Difference between revisions of "JSON Module"

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Revision as of 13:22, 17 September 2011

JSON (JavaScript Object Notation) is a popular data exchange format for applications written in JavaScript. This module contains XQuery functions to parse and serialize JSON documents. All functions are preceded by the json: prefix, which is linked to the http://www.basex.org/json namespace. The JSON module will be officially supported with the upcoming Version 6.8 of BaseX.

As there are notable differences between JSON and XML, no mapping exists that guarantees a lossless, bidirectional conversion between JSON and XML. For this reason, we offer two sets of functions in this module:

JSON

json:parse and json:serialize facilitate a lossless conversion from JSON to XML and back. The transformation is based on the following rules:

  1. The resulting document has a <json/> root node.
  2. Names (keys) of objects are represented as elements:
    1. Empty names are represented by a single underscore (<_>...</_>).
    2. Underscore characters are rewritten to two underscores (__).
    3. A character that cannot be represented as NCName character is rewritten to an underscore and its four-digit Unicode.
  3. As arrays have no names, <value/> is used as element name.
  4. JSON values are represented as text nodes.
  5. The types of values are represented in attributes:
    1. The value types number, boolean, null, object and array are represented by a type attribute.
    2. The string type is omitted, as it is treated as default type.
    3. If a name has the same type throughout the document, the type attribute will be omitted. Instead, the name will be listed in additional, type-specific attributes in the root node. The attributes are named by their type in the plural (numbers, booleans, nulls, objects and arrays), and the attribute value contains all names with that type, separated by whitespaces.

json:parse

Signatures json:parse($input as xs:string()) as element(json)
Summary Converts the JSON document specified by $input to XML, and returns the result as element(json) instance. The converted XML document is both well readable and lossless, i.e., the converted document can be serialized back to the original JSON representation.
Errors BASX0015 is raised if the specified input cannot be parsed as JSON document.

json:serialize

Signatures json:serialize($input as node()) as xs:string()
Summary Serializes the node specified by $input as JSON, and returns the result as xs:string instance. The serialized node must conform to the syntax specified by the json:parse() function.
XML documents can also be serialized as JSON if the Serialization Option "method" is set to "json".
Errors BASX0016 is raised if the specified node cannot be serialized as JSON document.

Examples

Example 1: Adds all JSON documents in a directory to a database

Query:

let $database := "database"
for $name in file:list('.', false(), '*.json')
let $file := file:read-text($name)
let $json := json:parse($file)
return db:add($database, document { $json }, $name) 

Example 2: Converts a simple JSON string to XML

Query:

json:parse('{}')

Result:

<json objects="json"/>

Example 3: Converts a JSON string with simple objects and arrays

Query:

json:parse('{
  "title": "Talk On Travel Pool",
  "link": "http://www.flickr.com/groups/talkontravel/pool/",
  "description": "Travel and vacation photos from around the world.",
  "modified": "2009-02-02T11:10:27Z",
  "generator": "http://www.flickr.com/"
}')

Result:

<json objects="json">
  <title>Talk On Travel Pool</title>
  <link>http://www.flickr.com/groups/talkontravel/pool/</link>
  <description>Travel and vacation photos from around the world.</description>
  <modified>2009-02-02T11:10:27Z</modified>
  <generator>http://www.flickr.com/</generator>
</json>

Example 4: Converts a JSON string with different data types

Query:

json:parse('{
  "first_name": "John",
  "last_name": "Smith",
  "age": 25,
  "address": {
    "street": "21 2nd Street",
    "city": "New York",
    "code": 10021
  },
  "phone": [
    {
      "type": "home",
      "number": "212 555-1234"
    },
    {
      "type": "mobile",
      "number": 001327724623
    }
  ]
}')

Result:

<json numbers="age code" arrays="phone" objects="json address value">
  <first__name>John</first__name>
  <last__name>Smith</last__name>
  <age>25</age>
  <address>
    <street>21 2nd Street</street>
    <city>New York</city>
    <code>10021</code>
  </address>
  <phone>
    <value>
      <type>home</type>
      <number>212 555-1234</number>
    </value>
    <value>
      <type>mobile</type>
      <number type="number">001327724623</number>
    </value>
  </phone>
</json>

JsonML

json:serialize-ml and json:parse-ml are used to transform XML to JSON and back, using the JsonML dialect. JsonML can be used to transform arbitrary XML documents, but namespaces, comments and processing instructions will be discarded in the transformation process. More details are found in the official JsonML documentation.

json:serialize-ml

Signatures json:serialize-ml($input as node()) as xs:string()
Summary Serializes the node specified by $input and returns the result as xs:string instance.
XML documents can also be output in the JsonML format by setting the Serialization Option "method" to "jsonml".
Errors BASX0016 is raised if the specified value cannot be serialized.

json:parse-ml

Signatures json:parse-ml($input as xs:string()) as element()
Summary Converts the JsonML document specified by $input to XML, and returns the result as element() instance. The JSON input must conform to the JsonML specification to be successfully converted.
Errors BASX0015 is raised if the specified input cannot be parsed as JsonML instance.

Examples

Example 1: Converts all XML documents in a database to JsonML and writes them to disk

Query:

for $doc in collection('json')
let $name := document-uri($doc)
let $json := json:serialize($doc)
return file:write($name, $json)

Example 2: Converts a simple XML fragment to the JsonML format

Query:

json:serialize-ml(<xml/>)

Result:

["xml"]

Example 3: Converts an XML document with elements and text

Query:

json:serialize-ml(doc('flickr.xml'))

flickr.xml:

<flickr>
  <title>Talk On Travel Pool</title>
  <link>http://www.flickr.com/groups/talkontravel/pool/</link>
  <description>Travel and vacation photos from around the world.</description>
  <modified>2009-02-02T11:10:27Z</modified>
  <generator>http://www.flickr.com/</generator>
</flickr>

Result:

["flickr",
  ["title",
    "Talk On Travel Pool"],
  ["link",
    "http:\/\/www.flickr.com\/groups\/talkontravel\/pool\/"],
  ["description",
    "Travel and vacation photos from around the world."],
  ["modified",
    "2009-02-02T11:10:27Z"],
  ["generator",
    "http:\/\/www.flickr.com\/"]]

Example 4: Converts a document with nested elements and attributes

Query:

json:serialize-ml(doc('input.xml'))

input.xml:

<address id='1'>
  <!-- comments will be discarded -->
  <last_name>Smith</last_name>
  <age>25</age>
  <address xmlns='will be dropped as well'>
    <street>21 2nd Street</street>
    <city>New York</city>
    <code>10021</code>
  </address>
  <phone type='home'>212 555-1234</phone>
</address>

Result:

["address", {"id":"1"},
  ["last_name",
    "Smith"],
  ["age",
    "25"],
  ["address",
    ["street",
      "21 2nd Street"],
    ["city",
      "New York"],
    ["code",
      "10021"]],
  ["phone", {"type":"home"},
    "212 555-1234"]]