Class ConfigurationInterpolator
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
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Field Summary
FieldsModifier and TypeFieldDescriptionstatic final StringName of the system property used to determine the lookups added by thegetDefaultPrefixLookups()method. -
Constructor Summary
ConstructorsConstructorDescriptionCreates a new instance ofConfigurationInterpolator. -
Method Summary
Modifier and TypeMethodDescriptionvoidaddDefaultLookup(Lookup defaultLookup) Adds a defaultLookupobject.voidaddDefaultLookups(Collection<? extends Lookup> lookups) Adds allLookupobjects in the given collection as default lookups.booleanderegisterLookup(String prefix) Deregisters theLookupobject for the specified prefix at this instance.protected LookupfetchLookupForPrefix(String prefix) Obtains the lookup object for the specified prefix.static ConfigurationInterpolatorCreates a newConfigurationInterpolatorinstance based on the passed in specification object.Gets a collection with the defaultLookupobjects added to thisConfigurationInterpolator.Gets a map containing the default prefix lookups.Gets a map with the currently registeredLookupobjects and their prefixes.Gets the parentConfigurationInterpolator.Gets the function used to convert interpolated values to strings.interpolate(Object value) Performs interpolation of the passed in value.booleanSets a flag that variable names can contain other variables.static LookupnullSafeLookup(Lookup lookup) Utility method for obtaining aLookupobject in a safe way.Returns an unmodifiable set with the prefixes, for whichLookupobjects are registered at this instance.voidregisterLookup(String prefix, Lookup lookup) Registers the givenLookupobject for the specified prefix at this instance.voidregisterLookups(Map<String, ? extends Lookup> lookups) Registers allLookupobjects in the given map with their prefixes at thisConfigurationInterpolator.booleanremoveDefaultLookup(Lookup lookup) Removes the specifiedLookupobject from the list of defaultLookups.Resolves the specified variable.voidsetEnableSubstitutionInVariables(boolean f) Sets the flag whether variable names can contain other variables.voidsetParentInterpolator(ConfigurationInterpolator parentInterpolator) Sets the parentConfigurationInterpolator.voidsetStringConverter(Function<Object, String> stringConverter) Sets the function used to convert interpolated values to strings.
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Field Details
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DEFAULT_PREFIX_LOOKUPS_PROPERTY
Name of the system property used to determine the lookups added by thegetDefaultPrefixLookups()method. Use of this property is only required in cases where the set of default lookups must be modified.- Since:
- 2.8.0
- See Also:
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Constructor Details
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ConfigurationInterpolator
public ConfigurationInterpolator()Creates a new instance ofConfigurationInterpolator.
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Method Details
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fromSpecification
Creates a newConfigurationInterpolatorinstance based on the passed in specification object. If theInterpolatorSpecificationalready contains aConfigurationInterpolatorobject, it is used directly. Otherwise, a new instance is created and initialized with the properties stored in the specification.- Parameters:
spec- theInterpolatorSpecification(must not be null)- Returns:
- the
ConfigurationInterpolatorobtained or created based on the given specification - Throws:
IllegalArgumentException- if the specification is null- Since:
- 2.0
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getDefaultPrefixLookups
Gets a map containing the default prefix lookups. Every configuration object derived fromAbstractConfigurationis by default initialized with aConfigurationInterpolatorcontaining theseLookupobjects 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 (for example,SCRIPT) and those that can result in contact with remote servers (for example,URLandDNS) 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 ofDefaultLookupsenum names to be included in the set of defaults. For example, setting this system property to"BASE64_ENCODER,ENVIRONMENT"will only include theBASE64_ENCODERandENVIRONMENTlookups. 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" XMLAdditional Lookups (not included by default) Prefix Lookup "dns" DNS"url" URL"script" SCRIPT- Returns:
- a map with the default prefix
Lookupobjects and their prefixes - Since:
- 2.0
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nullSafeLookup
Utility method for obtaining aLookupobject in a safe way. This method always returns a non-nullLookupobject. If the passed inLookupis not null, it is directly returned. Otherwise, result is a dummyLookupwhich does not provide any values.- Parameters:
lookup- theLookupto check- Returns:
- a non-null
Lookupobject - Since:
- 2.0
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addDefaultLookup
Adds a defaultLookupobject. DefaultLookupobjects are queried (in the order they were added) for all variables without a special prefix. If no defaultLookupobjects are present, such variables won't be processed.- Parameters:
defaultLookup- the defaultLookupobject to be added (must not be null)- Throws:
IllegalArgumentException- if theLookupobject is null
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addDefaultLookups
Adds allLookupobjects 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- theLookupobjects to be added as default lookups- Throws:
IllegalArgumentException- if the collection contains a null entry
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deregisterLookup
Deregisters theLookupobject 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
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fetchLookupForPrefix
Obtains the lookup object for the specified prefix. This method is called by thelookup()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
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getDefaultLookups
Gets a collection with the defaultLookupobjects added to thisConfigurationInterpolator. 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
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getLookups
Gets a map with the currently registeredLookupobjects 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
Lookupobjects
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getParentInterpolator
Gets the parentConfigurationInterpolator.- Returns:
- the parent
ConfigurationInterpolator(can be null)
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getStringConverter
Gets the function used to convert interpolated values to strings.- Returns:
- function used to convert interpolated values to strings
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interpolate
Performs interpolation of the passed in value. If the value is of typeString, 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 configuredstring 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
imaps to the integer value42.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
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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
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prefixSet
Returns an unmodifiable set with the prefixes, for whichLookupobjects are registered at this instance. This means that variables with these prefixes can be processed.- Returns:
- a set with the registered variable prefixes
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registerLookup
Registers the givenLookupobject 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- theLookupobject to be used for this prefix (must not be null)- Throws:
IllegalArgumentException- if either the prefix or theLookupobject is null
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registerLookups
Registers allLookupobjects in the given map with their prefixes at thisConfigurationInterpolator. Using this method multipleLookupobjects 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
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removeDefaultLookup
Removes the specifiedLookupobject from the list of defaultLookups.- Parameters:
lookup- theLookupobject to be removed- Returns:
- a flag whether this
Lookupobject actually existed and was removed
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resolve
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 parentConfigurationInterpolatoris 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
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setEnableSubstitutionInVariables
Sets the flag whether variable names can contain other variables. This flag corresponds to theenableSubstitutionInVariablesproperty of the underlyingStringSubstitutorobject.- Parameters:
f- the new value of the flag
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setParentInterpolator
Sets the parentConfigurationInterpolator. This object is used if theLookupobjects registered at this object cannot resolve a variable.- Parameters:
parentInterpolator- the parentConfigurationInterpolatorobject (can be null)
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setStringConverter
Sets the function used to convert interpolated values to strings. Passnullto use the default conversion function.- Parameters:
stringConverter- function used to convert interpolated values to strings ornullto use the default conversion function
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