/*
* Hibernate Search, full-text search for your domain model
*
* License: GNU Lesser General Public License (LGPL), version 2.1 or later
* See the lgpl.txt file in the root directory or <http://www.gnu.org/licenses/lgpl-2.1.html>.
*/
package org.hibernate.search.engine.metadata.impl;
import org.hibernate.search.exception.AssertionFailure;
/**
* A container allowing to build metadata with circular dependency.
*
* <p>It's useful when building {@link DocumentFieldMetadata}, for instance:
* when we build those, the parent type metadata hasn't been built yet (since
* it requires every fields to be created). To break the circular dependency,
* we provide the DocumentFieldMetadata with a {@code BackReference<TypeMetadata>},
* and we initialize the reference later.
*
* @author Yoann Rodiere
*/
public class BackReference<T> {
private static final BackReference<Object> EMPTY = new BackReference<>();
static {
EMPTY.initialize( null );
}
@SuppressWarnings("unchecked") // BackReference is covariant on T
public static <T> BackReference<T> empty() {
return (BackReference<T>) EMPTY;
}
private boolean initialized = false;
private T value;
public BackReference() {
}
public T get() {
if ( !initialized ) {
throw new AssertionFailure( "A reference has been accessed before having been initialized." );
}
return value;
}
void initialize(T value) {
if ( this.value != null ) {
throw new AssertionFailure( "A reference has been initialized more than once." );
}
this.value = value;
this.initialized = true;
}
@Override
public String toString() {
if ( initialized ) {
return String.valueOf( value );
}
else {
return "<not initialized>";
}
}
}