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/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.commons.lang3.concurrent;
import java.util.concurrent.atomic.AtomicReference;
import org.apache.commons.lang3.function.FailableConsumer;
import org.apache.commons.lang3.function.FailableSupplier;
/**
* A specialized {@link ConcurrentInitializer} implementation which is similar
* to {@link AtomicInitializer}, but ensures that the {@link #initialize()}
* method is called only once.
*
*
* As {@link AtomicInitializer} this class is based on atomic variables, so it
* can create an object under concurrent access without synchronization.
* However, it implements an additional check to guarantee that the
* {@link #initialize()} method which actually creates the object cannot be
* called multiple times.
*
*
* Because of this additional check this implementation is slightly less
* efficient than {@link AtomicInitializer}, but if the object creation in the
* {@code initialize()} method is expensive or if multiple invocations of
* {@code initialize()} are problematic, it is the better alternative.
*
*
* From its semantics this class has the same properties as
* {@link LazyInitializer}. It is a "save" implementation of the lazy
* initializer pattern. Comparing both classes in terms of efficiency is
* difficult because which one is faster depends on multiple factors. Because
* {@link AtomicSafeInitializer} does not use synchronization at all it probably
* outruns {@link LazyInitializer}, at least under low or moderate concurrent
* access. Developers should run their own benchmarks on the expected target
* platform to decide which implementation is suitable for their specific use
* case.
*
*
* @since 3.0
* @param the type of the object managed by this initializer class
*/
public class AtomicSafeInitializer extends AbstractConcurrentInitializer {
/**
* Builds a new instance.
*
* @param the type of the object managed by the initializer.
* @param the type of the initializer managed by this builder.
* @since 3.14.0
*/
public static class Builder, T> extends AbstractBuilder, ConcurrentException> {
/**
* Constructs a new instance.
*/
public Builder() {
// empty
}
@SuppressWarnings("unchecked")
@Override
public I get() {
return (I) new AtomicSafeInitializer(getInitializer(), getCloser());
}
}
private static final Object NO_INIT = new Object();
/**
* Creates a new builder.
*
* @param the type of object to build.
* @return a new builder.
* @since 3.14.0
*/
public static Builder, T> builder() {
return new Builder<>();
}
/** A guard which ensures that initialize() is called only once. */
private final AtomicReference> factory = new AtomicReference<>();
/** Holds the reference to the managed object. */
private final AtomicReference reference = new AtomicReference<>(getNoInit());
/**
* Constructs a new instance.
*/
public AtomicSafeInitializer() {
// empty
}
/**
* Constructs a new instance.
*
* @param initializer the initializer supplier called by {@link #initialize()}.
* @param closer the closer consumer called by {@link #close()}.
*/
private AtomicSafeInitializer(final FailableSupplier initializer, final FailableConsumer closer) {
super(initializer, closer);
}
/**
* Gets (and initialize, if not initialized yet) the required object
*
* @return lazily initialized object
* @throws ConcurrentException if the initialization of the object causes an
* exception
*/
@Override
public final T get() throws ConcurrentException {
T result;
while ((result = reference.get()) == getNoInit()) {
if (factory.compareAndSet(null, this)) {
reference.set(initialize());
}
}
return result;
}
/** Gets the internal no-init object cast for this instance. */
@SuppressWarnings("unchecked")
private T getNoInit() {
return (T) NO_INIT;
}
/**
* {@inheritDoc}
*/
@Override
protected ConcurrentException getTypedException(final Exception e) {
return new ConcurrentException(e);
}
/**
* Tests whether this instance is initialized. Once initialized, always returns true.
*
* @return whether this instance is initialized. Once initialized, always returns true.
* @since 3.14.0
*/
@Override
public boolean isInitialized() {
return reference.get() != NO_INIT;
}
}