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A Very High Performance Objective-C JSON Library

JSONKit is licensed under the terms of the BSD License. Copyright © 2011, John Engelhart.

JavaScript Object Notation, or JSON, is a lightweight, text-based, serialization format for structured data that is used by many web-based services and API's. It is defined by RFC 4627.

JSON provides the following primitive types:

  • null
  • Boolean true and false
  • Number
  • String
  • Array
  • Associative Arrays (Objects in RFC 4627 nomenclature, a.k.a. Key / Value Hash Tables, Maps, Dictionaries, etc.)

These primitive types are mapped to the following Objective-C Foundation classes:

true and falseNSNumber
Associative ArraysNSDictionary

JSONKit uses Core Foundation internally, and it is assumed that Core Foundation ≡ Foundation for every equivalent base type, i.e. CFStringNSString.

JSON To Objective-C Primitive Mapping Details

  • While the JSON specification specifies that the serialized JSON must be encoded in Unicode, it does not specify how Unicode encoding errors should be handled. In general, JSONKit will not accept JSON that contains ill-formed Unicode.

    When the JKParseOptionLooseUnicode option is used, JSONKit follows the specifications and recommendations given in The Unicode 5.2 standard, Chapter 3, section 3.9 Unicode Encoding Forms. As a general rule of thumb, the Unicode code point U+FFFD is substituted for any ill-formed Unicode encountered. JSONKit attempts to follow the recommended Best Practice for Using U+FFFD: Replace each maximal subpart of an ill-formed subsequence by a single U+FFFD. Additionally, the following Unicode code points are treated as ill-formed Unicode, and if JKParseOptionLooseUnicode is enabled, cause U+FFFD to be substituted in their place:

    U+D800 thru U+DFFF, inclusive.
    U+FDD0 thru U+FDEF, inclusive.
    U+nFFFE and U+nFFFF, where n is from 0x0 to 0x10

    The code points U+FDD0 thru U+FDEF, U+nFFFE, and U+nFFFF (where n is from 0x0 to 0x10), are defined as Noncharacters by the Unicode standard and "should never be interchanged".

    RFC 4627 allows for these limitations under section 4, Parsers: An implementation may set limits on the length and character contents of strings.

    The NSString class may place additional restrictions or otherwise transform the JSON String in such a way so that the JSON String is not bijective with the instantiated NSString object. In other words, JSONKit can not guarantee that when you round trip a JSON String to a NSString and then back to a JSON String that the two JSON Strings will be exactly the same, even though in practice they are. For clarity, "exactly" in this case means bit for bit identical. JSONKit can not even guarantee that the two JSON Strings will be Unicode equivalent, even though in practice they will be and would be the most likely cause for the two round tripped JSON Strings to no longer be bit for bit identical.

  • JSONKit maps true and false to the CFBoolean values kCFBooleanTrue and kCFBooleanFalse, respectively. Conceptually, CFBoolean values can be thought of, and treated as, NSNumber class objects. The benefit to using CFBoolean is that true and false JSON values can be round trip deserialized and serialized without conversion or promotion to a NSNumber with a value of 0 or 1.

  • The JSON specification does not specify the details or requirements for JSON Number values, nor does it specify how errors due to conversion should be handled. In general, JSONKit will not accept JSON that contains JSON Number values that it can not convert with out error or loss of precision.

    For non-floating-point numbers (i.e., JSON Number values that do not include a . or e|E), JSONKit uses a 64-bit C primitive type internally, regardless of whether the target architecture is 32-bit or 64-bit. For unsigned values (i.e., those that do not begin with a -), this allows for values up to 264-1 and up to -263 for negative values. As a special case, the JSON Number -0 is treated as a floating-point number since the underlying floating-point primitive type is capable of representing a negative zero, whereas the underlying twos-complement non-floating-point primitive type can not. JSON that contains Number values that exceed these limits will fail to parse and optionally return a NSError object. The functions strtoll() and strtoull() are used to perform the conversions.

    The C double primitive type, or IEEE 754 Double 64-bit floating-point, is used to represent floating-point JSON Number values. JSON that contains floating-point Number values that can not be represented as a double (i.e., due to over or underflow) will fail to parse and optionally return a NSError object. The function strtod() is used to perform the conversion. Note that the JSON standard does not allow for infinities or NaN (Not a Number).

  • For JSON Associative Arrays (or object in RFC 4627 nomenclature), RFC 4627 says The names within an object SHOULD be unique (note: name is a key in JSONKit nomenclature). At this time the JSONKit behavior is undefined for JSON that contains names within an object that are not unique. However, JSONKit currently tries to follow a "the last key / value pair parsed is the one chosen" policy. This behavior is not finalized and should not be depended on.

Objective-C To JSON Primitive Mapping Details

  • The NSDictionary class allows for any object, which can be of any class, to be used as a key. JSON, however, only permits Strings to be used as keys. Therefore JSONKit will fail with an error if it encounters a NSDictionary that contains keys that are not NSString objects during serialization.

  • JSON does not allow for Numbers that are ±Infinity or ±NaN. Therefore JSONKit will fail with an error if it encounters a NSNumber that contains such a value during serialization.

  • JSONKit will fail with an error if it encounters an object that is not a NSNull, NSNumber, NSString, NSArray, or NSDictionary class object during serialization.

  • Objects created with [NSNumber numberWithBool:YES] and [NSNumber numberWithBool:NO] will be mapped to the JSON values of true and false, respectively.

Important Details

  • JSONKit is not designed to be used with the Mac OS X Garbage Collection. The behavior of JSONKit when compiled with -fobj-gc is undefined. It is extremely unlikely that Mac OS X Garbage Collection will ever be supported.

  • The JSON to be parsed by JSONKit must be encoded as Unicode. In the unlikely event you end up with JSON that is not encoded as Unicode, you must first convert the JSON to Unicode, preferably as UTF8. One way to accomplish this is with the NSString methods -initWithBytes:length:encoding: and -initWithData:encoding:.

  • Internally, the low level parsing engine uses UTF8 exclusively. The JSONDecoder method -parseJSONData: takes a NSData object as its argument and it is assumed that the raw bytes contained in the NSData is UTF8 encoded, otherwise the behavior is undefined.

  • It is not safe to use the same instantiated JSONDecoder object from multiple threads at the same time. If you wish to share a JSONDecoder between threads, you are responsible for adding mutex barriers to ensure that only one thread is decoding JSON using the shared JSONDecoder object at a time.

Tips for speed

  • Enable the NS_BLOCK_ASSERTIONS pre-processor flag. JSONKit makes heavy use of NSCParameterAssert() internally to ensure that various arguments, variables, and other state contains only legal and expected values. If an assertion check fails it causes a run time exception that will normally cause a program to terminate. These checks and assertions come with a price: they take time to execute and do not contribute to the work being performed. While your mileage may vary, the author has found that adding -DNS_BLOCK_ASSERTIONS to an -O2 optimization setting can generally result in an approximate 7-12% increase in performance.

  • Since the very low level parsing engine works exclusively with UTF8 byte streams, anything that is not already encoded as UTF8 must first be converted to UTF8. While JSONKit provides additions to the NSString class which allows you to conveniently convert JSON contained in a NSString, this convenience does come with a price. JSONKit uses the -UTF8String method to obtain a UTF8 encoded version of a NSString, and while the details of how a strings performs that conversion are an internal implementation detail, it is likely that this conversion carries a cost both in terms of time and the memory needed to store the conversion result. Therefore, if speed is a priority, you should avoid using the NSString convenience methods if possible.

  • If you are receiving JSON data from a web server, and you are able to determine that the raw bytes returned by the web server is JSON encoded as UTF8, you should use the JSONDecoder method -parseUTF8String:length: which immediately begins parsing the pointers bytes. In practice, every JSONKit method that converts JSON to an Objective-C object eventually calls this method to perform the conversion.

  • If you are using one of the various ways provided by the NSURL family of classes to receive JSON results from a web server, which typically return the results in the form of a NSData object, and you are able to determine that the raw bytes contained in the NSData are encoded as UTF8, then you should use either the JSONDecoder method parseJSONData: or the NSData method -objectFromJSONData. If are going to be converting a lot of JSON, the better choice is to instantiate a JSONDecoder object once and use the same instantiated object to perform all your conversions. This has two benefits:

    1. The NSData method -objectFromJSONData creates an autoreleased JSONDecoder object to perform the one time conversion. By instantiating a JSONDecoder object once and using the parseJSONData: method repeatedly, you can avoid this overhead.
    2. The instantiated object cache from the previous JSON conversion is reused. This can result in both better performance and a reduction in memory usage if the JSON your are converting is very similar. A typical example might be if you are converting JSON at periodic intervals that consists of real time status updates.

Parsing Interface

JSONDecoder Interface

Note: The bytes contained in a NSData object must be UTF8 encoded.

Important: Methods will raise NSInvalidArgumentException if parseOptionFlags is not valid.
Important: parseUTF8String: will raise NSInvalidArgumentException if parseUTF8String is NULL.
Important: parseJSONData: will raise NSInvalidArgumentException if jsonData is NULL.

+ (id)decoder;
+ (id)decoderWithParseOptions:(JKParseOptionFlags)parseOptionFlags;
- (id)initWithParseOptions:(JKParseOptionFlags)parseOptionFlags;

- (void)clearCache;

- (id)parseUTF8String:(const unsigned char *)string length:(size_t)length;
- (id)parseUTF8String:(const unsigned char *)string length:(size_t)length error:(NSError **)error;

- (id)parseJSONData:(NSData *)jsonData;
- (id)parseJSONData:(NSData *)jsonData error:(NSError **)error;

NSString Interface

- (id)objectFromJSONString;
- (id)objectFromJSONStringWithParseOptions:(JKParseOptionFlags)parseOptionFlags;
- (id)objectFromJSONStringWithParseOptions:(JKParseOptionFlags)parseOptionFlags error:(NSError **)error;

NSData Interface

- (id)objectFromJSONData;
- (id)objectFromJSONDataWithParseOptions:(JKParseOptionFlags)parseOptionFlags;
- (id)objectFromJSONDataWithParseOptions:(JKParseOptionFlags)parseOptionFlags error:(NSError **)error;


Parsing OptionDescription
JKParseOptionNoneThis is the default if no other other parse option flags are specified, and the option used when a convenience method does not provide an argument for explicitly specifying the parse options to use. Synonymous with JKParseOptionStrict.
JKParseOptionStrictThe JSON will be parsed in strict accordance with the RFC 4627 specification.
JKParseOptionCommentsAllow C style // and /* … */ comments in JSON. This is a fairly common extension to JSON, but JSON that contains C style comments is not strictly conforming JSON.
JKParseOptionUnicodeNewlinesAllow Unicode recommended (?:\r\n|[\n\v\f\r\x85\p{Zl}\p{Zp}]) newlines in JSON. The JSON specification only allows the newline characters \r and \n, but this option allows JSON that contains the Unicode recommended newline characters to be parsed. JSON that contains these additional newline characters is not strictly conforming JSON.
JKParseOptionLooseUnicodeNormally the decoder will stop with an error at any malformed Unicode. This option allows JSON with malformed Unicode to be parsed without reporting an error. Any malformed Unicode is replaced with \uFFFD, or REPLACEMENT CHARACTER, as specified in The Unicode 5.2 standard, Chapter 3, section 3.9 Unicode Encoding Forms.
JKParseOptionPermitTextAfterValidJSONNormally, white-space that follows the JSON is interpreted as a parsing failure. This option allows for any trailing white-space to be ignored and not cause a parsing error.

Serializing Interface

Note: The bytes contained in the returned NSData object is UTF8 encoded.

NSArray and NSDictionary Interface

- (NSData *)JSONData;
- (NSData *)JSONDataWithOptions:(JKSerializeOptionFlags)serializeOptions error:(NSError **)error;
- (NSString *)JSONString;
- (NSString *)JSONStringWithOptions:(JKSerializeOptionFlags)serializeOptions error:(NSError **)error;


Serializing OptionDescription
JKSerializeOptionNoneThis is the default if no other other serialize option flags are specified, and the option used when a convenience method does not provide an argument for explicitly specifying the serialize options to use.
JKSerializeOptionEscapeUnicodeWhen JSONKit encounters Unicode characters in NSString objects, the default behavior is to encode those Unicode characters as UTF8. This option causes JSONKit to encode those characters as \uXXXX. For example,
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