@GeneratedValue源码解析

JPA要求每一个实体必须有且只有一个主键，而@GeneratedValue提供了主键的生成策略，这就是@GeneratedValue注解存在的意义。本文将浅析@GeneratedValue的源码。

@GeneratedValue的源码如下：

/*
 * 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;
import static javax.persistence.GenerationType.AUTO;

/**
 * Provides for the specification of generation strategies for the
 * values of primary keys.
 *
 * <p> The <code>GeneratedValue</code> annotation
 * may be applied to a primary key property or field of an entity or
 * mapped superclass in conjunction with the {@link Id} annotation.
 * The use of the <code>GeneratedValue</code> annotation is only
 * required to be supported for simple primary keys.  Use of the
 * <code>GeneratedValue</code> annotation is not supported for derived
 * primary keys.
 *
 * <pre>
 *
 *     Example 1:
 *
 *     @Id
 *     @GeneratedValue(strategy=SEQUENCE, generator="CUST_SEQ")
 *     @Column(name="CUST_ID")
 *     public Long getId() { return id; }
 *
 *     Example 2:
 *
 *     @Id
 *     @GeneratedValue(strategy=TABLE, generator="CUST_GEN")
 *     @Column(name="CUST_ID")
 *     Long id;
 * </pre>
 *
 * @see Id
 * @see TableGenerator
 * @see SequenceGenerator
 *
 * @since Java Persistence 1.0
 */
@Target({METHOD, FIELD})
@Retention(RUNTIME)

public @interface GeneratedValue {

    /**
     * (Optional) The primary key generation strategy
     * that the persistence provider must use to
     * generate the annotated entity primary key.
     */
    GenerationType strategy() default AUTO;

    /**
     * (Optional) The name of the primary key generator
     * to use as specified in the {@link SequenceGenerator}
     * or {@link TableGenerator} annotation.
     * <p> Defaults to the id generator supplied by persistence provider.
     */
    String generator() default "";
}

@Target({METHOD, FIELD})说明该注解可以用于方法声明和域声明。

@Retention(RUNTIME)说明该注解在VM运行期间也可以保留，可以通过反射机制读取注解的信息。

如果不是很清楚，可参考@Controller和@Restcontroller源码解析中有对元注解的介绍。

@GeneratedValue注解有两个属性,分别是strategy和generator,其中generator属性的值是一个字符串,默认为"",其声明了主键生成器的名称。

查看GenerationType的源码，可以看到定义了四种主键生成策略：TABLE，SEQUENCE，IDENTITY，AUTO。

/*
 * 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;

/**
 * Defines the types of primary key generation strategies.
 *
 * @see GeneratedValue
 *
 * @since Java Persistence 1.0
 */
public enum GenerationType {

    /**
     * Indicates that the persistence provider must assign
     * primary keys for the entity using an underlying
     * database table to ensure uniqueness.
     */
    TABLE,

    /**
     * Indicates that the persistence provider must assign
     * primary keys for the entity using a database sequence.
     */
    SEQUENCE,

    /**
     * Indicates that the persistence provider must assign
     * primary keys for the entity using a database identity column.
     */
    IDENTITY,

    /**
     * Indicates that the persistence provider should pick an
     * appropriate strategy for the particular database. The
     * <code>AUTO</code> generation strategy may expect a database
     * resource to exist, or it may attempt to create one. A vendor
     * may provide documentation on how to create such resources
     * in the event that it does not support schema generation
     * or cannot create the schema resource at runtime.
     */
    AUTO
}

TABLE

使用数据库表来生成主键。该策略一般与另外一个注解一起使用@TableGenerator,@TableGenerator注解指定了生成主键的表(可以在实体类上指定也可以在主键字段或属性上指定),然后JPA将会根据注解内容自动生成一张表作为序列表(或使用现有的序列表)。如果不指定序列表,则会生成一张默认的序列表,表中的列名也是自动生成,数据库上会生成一张名为sequence的表(SEQ_NAME,SEQ_COUNT)。序列表一般只包含两个字段:第一个字段是该生成策略的名称,第二个字段是该关系表的最大序号,它会随着数据的插入逐渐累加。该策略的好处就是不依赖于外部环境和数据库的具体实现,便于移植；但由于其不能充分利用数据库的特性,所以不会优先使用。

    @Id
    @GeneratedValue(strategy = GenerationType.TABLE, generator = "roleSeq")
    @TableGenerator(name = "roleSeq", allocationSize = 1, table = "seq_table", pkColumnName = "seq_id", valueColumnName = "seq_count")
    private Long id;

SEQUENCE

使用序列来生成主键。 该策略的不足之处正好与TABLE相反,由于只有部分数据库(Oracle,PostgreSQL,DB2)支持序列对象,所以该策略一般不应用于其他数据库。需要注意，MySQL不支持这种主键生成策略。该策略一般与另外一个注解一起使用@SequenceGenerator,@SequenceGenerator注解指定了生成主键的序列.然后JPA会根据注解内容创建一个序列(或使用一个现有的序列)。如果不指定序列,则会自动生成一个序列SEQ_GEN_SEQUENCE。

    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "menuSeq")
    @SequenceGenerator(name = "menuSeq", initialValue = 1, allocationSize = 1, sequenceName = "MENU_SEQUENCE")
    private Long id;

IDENTITY

使用数据库ID自增长的方式来生成主键。 该策略在大部分数据库中都提供了支持(指定方法或关键字可能不同),但还是有少数数据库（Oracle）不支持,所以可移植性略差。

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

AUTO

自动选择合适的主键生成策略。该策略比较常用，也是默认选项，@GeneratedValue(strategy = GenerationType.AUTO)相当于@GeneratedValue。

    @Id
    @GeneratedValue
    private Long id;

参考：JPA之@GeneratedValue注解。这篇介绍的挺详细的，推荐一下。