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

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);

    }
}

可能在这种情况下，您使用merge逻辑获得了帐户对象，而persist用于持久化新对象，如果层次结构有一个已经持久化的对象，它将报错。在这种情况下，应该使用saveOrUpdate，而不是持久化。

此错误来自JPA生命周期。 要解决，不需要使用特定的装饰器。只需要像这样使用merge来连接实体:

entityManager.merge(transaction);

不要忘记正确设置你的getter和setter，这样你的两边都是同步的。

在我的情况下，我正在提交事务时，持久化方法被使用。 在更改persist to save方法时，该问题得到了解决。

通过在下一个对象之前保存依赖对象来解决。

这发生在我身上，因为我没有设置Id(这不是自动生成的)。并试图拯救@ManytoOne的关系