HQL查询中List()和iterate()的区别

小灰灰 2022-08-21 09:45 203阅读 0赞

# １．采用List查询 #

执行一条sql语句，查出所需数据.

Result:

Hibernate: select student0_.id as id0_, student0_.createTime as createTime0_, student0_.name as name0_, student0_.classesid as classesid0_ from t_student student0_
    班级0的学生0
    班级0的学生1
    班级0的学生2
    班级1的学生3
    班级1的学生4
    班级1的学生5

**Code:**

@SuppressWarnings("unchecked")
    	public void testQuery1() {
    		Session session = null;
    		try {
    			session = HibernateUtils.getSession();
    			session.beginTransaction();
    			/**
    			 * 采用list查询实体对象会发出一条查询语句，取得实体对象数据
    			 * 
    			 */
    			List students = session.createQuery("from Student").list();
    			for (Iterator iter=students.iterator(); iter.hasNext();) {
    				Student student = (Student)iter.next();
    				System.out.println(student.getName());
    			}
    			session.getTransaction().commit();
    		}catch(Exception e) {
    			e.printStackTrace();
    			session.getTransaction().rollback();
    		}finally {
    			HibernateUtils.closeSession(session);
    		}
    	}

# **2.采用Iterate查询** #

\#.首先发出一条查询ID列表的sql. 然后会发出N条sql去加载相关的对象.若缓存中不存在相关对象.就会产生1+N次SQl查询问题.

\#.若缓存中存在相关对象时，就会根据Id列表去拿相关对象.并将数据存入缓存中.

Result:

Hibernate: select student0_.id as col_0_0_ from t_student student0_
    Hibernate: select student0_.id as id0_0_, student0_.createTime as createTime0_0_, student0_.name as name0_0_, student0_.classesid as classesid0_0_ from t_student student0_ where student0_.id=?
    班级0的学生0
    Hibernate: select student0_.id as id0_0_, student0_.createTime as createTime0_0_, student0_.name as name0_0_, student0_.classesid as classesid0_0_ from t_student student0_ where student0_.id=?
    班级0的学生1
    Hibernate: select student0_.id as id0_0_, student0_.createTime as createTime0_0_, student0_.name as name0_0_, student0_.classesid as classesid0_0_ from t_student student0_ where student0_.id=?
    班级0的学生2
    Hibernate: select student0_.id as id0_0_, student0_.createTime as createTime0_0_, student0_.name as name0_0_, student0_.classesid as classesid0_0_ from t_student student0_ where student0_.id=?
    班级1的学生3
    Hibernate: select student0_.id as id0_0_, student0_.createTime as createTime0_0_, student0_.name as name0_0_, student0_.classesid as classesid0_0_ from t_student student0_ where student0_.id=?
    班级1的学生4
    Hibernate: select student0_.id as id0_0_, student0_.createTime as createTime0_0_, student0_.name as name0_0_, student0_.classesid as classesid0_0_ from t_student student0_ where student0_.id=?
    班级1的学生5

**Code：**

@SuppressWarnings("unchecked")
    	public void testQuery2() {
    		Session session = null;
    		try {
    			session = HibernateUtils.getSession();
    			session.beginTransaction();
    			/**
    			 * 会出现N+1问题，所谓的N+1指的是发出了N+1条sql语句
    			 * 
    			 * 1:发出一条查询id列表的语句
    			 * 
    			 * N:根据id发出N条sql语句，加载相关的对象
    			 */
    			Iterator iter = session.createQuery("from Student").iterate();
    			while (iter.hasNext()) {
    				Student student = (Student)iter.next();
    				System.out.println(student.getName());
    			}
    			session.getTransaction().commit();
    		}catch(Exception e) {
    			e.printStackTrace();
    			session.getTransaction().rollback();
    		}finally {
    			HibernateUtils.closeSession(session);
    		}
    	}

# 总结: #

**\#1.** list()方法会一次取出所有的结果集对象，而且他会依据查询的结果初始化所有的结果集对象。如果在结果集非常庞大的时候会占据非常多的内存，甚至会造成内存溢出的情况发生.  
 iterate()方法在执行时不会一次初始化所有的对象，而是根据对结果集的访问情况来初始化对象。一次在访问中可以控制缓存中对象的数量，以避免占用过多的缓存，导致内存溢出情况的发生.

**\#2.** 返回类型不一样.

list()方法返回list.且list中每个对象都是原来的对象.

iterate()方法返回iterator,返回的是代理对象.

**\#3.** list()会直接查数据库; iterate()会先把数据库中所有id取出来,等真正需要某个对象时再根据id先去查缓存.若没有,再去执行一条sql去查数据库,这样子就会产生所说的1+n次的问题.