移除子关联级联

因此，您需要删除@CascadeType。这些都来自@ManyToOne协会。子实体不应该级联到父关联。只有父实体应该级联到子实体。

@ManyToOne(fetch= FetchType.LAZY)

注意，我将fetch属性设置为FetchType。LAZY，因为急切抓取对性能非常不利。

设置关联的双方

当你有一个双向关联时，你需要在父实体中使用addChild和removecchild方法来同步双方:

public void addTransaction(Transaction transaction) {
    transcations.add(transaction);
    transaction.setAccount(this);
}

public void removeTransaction(Transaction transaction) {
    transcations.remove(transaction);
    transaction.setAccount(null);
}

使用合并是有风险和棘手的，所以在您的情况下，这是一种肮脏的变通方法。您至少需要记住，当您将一个实体对象传递给merge时，它将停止附加到事务，而是返回一个新的、现在已附加的实体。这意味着如果任何人仍然拥有旧的实体对象，那么对它的更改将被无声地忽略并在提交时丢弃。

You are not showing the complete code here, so I cannot double-check your transaction pattern. One way to get to a situation like this is if you don't have a transaction active when executing the merge and persist. In that case persistence provider is expected to open a new transaction for every JPA operation you perform and immediately commit and close it before the call returns. If this is the case, the merge would be run in a first transaction and then after the merge method returns, the transaction is completed and closed and the returned entity is now detached. The persist below it would then open a second transaction, and trying to refer to an entity that is detached, giving an exception. Always wrap your code inside a transaction unless you know very well what you are doing.

使用容器管理的事务，它看起来像这样。注意:这假设方法在会话bean中，并通过本地或远程接口调用。

@TransactionAttribute(TransactionAttributeType.REQUIRED)
public void storeAccount(Account account) {
    ...

    if (account.getId()!=null) {
        account = entityManager.merge(account);
    }

    Transaction transaction = new Transaction(account,"other stuff");

    entityManager.persist(account);
}

这里的问题是缺乏控制。

当我们使用CrudRepository/JPARepository保存方法时，我们失去了事务控制。

为了解决这个问题，我们有了事务管理

我更喜欢@Transactional机制

进口

import javax.transaction.Transactional;

完整源代码:

package com.oracle.dto;

import lombok.*;

import javax.persistence.*;
import java.util.Date;
import java.util.List;

@Entity
@Data
@ToString(exclude = {"employee"})
@EqualsAndHashCode(exclude = {"employee"})
public class Project {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO,generator = "ps")
    @SequenceGenerator(name = "ps",sequenceName = "project_seq",initialValue = 1000,allocationSize = 1)
    @Setter(AccessLevel.NONE)
    @Column(name = "project_id",updatable = false,nullable = false)
    private Integer pId;
    @Column(name="project_name",nullable = false,updatable = true)
    private String projectName;
    @Column(name="team_size",nullable = true,updatable = true)
    private Integer teamSize;
    @Column(name="start_date")
    private Date startDate;
    @ManyToMany(cascade = CascadeType.ALL)
    @JoinTable(name="projectemp_join_table",
        joinColumns = {@JoinColumn(name = "project_id")},
        inverseJoinColumns = {@JoinColumn(name="emp_id")}
    )
    private List<Employee> employees;
}

package com.oracle.dto;

import lombok.*;

import javax.persistence.*;
import java.util.List;

@Entity
@Data
@EqualsAndHashCode(exclude = {"projects"})
@ToString(exclude = {"projects"})
public class Employee {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO,generator = "es")
    @SequenceGenerator(name = "es",sequenceName = "emp_seq",allocationSize = 1,initialValue = 2000)
    @Setter(AccessLevel.NONE)
    @Column(name = "emp_id",nullable = false,updatable = false)
    private Integer eId;
    @Column(name="fist_name")
    private String firstName;
    @Column(name="last_name")
    private String lastName;
    @ManyToMany(mappedBy = "employees")
    private List<Project> projects;
}

package com.oracle.repo;

import com.oracle.dto.Employee;
import org.springframework.data.jpa.repository.JpaRepository;

public interface EmployeeRepo extends JpaRepository<Employee,Integer> {
}

package com.oracle.repo;

import com.oracle.dto.Project;
import org.springframework.data.jpa.repository.JpaRepository;

public interface ProjectRepo extends JpaRepository<Project,Integer> {
}

package com.oracle.services;

import com.oracle.dto.Employee;
import com.oracle.dto.Project;
import com.oracle.repo.ProjectRepo;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;

import javax.transaction.Transactional;
import java.util.Date;
import java.util.LinkedList;
import java.util.List;

@Component
public class DBServices {
    @Autowired
    private ProjectRepo repo;
    @Transactional
    public void performActivity(){

        Project p1 = new Project();
        p1.setProjectName("Bank 2");
        p1.setTeamSize(20);
        p1.setStartDate(new Date(2020, 12, 22));

        Project p2 = new Project();
        p2.setProjectName("Bank 1");
        p2.setTeamSize(21);
        p2.setStartDate(new Date(2020, 12, 22));

        Project p3 = new Project();
        p3.setProjectName("Customs");
        p3.setTeamSize(11);
        p3.setStartDate(new Date(2010, 11, 20));

        Employee e1 = new Employee();
        e1.setFirstName("Pratik");
        e1.setLastName("Gaurav");

        Employee e2 = new Employee();
        e2.setFirstName("Ankita");
        e2.setLastName("Noopur");

        Employee e3 = new Employee();
        e3.setFirstName("Rudra");
        e3.setLastName("Narayan");

        List<Employee> empList1 = new LinkedList<Employee>();
        empList1.add(e2);
        empList1.add(e3);

        List<Employee> empList2 = new LinkedList<Employee>();
        empList2.add(e1);
        empList2.add(e2);

        List<Project> pl1=new LinkedList<Project>();
        pl1.add(p1);
        pl1.add(p2);

        List<Project> pl2=new LinkedList<Project>();
        pl2.add(p2);pl2.add(p3);

        p1.setEmployees(empList1);
        p2.setEmployees(empList2);

        e1.setProjects(pl1);
        e2.setProjects(pl2);

        repo.save(p1);
        repo.save(p2);
        repo.save(p3);

    }
}

这是一个典型的双向一致性问题。在这个链接和这个链接中都有很好的讨论。

根据前两个链接中的文章，您需要在双向关系的两侧修复您的setter。一方的示例setter在此链接中。

在此链接中有一个用于多方的示例setter。

在你纠正你的setter之后，你想要声明实体访问类型为“属性”。声明“Property”访问类型的最佳实践是将所有注释从成员属性移动到相应的getter。一个重要的警告是不要在实体类中混合使用“Field”和“Property”访问类型，否则JSR-317规范没有定义行为。

这是我的药。

Below is my Entity. Mark that the id is annotated with @GeneratedValue(strategy = GenerationType.AUTO), which means that the id would be generated by the Hibernate. Don't set it when entity object is created. As that will be auto generated by the Hibernate. Mind you if the entity id field is not marked with @GeneratedValue then not assigning the id a value manually is also a crime, which will be greeted with IdentifierGenerationException: ids for this class must be manually assigned before calling save()

@Entity
@Data
@NamedQuery(name = "SimpleObject.findAll", query="Select s FROM SimpleObject s")
public class SimpleObject {

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

    @Column
    private String key;

    @Column
    private String value;

}

这是我的主类。

public class SimpleObjectMain {

    public static void main(String[] args) {

        System.out.println("Hello Hello From SimpleObjectMain");

        SimpleObject simpleObject = new SimpleObject();
        simpleObject.setId(420L); // Not right, when id is a generated value then no need to set this.
        simpleObject.setKey("Friend");
        simpleObject.setValue("Bani");

        EntityManager entityManager = EntityManagerUtil.getEntityManager();
        entityManager.getTransaction().begin();
        entityManager.persist(simpleObject);
        entityManager.getTransaction().commit();

        List<SimpleObject> simpleObjectList = entityManager.createNamedQuery("SimpleObject.findAll").getResultList();
        for(SimpleObject simple : simpleObjectList){
            System.out.println(simple);
        }

        entityManager.close();
        
    }
}

我想救它的时候，它把它扔出去了

PersistentObjectException: detached entity passed to persist.

我所需要修复的是删除主方法中simpleObject的id设置行。

这是一个老问题，但最近又遇到了同样的问题。在这里分享我的经验。

实体

@Data
@Entity
@Table(name = "COURSE")
public class Course  {

    @Id
    @GeneratedValue
    private Long id;
}

保存实体(JUnit)

Course course = new Course(10L, "testcourse", "DummyCourse");
testEntityManager.persist(course);

Fix

Course course = new Course(null, "testcourse", "DummyCourse");
testEntityManager.persist(course);

结论:如果实体类的主键(id)有@GeneratedValue，那么确保您没有传递主键(id)的值