    001/*
    002 * Licensed to the Apache Software Foundation (ASF) under one or more
    003 * contributor license agreements.  See the NOTICE file distributed with
    004 * this work for additional information regarding copyright ownership.
    005 * The ASF licenses this file to You under the Apache License, Version 2.0
    006 * (the "License"); you may not use this file except in compliance with
    007 * the License.  You may obtain a copy of the License at
    008 *
    009 *      http://www.apache.org/licenses/LICENSE-2.0
    010 *
    011 * Unless required by applicable law or agreed to in writing, software
    012 * distributed under the License is distributed on an "AS IS" BASIS,
    013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    014 * See the License for the specific language governing permissions and
    015 * limitations under the License.
    016 */
    017package org.apache.commons.lang3.concurrent;
    018
    019import org.apache.commons.lang3.function.FailableConsumer;
    020import org.apache.commons.lang3.function.FailableSupplier;
    021
    022/**
    023 * This class provides a generic implementation of the lazy initialization pattern.
    024 *
    025 * <p>
    026 * 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
    027 * 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
    028 * creation of this object until it is really needed. This is a use case for the lazy initialization pattern.
    029 * </p>
    030 * <p>
    031 * 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
    032 * edition, item 71. The class already implements all necessary synchronization. A concrete subclass has to implement the {@code initialize()} method, which
    033 * actually creates the wrapped data object.
    034 * </p>
    035 * <p>
    036 * 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
    037 * this class, a subclass of {@link LazyInitializer} has to be created:
    038 * </p>
    039 *
    040 * <pre>
    041 * public class ComplexObjectInitializer extends LazyInitializer&lt;ComplexObject&gt; {
    042 *     &#064;Override
    043 *     protected ComplexObject initialize() {
    044 *         return new ComplexObject();
    045 *     }
    046 * }
    047 * </pre>
    048 *
    049 * <p>
    050 * 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
    051 * like this:
    052 * </p>
    053 *
    054 * <pre>
    055 * // Create an instance of the lazy initializer
    056 * ComplexObjectInitializer initializer = new ComplexObjectInitializer();
    057 * ...
    058 * // When the object is actually needed:
    059 * ComplexObject cobj = initializer.get();
    060 * </pre>
    061 *
    062 * <p>
    063 * If multiple threads call the {@code get()} method when the object has not yet been created, they are blocked until initialization completes. The algorithm
    064 * 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()}
    065 * method are pretty fast because no synchronization is needed (only an access to a <b>volatile</b> member field).
    066 * </p>
    067 *
    068 * @since 3.0
    069 * @param <T> the type of the object managed by the initializer.
    070 */
    071public class LazyInitializer<T> extends AbstractConcurrentInitializer<T, ConcurrentException> {
    072
    073    /**
    074     * Builds a new instance.
    075     *
    076     * @param <T> the type of the object managed by the initializer.
    077     * @param <I> the type of the initializer managed by this builder.
    078     * @since 3.14.0
    079     */
    080    public static class Builder<I extends LazyInitializer<T>, T> extends AbstractBuilder<I, T, Builder<I, T>, ConcurrentException> {
    081
    082        @SuppressWarnings("unchecked")
    083        @Override
    084        public I get() {
    085            return (I) new LazyInitializer(getInitializer(), getCloser());
    086        }
    087
    088    }
    089
    090    /**
    091     * A unique value indicating an un-initialzed instance.
    092     */
    093    private static final Object NO_INIT = new Object();
    094
    095    /**
    096     * Creates a new builder.
    097     *
    098     * @param <T> the type of object to build.
    099     * @return a new builder.
    100     * @since 3.14.0
    101     */
    102    public static <T> Builder<LazyInitializer<T>, T> builder() {
    103        return new Builder<>();
    104    }
    105
    106    /** Stores the managed object. */
    107    @SuppressWarnings("unchecked")
    108    private volatile T object = (T) NO_INIT;
    109
    110    /**
    111     * Constructs a new instance.
    112     */
    113    public LazyInitializer() {
    114        // empty
    115    }
    116
    117    /**
    118     * Constructs a new instance.
    119     *
    120     * @param initializer the initializer supplier called by {@link #initialize()}.
    121     * @param closer the closer consumer called by {@link #close()}.
    122     */
    123    private LazyInitializer(final FailableSupplier<T, ConcurrentException> initializer, final FailableConsumer<T, ConcurrentException> closer) {
    124        super(initializer, closer);
    125    }
    126
    127    /**
    128     * 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.
    129     *
    130     * @return the object initialized by this {@link LazyInitializer}
    131     * @throws ConcurrentException if an error occurred during initialization of the object
    132     */
    133    @Override
    134    public T get() throws ConcurrentException {
    135        // use a temporary variable to reduce the number of reads of the
    136        // volatile field
    137        T result = object;
    138
    139        if (result == NO_INIT) {
    140            synchronized (this) {
    141                result = object;
    142                if (result == NO_INIT) {
    143                    object = result = initialize();
    144                }
    145            }
    146        }
    147
    148        return result;
    149    }
    150
    151    /**
    152     * {@inheritDoc}
    153     */
    154    @Override
    155    protected ConcurrentException getTypedException(Exception e) {
    156        return new ConcurrentException(e);
    157    }
    158
    159    /**
    160     * Tests whether this instance is initialized. Once initialized, always returns true.
    161     *
    162     * @return whether this instance is initialized. Once initialized, always returns true.
    163     * @since 3.14.0
    164     */
    165    @Override
    166    public boolean isInitialized() {
    167        return object != NO_INIT;
    168    }
    169
    170}