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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;
/**
* 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 {@link 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 {@link 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;
}