JPA n次查询问题优化

问题：使用 Spring Data JPA 定义 Repo find方法时，发现会执行了N+2条SQL 。 … Read More

JPA 继承例子

问题： Entity Car 有两类Entity BigCar 及Entity SmallCar；数据库中对… Read More

JPA Resolved [java.lang.IllegalArgumentException: Id must be assignable to Serializable: null]

问题： JPA Entity 如下

@Table (name= "EMP") 
@Inheritance
@DiscriminatorColumn (name="EMP TPE") 
public abstract class Employee {...}

@Entity
@DiscriminatorValue ("CEmp")
public class contractemployee extenas Employee{...}

@MappedSuperclass
public abstract class CompanyEmployee extends Employee {...}

@Entity
@DiscriminatorValue ("FTEmp")
public class FullTimeEmployee extenas companyemployee {....}

@Entity (name= "PTEmp")
public class PartTimeEmployee extends CompanyEmployee {....}

@Table (name= "EMP")

@Inheritance

@DiscriminatorColumn (name="EMP TPE")

public abstract class Employee {...}

@Entity

@DiscriminatorValue ("CEmp")

public class contractemployee extenas Employee{...}

@MappedSuperclass

public abstract class CompanyEmployee extends Employee {...}

@Entity

@DiscriminatorValue ("FTEmp")

public class FullTimeEmployee extenas companyemployee {....}

@Entity (name= "PTEmp")

public class PartTimeEmployee extends CompanyEmployee {....}

&nbs… Read More

JPA 同一个项目persistent.xml有多个PersistentUnit 时报错

问题： JPA 同一个项目persistent.xml有多个PersistentUnit 时报错 Fatal … Read More

JPA 如何设置生成列字段

问题： JPA 如何设置生成列？如last_update_date 字段： [crayon-67efd9e6… Read More

JPA 如何使用乐观锁

问题： JPA 如何使用乐观锁解决：表中增加字段 veraaa int 类型实体增加 @Versio… Read More

JPA 事务中的自增id

问题：代码如下，e1.getId() 不行

EntityManager em = ****
EntityTransaction trans = em.getTransaction();
trans.begin();
em.persistence(e1);
System.out.println(e1.getId());//not work e1. getId() is null
e2.referId=e1.getId();
em.persistence(e2);
trans.commit();

EntityManager em = ****

EntityTransaction trans = em.getTransaction();

trans.begin();

em.persistence(e1);

System.out.println(e1.getId());//not work e1. getId() is null

e2.referId=e1.getId();

em.persistence(e2);

trans.commit();

JPA 事务经常出现锁

问题： JPA 事务经常出现锁解决：应该是有事务没有 commit / rollback… Read More

Maven JPA Fatal error compiling: java.lang.RuntimeException: javax.annotation.processing.FilerException: Attempt to recreate a file for type ***

问题： mvn clean assembly:assembly 时报错： Maven JPA Fatal er… Read More

JPA 禁止缓存

问题：解决：方法一：entity 加 @cacheable(value=false) [cr… Read More

IT问题

程序员的疑难杂症录

JPA