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/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.commons.configuration2.interpol;
/**
*
* Definition of an interface for looking up variables during interpolation.
*
*
* Objects implementing this interface can be assigned a variable prefix and
* added to a {@link ConfigurationInterpolator} object. Whenever the
* {@code ConfigurationInterpolator} encounters a property value referencing a
* variable, e.g. ${prefix:variableName}, it extracts the prefix
* and finds the matching {@code Lookup} object. Then this object is asked to
* resolve the variable name and provide the corresponding value.
*
*
* This interface defines a single method for performing variable lookup. It is
* passed the name of a variable and has to return the corresponding value. It
* is of course up to a specific implementation how this is done. If the
* variable name cannot be resolved, an implementation has to return
* null.
*
*
* Note: Implementations must be aware that they can be accessed concurrently.
* This is for instance the case if a configuration object is read by multiple
* threads or if a {@code Lookup} object is shared by multiple configurations.
*
*
* @version $Id: Lookup.java 1624601 2014-09-12 18:04:36Z oheger $
* @since 2.0
*/
public interface Lookup
{
/**
* Looks up the value of the specified variable. This method is called by
* {@link ConfigurationInterpolator} with the variable name extracted from
* the expression to interpolate (i.e. the prefix name has already been
* removed). A concrete implementation has to return the value of this
* variable or null if the variable name is unknown.
*
* @param variable the name of the variable to be resolved
* @return the value of this variable or null
*/
Object lookup(String variable);
}