hibernate 二级缓存(3)

hibernate 二级缓存(三)

转自http://terryjs.iteye.com/blog/751486

过这篇文章纪录hibernate二级缓存的一些使用经历,利用几个test case,从代码角度说明二级缓存在使用过程中一些需要注意的问题

使用到的Model类有两个，Author, Book, 两者之间为一对多的关系

@Entity@Cache(usage = CacheConcurrencyStrategy.READ_WRITE)public class Author { private Long id; private String name; private Set<Book> books = new HashSet<Book>(); // getter setter methods omitted｝

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@Entity@Cache(usage = CacheConcurrencyStrategy.READ_WRITE)public class Book { private Long id; private String title; private Author author; // getter setter methods omitted}

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主要的测试类为TestHibernateSecondLevelCache.java

public class TestHibernateSecondLevelCache { protected Logger logger = LoggerFactory.getLogger(getClass()); private static SessionFactory sessionFactory; @BeforeClass public static void setUpSessionFactory(){ sessionFactory = new AnnotationConfiguration().configure().buildSessionFactory(); } @After public void clearSecondLevelCache(){ logger.info("clear second level cache"); sessionFactory.evict(Author.class); sessionFactory.evict(Book.class); sessionFactory.getStatistics().clear(); } private Session openSession(){ return sessionFactory.openSession(); } private Statistics getStatistics(){ return sessionFactory.getStatistics(); }}

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方法setUpSessionFactory用于创建Hibernate SessionFactory,因为创建Session Factory是个相对比较耗时的操作,因此加上Junit4的@BeforeClass annotation,表示该Session Factory只会创建一次,被所有的test case共享.而clearSecondLevelCache方法会在每个test case结束时调用,用于清空二级缓存,防止前一个test case的結果影响后一个test case

测试使用的hibernate-core版本为:3.3.2.GA, hibernate-annotations版本为:3.4.0.GA,测试的数据库为hsqldb内存数据库

一. session.get()

先来看一下session.get是否会查找二级缓存

@Test public void testSessionGetCache(){ Author author = createAuthor(); assertGetMissCache(Author.class, author.getId()); assertGetHitCache(Author.class, author.getId()); updateAuthor(author); assertGetMissCache(Author.class, author.getId()); } private Author createAuthor(){ Session session = openSession(); Author author = new Author(); author.setName("septem"); session.save(author); session.close(); return author; } @SuppressWarnings("unchecked") private void assertGetMissCache(Class clazz, Serializable id){ Statistics stat = getStatistics(); long missCount = stat.getSecondLevelCacheMissCount(); Session session = openSession(); session.get(clazz, id); session.close(); assertEquals(missCount + 1, stat.getSecondLevelCacheMissCount()); } @SuppressWarnings("unchecked") private void assertGetHitCache(Class clazz, Serializable id){ Statistics stat = getStatistics(); long hitCount = stat.getSecondLevelCacheHitCount(); Session session = openSession(); session.get(clazz, id); session.close(); assertEquals(hitCount + 1, stat.getSecondLevelCacheHitCount()); } private void updateAuthor(Author author){ author.setName("new_name"); Session session = openSession(); session.update(author); session.flush(); session.close(); }

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testSessionGetCache首先通过createAuthor创建一个author对象,然后在 assertGetMissCache里面通过author.id使用get方法查出之前创建的author,因为这是每一次调用get方法,所以 hibernate从数据库取回author对象,并将它存入二级缓存.测试結果通过hibernate statistics统计信息里的second level cache miss count来判断这次的get查询未命中缓存

接着assertGetHitCache用同一个id通过get方法获取author对象,因为这个id的对象之前已存入二级缓存,所以这次操作命中缓存

最后通过updateAuthor更新之前的author对象,hibernate会自动将该对象从二级缓存中清除,因此第三次调用get方法时没有命中缓存

总结 : session.get方法会先中二级缓存中通过id做为key查找相应的对象,如果不存在,再发送SQL语句到数据库中查询

二. session.load()

第二步试一下session.load方法

@Test public void testSessionLoadCache(){ Author author = createAuthor(); assertLoadMissCache(Author.class, author.getId()); assertLoadHitCache(Author.class, author.getId()); updateAuthor(author); assertLoadMissCache(Author.class, author.getId()); } @SuppressWarnings("unchecked") private void assertLoadMissCache(Class clazz, Serializable id){ Statistics stat = getStatistics(); long missCount = stat.getSecondLevelCacheMissCount(); Session session = openSession(); Author author = (Author) session.load(clazz, id); author.getName(); session.close(); assertEquals(missCount + 1, stat.getSecondLevelCacheMissCount()); } @SuppressWarnings("unchecked") private void assertLoadHitCache(Class clazz, Serializable id){ Statistics stat = getStatistics(); long hitCount = stat.getSecondLevelCacheHitCount(); Session session = openSession(); session.load(clazz, id); Author author = (Author) session.load(clazz, id); author.getName(); session.close(); assertEquals(hitCount + 1, stat.getSecondLevelCacheHitCount()); }

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同样的結果,每一次通过id load未命中缓存,第二次通过相同的id调用load方法命中缓存,而更新过author对象后,缓存失效,第三次查询通过数据库获取author

有一点跟get方法不同:

Author author = (Author) session.load(clazz, id); author.getName();

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总结: 调用load方法的时候,hibernate一开始并没有查询二级缓存或是数据库,而是先返回一个代理对象,该对象只包含id,只有显示调用对象的非id 属性时,比如author.getName(),hibernate才会去二级缓存查找,如果没命中缓存再去数据库找,数据库还找不到则抛异常.load 方法会尽量推迟对象的查找工作,这是它跟get方法最大的区别.

这两者的测试用例如下:

@Test(expected=ObjectNotFoundException.class) public void testSessionLoadNonexistAuthor(){ Session session = openSession(); Author author = (Author) session.load(Author.class, -1L); assertEquals(Long.valueOf(-1), author.getId()); author.getName(); session.close(); } @Test public void testSessionGetNonexistAuthor(){ Session session = openSession(); Author author = (Author) session.get(Author.class, -1L); session.close(); assertNull(author); }

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三. session.createQuery().list()

@SuppressWarnings("unchecked") @Test public void testSessionList(){ Author author = createAuthor(); createAuthor(); Session session = openSession(); //hit database to select authors and populate the cache List<Author> authors = session.createQuery("from Author").list(); session.close(); assertEquals(authors.size(), getStatistics().getSecondLevelCachePutCount()); Session session2 = openSession(); //hit database again to select authors session2.createQuery("from Author").list(); session2.close(); assertEquals(authors.size(), getStatistics().getSecondLevelCachePutCount()); assertGetHitCache(Author.class, author.getId()); }

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首先创建2个author对象,使用HQL : "from Author"调用list方法,这时hibernate直接从数据库查询所有的author对象,并没有从缓存中查询,但是通过list方法查出的所有 author对象会存入二级缓存,这点通过getStatistics().getSecondLevelCachePutCount()可以看出来

接着再调用list方法一次,因为此时还没找开查询缓存,list方法重新从数据查了一次.因为第一次查询已将所有的author存入缓存,所以再调用get方法时会命中缓存,assertGetHitCache通过

总结: list方法不会从二级缓存中查找,但它从数据库中查找出来的对象会被存入cache

四. session.createQuery().iterate()

@SuppressWarnings("unchecked") @Test public void testSessionIterate(){ Author author = createAuthor(); createAuthor(); int authorCount = 0; Session session = openSession(); //hit database to get ids for all author Iterator<Author> it = session.createQuery("from Author").iterate(); while(it.hasNext()){ Author a = it.next(); a.getName(); authorCount++; } session.close(); assertEquals(authorCount, getStatistics().getEntityLoadCount()); assertGetHitCache(Author.class, author.getId()); }

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先创建2个author对象, 通过HQL: "from Author"调用iterate方法,此时hibernate并没有查author对象,而是先从数据库查出所有author的id,控制台会输入类似以下的SQL:

select id from author

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在对iterator里面遍历的时候,会根据id一个一个地从先中缓存中查找author,没找到再访问数据库

总结: iterate方法使用的是典型的N+1次查询,先从数据库查询出所有对象的ID,再根据ID一个一个地从二级缓存查找,二级缓存找不到再查询数据库

五. association cache

hibernate支持对关联进行缓存,先在Book.java加上books集合的缓存配置

@OneToMany(mappedBy = "author", fetch = FetchType.LAZY) @Cache(usage = CacheConcurrencyStrategy.READ_WRITE) public Set<Book> getBooks() { return books; }

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测试用例如下:

@Test public void testAssociationCache(){ Author author = createAuthorWith3Books(); assertGetBooksForAuthorMissCache(author, 1); assertGetBooksForAuthorHitCache(author, 4); updateOneBookForAuthor(author); assertGetBooksForAuthorMissCache(author, 1); addNewBookForAuthor(author); assertGetBooksForAuthorMissCache(author, 1); } private Author createAuthorWith3Books(){ Session session = openSession(); Author author = new Author(); author.setName("septem"); Book book1 = new Book(); book1.setTitle("book1"); book1.setAuthor(author); Book book2 = new Book(); book2.setTitle("book2"); book2.setAuthor(author); Book book3 = new Book(); book3.setTitle("book3"); book3.setAuthor(author); author.getBooks().add(book1); author.getBooks().add(book2); author.getBooks().add(book3); session.save(book1); session.save(book2); session.save(book3); session.close(); return author; } private void assertGetBooksForAuthorMissCache(Author author, long miss){ Session session = openSession(); Author a = (Author) session.get(Author.class, author.getId()); long missCount = getStatistics().getSecondLevelCacheMissCount(); a.getBooks().size(); session.close(); assertEquals(missCount + miss, getStatistics().getSecondLevelCacheMissCount()); } private void assertGetBooksForAuthorHitCache(Author author, long hit){ Session session = openSession(); Author a = (Author) session.get(Author.class, author.getId()); long hitCount = getStatistics().getSecondLevelCacheHitCount(); a.getBooks().size(); session.close(); assertEquals(hitCount + hit, getStatistics().getSecondLevelCacheHitCount()); } private void updateOneBookForAuthor(Author author){ Session session = openSession(); Author a = (Author) session.get(Author.class, author.getId()); Book book = (Book) session.get(Book.class, a.getBooks().iterator().next().getId()); book.setTitle("new_title"); session.flush(); session.close(); } private void addNewBookForAuthor(Author author){ Session session = openSession(); Author a = (Author) session.get(Author.class, author.getId()); Book book = new Book(); book.setTitle("new_book"); book.setAuthor(a); a.getBooks().add(book); session.save(book); session.update(a); session.flush(); session.close(); }

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先创建一个author,为该author添加3个book对象.在assertGetBooksForAuthorMissCache通过 author.getBooks访问关联的book集合,因为延迟加载的关系,此时并没有查询缓存也没有查询数据库,在调用 a.getBooks().size()也就是访问book集合的元素时,hibernate先中缓存中查找,没有发现关联缓存,重新数据库查询,生成的 SQL类似如下:

select * from book where author_id = ?

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此时statistics的missCount只增加了1,因为调用author.getBooks没有命中缓存.hibernate从数据库查询出books后,将books关联以及三个book对象都存入二级缓存.

关联的缓存是以什么样的形式存在呢?注意关联缓存没有保存books集合本身,而是保存所有book的id,假设3个book对象的id分别为1, 2, 3,则author缓存的格式类似于如下:

*---------------------------------*| Author Data Cache ||---------------------------------|| 1 -> [ "septem" , [ 1, 2, 3 ] ] |*---------------------------------*

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第二步执行assertGetBooksForAuthorHitCache(author, 4)的时候,我们看到hitCount增加了4.因为第二次调用author.getBooks的时候,命中了关联缓存,从缓存中取回3个id,又分别用 id一个一个地从二级缓存中取回3个book对象,一共命中缓存4次

接着通过updateOneBookForAuthor(author)更新了其中的一个book对象,假设更新的是id为1的book.接着的 assertGetBooksForAuthorMissCache(author, 1)方法里面missCount又增加了1.book虽然更新了,但是author.getBooks还是能命中缓存,因为book id列表还是[ 1, 2, 3 ].从缓存中取回book id列表,通过book id查找book的时候,因为id为1的book已经更新过了,它的二级缓存失效了,重新去数据库取,此时missCount增加了1,而id为2,3的 book还是从二级缓存中找到的.这个方法hibernate会生成类似如下的SQL:


select * from book where id = 1

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更新其中的一个book对象不会造成关联缓存的失效,但如果更新了集合id列表的话,缓存就会失效.先通过 addNewBookForAuthor为author增加一个books对象,此时books集合里面一共有4个book对象,最后的 assertGetBooksForAuthorMissCache(author, 1)我们可以看到缓存失效,missCount增加了1.此时同第一次调用author.getBooks一样,hibernate生成类似如下的SQL

select * from book where author_id = ?

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总结: 关联缓存保存的是集合的id列表,而不是集合本身,关联命中缓存的时候,会根据id一个一个地先从二级缓存查找,找不到再查询数据库.更新集合中的某个对象不会造成关联缓存失效,只有改变集合的id列表才会造成缓存失效

五. 查询缓存query cache

在hibernate.cfg.xml里面加上以下配置开启查询缓存:

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

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@Test public void testQueryCache(){ createAuthor(); createAuthor(); assertQueryMissCache(); assertQueryHitCache(); createAuthor(); assertQueryMissCache(); }

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先做准备工作,创建两个author对象,假设它们的id分别为1,2.assertQueryMissCache里面第一次调用list方法, 注意调用list前必须setCacheable(true)才会使用查询缓存,此时未命中查询缓存,hibernate从数据库查询Author对象, 将此次查询存入查询缓存,同时会将查询到的author对象存入二级缓存



查询缓存并不保存查询结果集,而只是保存结果集的id,它的结构类似以下数据:

*---------------------------*| Query Cache ||---------------------------|| [ ["from Author where name = ?", [ "septem"] ] -> [ 1, 2 ] ] |*---------------------------*

?注意缓存的key与HQL,参数以及分页参数有关



再调用assertQueryHitCache()用同样的HQL与参数重新查询Author此时会命中查询缓存,并根据结果集id一个一个地查询author对象,因为author对象之前已存入二级缓存,所以这次查询也会命中二级缓存

查询缓存的失效比较特殊,只要查询涉及的任何一张表的数据发生变化,缓存就会失效.比如我们再创建一个Author对象,此时Author表发生了变化,原来的查询缓存就失效了


总结: 查询缓存的key与HQL,查询参数以及分布参数有关,而且一旦查询涉及到的任何一张表的数据发生了变化,缓存就失效了,所以在生产环境中命中率较低.查询缓存保存的是结果集的id列表,而不是结果集本身,命中缓存的时候,会根据id一个一个地先从二级缓存查找,找不到再查询数据库.

涉及到的所有代码保存在google code上
svn checkout http://hibernate-cache-testcase.googlecode.com/svn/trunk/ hibernate-cache-testcase