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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.TYPE;
import static java.lang.annotation.RetentionPolicy.RUNTIME;
import static javax.persistence.DiscriminatorType.STRING;

/**
 * Specifies the discriminator column for the SINGLE_TABLE and JOINED
 * {@link Inheritance} mapping strategies.
 * 
 * The strategy and the discriminator column are only specified in the root of an entity class hierarchy or
 * subhierarchy in which a different inheritance strategy is applied
 * 

 * If the DiscriminatorColumn annotation is missing, and a discriminator column is required, the
 * name of the discriminator column defaults to "DTYPE" and the discriminator type to
 * {@link DiscriminatorType#STRING DiscriminatorType.STRING}.
 *
 * 
 *     Example:
 * 
 *     @Entity
 *     @Table(name="CUST")
 *     @Inheritance(strategy=SINGLE_TABLE)
 *     @DiscriminatorColumn(name="DISC", discriminatorType=STRING, length=20)
 *     public class Customer { ... }
 * 
 *     @Entity
 *     public class ValuedCustomer extends Customer { ... }
 * 
 * @see DiscriminatorValue
 * @since Java Persistence 1.0
 */
@Target({TYPE})
@Retention(RUNTIME)
public @interface DiscriminatorColumn
{

    /**
     * (Optional) The name of column to be used for the discriminator.
     * @return col name
     */
    String name() default "DTYPE";

    /**
     * (Optional) The type of object/column to use as a class discriminator. Defaults to
     * {@link DiscriminatorType#STRING DiscriminatorType.STRING}.
     * @return discrim type
     */
    DiscriminatorType discriminatorType() default STRING;

    /**
     * (Optional) The SQL fragment that is used when generating the DDL for the discriminator column.
     * 
     * Defaults to the provider-generated SQL to create a column of the specified discriminator type.
     * @return col def
     */
    String columnDefinition() default "";

    /**
     * (Optional) The column length for String-based discriminator types. Ignored for other discriminator
     * types.
     * @return length
     */
    int length() default 31;
}