    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 *      https://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>{@code
    041 * public class ComplexObjectInitializer extends LazyInitializer<ComplexObject> {
    042 *     &#064;Override
    043 *     protected ComplexObject initialize() {
    044 *         return new ComplexObject();
    045 *     }
    046 * }
    047 * }
    048 * </pre>
    049 *
    050 * <p>
    051 * 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
    052 * like this:
    053 * </p>
    054 *
    055 * <pre>
    056 * // Create an instance of the lazy initializer
    057 * ComplexObjectInitializer initializer = new ComplexObjectInitializer();
    058 * ...
    059 * // When the object is actually needed:
    060 * ComplexObject cobj = initializer.get();
    061 * </pre>
    062 *
    063 * <p>
    064 * If multiple threads call the {@code get()} method when the object has not yet been created, they are blocked until initialization completes. The algorithm
    065 * 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()}
    066 * method are pretty fast because no synchronization is needed (only an access to a <strong>volatile</strong> member field).
    067 * </p>
    068 *
    069 * @param <T> the type of the object managed by the initializer.
    070 * @since 3.0
    071 */
    072public class LazyInitializer<T> extends AbstractConcurrentInitializer<T, ConcurrentException> {
    073
    074    /**
    075     * Builds a new instance.
    076     *
    077     * @param <T> The type of results supplied by this builder.
    078     * @param <I> The type of the initializer managed by this builder.
    079     * @since 3.14.0
    080     */
    081    public static class Builder<I extends LazyInitializer<T>, T> extends AbstractBuilder<I, T, Builder<I, T>, ConcurrentException> {
    082
    083        /**
    084         * Constructs a new instance.
    085         */
    086        public Builder() {
    087            // empty
    088        }
    089
    090        @SuppressWarnings("unchecked")
    091        @Override
    092        public I get() {
    093            return (I) new LazyInitializer(getInitializer(), getCloser());
    094        }
    095
    096    }
    097
    098    /**
    099     * A unique value indicating an un-initialized instance.
    100     */
    101    private static final Object NO_INIT = new Object();
    102
    103    /**
    104     * Creates a new builder.
    105     *
    106     * @param <T> the type of object to build.
    107     * @return a new builder.
    108     * @since 3.14.0
    109     */
    110    public static <T> Builder<LazyInitializer<T>, T> builder() {
    111        return new Builder<>();
    112    }
    113
    114    /** Stores the managed object. */
    115    @SuppressWarnings("unchecked")
    116    private volatile T object = (T) NO_INIT;
    117
    118    /**
    119     * Constructs a new instance.
    120     */
    121    public LazyInitializer() {
    122        // empty
    123    }
    124
    125    /**
    126     * Constructs a new instance.
    127     *
    128     * @param initializer the initializer supplier called by {@link #initialize()}.
    129     * @param closer the closer consumer called by {@link #close()}.
    130     */
    131    private LazyInitializer(final FailableSupplier<T, ConcurrentException> initializer, final FailableConsumer<T, ConcurrentException> closer) {
    132        super(initializer, closer);
    133    }
    134
    135    /**
    136     * Gets the object wrapped by this instance. On first access the object is created. After that it is cached and can be accessed pretty fast.
    137     *
    138     * @return the object initialized by this {@link LazyInitializer}.
    139     * @throws ConcurrentException if an error occurred during initialization of the object.
    140     */
    141    @Override
    142    public T get() throws ConcurrentException {
    143        // use a temporary variable to reduce the number of reads of the
    144        // volatile field
    145        T result = object;
    146        if (result == NO_INIT) {
    147            synchronized (this) {
    148                result = object;
    149                if (result == NO_INIT) {
    150                    object = result = initialize();
    151                }
    152            }
    153        }
    154        return result;
    155    }
    156
    157    /**
    158     * {@inheritDoc}
    159     */
    160    @Override
    161    protected ConcurrentException getTypedException(final Exception e) {
    162        return new ConcurrentException(e);
    163    }
    164
    165    /**
    166     * Tests whether this instance is initialized. Once initialized, always returns true.
    167     *
    168     * @return whether this instance is initialized. Once initialized, always returns true.
    169     * @since 3.14.0
    170     */
    171    @Override
    172    public boolean isInitialized() {
    173        return object != NO_INIT;
    174    }
    175
    176}