| /* |
| * 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; |
| |
| /** |
| * <p> |
| * Definition of an interface for looking up variables during interpolation. |
| * </p> |
| * <p> |
| * 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. <code>${prefix:variableName}</code>, 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. |
| * </p> |
| * <p> |
| * 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 |
| * <b>null</b>. |
| * </p> |
| * <p> |
| * 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. |
| * </p> |
| * |
| * @version $Id$ |
| * @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 <b>null</b> if the variable name is unknown. |
| * |
| * @param variable the name of the variable to be resolved |
| * @return the value of this variable or <b>null</b> |
| */ |
| Object lookup(String variable); |
| } |