9.7. JSON Converter

The JSON converter handles JSON files. To use the JSON converter, specify type = "json" in your converter definition.

9.7.1. Configuration

The JSON converter supports parsing multiple JSON documents out of a single file. In order to support JSON path expressions, each JSON document is fully parsed into memory. For large documents, this may take considerable time and memory. Thus, it is usually better to have multiple smaller JSON documents per file, when possible.

Since a single JSON document may contain multiple features, the JSON parser supports a JSONPath expression pointing to each feature element. This can be specified using the feature-path element.

The fields element in a JSON converter supports two additional attributes, path and json-type. path should be a JSONPath expression, which is relative to the feature-path, if defined (above). For absolute paths, root-path may be used instead of path. json-type should specify the type of JSON field being read. Valid values are: string, float, double, integer, long, boolean, geometry, array and object. The value will be appropriately typed, and available in the transform element as $0. Geometry types can handle either WKT strings or GeoJSON geometry objects.

9.7.2. Handling Complex Elements

JSON can contain complex, nested elements that don’t necessarily map well to the flat attribute structure used by SimpleFeatureTypes. These type of elements can be easily handled using GeoMesa’s support for JSON Attributes. In your SimpleFeatureType schema, indicate a complex JSON string through the user data hint json=true. In your converter, select the outer element and then turn it into a JSON string through the toString transformer function. You will be able to filter and transform the data using JSONPath at query time. See JSON Attributes for more details.

9.7.3. JSON Composite Converters

Composite converters can handle processing different JSON formats in a single stream. To use a composite converter, specify type = "composite-json" in your converter definition.

Composite converters can define top-level options, fields, etc, the same as a normal JSON converter. These values will be inherited by each of the child converters. If each child is unique, then it is valid to not define anything at the top level.

Composite converters must define a converters element, which is an array of nested JSON converter definitions. In addition to the standard configuration, each nested converter must have a predicate element that determines which converter to use for each JSON document. The value passed into the predicate will be the parsed JSON document (available as $0), so generally the predicate will make use of the jsonPath function (below). See Predicates for more details on predicates.

9.7.4. JSON Transform Functions

The transform element supports referencing the JSON element through $0. Each column will initially be typed according to the field’s json-type. Most types will be converted to the equivalent Java class, e.g. java.lang.Integer, etc. array and object types will be raw JSON elements, and thus usually require further processing (e.g. jsonList or jsonMap, below).

In addition to the standard functions in Transformation Function Overview, the JSON converter provides the following JSON-specific functions:

9.7.4.1. emptyJsonToNull

This function converts empty JSON objects and arrays to null. A JSON object is also considered empty if all its values are null.

9.7.4.2. jsonArrayToObject

This function converts a JSON array into a JSON object, by using the index of each array element as the object key. This is useful for GeoMesa’s JSON attribute types, which currently require a top-level object and not an array.

9.7.4.3. jsonList

This function converts a JSON array element into a java.util.List. It requires two parameters; the first is the type of the list elements as a string, and the second is a JSON array. The type of list elements must be one of the types defined in GeoTools Feature Types. See below for an example.

9.7.4.4. jsonMap

This function converts a JSON object element into a java.util.Map. It requires three parameters; the first is the type of the map key elements as a string, the second is the type of the map value elements as a string, and the third is a JSON object. The type of keys and values must be one of the types defined in GeoTools Feature Types. See below for an example.

9.7.4.5. jsonPath

This function will evaluate a JSONPath expression against a given JSON element. Generally, it is better to use the path element of the fields element, but this method can be useful for composite predicates (see above). The first argument is the path to evaluate, and the second argument is the element to operate on.

9.7.4.6. mapToJson

This function converts a java.util.Map into a JSON string. It requires a single parameter, which must be a java.util.Map. It can be useful for storing complex JSON as a single attribute, which can then be queried using GeoMesa’s JSON attribute support. See JSON Attributes for more information.

9.7.4.7. newJsonObject

This function creates a new JSON object from key-value pairs. It can be useful for generating JSON text values.

Example:

fields = [
  { name = "foo", path = "$.foo", json-type = "String" }
  { name = "bar", path = "$.bar", json-type = "Array" }
  { name = "foobar", transform = "toString(newJsonObject('foo', $foo, 'bar', $bar))"
]

9.7.5. Example Usage

Assume the following SimpleFeatureType:

geomesa.sfts.example = {
  attributes = [
    { name = "name",    type = "String"          }
    { name = "age",     type = "Integer"         }
    { name = "weight",  type = "Double"          }
    { name = "hobbies", type = "List[String]"    }
    { name = "skills",  type = "Map[String,Int]" }
    { name = "source",  type = "String"          }
    { name = "geom",    type = "Point"           }
  ]
}

And the following JSON document:

{
  "DataSource": { "name": "myjson" },
  "Features": [
    {
      "id": 1,
      "name": "phil",
      "physicals": {
        "age": 32,
        "weight": 150.2
      },
      "hobbies": [ "baseball", "soccer" ],
      "languages": {
        "java": 100,
        "scala": 70
      },
      "geometry": { "type": "Point", "coordinates": [55, 56] }
    },
    {
      "id": 2,
      "name": "fred",
      "physicals": {
        "age": 33,
        "weight": 150.1
      },
      "hobbies": [ "archery", "tennis" ],
      "languages": {
        "c++": 10,
        "fortran": 50
      },
      "geometry": { "type": "Point", "coordinates": [45, 46] }
    }
  ]
}

You could ingest with the following converter:

geomesa.converters.myjson = {
  type         = "json"
  id-field     = "$id"
  feature-path = "$.Features[*]"
  fields = [
    { name = "id",      json-type = "integer",  path = "$.id",               transform = "toString($0)"                }
    { name = "name",    json-type = "string",   path = "$.name",             transform = "trim($0)"                    }
    { name = "age",     json-type = "integer",  path = "$.physicals.age",                                              }
    { name = "weight",  json-type = "double",   path = "$.physicals.weight"                                            }
    { name = "hobbies", json-type = "array",    path = "$.hobbies",          transform = "jsonList('string', $0)"      }
    { name = "skills",  json-type = "map",      path = "$.languages",        transform = "jsonMap('string','int', $0)" }
    { name = "geom",    json-type = "geometry", path = "$.geometry",         transform = "point($0)"                   }
    { name = "source",  json-type = "string",   root-path = "$.DataSource.name"                                        }
  ]
}