    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 java.util.concurrent.atomic.AtomicReference;
    020
    021import org.apache.commons.lang3.exception.ExceptionUtils;
    022import org.apache.commons.lang3.function.FailableConsumer;
    023import org.apache.commons.lang3.function.FailableSupplier;
    024
    025/**
    026 * A specialized {@link ConcurrentInitializer} implementation which is similar
    027 * to {@link AtomicInitializer}, but ensures that the {@link #initialize()}
    028 * method is called only once.
    029 *
    030 * <p>
    031 * As {@link AtomicInitializer} this class is based on atomic variables, so it
    032 * can create an object under concurrent access without synchronization.
    033 * However, it implements an additional check to guarantee that the
    034 * {@link #initialize()} method which actually creates the object cannot be
    035 * called multiple times.
    036 * </p>
    037 * <p>
    038 * Because of this additional check this implementation is slightly less
    039 * efficient than {@link AtomicInitializer}, but if the object creation in the
    040 * {@code initialize()} method is expensive or if multiple invocations of
    041 * {@code initialize()} are problematic, it is the better alternative.
    042 * </p>
    043 * <p>
    044 * From its semantics this class has the same properties as
    045 * {@link LazyInitializer}. It is a &quot;save&quot; implementation of the lazy
    046 * initializer pattern. Comparing both classes in terms of efficiency is
    047 * difficult because which one is faster depends on multiple factors. Because
    048 * {@link AtomicSafeInitializer} does not use synchronization at all it probably
    049 * outruns {@link LazyInitializer}, at least under low or moderate concurrent
    050 * access. Developers should run their own benchmarks on the expected target
    051 * platform to decide which implementation is suitable for their specific use
    052 * case.
    053 * </p>
    054 *
    055 * @param <T> the type of the object managed by this initializer class
    056 * @since 3.0
    057 */
    058public class AtomicSafeInitializer<T> extends AbstractConcurrentInitializer<T, ConcurrentException> {
    059
    060    /**
    061     * Builds a new instance.
    062     *
    063     * @param <T> The type of results supplied by this builder.
    064     * @param <I> The type of the initializer managed by this builder.
    065     * @since 3.14.0
    066     */
    067    public static class Builder<I extends AtomicSafeInitializer<T>, T> extends AbstractBuilder<I, T, Builder<I, T>, ConcurrentException> {
    068
    069        /**
    070         * Constructs a new instance.
    071         */
    072        public Builder() {
    073            // empty
    074        }
    075
    076        @SuppressWarnings("unchecked")
    077        @Override
    078        public I get() {
    079            return (I) new AtomicSafeInitializer(getInitializer(), getCloser());
    080        }
    081
    082    }
    083
    084    private static final Object NO_INIT = new Object();
    085
    086    /**
    087     * Creates a new builder.
    088     *
    089     * @param <T> the type of object to build.
    090     * @return a new builder.
    091     * @since 3.14.0
    092     */
    093    public static <T> Builder<AtomicSafeInitializer<T>, T> builder() {
    094        return new Builder<>();
    095    }
    096
    097    /** A guard which ensures that initialize() is called only once. */
    098    private final AtomicReference<AtomicSafeInitializer<T>> factory = new AtomicReference<>();
    099
    100    /** Holds the reference to the managed object. */
    101    private final AtomicReference<T> reference = new AtomicReference<>(getNoInit());
    102
    103    /**
    104     * Constructs a new instance.
    105     */
    106    public AtomicSafeInitializer() {
    107        // empty
    108    }
    109
    110    /**
    111     * Constructs a new instance.
    112     *
    113     * @param initializer the initializer supplier called by {@link #initialize()}.
    114     * @param closer the closer consumer called by {@link #close()}.
    115     */
    116    private AtomicSafeInitializer(final FailableSupplier<T, ConcurrentException> initializer, final FailableConsumer<T, ConcurrentException> closer) {
    117        super(initializer, closer);
    118    }
    119
    120    /**
    121     * Gets (and initialize, if not initialized yet) the required object.
    122     *
    123     * @return lazily initialized object.
    124     * @throws ConcurrentException if the initialization of the object causes an exception.
    125     */
    126    @Override
    127    public final T get() throws ConcurrentException {
    128        T result;
    129        while ((result = reference.get()) == getNoInit()) {
    130            if (factory.compareAndSet(null, this)) {
    131                try {
    132                    reference.set(initialize());
    133                } catch (final Throwable t) {
    134                    // Allow retry on failure; otherwise callers spin forever.
    135                    factory.set(null);
    136                    // Rethrow preserving original semantics: unchecked as-is, checked wrapped.
    137                    final Throwable checked = ExceptionUtils.throwUnchecked(t);
    138                    throw checked instanceof ConcurrentException ? (ConcurrentException) checked : new ConcurrentException(checked);
    139                }
    140            }
    141        }
    142        return result;
    143    }
    144
    145    /** Gets the internal no-init object cast for this instance. */
    146    @SuppressWarnings("unchecked")
    147    private T getNoInit() {
    148        return (T) NO_INIT;
    149    }
    150
    151    /**
    152     * {@inheritDoc}
    153     */
    154    @Override
    155    protected ConcurrentException getTypedException(final 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 reference.get() != NO_INIT;
    168    }
    169}