开发学院软件开发Java jpa抓取策略详解fetch(lazy ,eager) 阅读

jpa抓取策略详解fetch(lazy ,eager)

 2009-09-26 00:00:00 来源:WEB开发网   
核心提示: junit的测试方法Java代码@Testpublicvoidquery(){EntityManagerFactoryfactory=Persistence.createEntityManagerFactory("jpa-hibernate");EntityManagerem

junit的测试方法

Java代码   

 @Test public void query() { 
 EntityManagerFactory factory = Persistence.createEntityManagerFactory("jpa-hibernate"); 
 EntityManager em = factory.createEntityManager(); 
  
 Garage garage = em.find(Garage.class, 1); 
 
  
 em.close(); 
 factory.close(); 
 }

调用query方法的时候发出的sql语句是:

Sql代码 Hibernate: select garage0_.gid as gid1_0_, garage0_.garagenum as garagenum1_0_ from Garage garage0_ where garage0_.gid=?

也就是仅仅获取了garage对象,而没有获取与garage关联的auto对象

-----------------

(二)在Garage.java中添加fetch=FetchType.EAGER字段

@OneToMany(cascade={CascadeType.PERSIST},fetch=FetchType.EAGER,mappedBy="garage")
public Set<Auto> getAutos() {
return autos;
}

再次运行query方法,这一次发出的sql语句是:

Sql代码

Hibernate: select garage0_.gid as gid1_1_, garage0_.garagenum as garagenum1_1_, autos1_.garageid as garageid3_, autos1_.autoId as autoId3_, autos1_.autoId as autoId0_0_, autos1_.autonum as autonum0_0_, autos1_.autotype as autotype0_0_, autos1_.garageid as garageid0_0_ from Garage garage0_ left outer join Auto autos1_ on garage0_.gid=autos1_.garageid where garage0_.gid=? 

这一次由于将jpa默认的抓取策略改为fetch=FetchType.EAGER

所以jpa在加载数据的时候一次性的加载了和garage相关联的数据

说明:由于fetch=FetchType.EAGER加载数据的时候是一次性加载可能会造成不必要的性能浪费,使用是应该慎重考虑

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Tags:jpa 抓取 策略

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