Writing JSONPath Expressions
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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 JsonPath for the complete reference.