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 implementation of the {@link ConcurrentInitializer} interface
026 * based on an {@link AtomicReference} variable.
027 *
028 * <p>
029 * This class maintains a member field of type {@link AtomicReference}. It
030 * implements the following algorithm to create and initialize an object in its
031 * {@link #get()} method:
032 * </p>
033 * <ul>
034 * <li>First it is checked whether the {@link AtomicReference} variable contains
035 * already a value. If this is the case, the value is directly returned.</li>
036 * <li>Otherwise the {@link #initialize()} method is called. This method must be
037 * defined in concrete subclasses to actually create the managed object.</li>
038 * <li>After the object was created by {@link #initialize()} it is checked
039 * whether the {@link AtomicReference} variable is still undefined. This has to
040 * be done because in the meantime another thread may have initialized the
041 * object. If the reference is still empty, the newly created object is stored
042 * in it and returned by this method.</li>
043 * <li>Otherwise the value stored in the {@link AtomicReference} is returned.</li>
044 * </ul>
045 * <p>
046 * Because atomic variables are used this class does not need any
047 * synchronization. So there is no danger of deadlock, and access to the managed
048 * object is efficient. However, if multiple threads access the {@code
049 * AtomicInitializer} object before it has been initialized almost at the same
050 * time, it can happen that {@link #initialize()} is called multiple times. The
051 * algorithm outlined above guarantees that {@link #get()} always returns the
052 * same object though.
053 * </p>
054 * <p>
055 * Compared with the {@link LazyInitializer} class, this class can be more
056 * efficient because it does not need synchronization. The drawback is that the
057 * {@link #initialize()} method can be called multiple times which may be
058 * problematic if the creation of the managed object is expensive. As a rule of
059 * thumb this initializer implementation is preferable if there are not too many
060 * threads involved and the probability that multiple threads access an
061 * uninitialized object is small. If there is high parallelism,
062 * {@link LazyInitializer} is more appropriate.
063 * </p>
064 *
065 * @since 3.0
066 * @param <T> the type of the object managed by this initializer class
067 */
068public class AtomicInitializer<T> extends AbstractConcurrentInitializer<T, ConcurrentException> {
069
070    /**
071     * Builds a new instance.
072     *
073     * @param <T> the type of the object managed by the initializer.
074     * @param <I> the type of the initializer managed by this builder.
075     * @since 3.14.0
076     */
077    public static class Builder<I extends AtomicInitializer<T>, T> extends AbstractBuilder<I, T, Builder<I, T>, ConcurrentException> {
078
079        @SuppressWarnings("unchecked")
080        @Override
081        public I get() {
082            return (I) new AtomicInitializer(getInitializer(), getCloser());
083        }
084
085    }
086
087    private static final Object NO_INIT = new Object();
088
089    /**
090     * Creates a new builder.
091     *
092     * @param <T> the type of object to build.
093     * @return a new builder.
094     * @since 3.14.0
095     */
096    public static <T> Builder<AtomicInitializer<T>, T> builder() {
097        return new Builder<>();
098    }
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 AtomicInitializer() {
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 AtomicInitializer(final FailableSupplier<T, ConcurrentException> initializer, final FailableConsumer<T, ConcurrentException> closer) {
117        super(initializer, closer);
118    }
119
120    /**
121     * Returns the object managed by this initializer. The object is created if
122     * it is not available yet and stored internally. This method always returns
123     * the same object.
124     *
125     * @return the object created by this {@link AtomicInitializer}
126     * @throws ConcurrentException if an error occurred during initialization of
127     * the object
128     */
129    @Override
130    public T get() throws ConcurrentException {
131        T result = reference.get();
132
133        if (result == getNoInit()) {
134            result = initialize();
135            if (!reference.compareAndSet(getNoInit(), result)) {
136                // another thread has initialized the reference
137                result = reference.get();
138            }
139        }
140
141        return result;
142    }
143
144    /** Gets the internal no-init object cast for this instance. */
145    @SuppressWarnings("unchecked")
146    private T getNoInit() {
147        return (T) NO_INIT;
148    }
149
150    /**
151     * {@inheritDoc}
152     */
153    @Override
154    protected ConcurrentException getTypedException(Exception e) {
155        return new ConcurrentException(e);
156    }
157
158    /**
159     * Tests whether this instance is initialized. Once initialized, always returns true.
160     *
161     * @return whether this instance is initialized. Once initialized, always returns true.
162     * @since 3.14.0
163     */
164    @Override
165    public boolean isInitialized() {
166        return reference.get() != NO_INIT;
167    }
168}