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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 implementation of the {@link ConcurrentInitializer} interface
* based on an {@link AtomicReference} variable.
*
*
* This class maintains a member field of type {@link AtomicReference}. It
* implements the following algorithm to create and initialize an object in its
* {@link #get()} method:
*
*
* - First it is checked whether the {@link AtomicReference} variable contains
* already a value. If this is the case, the value is directly returned.
* - Otherwise the {@link #initialize()} method is called. This method must be
* defined in concrete subclasses to actually create the managed object.
* - After the object was created by {@link #initialize()} it is checked
* whether the {@link AtomicReference} variable is still undefined. This has to
* be done because in the meantime another thread may have initialized the
* object. If the reference is still empty, the newly created object is stored
* in it and returned by this method.
* - Otherwise the value stored in the {@link AtomicReference} is returned.
*
*
* Because atomic variables are used this class does not need any
* synchronization. So there is no danger of deadlock, and access to the managed
* object is efficient. However, if multiple threads access the {@code
* AtomicInitializer} object before it has been initialized almost at the same
* time, it can happen that {@link #initialize()} is called multiple times. The
* algorithm outlined above guarantees that {@link #get()} always returns the
* same object though.
*
*
* Compared with the {@link LazyInitializer} class, this class can be more
* efficient because it does not need synchronization. The drawback is that the
* {@link #initialize()} method can be called multiple times which may be
* problematic if the creation of the managed object is expensive. As a rule of
* thumb this initializer implementation is preferable if there are not too many
* threads involved and the probability that multiple threads access an
* uninitialized object is small. If there is high parallelism,
* {@link LazyInitializer} is more appropriate.
*
*
* @since 3.0
* @param the type of the object managed by this initializer class
*/
public class AtomicInitializer 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> {
@SuppressWarnings("unchecked")
@Override
public I get() {
return (I) new AtomicInitializer(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<>();
}
/** Holds the reference to the managed object. */
private final AtomicReference reference = new AtomicReference<>(getNoInit());
/**
* Constructs a new instance.
*/
public AtomicInitializer() {
// empty
}
/**
* Constructs a new instance.
*
* @param initializer the initializer supplier called by {@link #initialize()}.
* @param closer the closer consumer called by {@link #close()}.
*/
private AtomicInitializer(final FailableSupplier initializer, final FailableConsumer closer) {
super(initializer, closer);
}
/**
* Returns the object managed by this initializer. The object is created if
* it is not available yet and stored internally. This method always returns
* the same object.
*
* @return the object created by this {@link AtomicInitializer}
* @throws ConcurrentException if an error occurred during initialization of
* the object
*/
@Override
public T get() throws ConcurrentException {
T result = reference.get();
if (result == getNoInit()) {
result = initialize();
if (!reference.compareAndSet(getNoInit(), result)) {
// another thread has initialized the reference
result = reference.get();
}
}
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(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;
}
}