Hibernate 迫切左外连接 left join fetch

建议在一多对关联中在"一"方用延迟加载"多"方

可以在HQL中显式的"迫切左外连接" left join fetch
这样做Hibernate可以少访问数据库,也可以用"@BatchSize(size = 5)"来减少访问数据库的次数

 

User.java 一方

 

@Entity
@Table(name = "tbl_user")
@org.hibernate.annotations.Entity(dynamicUpdate=true,dynamicInsert=true)
public class User implements Serializable{

 @Id
 @GeneratedValue(generator="gen")
 @GenericGenerator(name="gen",strategy="increment")
 @Column(name="id")
 private Integer id;

 
 /**
  * username
  */
 @Column(name="username", length=50, insertable = true, updatable = true, nullable = false)
 private String username;
 
 
 /**
  * password
  */
 @Column(name="password", length=20, insertable = true, updatable = true, nullable = false)
 private String password;
 
 
 /**
  * 创建时间
  */
 @Column(name="createtime")
 @Temporal(value=TemporalType.TIMESTAMP)
 private Date createtime;
 
 
 
 @OneToMany(cascade=CascadeType.ALL,fetch=FetchType.LAZY,mappedBy = "user")
 @BatchSize(size = 5)
 private Set<Order> orders=new HashSet<Order>();

 
 /**
 * 省略get,set方法
 */

}

 

 

Order.java 多方

 

@Entity
@Table(name = "tbl_order")
@org.hibernate.annotations.Entity(dynamicUpdate=true,dynamicInsert=true)

public class Order implements Serializable{

 /**
  * id
  */
 @Id
 @GeneratedValue(generator="gen")
 @GenericGenerator(name="gen",strategy="increment")
 @Column(name="id")
 private Integer id;
 

 /**
  * orderNum
  */
 @Column(name="orderNum", length=50, insertable = true, updatable = true, nullable = false)
 private String orderNum;
 
 
 /**
  * 创建时间
  */
 @Column(name="createtime")
 @Temporal(value=TemporalType.TIMESTAMP)
 private Date createtime;
 
 
 
 @ManyToOne
 @JoinColumn(name = "userId")
 private User user;

 /**
 * 省略get,set方法
 */

 

}

 

UseerDaoImpl.java中部分代码

 public List<User> listUser(){
  
  List<User> list=this.getHibernateTemplate().find(" from User as u left join fetch u.orders ");
  
  return list;
  
 }