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/*
* 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;
import org.apache.commons.text.StringSubstitutor;
/**
*
* Looks up the value of a fully-qualified static final value.
*
*
* 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 must be in the format {@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. static final) member field. Then the class is loaded and the field's
* value is obtained using reflection.
*
*
* 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.
*
*
* Using a {@link StringLookup} from the {@link StringLookupFactory}:
*
*
*
* StringLookupFactory.INSTANCE.constantStringLookup().lookup("java.awt.event.KeyEvent.VK_ESCAPE");
*
*
* Using a {@link StringSubstitutor}:
*
*
*
* StringSubstitutor.createInterpolator().replace("... ${const:java.awt.event.KeyEvent.VK_ESCAPE} ..."));
*
*
* The above examples convert {@code java.awt.event.KeyEvent.VK_ESCAPE} to {@code "27"}.
*
*
* This class was adapted from Apache Commons Configuration.
*
*
* @since 1.5
*/
class ConstantStringLookup extends AbstractStringLookup {
/** An internally used cache for already retrieved values. */
private static final ConcurrentHashMap CONSTANT_CACHE = new ConcurrentHashMap<>();
/** Constant for the field separator. */
private static final char FIELD_SEPARATOR = '.';
/**
* Defines the singleton for this class.
*/
static final ConstantStringLookup INSTANCE = new ConstantStringLookup();
/**
* Clears the shared cache with the so far resolved constants.
*/
static void clear() {
CONSTANT_CACHE.clear();
}
/**
* 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
*
* ClassUtils
.
*
* @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);
}
/**
* Tries to resolve the specified variable. The passed in variable name is interpreted as the name of a static
* final 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 null if it cannot be resolved
*/
@Override
public synchronized String lookup(final String key) {
if (key == null) {
return null;
}
String result;
result = CONSTANT_CACHE.get(key);
if (result != null) {
return result;
}
final int fieldPos = key.lastIndexOf(FIELD_SEPARATOR);
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);
CONSTANT_CACHE.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 {@link 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 accessible.
*
* @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 ReflectiveOperationException if an error occurs
*/
protected Object resolveField(final String className, final String fieldName) throws ReflectiveOperationException {
final Class> clazz = fetchClass(className);
if (clazz == null) {
return null;
}
return clazz.getField(fieldName).get(null);
}
}