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        @SuppressWarnings("unchecked")
069        @Override
070        public I get() {
071            return (I) new AtomicSafeInitializer(getInitializer(), getCloser());
072        }
073
074    }
075
076    private static final Object NO_INIT = new Object();
077
078    /**
079     * Creates a new builder.
080     *
081     * @param <T> the type of object to build.
082     * @return a new builder.
083     * @since 3.14.0
084     */
085    public static <T> Builder<AtomicSafeInitializer<T>, T> builder() {
086        return new Builder<>();
087    }
088
089    /** A guard which ensures that initialize() is called only once. */
090    private final AtomicReference<AtomicSafeInitializer<T>> factory = new AtomicReference<>();
091
092    /** Holds the reference to the managed object. */
093    private final AtomicReference<T> reference = new AtomicReference<>(getNoInit());
094
095    /**
096     * Constructs a new instance.
097     */
098    public AtomicSafeInitializer() {
099        // empty
100    }
101
102    /**
103     * Constructs a new instance.
104     *
105     * @param initializer the initializer supplier called by {@link #initialize()}.
106     * @param closer the closer consumer called by {@link #close()}.
107     */
108    private AtomicSafeInitializer(final FailableSupplier<T, ConcurrentException> initializer, final FailableConsumer<T, ConcurrentException> closer) {
109        super(initializer, closer);
110    }
111
112    /**
113     * Gets (and initialize, if not initialized yet) the required object
114     *
115     * @return lazily initialized object
116     * @throws ConcurrentException if the initialization of the object causes an
117     * exception
118     */
119    @Override
120    public final T get() throws ConcurrentException {
121        T result;
122
123        while ((result = reference.get()) == getNoInit()) {
124            if (factory.compareAndSet(null, this)) {
125                reference.set(initialize());
126            }
127        }
128
129        return result;
130    }
131
132    /** Gets the internal no-init object cast for this instance. */
133    @SuppressWarnings("unchecked")
134    private T getNoInit() {
135        return (T) NO_INIT;
136    }
137
138    /**
139     * {@inheritDoc}
140     */
141    @Override
142    protected ConcurrentException getTypedException(Exception e) {
143        return new ConcurrentException(e);
144    }
145
146    /**
147     * Tests whether this instance is initialized. Once initialized, always returns true.
148     *
149     * @return whether this instance is initialized. Once initialized, always returns true.
150     * @since 3.14.0
151     */
152    @Override
153    public boolean isInitialized() {
154        return reference.get() != NO_INIT;
155    }
156}