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.

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