使用 @OneToOne 和 @OneToMany 时如何使 Hibernate OUTER JOIN

假设我有一张名为employee与领域employee_id, name and supervisor_id and employee_supervisor与领域supervisor_id and name。存在外键关系employee and employee_supervisor表关于supervisor_id柱子。

如果我在Employee类，我如何确保 Hibernate 使用 LEFT OUTER JOIN 来连接关联实体？

在我的测试中，Hibernate 默认执行 LEFT OUTER JOIN。但是，您可以确保它将始终使用带注释的 LEFT OUTER JOIN。我花了一些时间使用双向一对多映射来模拟您的情况我的模板 https://stackoverflow.com/q/24257449/2357233作为基础，然后更改类名称以符合您的情况。

我想出的课程如下：

职工监事班：

@Entity(name="EMPLOYEE_SUPERVISOR")
public class EmployeeSupervisor {
    @Id
    @GenericGenerator(name = "gen", strategy = "increment")
    @GeneratedValue(generator = "gen")
    @Column(name = "id")
    private int supervisorId;

    @Column
    private String name;

    @OneToMany(mappedBy = "supervisor")
    private List<Employee> employees;
    ....
}

员工类别：

@Entity
public class Employee {
    @Id
    @GenericGenerator(name = "gen", strategy = "increment")
    @GeneratedValue(generator = "gen")
    @Column(name = "id")
    private int employeeId;

    @Column
    private String name;

    @ManyToOne
    @Fetch(FetchMode.JOIN)
    @JoinColumn(name = "supervisorId")
    private EmployeeSupervisor supervisor;
    ....
}

确保您始终使用的注释LEFT OUTER JOIN is the @Fetch(FetchMode.JOIN注解。这告诉 Hibernate 使用 a 加载同一个 select 语句中的关联记录LEFT OUTER JOIN (see 文档 http://docs.jboss.org/hibernate/orm/4.3/manual/en-US/html/ch20.html#performance-fetching对于其他类型的FetchMode以及每个人的作用）。

然后我在 jUnit 中模拟了数据库，配置 Hibernate 以在 log4j 中打印所有生成的 SQL

log4j.logger.org.hibernate.SQL=DEBUG
log4j.logger.org.hibernate.type=TRACE

并运行了一个非常基本的单元测试。

public class EmployeeDAOTest extends SpringTest{

    @Autowired
    private EmployeeDAO dao;

    private Employee testLinkedEmployee;
    private Employee testUnlinkedEmployee;
    private EmployeeSupervisor testSupervisor;

    @BeforeClass
    public static void setUpBeforeClass() throws Exception {
        System.out.println("Starting DAO Test");
    }

    @AfterClass
    public static void tearDownAfterClass() throws Exception {
        System.out.println("Finished DAO Test");
    }

    @Before
    public void setUp() throws Exception {
        //Irrelevant horrible setup code snipped.
        /* Set up 2 employees and a EmployeeSupervisor in the database.
         * Link one employee to the EmployeeSupervisor and not the other
         */
    }

    @Test
    @Transactional
    public void test() {
        Employee actualLinkedEmployee = dao.getEmployee(testLinkedEmployee.getEmployeeId());
        Employee actualUnlinkedEmployee = dao.getEmployee(testUnlinkedEmployee.getEmployeeId());

        assertNotNull("The linked employee's supervisor didn't get selected.", actualLinkedEmployee.getSupervisor());
        assertNull("The unlinked employee's supervisor was not null.", actualUnlinkedEmployee.getSupervisor());
    }
}

我的 DAO 非常初级：

@Repository
public class EmployeeDAOImpl implements EmployeeDAO {

    @Autowired
    private SessionFactory sessionFactory;

    @Override
    public Employee getEmployee(int id) {
        Criteria query = sessionFactory.getCurrentSession().createCriteria(Employee.class);
        query.add(Restrictions.idEq(id));
        return (Employee) query.uniqueResult();
    }
}

SQL输出如下：

[junit]     /* criteria query */ select
[junit]         this_.id as id1_1_1_,
[junit]         this_.name as name2_1_1_,
[junit]         this_.supervisorId as supervis3_1_1_,
[junit]         employeesu2_.id as id1_0_0_,
[junit]         employeesu2_.name as name2_0_0_ 
[junit]     from
[junit]         Employee this_ 
[junit]     left outer join
[junit]         EMPLOYEE_SUPERVISOR employeesu2_ 
[junit]             on this_.supervisorId=employeesu2_.id 
[junit]     where
[junit]         this_.id = ? 01:23:54:0.668 
[junit] binding parameter [1] as [INTEGER] - 5 01:23:54:0.671 

[junit]     /* criteria query */ select
[junit]         this_.id as id1_1_1_,
[junit]         this_.name as name2_1_1_,
[junit]         this_.supervisorId as supervis3_1_1_,
[junit]         employeesu2_.id as id1_0_0_,
[junit]         employeesu2_.name as name2_0_0_ 
[junit]     from
[junit]         Employee this_ 
[junit]     left outer join
[junit]         EMPLOYEE_SUPERVISOR employeesu2_ 
[junit]             on this_.supervisorId=employeesu2_.id 
[junit]     where
[junit]         this_.id = ? 01:23:54:0.704 
[junit] binding parameter [1] as [INTEGER] - 6 01:23:54:0.704 

应该注意的是，默认情况下 Hibernate 似乎急切地获取这些实体并使用 LEFT OUTER JOIN 来执行此操作。但是，如果您尝试将默认获取类型设置为FetchType.LAZY然后连接类型更改为FetchMode.SELECT并且只为员工发出一个选择，而不选择主管。

Setting FetchType.LAZY with @Fetch(FetchMode.JOIN)但是，它会覆盖您的惰性获取并使用连接来急切地获取您的主管。