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 * @author <a
050 * href="http://commons.apache.org/configuration/team-list.html">Commons
051 * Configuration team</a>
052 */
053public class ConstantLookup implements Lookup
054{
055    /** Constant for the field separator. */
056    private static final char FIELD_SEPRATOR = '.';
057
058    /** An internally used cache for already retrieved values. */
059    private static Map<String, Object> constantCache = new HashMap<>();
060
061    /** The logger. */
062    private final Log log = LogFactory.getLog(getClass());
063
064    /**
065     * Tries to resolve the specified variable. The passed in variable name is
066     * interpreted as the name of a <b>static final</b> member field of a
067     * class. If the value has already been obtained, it can be retrieved from
068     * an internal cache. Otherwise this method will invoke the
069     * {@code resolveField()} method and pass in the name of the class
070     * and the field.
071     *
072     * @param var the name of the variable to be resolved
073     * @return the value of this variable or <b>null</b> if it cannot be
074     * resolved
075     */
076    @Override
077    public Object lookup(final String var)
078    {
079        if (var == null)
080        {
081            return null;
082        }
083
084        Object result;
085        synchronized (constantCache)
086        {
087            result = constantCache.get(var);
088        }
089        if (result != null)
090        {
091            return result;
092        }
093
094        final int fieldPos = var.lastIndexOf(FIELD_SEPRATOR);
095        if (fieldPos < 0)
096        {
097            return null;
098        }
099        try
100        {
101            final Object value = resolveField(var.substring(0, fieldPos), var
102                    .substring(fieldPos + 1));
103            if (value != null)
104            {
105                synchronized (constantCache)
106                {
107                    // In worst case, the value will be fetched multiple times
108                    // because of this lax synchronization, but for constant
109                    // values this shouldn't be a problem.
110                    constantCache.put(var, value);
111                }
112                result = value;
113            }
114        }
115        catch (final Exception ex)
116        {
117            log.warn("Could not obtain value for variable " + var, ex);
118        }
119
120        return result;
121    }
122
123    /**
124     * Clears the shared cache with the so far resolved constants.
125     */
126    public static void clear()
127    {
128        synchronized (constantCache)
129        {
130            constantCache.clear();
131        }
132    }
133
134    /**
135     * Determines the value of the specified constant member field of a class.
136     * This implementation will call {@code fetchClass()} to obtain the
137     * {@code java.lang.Class} object for the target class. Then it will
138     * use reflection to obtain the field's value. For this to work the field
139     * must be accessable.
140     *
141     * @param className the name of the class
142     * @param fieldName the name of the member field of that class to read
143     * @return the field's value
144     * @throws Exception if an error occurs
145     */
146    protected Object resolveField(final String className, final String fieldName)
147            throws Exception
148    {
149        final Class<?> clazz = fetchClass(className);
150        final Field field = clazz.getField(fieldName);
151        return field.get(null);
152    }
153
154    /**
155     * Loads the class with the specified name. If an application has special
156     * needs regarding the class loaders to be used, it can hook in here. This
157     * implementation delegates to the {@code getClass()} method of
158     * Commons Lang's
159     * <code><a href="http://commons.apache.org/lang/api-release/org/apache/commons/lang/ClassUtils.html">
160     * ClassUtils</a></code>.
161     *
162     * @param className the name of the class to be loaded
163     * @return the corresponding class object
164     * @throws ClassNotFoundException if the class cannot be loaded
165     */
166    protected Class<?> fetchClass(final String className) throws ClassNotFoundException
167    {
168        return ClassUtils.getClass(className);
169    }
170}