backrefpropertyaccessor.java

一个Java持久层类库

/*
 * Copyright (c) 2007, Red Hat Middleware, LLC. All rights reserved.
 *
 * 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, v. 2.1. This program is distributed in the
 * hope that it will be useful, but WITHOUT A 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, v.2.1 along with this
 * distribution; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 *
 * Red Hat Author(s): Gavin King, Steve Ebersole
 */
package org.hibernate.property;

import java.io.Serializable;
import java.lang.reflect.Method;
import java.util.Map;

import org.hibernate.engine.SessionFactoryImplementor;
import org.hibernate.engine.SessionImplementor;

/**
 * Represents a "back-reference" to the id of a collection owner.  A "back-reference" is pertinent in mapping scenarios
 * where we have a uni-directional one-to-many association in which only the many side is mapped.  In this case it is
 * the collection itself which is responsible for the FK value.
 * <p/>
 * In this scenario, the one side has no inherent knowledge of its "owner".  So we introduce a synthetic property into
 * the one side to represent the association; a so-called back-reference.
 *
 * @author Gavin King
 * @author Steve Ebersole
 */
public class BackrefPropertyAccessor implements PropertyAccessor {

	private final String propertyName;
	private final String entityName;

	// cache these since they are stateless
	private final BackrefSetter setter; // this one could even be static...
	private final BackrefGetter getter;

	/**
	 * A placeholder for a property value, indicating that
	 * we don't know the value of the back reference
	 */
	public static final Serializable UNKNOWN = new Serializable() {
		public String toString() {
			return "<unknown>";
		}

		public Object readResolve() {
			return UNKNOWN;
		}
	};

	/**
	 * Constructs a new instance of BackrefPropertyAccessor.
	 *
	 * @param collectionRole The collection role which this back ref references.
	 * @param entityName The owner's entity name.
	 */
	public BackrefPropertyAccessor(String collectionRole, String entityName) {
		this.propertyName = collectionRole.substring( entityName.length() + 1 );
		this.entityName = entityName;

		this.setter = new BackrefSetter();
		this.getter = new BackrefGetter();
	}

	/**
	 * {@inheritDoc}
	 */
	public Setter getSetter(Class theClass, String propertyName) {
		return setter;
	}

	/**
	 * {@inheritDoc}
	 */
	public Getter getGetter(Class theClass, String propertyName) {
		return getter;
	}


	/**
	 * Internal implementation of a property setter specific to these back-ref properties.
	 */
	public static final class BackrefSetter implements Setter {

		/**
		 * {@inheritDoc}
		 */
		public Method getMethod() {
			return null;
		}

		/**
		 * {@inheritDoc}
		 */
		public String getMethodName() {
			return null;
		}

		/**
		 * {@inheritDoc}
		 */
		public void set(Object target, Object value, SessionFactoryImplementor factory) {
			// this page intentionally left blank :)
		}

	}


	/**
	 * Internal implementation of a property getter specific to these back-ref properties.
	 */
	public class BackrefGetter implements Getter {

		/**
		 * {@inheritDoc}
		 */
		public Object getForInsert(Object target, Map mergeMap, SessionImplementor session) {
			if ( session == null ) {
				return UNKNOWN;
			}
			else {
				return session.getPersistenceContext().getOwnerId( entityName, propertyName, target, mergeMap );
			}
		}

		/**
		 * {@inheritDoc}
		 */
		public Object get(Object target) {
			return UNKNOWN;
		}

		/**
		 * {@inheritDoc}
		 */
		public Method getMethod() {
			return null;
		}

		/**
		 * {@inheritDoc}
		 */
		public String getMethodName() {
			return null;
		}

		/**
		 * {@inheritDoc}
		 */
		public Class getReturnType() {
			return Object.class;
		}
	}
}