Hibernate JPA Training-2

这篇blog将介绍hibernate 实体间one-to-one关系的单向和双向关联的差异,以及hibernate lazy loading问题。 ### 1- 准备工作 —

2- Hibernate One-To-One 介绍

2.1- 单向关联与双向关联差异：

单向关联:setup代码

@Before
public void setup(){
    // initial character
    character = new Character();
    character.setCharacterId(100000L);
    
    //initial wallet
    Wallet wallet = new Wallet();
    wallet.setCurrency("RMTC");
    wallet.setHardBalance(1L);
    wallet.setSoftBalance(1L);
    
    //set wallet to character
    character.setWallet(wallet);
}

双向关联:setup代码

@Before
public void setup(){
    // initial character
    character = new Character();
    character.setCharacterId(100000L);
    
    //initial wallet
    Wallet wallet = new Wallet();
    wallet.setCurrency("RMTC");
    wallet.setHardBalance(1L);
    wallet.setSoftBalance(1L);
    
    //set wallet to character
    character.setWallet(wallet);
    //set character to wallet
    wallet.setCharacter(character);
}

都运行测试

@Test
@Rollback(false)
public void testSave(){
   entityManager.persist(character);       
}

在log中可以看到单向关联执行了2条insert语句：

2013-03-29 08:35:16,434 DEBUG [org.hibernate.SQL] - <insert into Wallet (character_id, currency, hardBalance, softBalance, id) values (?, ?, ?, ?, ?)>
2013-03-29 08:35:16,441 DEBUG [org.hibernate.SQL] - <insert into Character (account_id, characterId, wallet_id, id) values (?, ?, ?, ?)>

而双向关联执行了2条insert语句和一条update语句用来更新关系,关系的主控方(inverse side)会主动更新关系:

2013-03-29 08:47:34,884 DEBUG [org.hibernate.SQL] - <insert into Wallet (character_id, currency, hardBalance, softBalance, id) values (?, ?, ?, ?, ?)>
2013-03-29 08:47:34,889 DEBUG [org.hibernate.SQL] - <insert into Character (account_id, characterId, wallet_id, id) values (?, ?, ?, ?)>
2013-03-29 08:47:34,891 DEBUG [org.hibernate.SQL] - <update Wallet set character_id=?, currency=?, hardBalance=?, softBalance=? where id=?>

在remove的时候也是一样双向关联的会多一条update语句，使用的时候要特别注意！

2.2- Hibernate Lazy Loading 问题介绍及避免

执行如下代码，找出某个character

@Test
@Rollback(false)
public void testGet(){
    List<Character> characters = entityManager.createQuery("from Character").getResultList();
    Character character = characters.get(0);
    Wallet wallet = null;
    if(character!=null){
        wallet = character.getWallet();
        wallet.getCurrency();
    }       
}

我们只执行了一条sql查询但是看log我们发现执行了2条select语句：

2013-03-29 08:53:36,401 DEBUG [org.hibernate.SQL] - <select character0_.id as id2_, character0_.account_id as account3_2_,...
2013-03-29 08:53:36,413 DEBUG [org.hibernate.SQL] - <select wallet0_.id as id6_3_, wallet0_.character_id as character5_6_3...

多出来这条sql是在character.getWallet()的时候去执行的。

以通过在第一次查询的时候把关联数据一起查出来，来解决这个问题，看如下代码

@Test
@Rollback(false)
public void testGetJoin(){
    List<Character> characters = entityManager.createQuery("from Character c join fetch c.wallet").getResultList();
    Character character = characters.get(0);
    Wallet wallet = null;
    if(character!=null){
        wallet = character.getWallet();
        wallet.getCurrency();
    } 
}

这样只会产生一条sql log

2013-03-29 09:02:13,344 DEBUG [org.hibernate.SQL] - <select character0_.id as id2_0_, wallet1_.id as id6_1_, character0_.account_id as account3_2_0_, character0_.characterId as characte2_2_0_, character0_.wallet_id as wallet4_2_0_, wallet1_.character_id as character5_6_1_, wallet1_.currency as currency6_1_, wallet1_.hardBalance as hardBala3_6_1_, wallet1_.softBalance as softBala4_6_1_ from Character character0_ inner join Wallet wallet1_ on character0_.wallet_id=wallet1_.id>

使用的时候要特别注意这个问题, Cheers!

372 Words 29 March 2013 Suzhou, China