hiberante中List和Iterate的区别

结论：

对于对象查询

1、使用List的时候会将对象全部取出，而使用Iterate则只先将对象主键取出，然后在使用的时候再一个个取出。

2、List第二次使用的时候会继续重新数据库中取出，而Iterate则会先成缓存中查找，如果没找到再去数据库中取出。

对于属性查询（条件：查询缓存关闭）：

两者没什么差别，根据查询对象的相应属性，在执行查询的时候将结果从数据库中取出（不同于上面据说的iterate在使用对象的时候取出），同时两者的第二次查询也是直接从数据库中取出数据，而不会从缓存中取得。

注意：一级缓存和二级缓存都只是存放实体对象的，如果查询实体对象的普通属性的数据，只能放到查询缓存里，查询缓存还存放查询实体对象的id。

hibernate的查询缓存默认是关闭的，如果要使用就要到hibernate.cfg.xml文件里配置：

<property name="hibernate.cache.use_query_cache">true</property>

并且必须在程序中手动启用查询缓存，在query接口中的setCacheable(true)方法来启用。

实践是检验真理的惟一标准：

Iterate实例（查询对象）:

SessionFactory factory = new Configuration().configure().buildSessionFactory();
Session session=null;
session=factory.openSession();
session.beginTransaction();
Iterator iter = session.createQuery
	("from User s where s.id<3").iterate();
while (iter.hasNext()) {
User age = (User) iter.next();
	System.out.println(age);
}
System.out.println("--------------------------------------");
iter = session.createQuery("from User s where s.id<3").iterate();
while (iter.hasNext()) {
	User age = (User) iter.next();
	System.out.println(age);
｝
session.getTransaction().commit();
session.close();

结果如下：

Hibernate: select user0_.id as col_0_0_ from User user0_ where user0_.id<3

Hibernate: select user0_.id as id0_0_, user0_.username as username0_0_, user0_.age as age0_0_, user0_.password as password0_0_ from User user0_ where user0_.id=?

User [id=1, username=JAY, age=8, password=JAY]

Hibernate: select user0_.id as id0_0_, user0_.username as username0_0_, user0_.age as age0_0_, user0_.password as password0_0_ from User user0_ where user0_.id=?

User [id=2, username=aa, age=3, password=aa]

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

Hibernate: select user0_.id as col_0_0_ from User user0_ where user0_.id<3

User [id=1, username=JAY, age=8, password=JAY]

User [id=2, username=aa, age=3, password=aa]

从结果可知，第一次运行查询语句时，只查询了对象的id(主键)，然后在每次使用具体对象时再进行查询取出（1+N）；由于hibrenate缓存机制，第二次运行查询语句是只运行了id(主键)查询，对象结果却是从缓存中直接取得。

List实例（查询对象）：

SessionFactory factory = new Configuration().configure().buildSessionFactory();
Session session=null;
session=factory.openSession();
session.beginTransaction();
List<User> list =(List<User>) session.createQuery
	("from User s where s.id<3").list();
for(User s:list){
	System.out.println(s.toString());
}

System.out.println("--------------------------------------");
		
list = session.createQuery("from User s where s.id<3").list();
for(User s:list){
	System.out.println(s.toString());
}
session.getTransaction().commit();
session.close();

结果如下：

Hibernate: select user0_.id as id0_, user0_.username as username0_, user0_.age as age0_, user0_.password as password0_ from User user0_ where user0_.id<3
User [id=1, username=JAY, age=8, password=JAY]
User [id=2, username=aa, age=3, password=aa]
--------------------------------------
Hibernate: select user0_.id as id0_, user0_.username as username0_, user0_.age as age0_, user0_.password as password0_ from User user0_ where user0_.id<3
User [id=1, username=JAY, age=8, password=JAY]
User [id=2, username=aa, age=3, password=aa]

从结果可知，两次执行查询语句时都是将所有对象一次取出，即使第二执行查询时也同样执行查询语句

混合情况实例（查询对象；第一次用list,第二次用iterate）：

SessionFactory factory = new Configuration().configure().buildSessionFactory();
Session session=null;
session=factory.openSession();
session.beginTransaction();
List<User> list =(List<User>) session.createQuery
	("from User s where s.id<3").list();
for(User s:list){
	System.out.println(s.toString());
}

System.out.println("--------------------------------------");
		
Iterator iter = session.createQuery("from User s where s.id<3").iterate();
while (iter.hasNext()) {
	User age = (User) iter.next();
	System.out.println(age);
}
session.getTransaction().commit();
session.close();

结果如下：

Hibernate: select user0_.id as id0_, user0_.username as username0_, user0_.age as age0_, user0_.password as password0_ from User user0_ where user0_.id<3
User [id=1, username=JAY, age=8, password=JAY]
User [id=2, username=aa, age=3, password=aa]
--------------------------------------
Hibernate: select user0_.id as col_0_0_ from User user0_ where user0_.id<3
User [id=1, username=JAY, age=8, password=JAY]
User [id=2, username=aa, age=3, password=aa]

从结果可知，第一次采用list查询，将对象全部取出，然后当第二次采用iterate查询时，只执行了对id(主键)的查询，然后通过主键从缓存中取出结果；可见采用list查询时在也会将结果放置缓存，只是它每次依旧从数据库中重新查询取得。

Iterate实例（查询对象属性）：

SessionFactory factory = new Configuration().configure().buildSessionFactory();
Session session=null;
session=factory.openSession();
session.beginTransaction();
Iterator iter= session.createQuery
	("select s.username from User s where s.id<3").iterate();
while (iter.hasNext()) {
	String age = (String) iter.next();
	System.out.println(age);
}
System.out.println("--------------------------------------");
iter= session.createQuery
		("select s.username from User s where s.id<3").iterate();
while (iter.hasNext()) {
	String age = (String) iter.next();
	System.out.println(age);
}
session.getTransaction().commit();
session.close();

结果如下：

Hibernate: select user0_.username as col_0_0_ from User user0_ where user0_.id<3
JAY
aa
--------------------------------------
Hibernate: select user0_.username as col_0_0_ from User user0_ where user0_.id<3
JAY
aa
从结果可知，当查询对象属性的时候，则是直接查询对象相应的属性，而不再查询对象id(主键)；并且第二查询的时候是从数据库中重新查询取出结果，而不是从缓存中直接取出。

List实例（查询对象属性）：

SessionFactory factory = new Configuration().configure().buildSessionFactory();
Session session=null;
session=factory.openSession();
session.beginTransaction();
List<String> list = session.createQuery
		("select s.username from User s where s.id<3").list();
for(String s:list){
	System.out.println(s.toString());
}
System.out.println("--------------------------------------");
list =(List) session.createQuery
		("select s.username from User s where s.id<3").list();
for(String s:list){
	System.out.println(s.toString());
}
session.getTransaction().commit();
session.close();

结果如下：

Hibernate: select user0_.username as col_0_0_ from User user0_ where user0_.id<3
JAY
aa
--------------------------------------
Hibernate: select user0_.username as col_0_0_ from User user0_ where user0_.id<3
JAY
aa

结论和Iterate实例（查询对象属性）的结论一样