| /* |
| * 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.text.lookup; |
| |
| import java.util.Objects; |
| import java.util.concurrent.ConcurrentHashMap; |
| |
| import org.apache.commons.lang3.ClassUtils; |
| |
| /** |
| * <p> |
| * A specialized lookup implementation that allows access to constant fields of classes. |
| * </p> |
| * <p> |
| * Sometimes it is necessary in a configuration file to refer to a constant defined in a class. This can be done with |
| * this lookup implementation. Variable names passed in must be of the form {@code apackage.AClass.AFIELD}. The |
| * {@code lookup(String)} method will split the passed in string at the last dot, separating the fully qualified class |
| * name and the name of the constant (i.e. <b>static final</b>) member field. Then the class is loaded and the field's |
| * value is obtained using reflection. |
| * </p> |
| * <p> |
| * Once retrieved values are cached for fast access. This class is thread-safe. It can be used as a standard (i.e. |
| * global) lookup object and serve multiple clients concurrently. |
| * </p> |
| * <p> |
| * This class was adapted from Apache Commons Configuration. |
| * </p> |
| * |
| * @since 1.5 |
| */ |
| class ConstantStringLookup extends AbstractStringLookup { |
| |
| /** Constant for the field separator. */ |
| private static final char FIELD_SEPRATOR = '.'; |
| |
| /** |
| * Defines the singleton for this class. |
| */ |
| static final ConstantStringLookup INSTANCE = new ConstantStringLookup(); |
| |
| /** An internally used cache for already retrieved values. */ |
| private static ConcurrentHashMap<String, String> constantCache = new ConcurrentHashMap<>(); |
| |
| /** |
| * Clears the shared cache with the so far resolved constants. |
| */ |
| static void clear() { |
| constantCache.clear(); |
| } |
| |
| /** |
| * Tries to resolve the specified variable. The passed in variable name is interpreted as the name of a <b>static |
| * final</b> member field of a class. If the value has already been obtained, it can be retrieved from an internal |
| * cache. Otherwise this method will invoke the {@code resolveField()} method and pass in the name of the class and |
| * the field. |
| * |
| * @param key |
| * the name of the variable to be resolved |
| * @return the value of this variable or <b>null</b> if it cannot be resolved |
| */ |
| @Override |
| public synchronized String lookup(final String key) { |
| if (key == null) { |
| return null; |
| } |
| String result; |
| result = constantCache.get(key); |
| if (result != null) { |
| return result; |
| } |
| final int fieldPos = key.lastIndexOf(FIELD_SEPRATOR); |
| if (fieldPos < 0) { |
| return null; |
| } |
| try { |
| final Object value = resolveField(key.substring(0, fieldPos), key.substring(fieldPos + 1)); |
| if (value != null) { |
| final String string = Objects.toString(value, null); |
| constantCache.put(key, string); |
| result = string; |
| } |
| } catch (final Exception ex) { |
| // TODO it would be nice to log |
| return null; |
| } |
| return result; |
| } |
| |
| /** |
| * Determines the value of the specified constant member field of a class. This implementation will call |
| * {@code fetchClass()} to obtain the {@code java.lang.Class} object for the target class. Then it will use |
| * reflection to obtain the field's value. For this to work the field must be accessable. |
| * |
| * @param className |
| * the name of the class |
| * @param fieldName |
| * the name of the member field of that class to read |
| * @return the field's value |
| * @throws Exception |
| * if an error occurs |
| */ |
| protected Object resolveField(final String className, final String fieldName) throws Exception { |
| final Class<?> clazz = fetchClass(className); |
| if (clazz == null) { |
| return null; |
| } |
| return clazz.getField(fieldName).get(null); |
| } |
| |
| /** |
| * Loads the class with the specified name. If an application has special needs regarding the class loaders to be |
| * used, it can hook in here. This implementation delegates to the {@code getClass()} method of Commons Lang's |
| * <code><a href="http://commons.apache.org/lang/api-release/org/apache/commons/lang/ClassUtils.html"> |
| * ClassUtils</a></code>. |
| * |
| * @param className |
| * the name of the class to be loaded |
| * @return the corresponding class object |
| * @throws ClassNotFoundException |
| * if the class cannot be loaded |
| */ |
| protected Class<?> fetchClass(final String className) throws ClassNotFoundException { |
| return ClassUtils.getClass(className); |
| } |
| } |