Java JPA插入错误

我正在学习JPA，我有一个问题。 我有以下实体：

雇员：

@Entity
@Table(name="employees")
@NamedQuery(name="Employee.findAll", query="SELECT e FROM Employee e")

public class Employee implements Serializable {

 @Id
 @Column(name="emp_no")
 private int empNo;

 @Temporal(TemporalType.DATE)
 @Column(name="birth_date")
 private Date birthDate;

 @Column(name="first_name")
 private String firstName;

 private String gender;

 @Temporal(TemporalType.DATE)
 @Column(name="hire_date")
 private Date hireDate;

 @Column(name="last_name")
 private String lastName;

 //bi-directional many-to-one association to DeptEmp
 @OneToMany(mappedBy="employee")
 private List<DeptEmp> deptEmps;

 public Employee() {
 }

 public int getEmpNo() {
    return this.empNo;
 }

 public void setEmpNo(int empNo) {
    this.empNo = empNo;
 }

 public Date getBirthDate() {
    return this.birthDate;
 }

 public void setBirthDate(Date birthDate) {
    this.birthDate = birthDate;
 }

 public String getFirstName() {
    return this.firstName;
 }

 public void setFirstName(String firstName) {
    this.firstName = firstName;
 }

 public String getGender() {
    return this.gender;
 }

 public void setGender(String gender) {
    this.gender = gender;
 }

 public Date getHireDate() {
    return this.hireDate;
 }

 public void setHireDate(Date hireDate) {
    this.hireDate = hireDate;
 }

 public String getLastName() {
    return this.lastName;
 }

 public void setLastName(String lastName) {
    this.lastName = lastName;
 }

 public List<DeptEmp> getDeptEmps() {
    return this.deptEmps;
 }

 public void setDeptEmps(List<DeptEmp> deptEmps) {
    this.deptEmps = deptEmps;
 }

 public DeptEmp addDeptEmp(DeptEmp deptEmp) {
    getDeptEmps().add(deptEmp);
    deptEmp.setEmployee(this);

    return deptEmp;
 }

 public DeptEmp removeDeptEmp(DeptEmp deptEmp) {
    getDeptEmps().remove(deptEmp);
    deptEmp.setEmployee(null);

    return deptEmp;
 }

这是我尝试在交叉表dept\u emp中插入员工和记录的代码：

    EntityManagerFactory fact=Persistence.createEntityManagerFactory("EmployeeJPA");
    EntityManager man=fact.createEntityManager();
    man.getTransaction().begin();
    Employee emp=new Employee();
    emp.setFirstName(request.getParameter("name"));
    emp.setLastName(request.getParameter("surname"));
    emp.setEmpNo(Integer.parseInt(request.getParameter("id")));
    emp.setBirthDate(Date.valueOf(request.getParameter("birth")));
    emp.setBirthDate(Date.valueOf(request.getParameter("hire")));
    emp.setGender(request.getParameter("gender"));
    HttpSession session=request.getSession();
    Department dept=man.find(Department.class, session.getAttribute("idDep"));
    man.persist(emp);
    man.getTransaction().commit();
    DeptEmp depEm=new DeptEmp();
    depEm.setEmployee(emp);
    depEm.setDepartment(dept);
    man.getTransaction().begin();
    man.persist(depEm);
    man.getTransaction().commit();
    man.close();
    fact.close();

这就是错误：

GRAVE: Servlet.service() for servlet [servlet.AddEmployeeServlet] in
context with path [/EmployeeJPA] threw exception
javax.persistence.RollbackException: Exception [EclipseLink-4002] (Eclipse 
Persistence Services - 2.5.0.v20130507-3faac2b):
org.eclipse.persistence.exceptions.DatabaseException
Internal Exception: 
com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Column 'dept_no' cannot be null Error Code: 1048 Call: INSERT INTO dept_emp (from_date, to_date, dept_no, emp_no) VALUES (?, ?, ?, ?)
bind => [null, null, null, null]

我不能理解，因为我有这个错误。我是否以不同的方式插入交叉表？ 有人能帮我吗？ 谢谢

看起来像一条线 Department dept=man.find（Department.class，session.getAttribute（“idDep”））； 返回NULL

---

但是在db表中，dept_no列上没有null约束。

不确定这是否是问题的真正原因，但是

Department

是在不同的事务中检索的，而

deep

实体是持久的。这通常会导致一些问题，因此可能需要开始将代码排序为以下内容：

HttpSession session = request.getSession();
Object deptId = session.getAttribute("idDep");

EntityManagerFactory entityManagerFactory = Persistence.createEntityManagerFactory("EmployeeJPA");
EntityManager entityManager = entityManagerFactory.createEntityManager();
entityManager.getTransaction().begin();

Employee emp = new Employee();
// fill employee's fields 
entityManager.persist(emp);

Department dept = entityManager.find(Department.class, deptId);

DeptEmp depEm = new DeptEmp();
depEm.setEmployee(emp);
depEm.setDepartment(dept);
entityManager.persist(depEm);

entityManager.getTransaction().commit();
entityManager.close();
entityManagerFactory.close();

您需要执行的三个操作应在同一事务的上下文中：

坚持新员工

找到部门

坚持员工与部门的关系

我检查了它，find（）返回正确的实体，事实上我无法理解，因为它不起作用

GRAVE: Servlet.service() for servlet [servlet.AddEmployeeServlet] in
context with path [/EmployeeJPA] threw exception
javax.persistence.RollbackException: Exception [EclipseLink-4002] (Eclipse 
Persistence Services - 2.5.0.v20130507-3faac2b):
org.eclipse.persistence.exceptions.DatabaseException
Internal Exception: 
com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Column 'dept_no' cannot be null Error Code: 1048 Call: INSERT INTO dept_emp (from_date, to_date, dept_no, emp_no) VALUES (?, ?, ?, ?)
bind => [null, null, null, null]

HttpSession session = request.getSession();
Object deptId = session.getAttribute("idDep");

EntityManagerFactory entityManagerFactory = Persistence.createEntityManagerFactory("EmployeeJPA");
EntityManager entityManager = entityManagerFactory.createEntityManager();
entityManager.getTransaction().begin();

Employee emp = new Employee();
// fill employee's fields 
entityManager.persist(emp);

Department dept = entityManager.find(Department.class, deptId);

DeptEmp depEm = new DeptEmp();
depEm.setEmployee(emp);
depEm.setDepartment(dept);
entityManager.persist(depEm);

entityManager.getTransaction().commit();
entityManager.close();
entityManagerFactory.close();