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

/**
 * Specifies the mapping for the key column of a map whose map key is a basic type. If the name
 * element is not specified, it defaults to the concatenation of the following: the name of the referencing
 * relationship field or property; "_"; "KEY".
 *
 *  *    Example:
 * 
 *    @Entity
 *    public class Item {
 *       @Id int id;
 *       ...
 *       @ElementCollection
 *       @MapKeyColumn(name="IMAGE_NAME")
 *       @Column(name="IMAGE_FILENAME")
 *       @CollectionTable(name="IMAGE_MAPPING")
 *       Map<String, String> images;  // map from image name to filename
 *       ...
 *    }
 * 
 * @since Java Persistence 2.0
 */
@Target({ANNOTATION_TYPE, METHOD, FIELD})
@Retention(RUNTIME)
public @interface MapKeyColumn
{
    /**
     * (Optional) The name of the map key column. The table in which it is found depends upon the context. If
     * the map key is for an element collection, the map key column is in the collection table for the map
     * value. If the map key is for a ManyToMany entity relationship or for a OneToMany entity relationship
     * using a join table, the map key column is in a join table. If the map key is for a OneToMany entity
     * relationship using a foreign key mapping strategy, the map key column is in the table of the entity
     * that is the value of the map.
     * 
     * Defaults to the concatenation of the following: the name of the referencing relationship field or
     * property; "_"; "KEY".
     * @return The name
     */
    String name() default "";

    /**
     * (Optional) Whether the column is a unique key. This is a shortcut for the UniqueConstraint
     * annotation at the table level and is useful for when the unique key constraint corresponds to only a
     * single column. This constraint applies in addition to any constraint entailed by primary key mapping
     * and to constraints specified at the table level.
     * @return Whether unique
     */
    boolean unique() default false;

    /** 
     * (Optional) Whether the database column is nullable.
     * @return Whether nullable
     */
    boolean nullable() default false;

    /**
     * (Optional) Whether the column is included in SQL INSERT statements generated by the persistence
     * provider.
     * @return Whether insertable
     */
    boolean insertable() default true;

    /**
     * (Optional) Whether the column is included in SQL UPDATE statements generated by the persistence
     * provider.
     * @return Whether updateable
     */
    boolean updatable() default true;

    /**
     * (Optional) The SQL fragment that is used when generating the DDL for the column.
     * 

     * Defaults to the generated SQL to create a column of the inferred type.
     * @return The column definition
     */
    String columnDefinition() default "";

    /**
     * (Optional) The name of the table that contains the column.
     * 

     * Defaults: If the map key is for an element collection, the name of the collection table for the map
     * value. If the map key is for a OneToMany or ManyToMany entity relationship using a join table, the name
     * of the join table for the map. If the map key is for a OneToMany entity relationship using a foreign
     * key mapping strategy, the name of the primary table of the entity that is the value of the map.
     * @return The table
     */
    String table() default "";

    /**
     * (Optional) The column length. (Applies only if a string-valued column is used.)
     * @return The length
     */
    int length() default 255;

    /**
     * (Optional) The precision for a decimal (exact numeric) column. (Applies only if a decimal column is
     * used.)
     * 
     * Default: 0. (The value must be set by the developer.)
     * @return The precision
     */
    int precision() default 0; // decimal precision

    /**
     * (Optional) The scale for a decimal (exact numeric) column. (Applies only if a decimal column is used.)
     * @return The scale
     */
    int scale() default 0; // decimal scale
}