@Column注解
用来标识实体类中属性与数据表中字段的对应关系
（1）源码：
/*
* 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.RetentionPolicy.RUNTIME;
/**
* Is used to specify the mapped column for a persistent property or field.
* If no <code>Column</code> annotation is specified, the default values apply.
*
* <blockquote><pre>
* 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; }
*
* </pre></blockquote>
*
*
* @since Java Persistence 1.0
*/
@Target({METHOD, FIELD})
@Retention(RUNTIME)
public @interface Column {
/**
* (Optional) The name of the column. Defaults to
* the property or field name.
*/
String name() default "";
/**
* (Optional) Whether the column is a unique key. This is a
* shortcut for the <code>UniqueConstraint</code> 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.
*/
boolean unique() default false;
/**
* (Optional) Whether the database column is nullable.
*/
boolean nullable() default true;
/**
* (Optional) Whether the column is included in SQL INSERT
* statements generated by the persistence provider.
*/
boolean insertable() default true;
/**
* (Optional) Whether the column is included in SQL UPDATE
* statements generated by the persistence provider.
*/
boolean updatable() default true;
/**
* (Optional) The SQL fragment that is used when
* generating the DDL for the column.
* <p> Defaults to the generated SQL to create a
* column of the inferred type.
*/
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.
*/
String table() default "";
/**
* (Optional) The column length. (Applies only if a
* string-valued column is used.)
*/
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.
*/
int precision() default 0;
/**
* (Optional) The scale for a decimal (exact numeric) column.
* (Applies only if a decimal column is used.)
*/
int scale() default 0;
}
（2）@Column属性详解：
name
定义了被标注字段在数据库表中所对应字段的名称；
unique
表示该字段是否为唯一标识，默认为false。如果表中有一个字段需要唯一标识，则既可以使用该标记，也可以使用@Table标记中的@UniqueConstraint。
nullable
表示该字段是否可以为null值，默认为true。
insertable
表示在使用“INSERT”脚本插入数据时，是否需要插入该字段的值。
updatable
表示在使用“UPDATE”脚本插入数据时，是否需要更新该字段的值。insertable和updatable属性一般多用于只读的属性，例如主键和外键等。这些字段的值通常是自动生成的。
columnDefinition（大多数情况，几乎不用）
表示创建表时，该字段创建的SQL语句，一般用于通过Entity生成表定义时使用。（也就是说，如果DB中表已经建好，该属性没有必要使用。）
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table
表示当映射多个表时，指定表的表中的字段。默认值为主表的表名。
length
表示字段的长度，当字段的类型为varchar时，该属性才有效，默认为255个字符。
precision和scale
precision属性和scale属性表示精度，当字段类型为double时，precision表示数值的总长度，scale表示小数点所占的位数。
（3）
@Column可以标注在属性前或getter方法前；
Ⅰ.@Column标注在属性前（建议使用这一种方式）
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Table;
/**
* 一卡通消费记录表
* @author Qian
*
*/
@Entity
@Table(name = "pb_op_card_consume")
public class CardConsume {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "id", length = 20, nullable = false)
private int id;
/**
* 交易编号
*/
@Column(name = "tradeNo" ,length = 50 , nullable = false)
private String tradeNo;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getTradeNo() {
return tradeNo;
}
public void setTradeNo(String tradeNo) {
this.tradeNo = tradeNo;
}
}
Ⅱ.@Column标注getter方法前
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Table;
/**
* 一卡通消费记录表
* @author Qian
*
*/
@Entity
@Table(name = "pb_op_card_consume")
public class CardConsume {
/**
* 交易编号
*/
private String tradeNo;
@Column(name = "tradeNo" ,length = 50 , nullable = false)
public String getTradeNo() {
return tradeNo;
}
public void setTradeNo(String tradeNo) {
this.tradeNo = tradeNo;
}
}
提示：JPA规范中并没有明确指定那种标注方法，只要两种标注方式任选其一都可以。
（4）示例（其中3、4不常用）
Example 1: 指定字段“tradeNo”交易编号的长度为50，且值不能为null
@Column(name = "tradeNo", length = 50, nullable = false) private String tradeNo;
Example 2: 指定字段“totalAmount”交易金额的精度（长度）为10，小数点位数为2位，且值不能为null
@Column(name = "totalAmount", precision = 10, scale = 2, nullable = false) private BigDecimal totalAmount;
Example 3: 字段“text”，指定建表时SQL语句 如“varchar(50) NOT NULL”
@Column(name = "text", columnDefinition = "varchar(50) not null")
private String text;
等同于SQL
CREATE TABLE [dbo].[my_test] (
[id] int NOT NULL IDENTITY(1,1) ,
[text] varchar(50) NOT NULL
)
columnDefinition，若不指定该属性，通常使用默认的类型建表，若此时需要自定义建表的类型时，可在该属性中设置。
Example 4: 字段值为只读的，不允许插入和修改。通常用于主键和外键
@Column(name = "id", insertable = false, updatable = false) private Integer id;