    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 java.util.concurrent.atomic.AtomicReference;
    020
    021import org.apache.commons.lang3.function.FailableConsumer;
    022import org.apache.commons.lang3.function.FailableSupplier;
    023
    024/**
    025 * A specialized {@link ConcurrentInitializer} implementation which is similar
    026 * to {@link AtomicInitializer}, but ensures that the {@link #initialize()}
    027 * method is called only once.
    028 *
    029 * <p>
    030 * As {@link AtomicInitializer} this class is based on atomic variables, so it
    031 * can create an object under concurrent access without synchronization.
    032 * However, it implements an additional check to guarantee that the
    033 * {@link #initialize()} method which actually creates the object cannot be
    034 * called multiple times.
    035 * </p>
    036 * <p>
    037 * Because of this additional check this implementation is slightly less
    038 * efficient than {@link AtomicInitializer}, but if the object creation in the
    039 * {@code initialize()} method is expensive or if multiple invocations of
    040 * {@code initialize()} are problematic, it is the better alternative.
    041 * </p>
    042 * <p>
    043 * From its semantics this class has the same properties as
    044 * {@link LazyInitializer}. It is a &quot;save&quot; implementation of the lazy
    045 * initializer pattern. Comparing both classes in terms of efficiency is
    046 * difficult because which one is faster depends on multiple factors. Because
    047 * {@link AtomicSafeInitializer} does not use synchronization at all it probably
    048 * outruns {@link LazyInitializer}, at least under low or moderate concurrent
    049 * access. Developers should run their own benchmarks on the expected target
    050 * platform to decide which implementation is suitable for their specific use
    051 * case.
    052 * </p>
    053 *
    054 * @since 3.0
    055 * @param <T> the type of the object managed by this initializer class
    056 */
    057public class AtomicSafeInitializer<T> extends AbstractConcurrentInitializer<T, ConcurrentException> {
    058
    059    /**
    060     * Builds a new instance.
    061     *
    062     * @param <T> the type of the object managed by the initializer.
    063     * @param <I> the type of the initializer managed by this builder.
    064     * @since 3.14.0
    065     */
    066    public static class Builder<I extends AtomicSafeInitializer<T>, T> extends AbstractBuilder<I, T, Builder<I, T>, ConcurrentException> {
    067
    068        /**
    069         * Constructs a new instance.
    070         */
    071        public Builder() {
    072            // empty
    073        }
    074
    075        @SuppressWarnings("unchecked")
    076        @Override
    077        public I get() {
    078            return (I) new AtomicSafeInitializer(getInitializer(), getCloser());
    079        }
    080
    081    }
    082
    083    private static final Object NO_INIT = new Object();
    084
    085    /**
    086     * Creates a new builder.
    087     *
    088     * @param <T> the type of object to build.
    089     * @return a new builder.
    090     * @since 3.14.0
    091     */
    092    public static <T> Builder<AtomicSafeInitializer<T>, T> builder() {
    093        return new Builder<>();
    094    }
    095
    096    /** A guard which ensures that initialize() is called only once. */
    097    private final AtomicReference<AtomicSafeInitializer<T>> factory = new AtomicReference<>();
    098
    099    /** Holds the reference to the managed object. */
    100    private final AtomicReference<T> reference = new AtomicReference<>(getNoInit());
    101
    102    /**
    103     * Constructs a new instance.
    104     */
    105    public AtomicSafeInitializer() {
    106        // empty
    107    }
    108
    109    /**
    110     * Constructs a new instance.
    111     *
    112     * @param initializer the initializer supplier called by {@link #initialize()}.
    113     * @param closer the closer consumer called by {@link #close()}.
    114     */
    115    private AtomicSafeInitializer(final FailableSupplier<T, ConcurrentException> initializer, final FailableConsumer<T, ConcurrentException> closer) {
    116        super(initializer, closer);
    117    }
    118
    119    /**
    120     * Gets (and initialize, if not initialized yet) the required object
    121     *
    122     * @return lazily initialized object
    123     * @throws ConcurrentException if the initialization of the object causes an
    124     * exception
    125     */
    126    @Override
    127    public final T get() throws ConcurrentException {
    128        T result;
    129
    130        while ((result = reference.get()) == getNoInit()) {
    131            if (factory.compareAndSet(null, this)) {
    132                reference.set(initialize());
    133            }
    134        }
    135
    136        return result;
    137    }
    138
    139    /** Gets the internal no-init object cast for this instance. */
    140    @SuppressWarnings("unchecked")
    141    private T getNoInit() {
    142        return (T) NO_INIT;
    143    }
    144
    145    /**
    146     * {@inheritDoc}
    147     */
    148    @Override
    149    protected ConcurrentException getTypedException(final Exception e) {
    150        return new ConcurrentException(e);
    151    }
    152
    153    /**
    154     * Tests whether this instance is initialized. Once initialized, always returns true.
    155     *
    156     * @return whether this instance is initialized. Once initialized, always returns true.
    157     * @since 3.14.0
    158     */
    159    @Override
    160    public boolean isInitialized() {
    161        return reference.get() != NO_INIT;
    162    }
    163}