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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;

/**
 * Is used to specify the mapped column for a persistent property or field. If no Column
 * annotation is specified, the default values apply. 
 * 
 *  *    Example 1:
 * 
 *    @Column(name="DESC", nullable=false, length=512)
 *    public String getDescription() { return description; }
 * 
 *    Example 2:
 * 
 *    @Column(name="DESC",
 *            columnDefinition="CLOB NOT NULL",
 *            table="EMP_DETAIL")
 *    @Lob
 *    public String getDescription() { return description; }
 * 
 *    Example 3:
 * 
 *    @Column(name="ORDER_COST", updatable=false, precision=12, scale=2)
 *    public BigDecimal getCost() { return cost; }
 *
 * 
 * 
 * 
 * @since Java Persistence 1.0
 */
@Target({ANNOTATION_TYPE, METHOD, FIELD})
@Retention(RUNTIME)
public @interface Column
{

    /**
     * (Optional) The name of the column. Defaults to the property or field name.
     * @return 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 this is unique
     */
    boolean unique() default false;

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

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

    /**
     * (Optional) Whether the column is included in SQL UPDATE statements generated by the persistence
     * provider.
     * @return whether this is 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 column def
     */
    String columnDefinition() default "";

    /**
     * (Optional) The name of the table that contains the column. If absent the column is assumed to be in the
     * primary table.
     * @return table
     */
    String table() default "";

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

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

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