JSON5

Query Language

Path expressions over a JSON5Value

The json5/query subpackage provides a tiny path expression language for pulling values out of a parsed JSON5Value tree. A query is parsed by QueryParser into a QueryExpression, then evaluated against a value by QueryEngine.

Syntax

A query is a chain of accessors starting from the root:

  • . - the root value (the parsed tree itself).
  • .field - field access on a JSON5Object.
  • [index] - element access on a JSON5Array (integer index) or JSON5Object (string or number index).

Combine accessors to drill in:

.user.address[0].zip

reads value.user.address[0].zip.

Expression types

Each accessor compiles to one of these QueryExpression nodes:

  • RootExpression - the leading .. Returns the value the query is run against.
  • FieldAccessExpression - the .name form. Returns the named field of an object, or null if absent.
  • IndexExpression - the [expr] form. The inner expression is one of:
    • NumberExpression - integer literal, used for array indexing.
    • StringExpression - quoted string, used for object key access.

Running a query

query = QueryParser.parseExpression(".user.name")
result = QueryEngine.eval(query, parsedValue)

result is itself a JSON5Value. To pull out a JVM-native string or number, downcast to the concrete type (JSON5String, JSON5Integer, etc.) and read its payload.

See also