001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *     http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.commons.configuration2.interpol;
018
019import java.lang.reflect.Field;
020import java.util.HashMap;
021import java.util.Map;
022
023import org.apache.commons.lang3.ClassUtils;
024import org.apache.commons.logging.Log;
025import org.apache.commons.logging.LogFactory;
026
027/**
028 * <p>
029 * A specialized lookup implementation that allows access to constant fields of
030 * classes.
031 * </p>
032 * <p>
033 * Sometimes it is necessary in a configuration file to refer to a constant
034 * defined in a class. This can be done with this lookup implementation.
035 * Variable names passed in must be of the form
036 * {@code mypackage.MyClass.FIELD}. The {@code lookup()} method
037 * will split the passed in string at the last dot, separating the fully
038 * qualified class name and the name of the constant (i.e. <strong>static final</strong>)
039 * member field. Then the class is loaded and the field's value is obtained
040 * using reflection.
041 * </p>
042 * <p>
043 * Once retrieved values are cached for fast access. This class is thread-safe.
044 * It can be used as a standard (i.e. global) lookup object and serve multiple
045 * clients concurrently.
046 * </p>
047 *
048 * @since 1.4
049 */
050public class ConstantLookup implements Lookup
051{
052    /** Constant for the field separator. */
053    private static final char FIELD_SEPRATOR = '.';
054
055    /** An internally used cache for already retrieved values. */
056    private static Map<String, Object> constantCache = new HashMap<>();
057
058    /** The logger. */
059    private final Log log = LogFactory.getLog(getClass());
060
061    /**
062     * Tries to resolve the specified variable. The passed in variable name is
063     * interpreted as the name of a <b>static final</b> member field of a
064     * class. If the value has already been obtained, it can be retrieved from
065     * an internal cache. Otherwise this method will invoke the
066     * {@code resolveField()} method and pass in the name of the class
067     * and the field.
068     *
069     * @param var the name of the variable to be resolved
070     * @return the value of this variable or <b>null</b> if it cannot be
071     * resolved
072     */
073    @Override
074    public Object lookup(final String var)
075    {
076        if (var == null)
077        {
078            return null;
079        }
080
081        Object result;
082        synchronized (constantCache)
083        {
084            result = constantCache.get(var);
085        }
086        if (result != null)
087        {
088            return result;
089        }
090
091        final int fieldPos = var.lastIndexOf(FIELD_SEPRATOR);
092        if (fieldPos < 0)
093        {
094            return null;
095        }
096        try
097        {
098            final Object value = resolveField(var.substring(0, fieldPos), var
099                    .substring(fieldPos + 1));
100            if (value != null)
101            {
102                synchronized (constantCache)
103                {
104                    // In worst case, the value will be fetched multiple times
105                    // because of this lax synchronization, but for constant
106                    // values this shouldn't be a problem.
107                    constantCache.put(var, value);
108                }
109                result = value;
110            }
111        }
112        catch (final Exception ex)
113        {
114            log.warn("Could not obtain value for variable " + var, ex);
115        }
116
117        return result;
118    }
119
120    /**
121     * Clears the shared cache with the so far resolved constants.
122     */
123    public static void clear()
124    {
125        synchronized (constantCache)
126        {
127            constantCache.clear();
128        }
129    }
130
131    /**
132     * Determines the value of the specified constant member field of a class.
133     * This implementation will call {@code fetchClass()} to obtain the
134     * {@code java.lang.Class} object for the target class. Then it will
135     * use reflection to obtain the field's value. For this to work the field
136     * must be accessable.
137     *
138     * @param className the name of the class
139     * @param fieldName the name of the member field of that class to read
140     * @return the field's value
141     * @throws Exception if an error occurs
142     */
143    protected Object resolveField(final String className, final String fieldName)
144            throws Exception
145    {
146        final Class<?> clazz = fetchClass(className);
147        final Field field = clazz.getField(fieldName);
148        return field.get(null);
149    }
150
151    /**
152     * Loads the class with the specified name. If an application has special
153     * needs regarding the class loaders to be used, it can hook in here. This
154     * implementation delegates to the {@code getClass()} method of
155     * Commons Lang's
156     * <code><a href="https://commons.apache.org/lang/api-release/org/apache/commons/lang/ClassUtils.html">
157     * ClassUtils</a></code>.
158     *
159     * @param className the name of the class to be loaded
160     * @return the corresponding class object
161     * @throws ClassNotFoundException if the class cannot be loaded
162     */
163    protected Class<?> fetchClass(final String className) throws ClassNotFoundException
164    {
165        return ClassUtils.getClass(className);
166    }
167}