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Apache Commons Lang, a package of Java utility classes for the classes that are in java.lang's hierarchy, or are considered to be so standard as to justify existence in java.lang.

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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.lang3.concurrent;

/**
 * 
 * Definition of an interface for the thread-safe initialization of objects.
 * 
 * 
 * The idea behind this interface is to provide access to an object in a
 * thread-safe manner. A {@code ConcurrentInitializer} can be passed to multiple
 * threads which can all access the object produced by the initializer. Through
 * the {@link #get()} method the object can be queried.
 * 
 * 
 * Concrete implementations of this interface will use different strategies for
 * the creation of the managed object, e.g. lazy initialization or
 * initialization in a background thread. This is completely transparent to
 * client code, so it is possible to change the initialization strategy without
 * affecting clients.
 * 
 *
 * @since 3.0
 * @param  the type of the object managed by this initializer class
 */
public interface ConcurrentInitializer {
    /**
     * Returns the fully initialized object produced by this {@code
     * ConcurrentInitializer}. A concrete implementation here returns the
     * results of the initialization process. This method may block until
     * results are available. Typically, once created the result object is
     * always the same.
     *
     * @return the object created by this {@code ConcurrentException}
     * @throws ConcurrentException if an error occurred during initialization of
     * the object
     */
    T get() throws ConcurrentException;
}