97 lines
3.9 KiB
Java
97 lines
3.9 KiB
Java
/*
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* Licensed to the Apache Software Foundation (ASF) under one or more
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* contributor license agreements. See the NOTICE file distributed with
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* this work for additional information regarding copyright ownership.
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* The ASF licenses this file to You under the Apache License, Version 2.0
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* (the "License"); you may not use this file except in compliance with
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* the License. You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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package org.apache.commons.lang3.concurrent;
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import java.util.concurrent.atomic.AtomicReference;
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/**
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* <p>
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* A specialized {@code ConcurrentInitializer} implementation which is similar
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* to {@link AtomicInitializer}, but ensures that the {@link #initialize()}
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* method is called only once.
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* </p>
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* <p>
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* As {@link AtomicInitializer} this class is based on atomic variables, so it
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* can create an object under concurrent access without synchronization.
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* However, it implements an additional check to guarantee that the
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* {@link #initialize()} method which actually creates the object cannot be
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* called multiple times.
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* </p>
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* <p>
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* Because of this additional check this implementation is slightly less
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* efficient than {@link AtomicInitializer}, but if the object creation in the
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* {@code initialize()} method is expensive or if multiple invocations of
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* {@code initialize()} are problematic, it is the better alternative.
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* </p>
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* <p>
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* From its semantics this class has the same properties as
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* {@link LazyInitializer}. It is a "save" implementation of the lazy
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* initializer pattern. Comparing both classes in terms of efficiency is
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* difficult because which one is faster depends on multiple factors. Because
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* {@code AtomicSafeInitializer} does not use synchronization at all it probably
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* outruns {@link LazyInitializer}, at least under low or moderate concurrent
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* access. Developers should run their own benchmarks on the expected target
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* platform to decide which implementation is suitable for their specific use
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* case.
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* </p>
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*
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* @since 3.0
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* @param <T> the type of the object managed by this initializer class
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*/
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public abstract class AtomicSafeInitializer<T> implements
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ConcurrentInitializer<T> {
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/** A guard which ensures that initialize() is called only once. */
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private final AtomicReference<AtomicSafeInitializer<T>> factory =
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new AtomicReference<>();
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/** Holds the reference to the managed object. */
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private final AtomicReference<T> reference = new AtomicReference<>();
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/**
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* Gets (and initialize, if not initialized yet) the required object
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*
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* @return lazily initialized object
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* @throws ConcurrentException if the initialization of the object causes an
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* exception
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*/
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@Override
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public final T get() throws ConcurrentException {
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T result;
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while ((result = reference.get()) == null) {
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if (factory.compareAndSet(null, this)) {
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reference.set(initialize());
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}
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}
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return result;
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}
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/**
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* Creates and initializes the object managed by this
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* {@code AtomicInitializer}. This method is called by {@link #get()} when
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* the managed object is not available yet. An implementation can focus on
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* the creation of the object. No synchronization is needed, as this is
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* already handled by {@code get()}. This method is guaranteed to be called
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* only once.
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*
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* @return the managed data object
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* @throws ConcurrentException if an error occurs during object creation
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*/
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protected abstract T initialize() throws ConcurrentException;
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}
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