Hibernate抓取策略以及如何避免cannot simultaneously fetch multiple bags异常(转)

在说解决cannot simultaneously fetch multiple bags异常之前，我先说下抓取策略

注解@Fetch(FetchMode.？)抓取策略有三种

1、FetchMode.JOIN（默认的抓取策略），采用外连接的形式，left outer join ... on

2、FetchMode.SELECT 会另外发送一条sql语句加载当前对象的关联实体

3、FetchMode.SUBSELECT 会另外发送一条select语句抓取前面查询到的所有实体对象的关联实体

通过Hibernate输出的SQL日志看成，个人感觉2、3的差别不是太大 ，都是另起select语句查询与当前某个实体相关联的其他实体。

下面是我写的一个简单DEMO，主要来验证抓取策略和异常的处理方法

表结构

人员表 tb_person

 

+-------------+

| Field |

+-------------+

| person_id |

| person_age |

| person_name |

+-------------+

一个人员可以有多个邮箱，邮箱表，人-邮箱是一对多的关系tb_person_email

 

+-----------+

| Field |

+-----------+

| id |

| person_id |

| address |

+-----------+

事件表，一个人可以有多个事件，一个事件也可有多个人处理，是多对多关系tb_event，tb_person_event

 

+-------------+

| Field |

+-------------+

| event_id |

| event_title |

| event_date |

+-------------+

 

 

+-----------+

| Field |

+-----------+

| event_id |

| person_id |

+-----------+

 

要执行的方法是：搜索人，以及相关的事件和邮件。

person.java

第一种写法，采用默认join抓取策略，进行强制抓取person所有关联对象的方式

 

@ManyToMany(fetch=FetchType.EAGER)
@JoinTable(name = "TB_PERSON_EVENT", joinColumns = {@JoinColumn(name = "PERSON_ID")},

@ManyToMany(fetch=FetchType.EAGER)
@JoinTable(name = "TB_PERSON_EVENT", joinColumns = {@JoinColumn(name = "PERSON_ID")}, 

inverseJoinColumns = {@JoinColumn(name = "EVENT_ID")})
public List<Event> getEvents() {
 return events;
}


@OneToMany(fetch=FetchType.EAGER)
@JoinTable(name = "TB_PERSON_EMAIL", joinColumns = {@JoinColumn(name = "PERSON_ID")},

inverseJoinColumns = {@JoinColumn(name = "EVENT_ID")})
public List<Event> getEvents() {
	return events;
}


@OneToMany(fetch=FetchType.EAGER)
@JoinTable(name = "TB_PERSON_EMAIL", joinColumns = {@JoinColumn(name = "PERSON_ID")},

inverseJoinColumns = {@JoinColumn(name = "ID")})
ublic List<Email> getEmails() {
return emails;

 inverseJoinColumns = {@JoinColumn(name = "ID")})
public List<Email> getEmails() {
	return emails;
}

如果这样写的情况下， 就会报cannot simultaneously fetch multiple bags异常，从SQL日志分析，hibernate是这样执行的

 

select *
from TB_PERSON tp left outer join TB_PERSON_EMAIL tpe1 on tp.person_id = tpe1.person_id
left outer join TB_PERSON_EMAIL tpe2 on tpe1.id = tpe2.id
left outer join TB_PERSON_EVENT tpet on tp.person_id = tpet.person_id
left outer join TB_EVENT te on tpet.event_id = te.event_id
where tp.person_id = 2;

select *
from TB_PERSON tp left outer join TB_PERSON_EMAIL tpe1 on tp.person_id = tpe1.person_id 
left outer join TB_PERSON_EMAIL tpe2 on tpe1.id = tpe2.id 
left outer join TB_PERSON_EVENT tpet on tp.person_id = tpet.person_id
left outer join TB_EVENT te on tpet.event_id = te.event_id
where tp.person_id = 2;

输出的记过，就会出现两个同样的email实体列，所以就报上述异常。

 

如果要避免这个异常，我在网上搜了许多资料都是说把fetch=FetchType.LAZY

变成懒加载模式，或者把List修改成Set集合，这两种我验证过了，是可以行的通的，但是如果不想采取这种两种办法，

怎么办。

 

其实还有一种，是看springside得到的一种方法，代码修改如下

 

@ManyToMany(fetch=FetchType.EAGER)
@JoinTable(name = "TB_PERSON_EVENT", joinColumns = {@JoinColumn(name = "PERSON_ID")},

@ManyToMany(fetch=FetchType.EAGER)
@JoinTable(name = "TB_PERSON_EVENT", joinColumns = {@JoinColumn(name = "PERSON_ID")},

 inverseJoinColumns = {@JoinColumn(name = "EVENT_ID")})
@Fetch(FetchMode.SUBSELECT)
public List<Event> getEvents() {
 return events;
}

@OneToMany(fetch=FetchType.EAGER)
@JoinTable(name = "TB_PERSON_EMAIL", joinColumns = {@JoinColumn(name = "PERSON_ID")},

 inverseJoinColumns = {@JoinColumn(name = "EVENT_ID")})
@Fetch(FetchMode.SUBSELECT)
public List<Event> getEvents() {
	return events;
}

@OneToMany(fetch=FetchType.EAGER)
@JoinTable(name = "TB_PERSON_EMAIL", joinColumns = {@JoinColumn(name = "PERSON_ID")},

 inverseJoinColumns = {@JoinColumn(name = "ID")})
@Fetch(FetchMode.SUBSELECT)
public List<Email> getEmails() {
 return emails;
}

 inverseJoinColumns = {@JoinColumn(name = "ID")})
@Fetch(FetchMode.SUBSELECT)
public List<Email> getEmails() {
        return emails;
}

这种方法就是在不修改原结构注释的情况下，可以修改一下抓取策略，分开select实体，这样就不会出现重复列现象。

个人感觉还是不错的方法，打算就用它了。

 

另外就是个人并不是太喜欢全抓取，密密麻麻一大堆的SQL打印的控制台上，排查SQL都麻烦，所以最终修改版，个人比较喜欢的注释方法：

 

@ManyToMany(fetch=FetchType.LAZY)
@JoinTable(name = "TB_PERSON_EVENT", joinColumns = {@JoinColumn(name = "PERSON_ID")},

@ManyToMany(fetch=FetchType.LAZY)
@JoinTable(name = "TB_PERSON_EVENT", joinColumns = {@JoinColumn(name = "PERSON_ID")},

 inverseJoinColumns = {@JoinColumn(name = "EVENT_ID")})
@Fetch(FetchMode.SUBSELECT)
public List<Event> getEvents() {
 return events;
}

@OneToMany(fetch=FetchType.LAZY)
@JoinTable(name = "TB_PERSON_EMAIL", joinColumns = {@JoinColumn(name = "PERSON_ID")},

 inverseJoinColumns = {@JoinColumn(name = "EVENT_ID")})
@Fetch(FetchMode.SUBSELECT)
public List<Event> getEvents() {
	return events;
}

@OneToMany(fetch=FetchType.LAZY)
@JoinTable(name = "TB_PERSON_EMAIL", joinColumns = {@JoinColumn(name = "PERSON_ID")},

 inverseJoinColumns = {@JoinColumn(name = "ID")})
@Fetch(FetchMode.SUBSELECT)
public List<Email> getEmails() {
 return emails;
}