原文：https://www.cnblogs.com/MrSi/p/8081811.html
懒加载LAZY和实时加载EAGER的概念,在各种开发语言中都有广泛应用。其目的是实现关联数据的选择性加载，懒加载是在属性被引用时，才生成查询语句，抽取相关联数据。而实时加载则是执行完主查询后,不管是否被引用,立马执行后续的关联数据查询。社区里有人认为懒加载这种功能比较鸡肋，这种事仁者见仁，智者见智啦，个人觉得依自己业务场景而定。
顺带说一句,使用懒加载来调用关联数据，必须要保证主查询session(数据库连接会话)的生命周期没有结束，否则，你是无法抽取到数据的。在Spring Data JPA中如何自己控制session的生命周期，这个功能我还没有研究，作为下一次的研究课题。
在今天解析懒加载与实时加载的实例时，需要借助实体关系映射功能，配置实体间的一对多(OneToMany)和多对一(ManyToOne)关系。当然，还有OneToOne、ManyToMany关系，这个如有兴趣大家就自己研究下了。
一、为实体类ProcessBlock和Node添加一对多的关联关系。在我这个示例中ProcessBlock是流程定义类，一个流程定义对应多个流程节点，即Node表。下面给出代码。
1、ProcessBlock实体定义
@Entity(name = "nbpm_processblock")
@NamedQuery(name = "ProcessBlock.findByName", query = "select p from nbpm_processblock p where p.name=?1")
public class ProcessBlock implements Serializable {
private static final long serialVersionUID = 1L;
@Id
@Column(name = "id")
long id;
@Column(name = "name")
String name;
@Column(name = "description")
String description;
@OneToMany(mappedBy="processblock",cascade=CascadeType.ALL,fetch = FetchType.EAGER)
Set<Node> nodeSet = new HashSet<Node>();
public long getId() {
return id;
}
……
定义Set<Node>属性 nodeSet,为其添加注解@OneToMany，意思就是一个ProcessBlock实体对应多个Node节点。注解属性 mappedBy,是指关联关系可以从Node类中的processblock属性上获取；cascade属性设置成CascadeType.ALL,是说主从表全面建立级联关系；fetch属性设置成FetchType.EAGER，是指加载规则是即时加载。查看源码会发现默认情况下，OneToMany注解的fetch属性设置的是FetchType.LAZY。
2、Node实体定义
@Entity(name = "nbpm_node")
public class Node implements Serializable {
private static final long serialVersionUID = 1L;
@Id
@Column(name = "id")
long id;
@Column(name = "name")
String name;
@Column(name = "description")
String description;
@Column(name = "subclass")
String subclass;
/**
* 关联关系必须存在
*/
@ManyToOne(optional = true)
@JoinColumn(name="processblock")
ProcessBlock processblock;
public long getId() {
return id;
}
……
定义ProcessBlock属性 processblock,为其添加注解@ManyToOne，意思就是一个Node实体对应一个ProcessBlock流程定义。添加注解@JoinColumn,属性name设置成"processblock"，意思是nbpm_node表中的processblock字段是外键，与ProcessBlock的主键建议关联关系。需要说明一下 ManyToOne注解的fetch属性 默认是FetchType.EAGER，查询时会立马抽取对应的主表数据。这块的考虑还是很细致的，大致就是关联数据是集合时就懒加载，是单条数据时就实时加载。
二、编写查询示例并展示结果。查询使用自定义的JpaUtil工具，工具实现请参考《自定义JpaUtil,快速完成Hql执行逻辑(一)》
1、查询ProcessBlock,主干流程数据，因为设置的nodeSet属性是实时加载加载，查询完成，断点查看确实拿到了流程对应的所有节点信息。
Map<String, Object> params = new HashMap<>();
params.put("name", "主干流程");
List<ProcessBlock> list = ApplicationContextUtil.instance.getJpaUtil().list(
"select u from simm.spring.entity.ProcessBlock u where u.name=:name", params, ProcessBlock.class); Set<Node> nodes = list.get(0).getNodeSet();
-- Hibernate框架生成的查询语句 -----------------
Hibernate: select processblo0_.id as id1_2_, processblo0_.description as descript2_2_, processblo0_.name as name3_2_ from nbpm_processblock processblo0_ where processblo0_.name=?
Hibernate: select nodeset0_.processblock as processb5_1_0_, nodeset0_.id as id1_1_0_, nodeset0_.id as id1_1_1_, nodeset0_.description as descript2_1_1_, nodeset0_.name as name3_1_1_,
nodeset0_.processblock as processb5_1_1_, nodeset0_.subclass as subclass4_1_1_
from nbpm_node nodeset0_ where nodeset0_.processblock=?
Hibernate: select nodeset0_.processblock as processb5_1_0_, nodeset0_.id as id1_1_0_, nodeset0_.id as id1_1_1_, nodeset0_.description as descript2_1_1_, nodeset0_.name as name3_1_1_,
nodeset0_.processblock as processb5_1_1_, nodeset0_.subclass as subclass4_1_1_
from nbpm_node nodeset0_ where nodeset0_.processblock=?
Hibernate: select nodeset0_.processblock as processb5_1_0_, nodeset0_.id as id1_1_0_, nodeset0_.id as id1_1_1_, nodeset0_.description as descript2_1_1_, nodeset0_.name as name3_1_1_,
nodeset0_.processblock as processb5_1_1_, nodeset0_.subclass as subclass4_1_1_
from nbpm_node nodeset0_ where nodeset0_.processblock=?
Hibernate: select nodeset0_.processblock as processb5_1_0_, nodeset0_.id as id1_1_0_, nodeset0_.id as id1_1_1_, nodeset0_.description as descript2_1_1_, nodeset0_.name as name3_1_1_,
nodeset0_.processblock as processb5_1_1_, nodeset0_.subclass as subclass4_1_1_
from nbpm_node nodeset0_ where nodeset0_.processblock=?
2、将ProcessBlock类的属性nodeSet设置成懒加载LAZY，查询后，发现对应的节点列表信息获取不到了。这里懒加载的数据读取不到是因为主查询的session已经结束。
@OneToMany(mappedBy="processblock",cascade=CascadeType.ALL,fetch = FetchType.LAZY)
//@JoinColumn(name="processblock_id")
Set<Node> nodeSet = new HashSet<Node>();
-- Hibernate框架生成的查询语句 -----------------
Hibernate: select processblo0_.id as id1_2_, processblo0_.description as descript2_2_, processblo0_.name as name3_2_ from nbpm_processblock processblo0_ where processblo0_.name=?
3、Hql直接查询关联属性
List<Node> nodeList = ApplicationContextUtil.instance.getJpaUtil().list(
"select u.nodeSet from simm.spring.entity.ProcessBlock u where u.id=1", null, Node.class);
-- Hibernate框架生成的查询语句 -----------------
Hibernate: select nodeset1_.id as id1_1_, nodeset1_.description as descript2_1_, nodeset1_.name as name3_1_, nodeset1_.processblock as processb5_1_, nodeset1_.subclass as subclass4_1_
nbpm_processblock processblo0_ inner join nbpm_node nodeset1_ on processblo0_.id=nodeset1_.processblock where processblo0_.id=1
Hibernate: select processblo0_.id as id1_2_0_, processblo0_.description as descript2_2_0_, processblo0_.name as name3_2_0_
from nbpm_processblock processblo0_ where processblo0_.id=?