Applications Area Working Group P. Bryan, Ed.
Intended status: Informational K. Zyp
Expires: April 23, 2013 SitePen (USA)
M. Nottingham, Ed.
October 22, 2012

JSON Pointer


JSON Pointer defines a string syntax for identifying a specific value within a JSON document.

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Table of Contents

1. Introduction

This specification defines JSON Pointer, a string syntax for identifying a specific value within a JavaScript Object Notation (JSON) [RFC4627] document. It 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 containing a sequence of zero or more reference tokens, each prefixed by a '/' (%x2F) character.

Since the characters '~' (%x7E) and '/' (%x2F) have a special meaning in JSON Pointer, they need to be encoded as '~0' and '~1' respectively, when appearing in a reference token.

Its ABNF syntax is:

json-pointer = *( "/" reference-token )
reference-token = *( unescaped / escaped )
unescaped = %x00-2E / %x30-7D / %x7F-10FFFF
escaped = "~" ( "0" / "1" )

It is an error condition if a JSON Pointer value does not conform to this syntax (see Section 7).

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 sequentially evaluated.

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 '/', then transforming any occurrence of the sequence '~0' to '~'.

The reference token then modifies which value is referenced according to the following scheme:

If a reference token is being evaluated against a JSON document, implementations will evaluate each token against the document's contents, and terminate evaluation with an error condition if it fails to resolve a concrete value for any of the JSON pointer's reference tokens. See Section 7 for details.

Note that the use of the "-" character to index an array will always result in such an error; applications of JSON Pointer thus need to specify how it is to be handled, if it is to be useful.

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

Then 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], percent-encoding those characters not allowed by the fragment rule in [RFC3986].

Note that a given media type needs to nominate JSON Pointer as its fragment identifier syntax explicitly (usually, in its registration [RFC4288]); i.e., 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.

This includes, but is not limited to:

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; others may attempt to recover from missing values by inserting default ones.

8. IANA Considerations

This document has no impact upon IANA.

9. Security Considerations

A given JSON Pointer is not guaranteed to reference an actual JSON value. Implementations should be aware of this and take appropriate precautions.

Note that JSON pointers can contain the NUL (Unicode U+0000) character, which may not be representable in all programming languages.

10. Acknowledgements

The following individuals contributed ideas, feedback and wording to this specification:

11. References

11.1. 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.
[Unicode] The Unicode Consortium, "The Unicode Standard, Version 6.0", October 2011.

11.2. Informative References

[RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and Registration Procedures", BCP 13, RFC 4288, December 2005.

Authors' Addresses

Paul C. Bryan (editor) Phone: +1 604 783 1481 EMail:
Kris Zyp SitePen (USA) Phone: +1 650 968 8787 EMail:
Mark Nottingham (editor) EMail: