TO_JSON method found) is being
encoded. Has no effect on decode.
If $enable is false (the default), then encode will throw an
exception when it encounters a blessed object.
$enable is true (or missing), then encode, upon encountering a
blessed object, will check for the availability of the TO_JSON method
on the object's class. If found, it will be called in scalar context
and the resulting scalar will be encoded instead of the object. If no
TO_JSON method is found, the value of allow_blessed will decide what
to do.
The TO_JSON method may safely call die if it wants. If TO_JSON
returns other blessed objects, those will be handled in the same
way. TO_JSON must take care of not causing an endless recursion cycle
(== crash) in this case. The name of TO_JSON was chosen because other
methods called by the Perl core (== not by the user of the object) are
usually in upper case letters and to avoid collisions with any to_json
function or method.
This setting does not yet influence decode in any way, but in the
future, global hooks might get installed that influence decode and are
enabled by this setting.
If $enable is false, then the allow_blessed setting will decide what
to do when a blessed object is found.
$coderef is specified, it will be called from decode each
time it decodes a JSON object. The only argument is a reference to the
newly-created hash. If the code references returns a single scalar (which
need not be a reference), this value (i.e. a copy of that scalar to avoid
aliasing) is inserted into the deserialised data structure. If it returns
an empty list (NOTE: not undef, which is a valid scalar), the
original deserialised hash will be inserted. This setting can slow down
decoding considerably.
When $coderef is omitted or undefined, any existing callback will
be removed and decode will not change the deserialised hash in any
way.
Example, convert all JSON objects into the integer 5:
my $js = JSON::XS->new->filter_json_object (sub { 5 });
# returns [5]
$js->decode ('[{}]')
# throw an exception because allow_nonref is not enabled
# so a lone 5 is not allowed.
$js->decode ('{"a":1, "b":2}');
filter_json_object, but is only called for
JSON objects having a single key named $key.
This $coderef is called before the one specified via
filter_json_object, if any. It gets passed the single value in the JSON
object. If it returns a single value, it will be inserted into the data
structure. If it returns nothing (not even undef but the empty list),
the callback from filter_json_object will be called next, as if no
single-key callback were specified.
If $coderef is omitted or undefined, the corresponding callback will be
disabled. There can only ever be one callback for a given key.
As this callback gets called less often then the filter_json_object
one, decoding speed will not usually suffer as much. Therefore, single-key
objects make excellent targets to serialise Perl objects into, especially
as single-key JSON objects are as close to the type-tagged value concept
as JSON gets (it's basically an ID/VALUE tuple). Of course, JSON does not
support this in any way, so you need to make sure your data never looks
like a serialised Perl hash.
Typical names for the single object key are __class_whatever__, or
$__dollars_are_rarely_used__$ or }ugly_brace_placement, or even
things like __class_md5sum(classname)__, to reduce the risk of clashing
with real hashes.
Example, decode JSON objects of the form { "__widget__" => <id> }
into the corresponding $WIDGET{<id>} object:
# return whatever is in $WIDGET{5}:
JSON::XS
->new
->filter_json_single_key_object (__widget__ => sub {
$WIDGET{ $_[0] }
})
->decode ('{"__widget__": 5')
# this can be used with a TO_JSON method in some "widget" class
# for serialisation to json:
sub WidgetBase::TO_JSON {
my ($self) = @_;
unless ($self->{id}) {
$self->{id} = ..get..some..id..;
$WIDGET{$self->{id}} = $self;
}
{ __widget__ => $self->{id} }
}
encode or decode to their minimum size possible. This can save
memory when your JSON texts are either very very long or you have many
short strings. It will also try to downgrade any strings to octet-form
if possible: perl stores strings internally either in an encoding called
UTF-X or in octet-form. The latter cannot store everything but uses less
space in general (and some buggy Perl or C code might even rely on that
internal representation being used).
The actual definition of what shrink does might change in future versions, but it will always try to save space at the expense of time.
If $enable is true (or missing), the string returned by encode will
be shrunk-to-fit, while all strings generated by decode will also be
shrunk-to-fit.
If $enable is false, then the normal perl allocation algorithms are used.
If you work with your data, then this is likely to be faster.
In the future, this setting might control other things, such as converting strings that look like integers or floats into integers or floats internally (there is no difference on the Perl level), saving space.
512) accepted while encoding
or decoding. If a higher nesting level is detected in JSON text or a Perl
data structure, then the encoder and decoder will stop and croak at that
point.
Nesting level is defined by number of hash- or arrayrefs that the encoder
needs to traverse to reach a given point or the number of { or [
characters without their matching closing parenthesis crossed to reach a
given character in a string.
Setting the maximum depth to one disallows any nesting, so that ensures that the object is only a single hash/object or array.
If no argument is given, the highest possible setting will be used, which is rarely useful.
Note that nesting is implemented by recursion in C. The default value has been chosen to be as large as typical operating systems allow without crashing.
See SECURITY CONSIDERATIONS, below, for more info on why this is useful.
0, meaning no limit. When decode
is called on a string that is longer then this many bytes, it will not
attempt to decode the string but throw an exception. This setting has no
effect on encode (yet).
If no argument is given, the limit check will be deactivated (same as when
0 is specified).
See SECURITY CONSIDERATIONS, below, for more info on why this is useful.
undef) become JSON null values. Neither true
nor false values will be generated.
encode: expects a JSON text and tries to parse it,
returning the resulting simple scalar or reference. Croaks on error.
JSON numbers and strings become simple Perl scalars. JSON arrays become
Perl arrayrefs and JSON objects become Perl hashrefs. true becomes
1, false becomes 0 and null becomes undef.
decode method, but instead of raising an exception
when there is trailing garbage after the first JSON object, it will
silently stop parsing there and return the number of characters consumed
so far.
This is useful if your JSON texts are not delimited by an outer protocol (which is not the brightest thing to do in the first place) and you need to know where the JSON text ends.
JSON::XS->new->decode_prefix ("[1] the tail")
=> ([], 3)
[This section and the API it details is still EXPERIMENTAL]
In some cases, there is the need for incremental parsing of JSON
texts. While this module always has to keep both JSON text and resulting
Perl data structure in memory at one time, it does allow you to parse a
JSON stream incrementally. It does so by accumulating text until it has
a full JSON object, which it then can decode. This process is similar to
using decode_prefix to see if a full JSON object is available, but is
much more efficient (JSON::XS will only attempt to parse the JSON text
once it is sure it has enough text to get a decisive result, using a very
simple but truly incremental parser).
The following two methods deal with this.
If $string is given, then this string is appended to the already
existing JSON fragment stored in the $json object.
After that, if the function is called in void context, it will simply return without doing anything further. This can be used to add more text in as many chunks as you want.
If the method is called in scalar context, then it will try to extract
exactly one JSON object. If that is successful, it will return this
object, otherwise it will return undef. If there is a parse error,
this method will croak just as decode would do (one can then use
incr_skip to skip the errornous part). This is the most common way of
using the method.
And finally, in list context, it will try to extract as many objects from the stream as it can find and return them, or the empty list otherwise. For this to work, there must be no separators between the JSON objects or arrays, instead they must be concatenated back-to-back. If an error occurs, an exception will be raised as in the scalar context case. Note that in this case, any previously-parsed JSON texts will be lost.
incr_parse in scalar context successfully returned an object. Under
all other circumstances you must not call this function (I mean it.
although in simple tests it might actually work, it will fail under
real world conditions). As a special exception, you can also call this
method before having parsed anything.
This function is useful in two cases: a) finding the trailing text after a JSON object or b) parsing multiple JSON objects separated by non-JSON text (such as commas).
incr_parse
died, in which case the input buffer and incremental parser state is left
unchanged, to skip the text parsed so far and to reset the parse state.
All options that affect decoding are supported, except
allow_nonref. The reason for this is that it cannot be made to
work sensibly: JSON objects and arrays are self-delimited, i.e. you can concatenate
them back to back and still decode them perfectly. This does not hold true
for JSON numbers, however.
For example, is the string 1 a single JSON number, or is it simply the
start of 12? Or is 12 a single JSON number, or the concatenation
of 1 and 2? In neither case you can tell, and this is why JSON::XS
takes the conservative route and disallows this case.
Some examples will make all this clearer. First, a simple example that
works similarly to decode_prefix: We want to decode the JSON object at
the start of a string and identify the portion after the JSON object:
my $text = "[1,2,3] hello";
my $json = new JSON::XS;
my $obj = $json->incr_parse ($text)
or die "expected JSON object or array at beginning of string";
my $tail = $json->incr_text; # $tail now contains " hello"
Easy, isn't it?
Now for a more complicated example: Imagine a hypothetical protocol where
you read some requests from a TCP stream, and each request is a JSON
array, without any separation between them (in fact, it is often useful to
use newlines as ``separators'', as these get interpreted as whitespace at
the start of the JSON text, which makes it possible to test said protocol
with telnet...).
Here is how you'd do it (it is trivial to write this in an event-based manner):
my $json = new JSON::XS;
# read some data from the socket
while (sysread $socket, my $buf, 4096) {
# split and decode as many requests as possible
for my $request ($json->incr_parse ($buf)) {
# act on the $request
}
}
Another complicated example: Assume you have a string with JSON objects
or arrays, all separated by (optional) comma characters (e.g. [1],[2],
[3]). To parse them, we have to skip the commas between the JSON texts,
and here is where the lvalue-ness of incr_text comes in useful:
my $text = "[1],[2], [3]"; my $json = new JSON::XS;
# void context, so no parsing done $json->incr_parse ($text);
# now extract as many objects as possible. note the
# use of scalar context so incr_text can be called.
while (my $obj = $json->incr_parse) {
# do something with $obj
# now skip the optional comma
$json->incr_text =~ s/^ \s* , //x;
}
Now lets go for a very complex example: Assume that you have a gigantic JSON array-of-objects, many gigabytes in size, and you want to parse it, but you cannot load it into memory fully (this has actually happened in the real world :).
Well, you lost, you have to implement your own JSON parser. But JSON::XS can still help you: You implement a (very simple) array parser and let JSON decode the array elements, which are all full JSON objects on their own (this wouldn't work if the array elements could be JSON numbers, for example):
my $json = new JSON::XS;
# open the monster
open my $fh, "<bigfile.json"
or die "bigfile: $!";
# first parse the initial "["
for (;;) {
sysread $fh, my $buf, 65536
or die "read error: $!";
$json->incr_parse ($buf); # void context, so no parsing
# Exit the loop once we found and removed(!) the initial "[".
# In essence, we are (ab-)using the $json object as a simple scalar
# we append data to.
last if $json->incr_text =~ s/^ \s* \[ //x;
}
# now we have the skipped the initial "[", so continue
# parsing all the elements.
for (;;) {
# in this loop we read data until we got a single JSON object
for (;;) {
if (my $obj = $json->incr_parse) {
# do something with $obj
last;
}
# add more data
sysread $fh, my $buf, 65536
or die "read error: $!";
$json->incr_parse ($buf); # void context, so no parsing
}
# in this loop we read data until we either found and parsed the
# separating "," between elements, or the final "]"
for (;;) {
# first skip whitespace
$json->incr_text =~ s/^\s*//;
# if we find "]", we are done
if ($json->incr_text =~ s/^\]//) {
print "finished.\n";
exit;
}
# if we find ",", we can continue with the next element
if ($json->incr_text =~ s/^,//) {
last;
}
# if we find anything else, we have a parse error!
if (length $json->incr_text) {
die "parse error near ", $json->incr_text;
}
# else add more data
sysread $fh, my $buf, 65536
or die "read error: $!";
$json->incr_parse ($buf); # void context, so no parsing
}
This is a complex example, but most of the complexity comes from the fact that we are trying to be correct (bear with me if I am wrong, I never ran the above example :).
This section describes how JSON::XS maps Perl values to JSON values and vice versa. These mappings are designed to ``do the right thing'' in most circumstances automatically, preserving round-tripping characteristics (what you put in comes out as something equivalent).
For the more enlightened: note that in the following descriptions, lowercase perl refers to the Perl interpreter, while uppercase Perl refers to the abstract Perl language itself.
If the number consists of digits only, JSON::XS will try to represent it as an integer value. If that fails, it will try to represent it as a numeric (floating point) value if that is possible without loss of precision. Otherwise it will preserve the number as a string value (in which case you lose roundtripping ability, as the JSON number will be re-encoded toa JSON string).
Numbers containing a fractional or exponential part will always be represented as numeric (floating point) values, possibly at a loss of precision (in which case you might lose perfect roundtripping ability, but the JSON number will still be re-encoded as a JSON number).
JSON::XS::true and JSON::XS::false,
respectively. They are overloaded to act almost exactly like the numbers
1 and 0. You can check whether a scalar is a JSON boolean by using
the JSON::XS::is_bool function.
undef in Perl.
The mapping from Perl to JSON is slightly more difficult, as Perl is a truly typeless language, so we can only guess which JSON type is meant by a Perl value.
0 and
1, which get turned into false and true atoms in JSON. You can
also use JSON::XS::false and JSON::XS::true to improve readability.
encode_json [\0,JSON::XS::true] # yields [false,true]
\1 and \0 directly if you want.
allow_blessed and convert_blessed methods on various options on
how to deal with this: basically, you can choose between throwing an
exception, encoding the reference as if it weren't blessed, or provide
your own serialiser method.
null values, scalars that have last been used in a string context
before encoding as JSON strings, and anything else as number value:
# dump as number encode_json [2] # yields [2] encode_json [-3.0e17] # yields [-3e+17] my $value = 5; encode_json [$value] # yields [5]
# used as string, so dump as string print $value; encode_json [$value] # yields ["5"]
# undef becomes null encode_json [undef] # yields [null]
You can force the type to be a JSON string by stringifying it:
my $x = 3.1; # some variable containing a number "$x"; # stringified $x .= ""; # another, more awkward way to stringify print $x; # perl does it for you, too, quite often
You can force the type to be a JSON number by numifying it:
my $x = "3"; # some variable containing a string $x += 0; # numify it, ensuring it will be dumped as a number $x *= 1; # same thing, the choice is yours.
You can not currently force the type in other, less obscure, ways. Tell me if you need this capability (but don't forget to explain why it's needed :).
The interested reader might have seen a number of flags that signify
encodings or codesets - utf8, latin1 and ascii. There seems to be
some confusion on what these do, so here is a short comparison:
utf8 controls whether the JSON text created by encode (and expected
by decode) is UTF-8 encoded or not, while latin1 and ascii only
control whether encode escapes character values outside their respective
codeset range. Neither of these flags conflict with each other, although
some combinations make less sense than others.
Care has been taken to make all flags symmetrical with respect to
encode and decode, that is, texts encoded with any combination of
these flag values will be correctly decoded when the same flags are used
- in general, if you use different flag settings while encoding vs. when
decoding you likely have a bug somewhere.
Below comes a verbose discussion of these flags. Note that a ``codeset'' is simply an abstract set of character-codepoint pairs, while an encoding takes those codepoint numbers and encodes them, in our case into octets. Unicode is (among other things) a codeset, UTF-8 is an encoding, and ISO-8859-1 (= latin 1) and ASCII are both codesets and encodings at the same time, which can be confusing.
utf8 flag disabledutf8 is disabled (the default), then encode/decode generate
and expect Unicode strings, that is, characters with high ordinal Unicode
values (> 255) will be encoded as such characters, and likewise such
characters are decoded as-is, no canges to them will be done, except
``(re-)interpreting'' them as Unicode codepoints or Unicode characters,
respectively (to Perl, these are the same thing in strings unless you do
funny/weird/dumb stuff).
This is useful when you want to do the encoding yourself (e.g. when you want to have UTF-16 encoded JSON texts) or when some other layer does the encoding for you (for example, when printing to a terminal using a filehandle that transparently encodes to UTF-8 you certainly do NOT want to UTF-8 encode your data first and have Perl encode it another time).
utf8 flag enabledutf8-flag is enabled, encode/decode will encode all
characters using the corresponding UTF-8 multi-byte sequence, and will
expect your input strings to be encoded as UTF-8, that is, no ``character''
of the input string must have any value > 255, as UTF-8 does not allow
that.
The utf8 flag therefore switches between two modes: disabled means you
will get a Unicode string in Perl, enabled means you get an UTF-8 encoded
octet/binary string in Perl.
latin1 or ascii flags enabledlatin1 (or ascii) enabled, encode will escape characters
with ordinal values > 255 (> 127 with ascii) and encode the remaining
characters as specified by the utf8 flag.
If utf8 is disabled, then the result is also correctly encoded in those
character sets (as both are proper subsets of Unicode, meaning that a
Unicode string with all character values < 256 is the same thing as a
ISO-8859-1 string, and a Unicode string with all character values < 128 is
the same thing as an ASCII string in Perl).
If utf8 is enabled, you still get a correct UTF-8-encoded string,
regardless of these flags, just some more characters will be escaped using
\uXXXX then before.
Note that ISO-8859-1-encoded strings are not compatible with UTF-8 encoding, while ASCII-encoded strings are. That is because the ISO-8859-1 encoding is NOT a subset of UTF-8 (despite the ISO-8859-1 codeset being a subset of Unicode), while ASCII is.
Surprisingly, decode will ignore these flags and so treat all input
values as governed by the utf8 flag. If it is disabled, this allows you
to decode ISO-8859-1- and ASCII-encoded strings, as both strict subsets of
Unicode. If it is enabled, you can correctly decode UTF-8 encoded strings.
So neither latin1 nor ascii are incompatible with the utf8 flag -
they only govern when the JSON output engine escapes a character or not.
The main use for latin1 is to relatively efficiently store binary data
as JSON, at the expense of breaking compatibility with most JSON decoders.
The main use for ascii is to force the output to not contain characters
with values > 127, which means you can interpret the resulting string
as UTF-8, ISO-8859-1, ASCII, KOI8-R or most about any character set and
8-bit-encoding, and still get the same data structure back. This is useful
when your channel for JSON transfer is not 8-bit clean or the encoding
might be mangled in between (e.g. in mail), and works because ASCII is a
proper subset of most 8-bit and multibyte encodings in use in the world.
You often hear that JSON is a subset of YAML. This is, however, a mass
hysteria(*) and very far from the truth (as of the time of this writing),
so let me state it clearly: in general, there is no way to configure
JSON::XS to output a data structure as valid YAML that works in all
cases.
If you really must use JSON::XS to generate YAML, you should use this algorithm (subject to change in future versions):
my $to_yaml = JSON::XS->new->utf8->space_after (1); my $yaml = $to_yaml->encode ($ref) . "\n";
This will usually generate JSON texts that also parse as valid
YAML. Please note that YAML has hardcoded limits on (simple) object key
lengths that JSON doesn't have and also has different and incompatible
unicode handling, so you should make sure that your hash keys are
noticeably shorter than the 1024 ``stream characters'' YAML allows and that
you do not have characters with codepoint values outside the Unicode BMP
(basic multilingual page). YAML also does not allow \/ sequences in
strings (which JSON::XS does not currently generate, but other JSON
generators might).
There might be other incompatibilities that I am not aware of (or the YAML specification has been changed yet again - it does so quite often). In general you should not try to generate YAML with a JSON generator or vice versa, or try to parse JSON with a YAML parser or vice versa: chances are high that you will run into severe interoperability problems when you least expect it.
In my opinion, instead of pressuring and insulting people who actually clarify issues with YAML and the wrong statements of some of its proponents, I would kindly suggest reading the JSON spec (which is not that difficult or long) and finally make YAML compatible to it, and educating users about the changes, instead of spreading lies about the real compatibility for many years and trying to silence people who point out that it isn't true.
It seems that JSON::XS is surprisingly fast, as shown in the following
tables. They have been generated with the help of the eg/bench program
in the JSON::XS distribution, to make it easy to compare on your own
system.
First comes a comparison between various modules using a very short single-line JSON string (also available at http://dist.schmorp.de/misc/json/short.json).
{"method": "handleMessage", "params": ["user1",
"we were just talking"], "id": null, "array":[1,11,234,-5,1e5,1e7,
true, false]}
It shows the number of encodes/decodes per second (JSON::XS uses the functional interface, while JSON::XS/2 uses the OO interface with pretty-printing and hashkey sorting enabled, JSON::XS/3 enables shrink). Higher is better:
module | encode | decode | -----------|------------|------------| JSON 1.x | 4990.842 | 4088.813 | JSON::DWIW | 51653.990 | 71575.154 | JSON::PC | 65948.176 | 74631.744 | JSON::PP | 8931.652 | 3817.168 | JSON::Syck | 24877.248 | 27776.848 | JSON::XS | 388361.481 | 227951.304 | JSON::XS/2 | 227951.304 | 218453.333 | JSON::XS/3 | 338250.323 | 218453.333 | Storable | 16500.016 | 135300.129 | -----------+------------+------------+
That is, JSON::XS is about five times faster than JSON::DWIW on encoding, about three times faster on decoding, and over forty times faster than JSON, even with pretty-printing and key sorting. It also compares favourably to Storable for small amounts of data.
Using a longer test string (roughly 18KB, generated from Yahoo! Locals search API (http://dist.schmorp.de/misc/json/long.json).
module | encode | decode | -----------|------------|------------| JSON 1.x | 55.260 | 34.971 | JSON::DWIW | 825.228 | 1082.513 | JSON::PC | 3571.444 | 2394.829 | JSON::PP | 210.987 | 32.574 | JSON::Syck | 552.551 | 787.544 | JSON::XS | 5780.463 | 4854.519 | JSON::XS/2 | 3869.998 | 4798.975 | JSON::XS/3 | 5862.880 | 4798.975 | Storable | 4445.002 | 5235.027 | -----------+------------+------------+
Again, JSON::XS leads by far (except for Storable which non-surprisingly decodes faster).
On large strings containing lots of high Unicode characters, some modules (such as JSON::PC) seem to decode faster than JSON::XS, but the result will be broken due to missing (or wrong) Unicode handling. Others refuse to decode or encode properly, so it was impossible to prepare a fair comparison table for that case.
When you are using JSON in a protocol, talking to untrusted potentially hostile creatures requires relatively few measures.
First of all, your JSON decoder should be secure, that is, should not have any buffer overflows. Obviously, this module should ensure that and I am trying hard on making that true, but you never know.
Second, you need to avoid resource-starving attacks. That means you should limit the size of JSON texts you accept, or make sure then when your resources run out, that's just fine (e.g. by using a separate process that can crash safely). The size of a JSON text in octets or characters is usually a good indication of the size of the resources required to decode it into a Perl structure. While JSON::XS can check the size of the JSON text, it might be too late when you already have it in memory, so you might want to check the size before you accept the string.
Third, JSON::XS recurses using the C stack when decoding objects and
arrays. The C stack is a limited resource: for instance, on my amd64
machine with 8MB of stack size I can decode around 180k nested arrays but
only 14k nested JSON objects (due to perl itself recursing deeply on croak
to free the temporary). If that is exceeded, the program crashes. To be
conservative, the default nesting limit is set to 512. If your process
has a smaller stack, you should adjust this setting accordingly with the
max_depth method.
Something else could bomb you, too, that I forgot to think of. In that case, you get to keep the pieces. I am always open for hints, though...
Also keep in mind that JSON::XS might leak contents of your Perl data structures in its error messages, so when you serialise sensitive information you might want to make sure that exceptions thrown by JSON::XS will not end up in front of untrusted eyes.
If you are using JSON::XS to return packets to consumption by JavaScript scripts in a browser you should have a look at http://jpsykes.com/47/practical-csrf-and-json-security to see whether you are vulnerable to some common attack vectors (which really are browser design bugs, but it is still you who will have to deal with it, as major browser developers care only for features, not about getting security right).
This module is not guaranteed to be thread safe and there are no plans to change this until Perl gets thread support (as opposed to the horribly slow so-called ``threads'' which are simply slow and bloated process simulations - use fork, it's much faster, cheaper, better).
(It might actually work, but you have been warned).
While the goal of this module is to be correct, that unfortunately does not mean it's bug-free, only that I think its design is bug-free. It is still relatively early in its development. If you keep reporting bugs they will be fixed swiftly, though.
Please refrain from using rt.cpan.org or any other bug reporting service. I put the contact address into my modules for a reason.
The json_xs command line utility for quick experiments.
Marc Lehmann <schmorp@schmorp.de> http://home.schmorp.de/
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