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package com.bjsxt.hibernate;import java.util.ArrayList;import java.util.Date;import java.util.List;import javax.persistence.Entity;import javax.persistence.FetchType;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.ManyToOne;import javax.persistence.NamedQueries;import javax.persistence.NamedQuery;import javax.persistence.OneToMany;import javax.persistence.Temporal;import javax.persistence.TemporalType;import org.hibernate.annotations.BatchSize;@Entitypublic class Topic { private int id; private String title; private Category category; private Date createDate; private List msgs = new ArrayList (); @OneToMany(mappedBy="topic") public List getMsgs() { return msgs; } public void setMsgs(List msgs) { this.msgs = msgs; } @Temporal(TemporalType.TIME) public Date getCreateDate() { return createDate; } public void setCreateDate(Date createDate) { this.createDate = createDate; } @ManyToOne(fetch=FetchType.LAZY) public Category getCategory() { return category; } public void setCategory(Category category) { this.category = category; } @Id @GeneratedValue public int getId() { return id; } public void setId(int id) { this.id = id; } public String getTitle() { return title; } public void setTitle(String title) { this.title = title; } } Category.java: package com.bjsxt.hibernate;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.Id;import org.hibernate.annotations.BatchSize;@Entitypublic class Category { private int id; private String name; @Id @GeneratedValue public int getId() { return id; } public void setId(int id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; }} 添加数据(每一个板块ci下有一个话题ti) @Test public void testSave() { Session session = sf.openSession(); session.beginTransaction(); //每一个版块ci下面有一个话题ti for(int i=0; i<10; i++) { Category c = new Category(); c.setName("c" + i); Topic t=new Topic(); t.setCategory(c); t.setTitle("t"+i); t.setCreateDate(new Date()); session.save(c); session.save(t); } session.getTransaction().commit(); session.close(); } 下面进行list与iterate的不同测试 1.list取所有 @Test public void testOneAddNProblem1(){ Session session = sf.openSession(); session.beginTransaction(); List topics=(List )session.createQuery("from Topic").list(); for(Topic t:topics){ System.out.println(t.getId()+"-"+t.getTitle()); } session.getTransaction().commit(); session.close(); } 结果： Hibernate: select topic0_.id as id2_, topic0_.category_id as category4_2_, topic0_.createDate as createDate2_, topic0_.title as title2_ from Topic topic0_ 1-t0 2-t1 3-t2 4-t3 5-t4 6-t5 7-t6 8-t7 9-t8 10-t9 2.iterate先取ID，等用到的时候在根据ID来取对象。 @Test public void testListAndIterate(){ Session session = sf.openSession(); session.beginTransaction(); Iterator topics=(Iterator )session.createQuery("from Topic").iterate(); for(Topic t=topics.next();t!=null;t=topics.next()){ System.out.println(t.getId()+"-"+t.getTitle()); } session.getTransaction().commit(); session.close(); } 测试结果： Hibernate: select topic0_.id as col_0_0_ from Topic topic0_ Hibernate: select topic0_.id as id2_0_, topic0_.category_id as category4_2_0_, topic0_.createDate as createDate2_0_, topic0_.title as title2_0_ from Topic topic0_ where topic0_.id=? 1-t0 Hibernate: select topic0_.id as id2_0_, topic0_.category_id as category4_2_0_, topic0_.createDate as createDate2_0_, topic0_.title as title2_0_ from Topic topic0_ where topic0_.id=? 2-t1 Hibernate: select topic0_.id as id2_0_, topic0_.category_id as category4_2_0_, topic0_.createDate as createDate2_0_, topic0_.title as title2_0_ from Topic topic0_ where topic0_.id=? 3-t2 Hibernate: select topic0_.id as id2_0_, topic0_.category_id as category4_2_0_, topic0_.createDate as createDate2_0_, topic0_.title as title2_0_ from Topic topic0_ where topic0_.id=? 4-t3 Hibernate: select topic0_.id as id2_0_, topic0_.category_id as category4_2_0_, topic0_.createDate as createDate2_0_, topic0_.title as title2_0_ from Topic topic0_ where topic0_.id=? 5-t4 Hibernate: select topic0_.id as id2_0_, topic0_.category_id as category4_2_0_, topic0_.createDate as createDate2_0_, topic0_.title as title2_0_ from Topic topic0_ where topic0_.id=? 6-t5 Hibernate: select topic0_.id as id2_0_, topic0_.category_id as category4_2_0_, topic0_.createDate as createDate2_0_, topic0_.title as title2_0_ from Topic topic0_ where topic0_.id=? 7-t6 Hibernate: select topic0_.id as id2_0_, topic0_.category_id as category4_2_0_, topic0_.createDate as createDate2_0_, topic0_.title as title2_0_ from Topic topic0_ where topic0_.id=? 8-t7 Hibernate: select topic0_.id as id2_0_, topic0_.category_id as category4_2_0_, topic0_.createDate as createDate2_0_, topic0_.title as title2_0_ from Topic topic0_ where topic0_.id=? 9-t8 Hibernate: select topic0_.id as id2_0_, topic0_.category_id as category4_2_0_, topic0_.createDate as createDate2_0_, topic0_.title as title2_0_ from Topic topic0_ where topic0_.id=? 10-t9 所以我们可以看出，先取ID，等用到的时候在根据ID来取对象。 3.session中list第二次发出，仍会到数据库查询。 测试代码： @Test public void testListAndIterate2(){ Session session = sf.openSession(); session.beginTransaction(); //Iterator topics=(Iterator )session.createQuery("from Topic").iterate(); Query q=session.createQuery("from Topic"); List topics=q.list(); for(Topic t:topics){ System.out.println(t.getId()+"-"+t.getTitle()); } List topics2=q.list(); for(Topic t:topics2){ System.out.println(t.getId()+"-"+t.getTitle()); } session.getTransaction().commit(); session.close(); } 测试结果： Hibernate: select topic0_.id as id2_, topic0_.category_id as category4_2_, topic0_.createDate as createDate2_, topic0_.title as title2_ from Topic topic0_ 1-t0 2-t1 3-t2 4-t3 5-t4 6-t5 7-t6 8-t7 9-t8 10-t9 Hibernate: select topic0_.id as id2_, topic0_.category_id as category4_2_, topic0_.createDate as createDate2_, topic0_.title as title2_ from Topic topic0_ 1-t0 2-t1 3-t2 4-t3 5-t4 6-t5 7-t6 8-t7 9-t8 10-t9 4.iterate第二次，首先找session级缓存 测试代码： @Test public void testListAndIterate3(){ Session session = sf.openSession(); session.beginTransaction(); Iterator categories = (Iterator )session.createQuery("from Category").iterate(); while(categories.hasNext()) { Category c = categories.next(); System.out.println(c.getName()); } Iterator categories2 = (Iterator )session.createQuery("from Category").iterate(); while(categories2.hasNext()) { Category c = categories2.next(); System.out.println(c.getName()); } session.getTransaction().commit(); session.close(); } 测试结果： Hibernate: select category0_.id as col_0_0_ from Category category0_ Hibernate: select category0_.id as id0_0_, category0_.name as name0_0_ from Category category0_ where category0_.id=? c0 Hibernate: select category0_.id as id0_0_, category0_.name as name0_0_ from Category category0_ where category0_.id=? c1 Hibernate: select category0_.id as id0_0_, category0_.name as name0_0_ from Category category0_ where category0_.id=? c2 Hibernate: select category0_.id as id0_0_, category0_.name as name0_0_ from Category category0_ where category0_.id=? c3 Hibernate: select category0_.id as id0_0_, category0_.name as name0_0_ from Category category0_ where category0_.id=? c4 Hibernate: select category0_.id as id0_0_, category0_.name as name0_0_ from Category category0_ where category0_.id=? c5 Hibernate: select category0_.id as id0_0_, category0_.name as name0_0_ from Category category0_ where category0_.id=? c6 Hibernate: select category0_.id as id0_0_, category0_.name as name0_0_ from Category category0_ where category0_.id=? c7 Hibernate: select category0_.id as id0_0_, category0_.name as name0_0_ from Category category0_ where category0_.id=? c8 Hibernate: select category0_.id as id0_0_, category0_.name as name0_0_ from Category category0_ where category0_.id=? c9 Hibernate: select category0_.id as col_0_0_ from Category category0_ c0 c1 c2 c3 c4 c5 c6 c7 c8 c9 看出了第二次使用的session缓存里面的数据，而没有去数据库再查。 总结： 原因是：先看list，list取完10个数据以后session中有了10个数据的缓存，第二次list再去取的时候，它仍然要再去数据库里加载一次，然后再刷新缓存，再把原来对象重新刷一遍。也就是list不会去读session的缓存，原因是作为一个查询来说，查询条件很难认定，不是list不想利用，而是很难利用。 iterate就不一样了，首先在第一遍的时候，他首先把id取出来，由于后面我们做了遍历，所以他把id对应的对象全都放在了缓存中，第二次再做遍历的时候，这个时候只要去缓存中查就行了。 什么时候用list？什么时候用iterate？工作的时候用list就行了。 小技巧:第一次用list，第二次再去访问的时候就可以用iterate了。因为iterate会首先检查缓存，缓存中有了，就不会再去数据库里去查了。 主要应付面试
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