LazyInitializer.java
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* 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
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*
* 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,
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* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.commons.lang3.concurrent;
import org.apache.commons.lang3.function.FailableConsumer;
import org.apache.commons.lang3.function.FailableSupplier;
/**
* This class provides a generic implementation of the lazy initialization pattern.
*
* <p>
* Sometimes an application has to deal with an object only under certain circumstances, e.g. when the user selects a specific menu item or if a special event
* is received. If the creation of the object is costly or the consumption of memory or other system resources is significant, it may make sense to defer the
* creation of this object until it is really needed. This is a use case for the lazy initialization pattern.
* </p>
* <p>
* This abstract base class provides an implementation of the double-check idiom for an instance field as discussed in Joshua Bloch's "Effective Java", 2nd
* edition, item 71. The class already implements all necessary synchronization. A concrete subclass has to implement the {@code initialize()} method, which
* actually creates the wrapped data object.
* </p>
* <p>
* As an usage example consider that we have a class {@code ComplexObject} whose instantiation is a complex operation. In order to apply lazy initialization to
* this class, a subclass of {@link LazyInitializer} has to be created:
* </p>
*
* <pre>
* public class ComplexObjectInitializer extends LazyInitializer<ComplexObject> {
* @Override
* protected ComplexObject initialize() {
* return new ComplexObject();
* }
* }
* </pre>
*
* <p>
* Access to the data object is provided through the {@code get()} method. So, code that wants to obtain the {@code ComplexObject} instance would simply look
* like this:
* </p>
*
* <pre>
* // Create an instance of the lazy initializer
* ComplexObjectInitializer initializer = new ComplexObjectInitializer();
* ...
* // When the object is actually needed:
* ComplexObject cobj = initializer.get();
* </pre>
*
* <p>
* If multiple threads call the {@code get()} method when the object has not yet been created, they are blocked until initialization completes. The algorithm
* guarantees that only a single instance of the wrapped object class is created, which is passed to all callers. Once initialized, calls to the {@code get()}
* method are pretty fast because no synchronization is needed (only an access to a <b>volatile</b> member field).
* </p>
*
* @since 3.0
* @param <T> the type of the object managed by the initializer.
*/
public class LazyInitializer<T> extends AbstractConcurrentInitializer<T, ConcurrentException> {
/**
* Builds a new instance.
*
* @param <T> the type of the object managed by the initializer.
* @param <I> the type of the initializer managed by this builder.
* @since 3.14.0
*/
public static class Builder<I extends LazyInitializer<T>, T> extends AbstractBuilder<I, T, Builder<I, T>, ConcurrentException> {
@SuppressWarnings("unchecked")
@Override
public I get() {
return (I) new LazyInitializer(getInitializer(), getCloser());
}
}
/**
* A unique value indicating an un-initialzed instance.
*/
private static final Object NO_INIT = new Object();
/**
* Creates a new builder.
*
* @param <T> the type of object to build.
* @return a new builder.
* @since 3.14.0
*/
public static <T> Builder<LazyInitializer<T>, T> builder() {
return new Builder<>();
}
/** Stores the managed object. */
@SuppressWarnings("unchecked")
private volatile T object = (T) NO_INIT;
/**
* Constructs a new instance.
*/
public LazyInitializer() {
// empty
}
/**
* Constructs a new instance.
*
* @param initializer the initializer supplier called by {@link #initialize()}.
* @param closer the closer consumer called by {@link #close()}.
*/
private LazyInitializer(final FailableSupplier<T, ConcurrentException> initializer, final FailableConsumer<T, ConcurrentException> closer) {
super(initializer, closer);
}
/**
* Returns the object wrapped by this instance. On first access the object is created. After that it is cached and can be accessed pretty fast.
*
* @return the object initialized by this {@link LazyInitializer}
* @throws ConcurrentException if an error occurred during initialization of the object
*/
@Override
public T get() throws ConcurrentException {
// use a temporary variable to reduce the number of reads of the
// volatile field
T result = object;
if (result == NO_INIT) {
synchronized (this) {
result = object;
if (result == NO_INIT) {
object = result = initialize();
}
}
}
return result;
}
/**
* {@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 object != NO_INIT;
}
}