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The Java Persistence API (JPA) : a standard interface-based Java model abstraction of persistence, developed by the JCP.
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/*
 * Copyright (c) 2008, 2009, 2011 Oracle, Inc. All rights reserved.
 *
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Public License v1.0 and Eclipse Distribution License v. 1.0
 * which accompanies this distribution.  The Eclipse Public License is available
 * at http://www.eclipse.org/legal/epl-v10.html and the Eclipse Distribution License
 * is available at http://www.eclipse.org/org/documents/edl-v10.php.
 */
package javax.persistence;

import java.lang.annotation.Retention;
import java.lang.annotation.Target;

import static java.lang.annotation.ElementType.FIELD;
import static java.lang.annotation.ElementType.METHOD;
import static java.lang.annotation.ElementType.TYPE;
import static java.lang.annotation.RetentionPolicy.RUNTIME;

/**
 * Specifies a primary key column that is used as a foreign key to join to another table.
 * 
 * It is used to join the primary table of an entity subclass in the {@link InheritanceType#JOINED JOINED}
 * mapping strategy to the primary table of its superclass; it is used within a {@link SecondaryTable}
 * annotation to join a secondary table to a primary table; and it may be used in a {@link OneToOne} mapping
 * in which the primary key of the referencing entity is used as a foreign key to the referenced entity.
 * 

 * If no PrimaryKeyJoinColumn annotation is specified for a subclass in the JOINED
 * mapping strategy, the foreign key columns are assumed to have the same names as the primary key columns of
 * the primary table of the superclass.
 *
 * 
 *
 *    Example: Customer and ValuedCustomer subclass
 * 
 *    @Entity
 *    @Table(name="CUST")
 *    @Inheritance(strategy=JOINED)
 *    @DiscriminatorValue("CUST")
 *    public class Customer { ... }
 * 
 *    @Entity
 *    @Table(name="VCUST")
 *    @DiscriminatorValue("VCUST")
 *    @PrimaryKeyJoinColumn(name="CUST_ID")
 *    public class ValuedCustomer extends Customer { ... }
 * 
 * @see SecondaryTable
 * @see Inheritance
 * @see OneToOne
 * @since Java Persistence 1.0
 */
@Target({TYPE, METHOD, FIELD})
@Retention(RUNTIME)
public @interface PrimaryKeyJoinColumn
{

    /**
     * (Optional) The name of the primary key column of the current table. Defaults to the same name as the
     * primary key column of the primary table of the superclass (JOINED mapping strategy); the
     * same name as the primary key column of the primary table (SecondaryTable mapping); or the
     * same name as the primary key column for the table for the referencing entity (OneToOne
     * mapping).
     * @return name
     */
    String name() default "";

    /**
     * (Optional) The name of the primary key column of the table being joined to. Defaults to the same name
     * as the primary key column of the primary table of the superclass (JOINED mapping
     * strategy); the same name as the primary key column of the primary table (SecondaryTable
     * mapping); or the same name as the primary key column for the table for the referencing entity (
     * OneToOne mapping).
     * @return ref col name
     */
    String referencedColumnName() default "";

    /**
     * (Optional) The SQL fragment that is used when generating the DDL for the column. This should not be
     * specified for a OneToOne primary key association. Defaults to the generated SQL to create
     * a column of the inferred type.
     * @return column def
     */
    String columnDefinition() default "";

    /**
     * (Optional) The foreign key constraint specification for the join column. This is used only if table
     * generation is in effect. Default is provider defined.
     * @return The foreign key specification
     */
    ForeignKey foreignKey() default @ForeignKey();
}