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Google App Engine compliant variation of FreeMarker. FreeMarker is a "template engine"; a generic tool to generate text output based on templates.

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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 freemarker.ext.beans;

import freemarker.template.ObjectWrapper;

/**
 * Marker interface useful when used together with {@link MethodAppearanceFineTuner} and such customizer objects, to
 * indicate that it doesn't contain reference to the {@link ObjectWrapper} (so beware with non-static inner
 * classes) and can be and should be used in call introspection cache keys. This also implies that you won't
 * create many instances of the class, rather just reuse the same (or same few) instances over and over. Furthermore,
 * the instances must be thread-safe. The typical pattern in which this instance should be used is like this:
 * 
 * static class MyMethodAppearanceFineTuner implements MethodAppearanceFineTuner, SingletonCustomizer {
 *      
 *    // This is the singleton:
 *    static final MyMethodAppearanceFineTuner INSTANCE = new MyMethodAppearanceFineTuner();
 *     
 *    // Private, so it can't be constructed from outside this class.
 *    private MyMethodAppearanceFineTuner() { }
 *
 *    @Override
 *    public void fineTuneMethodAppearance(...) {
 *       // Do something here, only using the parameters and maybe some other singletons. 
 *       ...
 *    }
 *     
 * }
 *
 * @since 2.3.21
 */
public interface SingletonCustomizer {

}