如何在 JPA 中进行 JOIN？

Question

我的 JPA (Hibernate) 和 Spring 应用程序。我的实体是：

部门

@Entity
@Table(schema = "myschema")
public class Department {

    @Id
    @GeneratedValue(generator = "uuid")
    @GenericGenerator(name = "uuid", strategy = "uuid2")
    @Column(columnDefinition = "BINARY(16)", length = 16 )
    private UUID uuid;

    @Column(name = "name", length = 200, nullable = false)
    private String name;

    @OneToMany(fetch = FetchType.LAZY, mappedBy = "department", cascade = CascadeType.ALL, orphanRemoval = true)
    List<User> users = new ArrayList<>();
}

用户

@Entity
@Table(schema = "myschema")
public class User {

    @Id
    @GeneratedValue(generator = "uuid")
    @GenericGenerator(name = "uuid", strategy = "uuid2")
    @Column(columnDefinition = "BINARY(16)", length = 16 )
    private UUID uuid;

    @NotNull
    @Column(name = "login", length = 100, nullable = false)
    private String login;

    @ManyToOne
    @JoinColumn(name = "department_fk", nullable = false)
    private Department department;

}

DAO 用于部门：

@Repository("departmentDao")
public class DepartmentDao {

    @PersistenceContext
    private EntityManager entityManager;

    @Transactional
    public UUID save(Department entity) {
        log.info("Department entity will be persisted", entity);
        entityManager.persist(entity);
        return entity.getUuid();
    }

    public List<Department> getAllWithUsers() {
        log.info("Select all departments");
        CriteriaQuery<Department> criteria = entityManager.getCriteriaBuilder().createQuery(Department.class);
        criteria.from(Department.class).fetch("users");
        return entityManager.createQuery(criteria).getResultList();
    }
}

当我在 Main 类中运行代码时，它会加入实体（但会重复它们，为什么？）：

主要

ApplicationContext context = new ClassPathXmlApplicationContext("META-INF/spring.xml");
UserDao userDao = (UserDao) context.getBean("userDao");
DepartmentDao departmentDao = (DepartmentDao) context.getBean("departmentDao");

Department newDep = new Department("WebDepartment", new Date());
departmentDao.save(newDep);

User newUser1 = new User("Test1", newDep);
User newUser2 = new User("Test2", newDep);
userDao.save(newUser1);
userDao.save(newUser2);

List<Department> deps = departmentDao.getAllWithUsers();

//deps
//[Department{name='WebDepartment', users = ['Test1', 'Test2']}, 
// Department{name='WebDepartment', users = ['Test1', 'Test2']}]

当我使用相同的代码运行单元测试时，部门列表仍然包含两个实体，每个部门都有空的用户列表：

@ContextConfiguration(locations = "classpath:META-INF/spring-test.xml")
@RunWith(SpringJUnit4ClassRunner.class)
public class DaoTests {

    @Autowired
    private DepartmentDao departmentDao;

    @Autowired
    private UserDao userDao;

    @Test
    @Transactional
    public void daoTests() throws NoSuchAlgorithmException {

        Department newDep = new Department("WebDepartment", new Date());
        departmentDao.save(newDep);

        User newUser1 = new User("Test1", newDep);
        User newUser2 = new User("Test2", newDep);
        userDao.save(newUser1);
        userDao.save(newUser2);
        List<Department> deps = departmentDao.getAllWithUsers();

        //deps
        //[Department{name='WebDepartment', users = []}, 
        // Department{name='WebDepartment', users = []}]
    }
}

所以，我有两个问题。 1. 为什么我得到两个部门实体？ 2. 为什么 join 在 Main 中有效，而在 test 中却无效？

Answer 1

1. 为什么我得到两个部门实体？

这是因为您在 DAO 中执行JOIN FETCH 查询：

criteria.from(Department.class).fetch("users")

这会将部门与用户连接起来，从而为每个用户生成一个部门行。由于该部门有两个用户，因此结果将包含具有相同部门的 2 行。你想要的是where clausle 而不是fetch join。

1. 为什么 join 在 Main 中有效，但在测试中却无效？

它确实有效，但您在部门内的用户集合上使用FetchType.LAZY。这意味着只有在您访问用户时才会从数据库中加载用户。您可以使用FetchType.EAGER 即时加载用户。