多对一关系中的休眠刷新

Question

我有这个 2 域模型：

员工

public class Employee {

    private Long    id; 
    private String  name;
    private String  family;
    private Company company;

    //getter & setter

}

和公司：

公共类公司{

private Long            id; 
private String          name; 
private Set<Employee>   employees;

//getter & setter 

}

Employee 和 Company 是多对一的关系，从 Company 端来看是一对多的关系。

我在交易中保存了一个公司对象，然后在该公司保存了员工：

@Transactional
puplic Long Save (){
        Company cmp=new Company(); 
        cmp.setName("Oracle");
        session.save(cmp);
        Employee employee=new Employee();
        employee.setName("james");
        employee.setFamily("dep");
        employee.setCompany(cmp);
        cmp.getEmployees();//will raise NullPointerException
        cmp=session.get(cmp.getId());//because of first level cache it doesnt load it 
    }

保存员工后，如果我想从公司获取员工的集合，它将引发空指针异常，如果我想加载它，因为一级缓存休眠将不会触及数据库并且员工集仍然为空。

第一个解决方案是我可以在保存员工后刷新 Company 对象，但我认为 hibernate 必须有除此之外的解决方案。

Answer 1

您永远不应该使用null ，因此您应该将映射更改为：

private Set<Employee>   employees = new HashSet<>();

这样，您甚至不需要 setter，因为 getter 就足够了。 另外，请确保同步双向关联的双方：

public void addEmployee(Employee employee) {
     employees.add(employee);
     employee.setCompany(this);
}

public void removeEmployee(Employee employee) {
     employees.remove(employee);
     employee.setCompany(null);
}

您还需要在子实体类中正确实现equals和hashCode 。

Answer 2

看看我的代码，

在 commit() 事务之后，只有你可以获得持久对象，

public class Employee {

    private Long    id; 
    private String  name;
    private String  family;

    @ManyToOne
    @JoinColumn(name = "company_id_fk")
    private Company company;

    //getter & setter

}


public class Company {

    private Long            id; 
    private String          name; 

    @OneToMany(mappedBy="company")
    List<Employee> listOfEmployees = new ArrayList<Employee>();

    //getter & setter 
}


puplic Long Save (){

    Session session = sessionFactory.openSession();
    Transaction transaction = session.beginTransaction();
    try{
        Company cmp=new Company(); 
        cmp.setName("Oracle");
        session.save(cmp);

        Employee employee=new Employee();
        employee.setName("james");
        employee.setFamily("dep");
        employee.setCompany(cmp);
        session.save(employee);

        transaction.commit();

        List<Employee> listOfEmployees = cmp.getListOfEmployees();
        if(listOfEmployees !=null && !listOfEmployees .isEmpty()){
            for(Employee employ:listOfEmployees){
              //You can wt you want
            }
        }


    }catch(Exception e){
        transaction.rollback();
    }finally{
        session.close();
    }
}