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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
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 */
package org.apache.commons.lang3.concurrent;

/**
 * 

* This class provides a generic implementation of the lazy initialization * pattern. *

*

* Sometimes an application has to deal with an object only under certain * circumstances, e.g. when the user selects a specific menu item or if a * special event is received. If the creation of the object is costly or the * consumption of memory or other system resources is significant, it may make * sense to defer the creation of this object until it is really needed. This is * a use case for the lazy initialization pattern. *

*

* This abstract base class provides an implementation of the double-check idiom * for an instance field as discussed in Joshua Bloch's "Effective Java", 2nd * edition, item 71. The class already implements all necessary synchronization. * A concrete subclass has to implement the {@code initialize()} method, which * actually creates the wrapped data object. *

*

* As an usage example consider that we have a class {@code ComplexObject} whose * instantiation is a complex operation. In order to apply lazy initialization * to this class, a subclass of {@code LazyInitializer} has to be created: *

* *
 * public class ComplexObjectInitializer extends LazyInitializer<ComplexObject> {
 *     @Override
 *     protected ComplexObject initialize() {
 *         return new ComplexObject();
 *     }
 * }
 * 
* *

* Access to the data object is provided through the {@code get()} method. So, * code that wants to obtain the {@code ComplexObject} instance would simply * look like this: *

* *
 * // Create an instance of the lazy initializer
 * ComplexObjectInitializer initializer = new ComplexObjectInitializer();
 * ...
 * // When the object is actually needed:
 * ComplexObject cobj = initializer.get();
 * 
* *

* If multiple threads call the {@code get()} method when the object has not yet * been created, they are blocked until initialization completes. The algorithm * guarantees that only a single instance of the wrapped object class is * created, which is passed to all callers. Once initialized, calls to the * {@code get()} method are pretty fast because no synchronization is needed * (only an access to a volatile member field). *

* * @since 3.0 * @param the type of the object managed by this initializer class */ public abstract class LazyInitializer implements ConcurrentInitializer { private static final Object NO_INIT = new Object(); @SuppressWarnings("unchecked") // Stores the managed object. private volatile T object = (T) NO_INIT; /** * Returns the object wrapped by this instance. On first access the object * is created. After that it is cached and can be accessed pretty fast. * * @return the object initialized by this {@code LazyInitializer} * @throws ConcurrentException if an error occurred during initialization of * the object */ @Override public T get() throws ConcurrentException { // use a temporary variable to reduce the number of reads of the // volatile field T result = object; if (result == NO_INIT) { synchronized (this) { result = object; if (result == NO_INIT) { object = result = initialize(); } } } return result; } /** * Creates and initializes the object managed by this {@code * LazyInitializer}. This method is called by {@link #get()} when the object * is accessed for the first time. An implementation can focus on the * creation of the object. No synchronization is needed, as this is already * handled by {@code get()}. * * @return the managed data object * @throws ConcurrentException if an error occurs during object creation */ protected abstract T initialize() throws ConcurrentException; }




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