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}