/*
 * Copyright (C) 2012-present the original author or authors.
 *
 * Licensed 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.pf4j;

import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * An {@link ExtensionFactory} that always returns a specific instance.
 * Optional, you can specify the extension classes for which you want singletons.
 *
 * @author Decebal Suiu
 */
public class SingletonExtensionFactory extends DefaultExtensionFactory {

    private final List<String> extensionClassNames;

    private Map<String, Object> cache;

    public SingletonExtensionFactory(String... extensionClassNames) {
        this.extensionClassNames = Arrays.asList(extensionClassNames);

        cache = new HashMap<>(); // simple cache implementation
    }

    @Override
    public Object create(Class<?> extensionClass) {
        String extensionClassName = extensionClass.getName();
        if (cache.containsKey(extensionClassName)) {
            return cache.get(extensionClassName);
        }

        Object extension = super.create(extensionClass);
        if (extensionClassNames.isEmpty() || extensionClassNames.contains(extensionClassName)) {
            cache.put(extensionClassName, extension);
        }

        return extension;
    }

}

Add SingletonExtensionFactory (see #232)