JPA结合Hibernate定义主键

实际工作中遇到类似这样的问题，问题如下面StackOverflow中描述的

https://stackoverflow.com/questions/43424203/org-hibernate-annotationexception-unknown-id-generator-id-generator

1. 下面是更改过后的代码（员工信息表对应的实体类），注意id的定义：

import lombok.AllArgsConstructor;import lombok.Data;import lombok.NoArgsConstructor;import org.hibernate.annotations.GenericGenerator;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.Id;/** * <p> * <code>Employee</code> * </p> * Description: * * @author Mcchu * @date 2017/10/13 14:54 */@Entity@Data@AllArgsConstructor@NoArgsConstructorpublic class Employee {    @GenericGenerator(name = "employeeSeqGenerator",            strategy = "org.hibernate.id.enhanced.SequenceStyleGenerator",            parameters = {                    @org.hibernate.annotations.Parameter(name = "sequence_name", value = "EMPLOYEE_SEQUENCE"),                    @org.hibernate.annotations.Parameter(name = "initial_value", value = "1000"),                    @org.hibernate.annotations.Parameter(name = "increment_size", value = "1")            })    @Id    @GeneratedValue(generator = "employeeSeqGenerator")    private Long id;    private String employeeName;    //... ...}

2. 注解说明：

① @Id注解：JPA提供

顾名思义，该属性字段定义为主键，注意在源码注释中有讲只有下面几种类型才可以定义为主键

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;/** * Specifies the primary key of an entity. * The field or property to which the <code>Id</code> annotation is applied * should be one of the following types: any Java primitive type; * any primitive wrapper type; * <code>String</code>; * <code>java.util.Date</code>; * <code>java.sql.Date</code>; * <code>java.math.BigDecimal</code>; * <code>java.math.BigInteger</code>. * * <p>The mapped column for the primary key of the entity is assumed * to be the primary key of the primary table. If no <code>Column</code> annotation * is specified, the primary key column name is assumed to be the name * of the primary key property or field. * * <pre> *   Example: * *   @Id *   public Long getId() { return id; } * </pre> * * @see Column * @see GeneratedValue * * @since Java Persistence 1.0 */@Target({METHOD, FIELD})@Retention(RUNTIME)public @interface Id {}

@GeneratedValue注解：JPA提供

文档参考：http://docs.oracle.com/javaee/5/api/javax/persistence/GeneratedValue.html

源码定义如下：

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 "";}

如上，包含两个属性，GenerationType和generator，其中GenerationType默认是AUTO

  GenerationType几个属性的定义可以查看文档：http://docs.oracle.com/javaee/5/api/javax/persistence/GenerationType.html

  generator自己定义

四种数据库的对GeneratorValue4种策略的支持情况如下：来自http://blog.csdn.net/tianxiezuomaikong/article/details/64930151mysql GenerationType.TABLE GenerationType.AUTO GenerationType.IDENTITY 不支持GenerationType.SEQUENCEoracle strategy=GenerationType.AUTO GenerationType.SEQUENCE GenerationType.TABLE 不支持GenerationType.IDENTITYpostgreSQL GenerationType.TABLE GenerationType.AUTO GenerationType.IDENTITY GenerationType.SEQUENCE 都支持kingbase GenerationType.TABLE GenerationType.SEQUENCE GenerationType.IDENTITY GenerationType.AUTO 都支持

@GenericGenerator注解：Hibernate提供

源码：

package org.hibernate.annotations;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.ElementType.PACKAGE;import static java.lang.annotation.ElementType.TYPE;import static java.lang.annotation.RetentionPolicy.RUNTIME;/** * Generator annotation describing any kind of Hibernate generator in a generic (de-typed) manner. * * @author Emmanuel Bernard */@Target({PACKAGE, TYPE, METHOD, FIELD})@Retention(RUNTIME)public @interface GenericGenerator {/** * unique generator name. */String name();/** * Generator strategy either a predefined Hibernate strategy or a fully qualified class name. */String strategy();/** * Optional generator parameters. */Parameter[] parameters() default {};}

参考文档：

http://docs.jboss.org/hibernate/annotations/3.4/api/org/hibernate/annotations/GenericGenerator.html
http://blog.csdn.net/tianxiezuomaikong/article/details/64930151  13种策略