Hibernate 小知识

1. get/load区别

如果记录存在，get返回对象（或代理对象）；否则返回null。
如果记录存在，load返回对象；否则返回 NotObjectFoundException



2.hibernate Cache
2.1 一级缓存，在当前session内
2.2 Query Cache
Cache queries and results.
方法：
1. 在hiberante Configuration 文件中：
<property name="hibernate.cache.use_query_cache">true</property>
2. 查询时
Query query = session.createQuery("from Person p where p.id=1");
query.setCacheable(true);



2.3 二级缓存, 跨session . 如EHCache


3. load 与 CreateQuery 在处理Cache的区别

首先,load时在一个session中，一级缓存总是有效的。比如下面的只issue sql操作一次

 

Session session = getSessionFactory().openSession();
  Transaction tx = session.beginTransaction();
  Person person1 = (Person) session.load(Person.class, 1L);
  System.out.println(person1.getFirstName());
  Person person2 = (Person) session.load(Person.class, 1L);
  System.out.println(person2.getFirstName());
  tx.commit();
  session.close();

Session session = getSessionFactory().openSession();
Transaction tx = session.beginTransaction();
Person person1 = (Person) session.load(Person.class, 1L);
System.out.println(person1.getFirstName());
Person person2 = (Person) session.load(Person.class, 1L);
System.out.println(person2.getFirstName());
tx.commit();
session.close();

其次,如果用CreateQuery有点不同，比若下面的需要issue sql操作两次

Session session = getSessionFactory().openSession();
 Transaction tx = session.beginTransaction();
 Query query = session.createQuery("from Person p where p.id=1");
 Iterator it = query.list().iterator();
 while (it.hasNext ()){
   Person p = (Person) it.next();
   System.out.println(p.getFirstName());
 }
 query = session.createQuery("from Person p where p.id=1");
 it = query.list().iterator();
 while (it.hasNext ()){
   Person p = (Person) it.next();
   System.out.println(p.getFirstName());
 }
 tx.commit();
 session.close();

Session session = getSessionFactory().openSession();
Transaction tx = session.beginTransaction();
Query query = session.createQuery("from Person p where p.id=1");
Iterator it = query.list().iterator();
while (it.hasNext ()){
Person p = (Person) it.next();
System.out.println(p.getFirstName());
}
query = session.createQuery("from Person p where p.id=1");
it = query.list().iterator();
while (it.hasNext ()){
Person p = (Person) it.next();
System.out.println(p.getFirstName());
}
tx.commit();
session.close();

原因是session cache时是根据key(load方法中传入的ID)来缓存的。解决的方法是：
1. 用load，如上面的代码
2. 用查询缓存，如上面的介绍

Session session = getSessionFactory().openSession();
 Transaction tx = session.beginTransaction();
 Query query = session.createQuery("from Person p where p.id=1");
 query.setCacheable(true);
 Iterator it = query.list().iterator();
 while (it.hasNext ()){
    Person p = (Person) it.next();
    System.out.println(p.getFirstName());
 }
 query = session.createQuery("from Person p where p.id=1");
 query.setCacheable(true);
 it = query.list().iterator();
 while (it.hasNext ()){
    Person p = (Person) it.next();
    System.out.println(p.getFirstName());
 }
 tx.commit();
 session.close();