# Package org.apache.commons.configuration2.interpol

A package with helper classes used for interpolation (variable substitution).

- Interface Summary
  Interface | Description
  --- | ---
  Lookup | Definition of an interface for looking up variables during interpolation.

- Class Summary
  Class | Description
  --- | ---
  ConfigurationInterpolator | A class that handles interpolation (variable substitution) for configuration objects.
  ConstantLookup | Looks up constant fields in classes.
  EnvironmentLookup | Deprecated. Use `DefaultLookups.ENVIRONMENT`.
  ExprLookup | Lookup that allows expressions to be evaluated.
  ExprLookup.Variable | The key and corresponding object that will be made available to the JexlContext for use in expressions.
  ExprLookup.Variables | List wrapper used to allow the Variables list to be created as beans in DefaultConfigurationBuilder.
  InterpolatorSpecification | A simple value class defining a `ConfigurationInterpolator`.
  InterpolatorSpecification.Builder | A builder class for creating instances of `InterpolatorSpecification`.
  SystemPropertiesLookup | Deprecated. Use `DefaultLookups.SYSTEM_PROPERTIES`.

- Enum Summary
  Enum | Description
  --- | ---
  DefaultLookups | An enumeration class defining constants for built-in `Lookup` objects available for `Configuration` instances.
  DummyLookup | A simple dummy `Lookup` implementation.