Spring Hibernate：尽管被配置，@Transactional在Spring Data JPA中不起作用

问题描述

我知道使用Eager获取类型可以正常工作，但这将是一个头痛和性能滞后于应用程序，所以，我必须继续考虑Lazy fetch类型。

我读到使用Spring Data JPA时，

您有多个选择：
$ b

1. 访问交易中的关系


2. 使用EAGER提取


3. 对 FETCH JOIN使用自定义查询


4. 使用投影

另请阅读我的文章以了解更多详情：

I have been receiving this error all the time when using Lazy evaluation in a ManyToMany relationship:

I know that using Eager fetch type will work fine, but this will be a headache and a performance lag to the application, so, I have to continue considering the Lazy fetch type.

I read that when using Spring Data JPA, @Transactional will initialize the lazy relations, but it doesn't seem to take effect with me.

Here is how my app look like:

Entities

Course Entity

@Entity
@Table(name = "course")
@Transactional
public class Course {


@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Integer id;

@Column(name = "course_name")
private String courseName;

@Column(name = "course_description")
private String courseDescription;

@ManyToMany(cascade = {CascadeType.MERGE,CascadeType.DETACH,CascadeType.REFRESH,CascadeType.PERSIST})
@JoinTable(
        name = "course_student",
        joinColumns = @JoinColumn(name = "course_id"),
        inverseJoinColumns = @JoinColumn(name = "student_id") )
private List<Student> students;

Student Entity

@Entity
@Table(name = "students")
public class Student {

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Integer id;

private String name;

private String grade;

@ManyToMany(cascade = {CascadeType.MERGE,CascadeType.DETACH,CascadeType.REFRESH,CascadeType.PERSIST},fetch = FetchType.EAGER)
@JoinTable(
        name = "course_student",
        joinColumns = @JoinColumn(name = "student_id"),
        inverseJoinColumns = @JoinColumn(name = "course_id") )
List<Course> courses;

Controller

@Controller
@RequestMapping("/courses")
@Transactional
public class CourseController {

....

@GetMapping("get-joined-students")
ModelAndView getJoined() {

    Course course = courseRepo.findById(6).get();

    for (Student student :
            course.getStudents()) {

        System.out.println("Student: " + student);
    }


    ModelAndView modelAndView = new ModelAndView("redirect:list");

    return modelAndView;
}

}

Repositories

Course Repository

@Repository
public interface CourseRepo extends JpaRepository<Course,Integer> {}

Student Repository

@Repository
public interface StudentRepo extends JpaRepository<Student,Integer> { }

Servlet Configuration File

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"  ....    >

<context:annotation-config/>

<context:component-scan base-package="com.company.springdemo"/>

<mvc:annotation-driven/>

....

<jpa:repositories transaction-manager-ref="transactionManagerJPA" base-package="com.company.springdemo" entity-manager-factory-ref="EMF" />

<bean id="transactionManagerJPA" class="org.springframework.orm.jpa.JpaTransactionManager" >
    <property name="dataSource" ref="myDataSource" />
</bean>

<tx:annotation-driven transaction-manager="transactionManagerJPA" />


<bean id="EMF"
      class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean">
    <property name="packagesToScan" value="com.company.springdemo.entity" />
    <property name="dataSource" ref="myDataSource" />

    <property name="jpaProperties">
        <props>
            <prop key="hibernate.show_sql">true</prop>
            <prop key="hibernate.hbm2ddl.auto">update</prop>
            <prop key="hibernate.dialect">org.hibernate.dialect.MySQL57Dialect</prop>
            <prop key="hibernate.show_sql">false</prop>
            <prop key="hibernate.format_sql">true</prop>
        </props>
    </property>

    <property name="persistenceProvider">
        <bean class="org.hibernate.jpa.HibernatePersistenceProvider" />
    </property>
</bean>

What potentially could I be doing wrong?

解决方案

I think you are mixing up things here. LazyInitializationException is popping up as you are trying to fetch the relation when the transaction is already over.

You have multiple choices:

1. Access the relation within the transaction
2. Use EAGER fetching
3. Use custom queries with FETCH JOIN
4. Use Projections

Also read my article for more details: https://blog.arnoldgalovics.com/2017/02/27/lazyinitializationexception-demystified/https://blog.arnoldgalovics.com/2017/03/14/using-projections-in-your-data-access-layer/

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