Spring 为什么JPA Hibernate在持久化时无法为对象生成标识符？

考虑一下这个片段-

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:p="http://www.springframework.org/schema/p"
    xmlns:tx="http://www.springframework.org/schema/tx" xmlns:context="http://www.springframework.org/schema/context"
    xmlns:task="http://www.springframework.org/schema/task"
    xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-3.0.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.xsd
http://www.springframework.org/schema/task http://www.springframework.org/schema/task/spring-task-3.0.xsd">

    <description>nes dao context configuration.</description>

    <!-- JPA Configuration -->
    <tx:annotation-driven />

    <!-- UAT -->
    <bean id="dataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource">
        <property name="driverClass" value="org.postgresql.Driver" />
        <property name="jdbcUrl"
            value="jdbc:postgresql://localhost:5432/postgres" />
        <property name="properties">
            <props>
                <prop key="c3p0.acquire_increment">5</prop>
                <prop key="c3p0.maxStatementsPerConnection">20</prop>
                <prop key="c3p0.maxStatements ">100</prop>
                <prop key="c3p0.maxPoolSize">500</prop>
                <prop key="c3p0.max_statements">0</prop>
                <prop key="c3p0.minPoolSize">5</prop>
                <prop key="user">postgres</prop>
                <prop key="password">system</prop>
            </props>
        </property>
    </bean>

    <bean id="entityManagerFactory"
        class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean">
        <property name="dataSource" ref="dataSource" />
        <property name="jpaVendorAdapter">
            <bean class="org.springframework.orm.jpa.vendor.HibernateJpaVendorAdapter">
                <property name="showSql" value="true" />
                <!-- <property name="generateDdl" value="true" /> -->
                <property name="databasePlatform" value="org.hibernate.dialect.PostgreSQLDialect" />
            </bean>
        </property>
        <property name="persistenceUnitName" value="" />
        <property name="persistenceUnitManager">
            <bean
                class="org.springframework.orm.jpa.persistenceunit.DefaultPersistenceUnitManager">
                <property name="defaultDataSource" ref="dataSource" />
            </bean>
        </property>
    </bean>

    <bean id="sessionFactory"
        class="org.springframework.orm.hibernate3.annotation.AnnotationSessionFactoryBean"
        p:dataSource-ref="dataSource" />

    <bean id="transactionManager" class="org.springframework.orm.jpa.JpaTransactionManager">
        <property name="entityManagerFactory" ref="entityManagerFactory" />
    </bean>
    <bean
        class="org.springframework.orm.jpa.support.PersistenceAnnotationBeanPostProcessor">
    </bean>

</beans>

在日志中，它显示为-

org.postgresql.util.PSQLException:错误：列中的值为null “员工id”违反了非空约束

在PostgreSQL端创建的表为-

CREATE TABLE employee
(
  employee_id integer NOT NULL,
  employee_name text NOT NULL,
  employee_address text NOT NULL,
  designation text NOT NULL,
  CONSTRAINT employee_pkey PRIMARY KEY (employee_id)
)

---

请建议。

它在Postgresql端使用自定义的序列-

CREATE SEQUENCE employee_id_seq
  INCREMENT 1
  MINVALUE 1
  MAXVALUE 9223372036854775807
  START 1
  CACHE 1;

id列注释为-

@Id
@GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "employee_id_seq")
@SequenceGenerator(name = "employee_id_seq", sequenceName = "employee_id_seq", allocationSize = 1)
@Column(name = "EMPLOYEE_ID", unique = true, nullable = false)
public long id;

---

您应该在create table script

employee_id SERIAL

中定义，以便让dbms处理id生成。您可能需要将employee_id设置为串行类型或使用序列生成标识。Postgresql使用序列生成器，因此会出现使用JPA自动生成主键的问题。这可能是解决此问题的方法。如果执行

ALTER表employee ALTER列employee\u id SET DEFAULT nextval（'employee\u id\u seq'：：regclass）

，您仍然可以使用

strategy=GenerationType.IDENTITY。
CREATE SEQUENCE employee_id_seq
  INCREMENT 1
  MINVALUE 1
  MAXVALUE 9223372036854775807
  START 1
  CACHE 1;

@Id
@GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "employee_id_seq")
@SequenceGenerator(name = "employee_id_seq", sequenceName = "employee_id_seq", allocationSize = 1)
@Column(name = "EMPLOYEE_ID", unique = true, nullable = false)
public long id;