107 lines
4.7 KiB
Java
107 lines
4.7 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 implementation of the {@code ConcurrentInitializer} interface
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* based on an {@link AtomicReference} variable.
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* </p>
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* <p>
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* This class maintains a member field of type {@code AtomicReference}. It
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* implements the following algorithm to create and initialize an object in its
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* {@link #get()} method:
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* </p>
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* <ul>
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* <li>First it is checked whether the {@code AtomicReference} variable contains
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* already a value. If this is the case, the value is directly returned.</li>
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* <li>Otherwise the {@link #initialize()} method is called. This method must be
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* defined in concrete subclasses to actually create the managed object.</li>
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* <li>After the object was created by {@link #initialize()} it is checked
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* whether the {@code AtomicReference} variable is still undefined. This has to
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* be done because in the meantime another thread may have initialized the
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* object. If the reference is still empty, the newly created object is stored
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* in it and returned by this method.</li>
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* <li>Otherwise the value stored in the {@code AtomicReference} is returned.</li>
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* </ul>
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* <p>
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* Because atomic variables are used this class does not need any
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* synchronization. So there is no danger of deadlock, and access to the managed
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* object is efficient. However, if multiple threads access the {@code
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* AtomicInitializer} object before it has been initialized almost at the same
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* time, it can happen that {@link #initialize()} is called multiple times. The
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* algorithm outlined above guarantees that {@link #get()} always returns the
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* same object though.
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* </p>
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* <p>
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* Compared with the {@link LazyInitializer} class, this class can be more
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* efficient because it does not need synchronization. The drawback is that the
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* {@link #initialize()} method can be called multiple times which may be
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* problematic if the creation of the managed object is expensive. As a rule of
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* thumb this initializer implementation is preferable if there are not too many
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* threads involved and the probability that multiple threads access an
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* uninitialized object is small. If there is high parallelism,
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* {@link LazyInitializer} is more appropriate.
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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 AtomicInitializer<T> implements ConcurrentInitializer<T> {
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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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* Returns the object managed by this initializer. The object is created if
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* it is not available yet and stored internally. This method always returns
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* the same object.
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*
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* @return the object created by this {@code AtomicInitializer}
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* @throws ConcurrentException if an error occurred during initialization of
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* the object
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*/
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@Override
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public T get() throws ConcurrentException {
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T result = reference.get();
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if (result == null) {
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result = initialize();
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if (!reference.compareAndSet(null, result)) {
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// another thread has initialized the reference
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result = reference.get();
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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 {@code
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* AtomicInitializer}. This method is called by {@link #get()} when the
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* managed object is not available yet. An implementation can focus on the
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* creation of the object. No synchronization is needed, as this is already
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* handled by {@code get()}. As stated by the class comment, it is possible
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* that this method is called multiple times.
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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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