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/*******************************************************************************
* Copyright (c) 2010 Oracle. All rights reserved.
* This program and the accompanying materials are made available under the
* terms of the Eclipse Public License v1.0, which accompanies this distribution
* and is available at http://www.eclipse.org/legal/epl-v10.html.
*
* Contributors:
* Oracle - initial API and implementation
******************************************************************************/
package org.eclipse.jpt.common.utility.internal;
import java.io.Serializable;
import org.eclipse.jpt.common.utility.ReadOnlyObjectReference;
/**
* Provide a thread-safe, reasonably performing container for holding an
* object that will be "lazy-initialized" upon its first reference. This is
* also useful for preventing direct references (accidental or otherwise) to
* lazy-initialized state.
* <p>
* There are some penalties:<ul>
* <li>The reference's use of generics will require casting (and the requisite
* VM testing) with every access
* <li>If the value calculated during lazy initialization is <code>null</code>,
* access will be <code>synchronized</code> <em>every</em> time.
* </ul>
* @see SimpleObjectReference
* @see SynchronizedObject
*/
public abstract class LazyReadOnlyObjectReference<V>
implements ReadOnlyObjectReference<V>, Cloneable, Serializable
{
/** Backing value. */
private volatile V value = null;
private static final long serialVersionUID = 1L;
// ********** constructors **********
/**
* Create a lazy object reference.
*/
public LazyReadOnlyObjectReference() {
super();
}
// ********** value **********
/**
* In JDK 5 and later, this "double-checked locking" idiom works as long
* as the instance variable is marked <code>volatile</code>.
*/
public V getValue() {
V result = this.value;
if (result == null) {
synchronized (this) {
result = this.value;
if (result == null) {
this.value = result = this.buildValue();
}
}
}
return result;
}
protected abstract V buildValue();
public boolean valueEquals(Object object) {
return Tools.valuesAreEqual(this.getValue(), object);
}
public boolean valueNotEqual(Object object) {
return Tools.valuesAreDifferent(this.getValue(), object);
}
public boolean isNull() {
return this.getValue() == null;
}
public boolean isNotNull() {
return this.getValue() != null;
}
// ********** standard methods **********
@Override
public LazyReadOnlyObjectReference<V> clone() {
try {
@SuppressWarnings("unchecked")
LazyReadOnlyObjectReference<V> clone = (LazyReadOnlyObjectReference<V>) super.clone();
return clone;
} catch (CloneNotSupportedException ex) {
throw new InternalError();
}
}
/**
* This method will <em>not</em> trigger the "lazy-initialization".
*/
@Override
public String toString() {
return '[' + String.valueOf(this.value) + ']';
}
}
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