org.hibernate.transform.DistinctResultTransformer Maven / Gradle / Ivy
/*
* Hibernate, Relational Persistence for Idiomatic Java
*
* Copyright (c) 2008, Red Hat Middleware LLC or third-party contributors as
* indicated by the @author tags or express copyright attribution
* statements applied by the authors. All third-party contributions are
* distributed under license by Red Hat Middleware LLC.
*
* This copyrighted material is made available to anyone wishing to use, modify,
* copy, or redistribute it subject to the terms and conditions of the GNU
* Lesser General Public License, as published by the Free Software Foundation.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
* or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License
* for more details.
*
* You should have received a copy of the GNU Lesser General Public License
* along with this distribution; if not, write to:
* Free Software Foundation, Inc.
* 51 Franklin Street, Fifth Floor
* Boston, MA 02110-1301 USA
*
*/
package org.hibernate.transform;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
import org.jboss.logging.Logger;
import org.hibernate.internal.CoreMessageLogger;
/**
* Distinctions the result tuples in the final result based on the defined
* equality of the tuples.
*
* Since this transformer is stateless, all instances would be considered equal.
* So for optimization purposes we limit it to a single, singleton {@link #INSTANCE instance}.
*
* @author Steve Ebersole
*/
public class DistinctResultTransformer extends BasicTransformerAdapter {
public static final DistinctResultTransformer INSTANCE = new DistinctResultTransformer();
private static final CoreMessageLogger LOG = Logger.getMessageLogger(CoreMessageLogger.class,
DistinctResultTransformer.class.getName());
/**
* Helper class to handle distincting
*/
private static final class Identity {
final Object entity;
private Identity(Object entity) {
this.entity = entity;
}
/**
* {@inheritDoc}
*/
@Override
public boolean equals(Object other) {
return Identity.class.isInstance( other )
&& this.entity == ( ( Identity ) other ).entity;
}
/**
* {@inheritDoc}
*/
@Override
public int hashCode() {
return System.identityHashCode( entity );
}
}
/**
* Disallow instantiation of DistinctResultTransformer.
*/
private DistinctResultTransformer() {
}
/**
* Uniquely distinct each tuple row here.
*/
@Override
public List transformList(List list) {
List result = new ArrayList( list.size() );
Set distinct = new HashSet();
for ( int i = 0; i < list.size(); i++ ) {
Object entity = list.get( i );
if ( distinct.add( new Identity( entity ) ) ) {
result.add( entity );
}
}
LOG.debugf( "Transformed: %s rows to: %s distinct results", list.size(), result.size() );
return result;
}
/**
* Serialization hook for ensuring singleton uniqueing.
*
* @return The singleton instance : {@link #INSTANCE}
*/
private Object readResolve() {
return INSTANCE;
}
}