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## org.apache.commons.configuration.interpol
Class ConfigurationInterpolator

    java.lang.Object
      org.apache.commons.lang.text.StrLookup
          org.apache.commons.configuration.interpol.ConfigurationInterpolator

    public class ConfigurationInterpolator

extends StrLookup

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` works together with the `StrSubstitutor` class from Commons Lang. By extending `StrLookup` it is able to provide values for variables that appear in expressions.

The basic idea of this class is that it can maintain a set of primitive `StrLookup` 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 a default lookup object, which will be used for variables that do not have a prefix or that cannot be resolved by their associated lookup object.

When a new instance of this class is created it is initialized with a default set of primitive lookup objects. This set can be customized using the static methods `registerGlobalLookup()` and `deregisterGlobalLookup()`. Per default it contains the following standard lookup objects:

Prefix | Lookup object
--- | ---
sys | With this prefix a lookup object is associated that is able to resolve system properties.
const | The `const` prefix indicates that a variable is to be interpreted as a constant member field of a class (i.e. a field with the static final modifiers). The name of the variable must be of the form `<full qualified class name>.<field name>`, e.g. `org.apache.commons.configuration.interpol.ConfigurationInterpolator.PREFIX_CONSTANTS` .

After an instance has been created the current set of lookup objects can be modified using the `registerLookup()` and `deregisterLookup()` methods. The default lookup object (that is invoked for variables without a prefix) can be set with the `setDefaultLookup()` method. (If a `ConfigurationInterpolator` instance is created by a configuration object, this lookup points to the configuration itself, so that variables are resolved using the configuration's properties. This ensures backward compatibility to earlier version of Commons Configuration.)

Implementation node: Instances of this class are not thread-safe related to modifications of their current set of registered lookup objects. It is intended that each instance is associated with a single `Configuration` object and used for its interpolation tasks.

Since:
  1.4
Version:
  $Id: ConfigurationInterpolator.java 561230 2007-07-31 04:17:09Z rahul $
Author:
  Commons Configuration team

Field Summary
`static String` | `PREFIX_CONSTANTS` Constant for the prefix of the standard lookup object for resolving constant values.
`static String` | `PREFIX_SYSPROPERTIES` Constant for the prefix of the standard lookup object for resolving system properties.

Constructor Summary
`ConfigurationInterpolator()` Creates a new instance of `ConfigurationInterpolator`.

Method Summary
`static boolean` | `deregisterGlobalLookup(String prefix)` Deregisters the global lookup object for the specified prefix.
`boolean` | `deregisterLookup(String prefix)` Deregisters the lookup object for the specified prefix at this instance.
`protected StrLookup` | `fetchLookupForPrefix(String prefix)` Obtains the lookup object for the specified prefix.
`protected StrLookup` | `fetchNoPrefixLookup()` Returns the lookup object to be used for variables without a prefix.
`StrLookup` | `getDefaultLookup()` Returns the default lookup object.
`String` | `lookup(String var)` Resolves the specified variable.
`Set` | `prefixSet()` Returns a set with the prefixes, for which lookup objects are registered at this instance.
`static void` | `registerGlobalLookup(String prefix, StrLookup lookup)` Registers the given lookup object for the specified prefix globally.
`void` | `registerLookup(String prefix, StrLookup lookup)` Registers the given lookup object for the specified prefix at this instance.
`void` | `setDefaultLookup(StrLookup defaultLookup)` Sets the default lookup object.

Methods inherited from class org.apache.commons.lang.text.StrLookup
`mapLookup, noneLookup, systemPropertiesLookup`

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Field Detail

### PREFIX_SYSPROPERTIES

    public static final String PREFIX_SYSPROPERTIES

  Constant for the prefix of the standard lookup object for resolving system properties.

  See Also:
    Constant Field Values

### PREFIX_CONSTANTS

    public static final String PREFIX_CONSTANTS

  Constant for the prefix of the standard lookup object for resolving constant values.

  See Also:
    Constant Field Values

Constructor Detail

### ConfigurationInterpolator

    public ConfigurationInterpolator()

  Creates a new instance of `ConfigurationInterpolator`.

Method Detail

### registerGlobalLookup

    public static void registerGlobalLookup(String prefix,
                                            StrLookup lookup)

  Registers the given lookup object for the specified prefix globally. This means that all instances that are created later will use this lookup object for this prefix. If for this prefix a lookup object is already registered, the new lookup object will replace the old one. Note that the lookup objects registered here will be shared between multiple clients. So they should be thread-safe.

  Parameters:
    `prefix` - the variable prefix (must not be null)
    `lookup` - the lookup object to be used for this prefix (must not be null)

### deregisterGlobalLookup

    public static boolean deregisterGlobalLookup(String prefix)

  Deregisters the global lookup object for the specified prefix. This means that this lookup object won't be available for later created instances any more. For already existing instances this operation does not have any impact.

  Parameters:
    `prefix` - the variable prefix
  Returns:
    a flag whether for this prefix a lookup object had been registered

### registerLookup

    public void registerLookup(String prefix,
                               StrLookup 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)

### 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

### prefixSet

    public Set prefixSet()

  Returns a 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

### getDefaultLookup

    public StrLookup getDefaultLookup()

  Returns the default lookup object.

  Returns:
    the default lookup object

### setDefaultLookup

    public void setDefaultLookup(StrLookup defaultLookup)

  Sets the default lookup object. This lookup object will be used for all variables without a special prefix. If it is set to null, such variables won't be processed.

  Parameters:
    `defaultLookup` - the new default lookup object

### lookup

    public String lookup(String var)

  Resolves the specified variable. This implementation will try 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 object will be used.

  Specified by:
    `lookup` in class `StrLookup`
  Parameters:
    `var` - the name of the variable whose value is to be looked up
  Returns:
    the value of this variable or null if it cannot be resolved

### fetchNoPrefixLookup

    protected StrLookup fetchNoPrefixLookup()

  Returns the lookup object to be used for variables without a prefix. This implementation will check whether a default lookup object was set. If this is the case, it will be returned. Otherwise a null lookup object will be returned.

  Returns:
    the lookup object to be used for variables without a prefix

### fetchLookupForPrefix

    protected StrLookup 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.

  Parameters:
    `prefix` - the prefix
  Returns:
    the lookup object to be used for this prefix

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