    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}