JsonPath

JsonPath — JSONPath implementation

Types and Values

Object Hierarchy

    GObject
    ╰── JsonPath

Description

JsonPath is a simple class implementing the JSONPath syntax for extracting data out of a JSON tree. While the semantics of the JSONPath expressions are heavily borrowed by the XPath specification for XML, the syntax follows the ECMAScript origins of JSON.

Once a JsonPath instance has been created, it has to compile a JSONPath expression using json_path_compile() before being able to match it to a JSON tree; the same JsonPath instance can be used to match multiple JSON trees. It it also possible to compile a new JSONPath expression using the same JsonPath instance; the previous expression will be discarded only if the compilation of the new expression is successful.

The simple convenience function json_path_query() can be used for one-off matching.

Syntax of the JSONPath expressions

A JSONPath expression is composed by path indices and operators. Each path index can either be a member name or an element index inside a JSON tree. A JSONPath expression must start with the '$' operator; each path index is separated using either the dot notation or the bracket notation, e.g.:

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// dot notation
$.store.book[0].title

// bracket notation
$['store']['book'][0]['title']

The available operators are:

  • Root node The '$' character represents the root node of the JSON tree, and matches the entire document.

  • Child nodes can either be matched using '.' or '[]'. For instance, both $.store.book and `$'store' match the contents of the book member of the store object.

  • Child nodes can be reached without specifying the whole tree structure through the recursive descent operator, or '..'. For instance, $..author matches all author member in every object.

  • Child nodes can grouped through the wildcard operator, or '*'. For instance, $.store.book[*].author matches all author members of any object element contained in the book array of the store object.

  • Element nodes can be accessed using their index (starting from zero) in the subscript operator '[]'. For instance, $.store.book[0] matches the first element of the book array of the store object.

  • Subsets of element nodes can be accessed using the set notation operator '[start,end]'. For instance, $.store.book[0,2] matches the first, second, and third elements of the book array of the store object.

  • Slices of element nodes can be accessed using the slice notation operation '[start:end:step]'. If start is omitted, the starting index of the slice is implied to be zero; if end is omitted, the ending index of the slice is implied to be the length of the array; if step is omitted, the step of the slice is implied to be 1. For instance, $.store.book[:2] matches the first two elements of the book array of the store object.

More information about JSONPath is available on Stefan Gössner's JSONPath website.

Example of JSONPath matches

The following example shows some of the results of using JsonPath on a JSON tree. We use the following JSON description of a bookstore:

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{ "store": {
    "book": [
      { "category": "reference", "author": "Nigel Rees",
        "title": "Sayings of the Century", "price": "8.95"  },
      { "category": "fiction", "author": "Evelyn Waugh",
        "title": "Sword of Honour", "price": "12.99" },
      { "category": "fiction", "author": "Herman Melville",
        "title": "Moby Dick", "isbn": "0-553-21311-3",
        "price": "8.99" },
      { "category": "fiction", "author": "J. R. R. Tolkien",
        "title": "The Lord of the Rings", "isbn": "0-395-19395-8",
        "price": "22.99" }
    ],
    "bicycle": { "color": "red", "price": "19.95" }
  }
}

We can parse the JSON using JsonParser:

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JsonParser *parser = json_parser_new ();
json_parser_load_from_data (parser, json_data, -1, NULL);

If we run the following code:

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JsonNode *result;
JsonPath *path = json_path_new ();
json_path_compile (path, "$.store..author", NULL);
result = json_path_match (path, json_parser_get_root (parser));

The result JsonNode will contain an array with all values of the author member of the objects in the JSON tree. If we use a JsonGenerator to convert the JsonNode to a string and print it:

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JsonGenerator *generator = json_generator_new ();
json_generator_set_root (generator, result);
char *str = json_generator_to_data (generator, NULL);
g_print ("Results: %s\n", str);

The output will be:

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["Nigel Rees","Evelyn Waugh","Herman Melville","J. R. R. Tolkien"]

JsonPath is available since JSON-GLib 0.14

Functions

json_path_new ()

JsonPath *
json_path_new (void);

Creates a new JsonPath instance.

Once created, the JsonPath object should be used with json_path_compile() and json_path_match().

Returns

the newly created JsonPath instance. Use g_object_unref() to free the allocated resources when done.

[transfer full]

Since: 0.14


json_path_compile ()

gboolean
json_path_compile (JsonPath *path,
                   const char *expression,
                   GError **error);

Validates and decomposes expression .

A JSONPath expression must be compiled before calling json_path_match().

Parameters

path

a JsonPath

 

expression

a JSONPath expression

 

error

return location for a GError, or NULL

 

Returns

TRUE on success; on error, error will be set with the JSON_PATH_ERROR domain and a code from the JsonPathError enumeration, and FALSE will be returned

Since: 0.14


json_path_match ()

JsonNode *
json_path_match (JsonPath *path,
                 JsonNode *root);

Matches the JSON tree pointed by root using the expression compiled into the JsonPath.

The matching JsonNodes will be copied into a JsonArray and returned wrapped in a JsonNode.

Parameters

path

a compiled JsonPath

 

root

a JsonNode

 

Returns

a newly-created JsonNode of type JSON_NODE_ARRAY containing an array of matching JsonNodes. Use json_node_unref() when done.

[transfer full]

Since: 0.14


json_path_query ()

JsonNode *
json_path_query (const char *expression,
                 JsonNode *root,
                 GError **error);

Queries a JSON tree using a JSONPath expression.

This function is a simple wrapper around json_path_new(), json_path_compile() and json_path_match(). It implicitly creates a JsonPath instance, compiles expression and matches it against the JSON tree pointed by root .

Parameters

expression

a JSONPath expression

 

root

the root of a JSON tree

 

error

return location for a GError, or NULL

 

Returns

a newly-created JsonNode of type JSON_NODE_ARRAY containing an array of matching JsonNodes. Use json_node_unref() when done.

[transfer full]

Since: 0.14

Types and Values

JsonPath

typedef struct _JsonPath JsonPath;

The JsonPath structure is an opaque object whose members cannot be directly accessed except through the provided API.

Since: 0.14


JsonPathClass

typedef struct _JsonPathClass JsonPathClass;

The JsonPathClass structure is an opaque object class whose members cannot be directly accessed.

Since: 0.14


JSON_PATH_ERROR

#define JSON_PATH_ERROR         (json_path_error_quark ())

Error domain for JsonPath errors

Since: 0.14


enum JsonPathError

Error code enumeration for the JSON_PATH_ERROR domain.

Members

JSON_PATH_ERROR_INVALID_QUERY

Invalid query

 

Since: 0.14