1 /*
2 * Licensed to the Apache Software Foundation (ASF) under one or more
3 * contributor license agreements. See the NOTICE file distributed with
4 * this work for additional information regarding copyright ownership.
5 * The ASF licenses this file to You under the Apache License, Version 2.0
6 * (the "License"); you may not use this file except in compliance with
7 * the License. You may obtain a copy of the License at
8 *
9 * http://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 */
17 package org.apache.commons.configuration2.interpol;
18
19 /**
20 * <p>
21 * Definition of an interface for looking up variables during interpolation.
22 * </p>
23 * <p>
24 * Objects implementing this interface can be assigned a variable prefix and added to a
25 * {@link ConfigurationInterpolator} object. Whenever the {@code ConfigurationInterpolator} encounters a property value
26 * referencing a variable, e.g. {@code ${prefix:variableName}}, it extracts the prefix and finds the matching
27 * {@code Lookup} object. Then this object is asked to resolve the variable name and provide the corresponding value.
28 * </p>
29 * <p>
30 * This interface defines a single method for performing variable lookup. It is passed the name of a variable and has to
31 * return the corresponding value. It is of course up to a specific implementation how this is done. If the variable
32 * name cannot be resolved, an implementation has to return <b>null</b>.
33 * </p>
34 * <p>
35 * Note: Implementations must be aware that they can be accessed concurrently. This is for instance the case if a
36 * configuration object is read by multiple threads or if a {@code Lookup} object is shared by multiple configurations.
37 * </p>
38 *
39 * @since 2.0
40 */
41 public interface Lookup {
42 /**
43 * Looks up the value of the specified variable. This method is called by {@link ConfigurationInterpolator} with the
44 * variable name extracted from the expression to interpolate (i.e. the prefix name has already been removed). A
45 * concrete implementation has to return the value of this variable or <b>null</b> if the variable name is unknown.
46 *
47 * @param variable the name of the variable to be resolved
48 * @return the value of this variable or <b>null</b>
49 */
50 Object lookup(String variable);
51 }