Package org.apache.commons.configuration2.interpol

Class ConfigurationInterpolator

java.lang.Object
org.apache.commons.configuration2.interpol.ConfigurationInterpolator
public class ConfigurationInterpolator extends Object

A class that handles interpolation (variable substitution) for configuration objects.

Each instance of AbstractConfiguration is associated with an object of this class. All interpolation tasks are delegated to this object.

ConfigurationInterpolator internally uses the StringSubstitutor class from Commons Text. Thus it supports the same syntax of variable expressions.

The basic idea of this class is that it can maintain a set of primitive Lookup objects, each of which is identified by a special prefix. The variables to be processed have the form ${prefix:name}. ConfigurationInterpolator will extract the prefix and determine, which primitive lookup object is registered for it. Then the name of the variable is passed to this object to obtain the actual value. It is also possible to define an arbitrary number of default lookup objects, which are used for variables that do not have a prefix or that cannot be resolved by their associated lookup object. When adding default lookup objects their order matters; they are queried in this order, and the first non-null variable value is used.

After an instance has been created it does not contain any Lookup objects. The current set of lookup objects can be modified using the registerLookup() and deregisterLookup() methods. Default lookup objects (that are invoked for variables without a prefix) can be added or removed with the addDefaultLookup() and removeDefaultLookup() methods respectively. (When a ConfigurationInterpolator instance is created by a configuration object, a default lookup object is added pointing to the configuration itself, so that variables are resolved using the configuration's properties.)

The default usage scenario is that on a fully initialized instance the interpolate() method is called. It is passed an object value which may contain variables. All these variables are substituted if they can be resolved. The result is the passed in value with variables replaced. Alternatively, the resolve() method can be called to obtain the values of specific variables without performing interpolation.

String Conversion

When variables are part of larger interpolated strings, the variable values, which can be of any type, must be converted to strings to produce the full result. Each interpolator instance has a configurable string converter to perform this conversion. The default implementation of this function simply uses the value's toString method in the majority of cases. However, for maximum consistency with DefaultConversionHandler, when a variable value is a container type (such as a collection or array), then only the first element of the container is converted to a string instead of the container itself. For example, if the variable x resolves to the integer array [1, 2, 3], then the string "my value = ${x}" will by default be interpolated to "my value = 1".

Implementation note: This class is thread-safe. Lookup objects can be added or removed at any time concurrent to interpolation operations.

Since:
1.4

  • Field Details

  • Constructor Details

  • Method Details

    • fromSpecification

      public static ConfigurationInterpolator fromSpecification(InterpolatorSpecification spec)
      Creates a new ConfigurationInterpolator instance based on the passed in specification object. If the InterpolatorSpecification already contains a ConfigurationInterpolator object, it is used directly. Otherwise, a new instance is created and initialized with the properties stored in the specification.
      Parameters:
      spec - the InterpolatorSpecification (must not be null)
      Returns:
      the ConfigurationInterpolator obtained or created based on the given specification
      Throws:
      IllegalArgumentException - if the specification is null
      Since:
      2.0

    • getDefaultPrefixLookups

      public static Map<String,Lookup> getDefaultPrefixLookups()
      Gets a map containing the default prefix lookups. Every configuration object derived from AbstractConfiguration is by default initialized with a ConfigurationInterpolator containing these Lookup objects and their prefixes. The map cannot be modified.

      All of the lookups present in the returned map are from DefaultLookups. However, not all of the available lookups are included by default. Specifically, lookups that can execute code (e.g., SCRIPT) and those that can result in contact with remote servers (e.g., URL and DNS) are not included. If this behavior must be modified, users can define the "org.apache.commons.configuration2.interpol.ConfigurationInterpolator.defaultPrefixLookups" system property with a comma-separated list of DefaultLookups enum names to be included in the set of defaults. For example, setting this system property to "BASE64_ENCODER,ENVIRONMENT" will only include the BASE64_ENCODER and ENVIRONMENT lookups. Setting the property to the empty string will cause no defaults to be configured.

      Default Lookups
      Prefix Lookup
      "base64Decoder" BASE64_DECODER
      "base64Encoder" BASE64_ENCODER
      "const" CONST
      "date" DATE
      "env" ENVIRONMENT
      "file" FILE
      "java" JAVA
      "localhost" LOCAL_HOST
      "properties" PROPERTIES
      "resourceBundle" RESOURCE_BUNDLE
      "sys" SYSTEM_PROPERTIES
      "urlDecoder" URL_DECODER
      "urlEncoder" URL_ENCODER
      "xml" XML
      Additional Lookups (not included by default)
      Prefix Lookup
      "dns" DNS
      "url" URL
      "script" SCRIPT
      Returns:
      a map with the default prefix Lookup objects and their prefixes
      Since:
      2.0

    • nullSafeLookup

      public static Lookup nullSafeLookup(Lookup lookup)
      Utility method for obtaining a Lookup object in a safe way. This method always returns a non-null Lookup object. If the passed in Lookup is not null, it is directly returned. Otherwise, result is a dummy Lookup which does not provide any values.
      Parameters:
      lookup - the Lookup to check
      Returns:
      a non-null Lookup object
      Since:
      2.0

    • addDefaultLookup

      public void addDefaultLookup(Lookup defaultLookup)
      Adds a default Lookup object. Default Lookup objects are queried (in the order they were added) for all variables without a special prefix. If no default Lookup objects are present, such variables won't be processed.
      Parameters:
      defaultLookup - the default Lookup object to be added (must not be null)
      Throws:
      IllegalArgumentException - if the Lookup object is null

    • addDefaultLookups

      public void addDefaultLookups(Collection<? extends Lookup> lookups)
      Adds all Lookup objects in the given collection as default lookups. The collection can be null, then this method has no effect. It must not contain null entries.
      Parameters:
      lookups - the Lookup objects to be added as default lookups
      Throws:
      IllegalArgumentException - if the collection contains a null entry

    • deregisterLookup

      public boolean deregisterLookup(String prefix)
      Deregisters the Lookup object for the specified prefix at this instance. It will be removed from this instance.
      Parameters:
      prefix - the variable prefix
      Returns:
      a flag whether for this prefix a lookup object had been registered

    • fetchLookupForPrefix

      protected Lookup fetchLookupForPrefix(String prefix)
      Obtains the lookup object for the specified prefix. This method is called by the lookup() method. This implementation will check whether a lookup object is registered for the given prefix. If not, a null lookup object will be returned (never null).
      Parameters:
      prefix - the prefix
      Returns:
      the lookup object to be used for this prefix

    • getDefaultLookups

      public List<Lookup> getDefaultLookups()
      Gets a collection with the default Lookup objects added to this ConfigurationInterpolator. These objects are not associated with a variable prefix. The returned list is a snapshot copy of the internal collection of default lookups; so manipulating it does not affect this instance.
      Returns:
      the default lookup objects

    • getLookups

      public Map<String,Lookup> getLookups()
      Gets a map with the currently registered Lookup objects and their prefixes. This is a snapshot copy of the internally used map. So modifications of this map do not effect this instance.
      Returns:
      a copy of the map with the currently registered Lookup objects

    • getParentInterpolator

      public ConfigurationInterpolator getParentInterpolator()
      Gets the parent ConfigurationInterpolator.
      Returns:
      the parent ConfigurationInterpolator (can be null)

    • getStringConverter

      public Function<Object,String> getStringConverter()
      Gets the function used to convert interpolated values to strings.
      Returns:
      function used to convert interpolated values to strings

    • interpolate

      public Object interpolate(Object value)
      Performs interpolation of the passed in value. If the value is of type String, this method checks whether it contains variables. If so, all variables are replaced by their current values (if possible). For non string arguments, the value is returned without changes. In the special case where the value is a string consisting of a single variable reference, the interpolated variable value is not converted to a string before returning, so that callers can access the raw value. However, if the variable is part of a larger interpolated string, then the variable value is converted to a string using the configured string converter. (See the discussion on string conversion in the class documentation for more details.)

      Examples

      For the following examples, assume that the default string conversion function is in place and that the variable i maps to the integer value 42. 

            interpolator.interpolate(1) → 1 // non-string argument returned unchanged
      interpolator.interpolate("${i}") → 42 // single variable value returned with raw type
      interpolator.interpolate("answer = ${i}") → "answer = 42" // variable value converted to string
      Parameters:
      value - the value to be interpolated
      Returns:
      the interpolated value

    • isEnableSubstitutionInVariables

      public boolean isEnableSubstitutionInVariables()
      Sets a flag that variable names can contain other variables. If enabled, variable substitution is also done in variable names.
      Returns:
      the substitution in variables flag

    • prefixSet

      public Set<String> prefixSet()
      Returns an unmodifiable set with the prefixes, for which Lookup objects are registered at this instance. This means that variables with these prefixes can be processed.
      Returns:
      a set with the registered variable prefixes

    • registerLookup

      public void registerLookup(String prefix, Lookup lookup)
      Registers the given Lookup object for the specified prefix at this instance. From now on this lookup object will be used for variables that have the specified prefix.
      Parameters:
      prefix - the variable prefix (must not be null)
      lookup - the Lookup object to be used for this prefix (must not be null)
      Throws:
      IllegalArgumentException - if either the prefix or the Lookup object is null

    • registerLookups

      public void registerLookups(Map<String,? extends Lookup> lookups)
      Registers all Lookup objects in the given map with their prefixes at this ConfigurationInterpolator. Using this method multiple Lookup objects can be registered at once. If the passed in map is null, this method does not have any effect.
      Parameters:
      lookups - the map with lookups to register (may be null)
      Throws:
      IllegalArgumentException - if the map contains entries

    • removeDefaultLookup

      public boolean removeDefaultLookup(Lookup lookup)
      Removes the specified Lookup object from the list of default Lookups.
      Parameters:
      lookup - the Lookup object to be removed
      Returns:
      a flag whether this Lookup object actually existed and was removed

    • resolve

      public Object resolve(String var)
      Resolves the specified variable. This implementation tries to extract a variable prefix from the given variable name (the first colon (':') is used as prefix separator). It then passes the name of the variable with the prefix stripped to the lookup object registered for this prefix. If no prefix can be found or if the associated lookup object cannot resolve this variable, the default lookup objects are used. If this is not successful either and a parent ConfigurationInterpolator is available, this object is asked to resolve the variable.
      Parameters:
      var - the name of the variable whose value is to be looked up which may contain a prefix.
      Returns:
      the value of this variable or null if it cannot be resolved

    • setEnableSubstitutionInVariables

      public void setEnableSubstitutionInVariables(boolean f)
      Sets the flag whether variable names can contain other variables. This flag corresponds to the enableSubstitutionInVariables property of the underlying StringSubstitutor object.
      Parameters:
      f - the new value of the flag

    • setParentInterpolator

      public void setParentInterpolator(ConfigurationInterpolator parentInterpolator)
      Sets the parent ConfigurationInterpolator. This object is used if the Lookup objects registered at this object cannot resolve a variable.
      Parameters:
      parentInterpolator - the parent ConfigurationInterpolator object (can be null)

    • setStringConverter

      public void setStringConverter(Function<Object,String> stringConverter)
      Sets the function used to convert interpolated values to strings. Pass null to use the default conversion function.
      Parameters:
      stringConverter - function used to convert interpolated values to strings or null to use the default conversion function