How to write JSONPath expressions

JSONPath tool lets you analyse and selectively extract data from a JSON structure. JSONPath is similar to XPath for XML.

JSONPath uses JSONPath expressions to select elements from a JSON structure.

Let us take a very simple use case where we want to extract the event name from the following JSON document.

{  
  "event": {  
    "name": "agent",  
    "data": {  
      "name": "James Bond"  
    }  
  }  
}

You can query for event name using following JSONPath expression

event.name

Supported Operators

JSONPath expressions support following operators

Operator Description
$ The root element to query. This starts all path expressions.
@ The current node being processed by a filter predicate.
* Wildcard. Available anywhere a name or numeric are required.
.. Deep scan. Available anywhere a name is required.
.<name> Dot-notated child
[’<name>’ (, ‘<name>’)] Bracket-notated child or children
[<number> (, <number>)] Array index or indexes
[start:end] Array slice operator
[?(<expression>)] Filter expression. Expression must evaluate to a boolean value.

Functions

Functions can be invoked at the tail end of a path - the input to a function is the output of the path expression.

Following functions are supported

Function Description Output
min() Provides the min value of an array of numbers Double
max() Provides the max value of an array of numbers Double
avg() Provides the average value of an array of numbers Double
stddev() Deep scan. Available anywhere a name is required. Double
length() Dot-notated child Double

Filter Operators

Filters are logical expressions used to filter arrays. A typical filter would be [?(@.age > 18)] where @ represents the current item being processed. More complex filters can be created with logical operators && and ||. String literals must be enclosed by single or double quotes ([?(@.color == 'blue')] or [?(@.color == "blue")]).

Operator Description
== left is equal to right (note that 1 is not equal to ‘1’)
!= left is not equal to right
< left is less than right
> left is greater than right
>= left is greater than or equal to right
=~ left matches regular expression [?(@.name =~ /foo.*?/i)]
in left exists in right [?(@.size in [‘S’, ‘M’])]
nin left does not exists in right
subsetof left is a subset of right [?(@.sizes subsetof [‘S’, ‘M’, ‘L’])]
size size of left (array or string) should match right
empty left (array or string) should be empty

JSONPath expression examples

For the following JSON:

{
  "event": {
    "name": "Bond Movies",
    "movies": [
      {
        "name": "Licence to Kill",
        "star": "Timothy Dalton",
        "rating": 6.6
      },
      {
        "name": "GoldenEye",
        "star": "Pierce Brosnan",
        "rating": 7.2
      },
      {
        "name": "Tomorrow Never Dies",
        "star": "Pierce Brosnan",
        "rating": 6.5
      },
      {
        "name": "Skyfall",
        "star": "Daniel Craig",
        "rating": 7.8
      }
    ]
  }
}
JSONPath Result
$ Entire object
$.event Event object
$.event[‘name’] Event name
$.event.name Event name
$..name All names
$.event.movies[0] First movie
$.event.[‘movies’]][0] First movie
$.event.movies[0,2] First three movies
$.event.movies[:2] First two movies
$.event.movies[-2:] Last two movies
$.event.movies[?(@.rating > 7)] All movies with rating greater than 7
$.event.movies[?(@.star == ‘Pierce Brosnan’)] All movies by Pierce Brosnan
$.event.movies.length() Number of movies

What is Returned When?

JsonPath tries to automatically cast the result to the type expected by the invoker.

Following operators always return a list

  • .. - a deep scan operator
  • ?() - an expression
  • [, (, )\] - multiple array indexes

Further Reading

Hevo uses Jayway JSONPath to evaluate JSONPath expressions. You can refer to https://github.com/json-path/JsonPath for the complete reference.