Internet Engineering Task Force (IETF) P. Bryan, Ed.
Request for Comments: 6901 Salesforce.com
Category: Standards Track K. Zyp
ISSN: 2070-1721 SitePen (USA)
M. Nottingham, Ed.
Akamai
March
April 2013
JavaScript Object Notation (JSON) Pointer
Abstract
JSON Pointer defines a string syntax for identifying a specific value
within a JavaScript Object Notation (JSON) document.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6901.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2
3. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2
4. Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . 3
5. JSON String Representation . . . . . . . . . . . . . . . . . . 5 4
6. URI Fragment Identifier Representation . . . . . . . . . . . . 6 5
7. Error Handling . . . . . . . . . . . . . . . . . . . . . . . . 6
8. Security Considerations . . . . . . . . . . . . . . . . . . . . 7 6
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
10.1. Normative References . . . . . . . . . . . . . . . . . . . 7
10.2. Informative References . . . . . . . . . . . . . . . . . . 7
1. Introduction
This specification defines JSON Pointer, a string syntax for
identifying a specific value within a JavaScript Object Notation
(JSON) document [RFC4627]. JSON Pointer is intended to be easily
expressed in JSON string values as well as Uniform Resource
Identifier (URI) [RFC3986] fragment identifiers.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This specification expresses normative syntax rules using Augmented
Backus-Naur Form (ABNF) [RFC5234] notation.
3. Syntax
A JSON Pointer is a Unicode string (see [RFC4627], Section 3)
containing a sequence of zero or more reference tokens, each prefixed
by a '/' (%x2F) character.
Because the characters '~' (%x7E) and '/' (%x2F) have special
meanings in JSON Pointer, '~' needs to be encoded as '~0' and '/'
needs to be encoded as '~1' when these characters appear in a
reference token.
The ABNF syntax of a JSON Pointer is:
json-pointer = *( "/" reference-token )
reference-token = *( unescaped / escaped )
unescaped = %x00-2E / %x30-7D / %x7F-10FFFF
; %x2F ('/') and %x7E ('~') are excluded from 'unescaped'
escaped = "~" ( "0" / "1" )
; representing '~' and '/', respectively
It is an error condition if a JSON Pointer value does not conform to
this syntax (see Section 7).
Note that JSON Pointers are specified in characters, not as bytes.
4. Evaluation
Evaluation of a JSON Pointer begins with a reference to the root
value of a JSON document and completes with a reference to some value
within the document. Each reference token in the JSON Pointer is
evaluated sequentially.
Evaluation of each reference token begins by decoding any escaped
character sequence. This is performed by first transforming any
occurrence of the sequence '~1' to '/', and then transforming any
occurrence of the sequence '~0' to '~'. By performing the
substitutions in this order, an implementation avoids the error of
turning '~01' first into '~1' and then into '/', which would be
incorrect (the string '~01' correctly becomes '~1' after
transformation).
The reference token then modifies which value is referenced according
to the following scheme:
o If the currently referenced value is a JSON object, the new
referenced value is the object member with the name identified by
the reference token. The member name is equal to the token if it
has the same number of Unicode characters as the token and their
code points are byte-by-byte equal. No Unicode character
normalization is performed. If a referenced member name is not
unique in an object, the member that is referenced is undefined,
and evaluation fails (see below).
o If the currently referenced value is a JSON array, the reference
token MUST contain either:
* characters comprised of digits (see ABNF below; note that
leading zeros are not allowed) that represent an unsigned
base-10 integer value, making the new referenced value the
array element with the zero-based index identified by the
token, or
* exactly the single character "-", making the new referenced
value the (nonexistent) member after the last array element.
The ABNF syntax for array indices is:
array-index = %x30 / ( %x31-39 *(%x30-39) )
; "0", or digits without a leading "0"
Implementations will evaluate each reference token against the
document's contents and will raise an error condition if it fails to
resolve a concrete value for any of the JSON pointer's reference
tokens. For example, if an array is referenced with a non-numeric
token, an error condition will be raised. See Section 7 for details.
Note that the use of the "-" character to index an array will always
result in such an error condition because by definition it refers to
a nonexistent array element. Thus, applications of JSON Pointer need
to specify how that character is to be handled, if it is to be
useful.
Any error condition for which a specific action is not defined by the
JSON Pointer application results in termination of evaluation.
5. JSON String Representation
A JSON Pointer can be represented in a JSON string value. Per
[RFC4627], Section 2.5, all instances of quotation mark '"' (%x22),
reverse solidus '\' (%x5C), and control (%x00-1F) characters MUST be
escaped.
Note that before processing a JSON string as a JSON Pointer,
backslash escape sequences must be unescaped.
For example, given the JSON document
{
"foo": ["bar", "baz"],
"": 0,
"a/b": 1,
"c%d": 2,
"e^f": 3,
"g|h": 4,
"i\\j": 5,
"k\"l": 6,
" ": 7,
"m~n": 8
}
The following JSON strings evaluate to the accompanying values:
"" // the whole document
"/foo" ["bar", "baz"]
"/foo/0" "bar"
"/" 0
"/a~1b" 1
"/c%d" 2
"/e^f" 3
"/g|h" 4
"/i\\j" 5
"/k\"l" 6
"/ " 7
"/m~0n" 8
6. URI Fragment Identifier Representation
A JSON Pointer can be represented in a URI fragment identifier by
encoding it into octets using UTF-8 [RFC3629], while percent-encoding
those characters not allowed by the fragment rule in [RFC3986].
Note that a given media type needs to specify JSON Pointer as its
fragment identifier syntax explicitly (usually, in its registration
[RFC6838]). That is, just because a document is JSON does not imply
that JSON Pointer can be used as its fragment identifier syntax. In
particular, the fragment identifier syntax for application/json is
not JSON Pointer.
Given the same example document as above, the following URI fragment
identifiers evaluate to the accompanying values:
# // the whole document
#/foo ["bar", "baz"]
#/foo/0 "bar"
#/ 0
#/a~1b 1
#/c%25d 2
#/e%5Ef 3
#/g%7Ch 4
#/i%5Cj 5
#/k%22l 6
#/%20 7
#/m~0n 8
7. Error Handling
In the event of an error condition, evaluation of the JSON Pointer
fails to complete.
Error conditions include, but are not limited to:
o Invalid pointer syntax
o A pointer that references a nonexistent value
This specification does not define how errors are handled. An
application of JSON Pointer SHOULD specify the impact and handling of
each type of error.
For example, some applications might stop pointer processing upon an
error, while others may attempt to recover from missing values by
inserting default ones.
8. Security Considerations
A given JSON Pointer is not guaranteed to reference an actual JSON
value. Therefore, applications using JSON Pointer should anticipate
this situation by defining how a pointer that does not resolve ought
to be handled.
Note that JSON pointers can contain the NUL (Unicode U+0000)
character. Care is needed not to misinterpret this character in
programming languages that use NUL to mark the end of a string.
9. Acknowledgements
The following individuals contributed ideas, feedback, and wording to
this specification:
Mike Acar, Carsten Bormann, Tim Bray, Jacob Davies, Martin J.
Duerst, Bjoern Hoehrmann, James H. Manger, Drew Perttula, and
Julian Reschke.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC4627] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627, July 2006.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
10.2. Informative References
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, January 2013.
Authors' Addresses
Paul C. Bryan (editor)
Salesforce.com
Phone: +1 604 783 1481
EMail: pbryan@anode.ca
Kris Zyp
SitePen (USA)
Phone: +1 650 968 8787
EMail: kris@sitepen.com
Mark Nottingham (editor)
Akamai
EMail: mnot@mnot.net