Hibernate從零開始_09_檢索策略

    Hibernate4的檢索策略分爲：類級別檢索和關聯級別檢索兩大類。

    一、類級別檢索

        類級別檢索分爲當即檢索、延遲檢索。對於Session的檢索方式，類級別檢索策略僅適用於load方法。是經過<class>中的 'lazy'屬性來肯定的，而類級別的lazy屬性默認是true。

  <!-- lazy 的默認屬性 true -->
  <class name="com.study.hibernate.domain.Customer" table="t_customer" catalog="db_hibernate" lazy="false">

@Test
 public void queryTest(){
  Configuration configuration = new Configuration().configure();
  ServiceRegistry serviceRegistry = new ServiceRegistryBuilder().applySettings(configuration.getProperties()).buildServiceRegistry();
  SessionFactory sessionFactory = configuration.buildSessionFactory(serviceRegistry);
  Session session = sessionFactory.openSession();
  Transaction tracsaction = session.beginTransaction();
  //lazy=true時
  //Customer customer = (Customer) session.get(Customer.class, new Long(1));
  //結果
  //Hibernate: /* load com.study.hibernate.domain.Customer */ select customer0_.id as id1_0_0_, customer0_.name as name2_0_0_ from db_hibernate.t_customer customer0_ where customer0_.id=?
  
  Customer customer1 = (Customer) session.load(Customer.class, new Long(1));
  //結果
  //沒有執行查詢語句
  
  //當觸發了customer1對象裏的get()方法時才執行了查詢
  //customer1.getName();
  //Hibernate: /* load com.study.hibernate.domain.Customer */ select customer0_.id as id1_0_0_, customer0_.name as name2_0_0_ from db_hibernate.t_customer customer0_ where customer0_.id=?
  
  tracsaction.commit();
  session.close();
  sessionFactory.close();
 }

 

    二、關聯級別檢索

        關聯級別檢索分爲：當即檢索、延遲檢索、迫切左外鏈接檢索。

   1） 當即檢索

        get()方法和query查詢方式都爲當即檢索。

    2）延遲檢索

<set name="orders" cascade="all" lazy="false"> <!-- lazy默認爲 true -->

  Customer customer1 = (Customer) session.load(Customer.class, new Long(1));
  customer1.getName(); 
  
  //結果
  Hibernate: /* load com.study.hibernate.domain.Customer */ select customer0_.id as id1_0_0_, customer0_.name as name2_0_0_ from db_hibernate.t_customer customer0_ where customer0_.id=?
Hibernate: /* load one-to-many com.study.hibernate.domain.Customer.orders */ select orders0_.customer_id as customer3_0_1_, orders0_.id as id1_1_1_, orders0_.id as id1_1_0_, orders0_.name as name2_1_0_, orders0_.customer_id as customer3_1_0_ from db_hibernate.t_orders orders0_ where orders0_.customer_id=?

    當lazy=false時customer對象被加載的同時，orders對象也被加載

    3）迫切左外鏈接檢索

<set name="orders" cascade="all" fetch="join">

  Customer customer1 = (Customer) session.load(Customer.class, new Long(1));
  System.out.println(customer1);
  
  //結果
  Hibernate: /* load com.study.hibernate.domain.Customer */ select customer0_.id as id1_0_1_, customer0_.name as name2_0_1_, orders1_.customer_id as customer3_0_3_, orders1_.id as id1_1_3_,orders1_.id as id1_1_0_, orders1_.name as name2_1_0_, orders1_.customer_id as customer3_1_0_ from db_hibernate.t_customer customer0_ left outer join db_hibernate.t_orders orders1_ on customer0_.id=orders1_.customer_id where customer0_.id=?
Customer [id=1, name=zhangsan, orders=[Orders [id=2, name=apple], Orders [id=1, name=orange]]]

    當fetch="join" 時 爲當即檢索其SQL語句爲左外鏈接，忽略lazy="true"。