我正在尝试使用JPA（Hibernate 4）+ Spring实现简单的多对多关联。已经看到了大量类似helloworld的示例，其中当从2个关联表中保存实体时，连接表会自动更新。
然而，在我的情况下并没有发生这种情况 - 即使我设置了双向关联和级联，联接表也没有在 em.persist（）上更新。在查看原因时，我来到this answer here at SO，建议使用 em.persist（）; em.flush（）; 来解决这个问题。我试过 - 奇迹，坚持做得很好！但为什么？？？
问题：
以下是相关代码。
实体类
@Entity
@Table(name="ROLE")
public class Role extends EntityBase implements Comparable
{
@ManyToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
@JoinTable(name = "ROLE_PERMISSION",
joinColumns = @JoinColumn(name="role_id", referencedColumnName="id"),
inverseJoinColumns = @JoinColumn(name="permission_id", referencedColumnName="id"))
private Set permissions = new HashSet();
//... other code (getters/setters/extra columns) is omitted ...
}
@Entity
@Table(name="PERMISSION")
public class Permission extends EntityBase
{
@ManyToMany(mappedBy = "permissions", fetch = FetchType.LAZY)
private Set roles = new HashSet();
//... other code (getters/setters/extra columns) is omitted ...
}
通用DAO实现（由我的具体DAO使用）：
@Repository
@Transactional(value="transactionManager")
public abstract class GenericDaoImpl implements GenericDao
{
@PersistenceContext(unitName = "entityManagerFactory")
protected EntityManager em;
public T create( final T t )
{
em.persist(t);
// em.flush(); - if I put this here, all works well (and fails if I'm not)
return t;
}
// ... other code is omitted ...
}
我正在尝试测试的服务层方法：
@Component("securityService")
public class SecurityServiceImpl implements SecurityService
{
// ... other code is omitted ...
@Transactional(value="transactionManager",
rollbackFor = Exception.class, readOnly = false)
public void createRole( Role role )
{
Validate.notNull( role, "Role should not be null" );
roleDao.create( role );
}
}
最后我的TestNG集成测试（使用内存H2 DB）：
@ContextConfiguration(
locations={"/META-INF/beans-test.xml"})
@TransactionConfiguration(
transactionManager = "transactionManager", defaultRollback = true)
public class SecurityServiceImplIT
extends AbstractTransactionalTestNGSpringContextTests
{
@Autowired
@Qualifier("securityService")
private SecurityService securityService;
@Test
@Transactional(value = "transactionManager")
public void createRole_createRoleWithPermissions()
{
// Add test data to DB.
super.executeSqlScript( TESTDATA_PATH, false);
// Remove all associations between permissions and roles, need
// clear intermediate table for this test case.
super.simpleJdbcTemplate.update(
"delete from DB_TEST.ROLE_PERMISSION;" );
super.simpleJdbcTemplate.update(
"delete from DB_TEST.ROLE;" );
final Role role = new Role();
role.setName( "Test role" );
role.addPermission( securityService.getAllPermissions().get(0) );
final int expectedPermissionCount = 1;
securityService.createRole(role);
// This is always passed
Assert.assertEquals( super.countRowsInTable( "DB_TEST.ROLE" ), 1,
"New role should be added, so table should contain 1 row" );
// This is failed if I'm not using flush() in my DAO.
Assert.assertEquals( super.countRowsInTable( "DB_TEST.ROLE_PERMISSION" ),
expectedPermissionCount, "Role-permission associations should be added" );
}
// ... other code is omitted ...
}
Hibernate调试日志（在DAO中没有flush（）调用）：
Hibernate: insert into DB_TEST.ROLE (id, version, description, name) values (null, ?, ?, ?) ... aa TRACE org.hibernate.engine.jdbc.internal.LogicalConnectionImpl: Starting after statement execution processing [ON_CLOSE] aa TRACE org.hibernate.action.internal.UnresolvedEntityInsertActions: No unresolved entity inserts that depended on [[xxx.logic.db.model.Role#4]] aa TRACE org.hibernate.engine.internal.Cascade: Processing cascade ACTION_PERSIST_SKIPLAZY for: xxx.logic.db.model.Role aa TRACE org.hibernate.engine.internal.Cascade: Cascade ACTION_PERSIST_SKIPLAZY for collection: xxx.logic.db.model.Role.permissions aa TRACE org.hibernate.engine.spi.EJB3CascadingAction: Cascading to persist: xxx.logic.db.model.Permission aa TRACE org.hibernate.event.internal.AbstractSaveEventListener: Persistent instance of: xxx.logic.db.model.Permission aa TRACE org.hibernate.event.internal.DefaultPersistEventListener: Ignoring persistent instance aa TRACE org.hibernate.engine.internal.Cascade: Done cascade ACTION_PERSIST_SKIPLAZY for collection: xxx.logic.db.model.Role.permissions aa TRACE org.hibernate.engine.internal.Cascade: Done processing cascade ACTION_PERSIST_SKIPLAZY for: xxx.logic.db.model.Role aa TRACE org.hibernate.action.internal.UnresolvedEntityInsertActions: No entity insert actions have non-nullable, transient entity dependencies.aa TRACE org.hibernate.engine.jdbc.internal.LogicalConnectionImpl: Starting after statement execution processing [ON_CLOSE] aa TRACE org.hibernate.action.internal.UnresolvedEntityInsertActions: No unresolved entity inserts that depended on [[xxx.logic.db.model.Role#4]] aa TRACE org.hibernate.engine.internal.Cascade: Processing cascade ACTION_PERSIST_SKIPLAZY for: xxx.logic.db.model.Role aa TRACE org.hibernate.engine.internal.Cascade: Cascade ACTION_PERSIST_SKIPLAZY for collection: xxx.logic.db.model.Role.permissions aa TRACE org.hibernate.engine.spi.EJB3CascadingAction: Cascading to persist: xxx.logic.db.model.Permission aa TRACE org.hibernate.event.internal.AbstractSaveEventListener: Persistent instance of: xxx.logic.db.model.Permission aa TRACE org.hibernate.event.internal.DefaultPersistEventListener: Ignoring persistent instance aa TRACE org.hibernate.engine.internal.Cascade: Done cascade ACTION_PERSIST_SKIPLAZY for collection: xxx.logic.db.model.Role.permissions aa TRACE org.hibernate.engine.internal.Cascade: Done processing cascade ACTION_PERSIST_SKIPLAZY for: xxx.logic.db.model.Role aa TRACE org.hibernate.action.internal.UnresolvedEntityInsertActions: No entity insert actions have non-nullable, transient entity dependencies.
这个日志说Hibernate正在通过Permissions集合，但由于某种原因忽略了那里的项目。我完全不明白为什么调用flush（）会在这里产生任何不同......通常，flush（）只是一种明确告诉Hibernate何时将SQL查询发布到DB的能力。
任何人都可以解释这个或者至少指出我正确的文档吗？
答案 0 :(得分：1)
Hibernate延迟SQL语句的执行，直到绝对必要。这可以避免在事务最终回滚时执行不必要的语句，从而节省时间。
最后，当提交事务时，Hibernate会刷新挂起的修改和提交。但是，由于您已将测试配置为回滚而未提交事务，因此此自动刷新永远不会发生，并且您需要显式调用flush()以使其执行插入。