1 hibernate 缓存
1.1 一级缓存（session 级别的缓存）默认开启，不能关闭
缓存的是id，和对象。
Session session=demo.openSession(); //直接对数据库发出语句进行查询，并生成对象 List<User> user1s= session.createQuery("from User",User.class).setFirstResult(0).setMaxResults(5).list(); //直接对数据库发出语句进行查询，拿id去缓存中查找到了对象，就不生成对象 List<User> user2s= session.createQuery("from User",User.class).setFirstResult(0).setMaxResults(5).list(); System.out.println(user1s.get(0)==user2s.get(0)); //获取缓存中的数据 User user =session.load(User.class, 2l); System.out.println(user.getName()); session.close();
输出结果：
[11:15:02.888] [DEBUG] [] SQL: select user0_.id as id1_2_, user0_.name as name2_2_ from User user0_ limit ? Hibernate: select user0_.id as id1_2_, user0_.name as name2_2_ from User user0_ limit ? [11:15:02.912] [TRACE] [] BasicExtractor: extracted value ([id1_2_] : [BIGINT]) - [2] [11:15:02.925] [TRACE] [] BasicExtractor: extracted value ([name2_2_] : [VARCHAR]) - [Brett Meyer0] [11:15:02.929] [TRACE] [] BasicExtractor: extracted value ([id1_2_] : [BIGINT]) - [5] [11:15:02.930] [TRACE] [] BasicExtractor: extracted value ([name2_2_] : [VARCHAR]) - [Brett Meyer1] [11:15:02.931] [TRACE] [] BasicExtractor: extracted value ([id1_2_] : [BIGINT]) - [8] [11:15:02.932] [TRACE] [] BasicExtractor: extracted value ([name2_2_] : [VARCHAR]) - [Brett Meyer2] [11:15:02.934] [TRACE] [] BasicExtractor: extracted value ([id1_2_] : [BIGINT]) - [11] [11:15:02.935] [TRACE] [] BasicExtractor: extracted value ([name2_2_] : [VARCHAR]) - [Brett Meyer3] [11:15:02.940] [TRACE] [] BasicExtractor: extracted value ([id1_2_] : [BIGINT]) - [14] [11:15:02.940] [TRACE] [] BasicExtractor: extracted value ([name2_2_] : [VARCHAR]) - [Brett Meyer4] [11:15:02.969] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.projects#2] [11:15:02.970] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.skills#2] [11:15:03.067] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.projects#5] [11:15:03.070] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.skills#5] [11:15:03.071] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.projects#8] [11:15:03.072] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.skills#8] [11:15:03.073] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.projects#11] [11:15:03.074] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.skills#11] [11:15:03.075] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.projects#14] [11:15:03.076] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.skills#14] [11:15:04.779] [DEBUG] [] SQL: select user0_.id as id1_2_, user0_.name as name2_2_ from User user0_ limit ? Hibernate: select user0_.id as id1_2_, user0_.name as name2_2_ from User user0_ limit ? [11:15:04.783] [TRACE] [] BasicExtractor: extracted value ([id1_2_] : [BIGINT]) - [2] [11:15:04.784] [TRACE] [] BasicExtractor: extracted value ([id1_2_] : [BIGINT]) - [5] [11:15:04.785] [TRACE] [] BasicExtractor: extracted value ([id1_2_] : [BIGINT]) - [8] [11:15:04.786] [TRACE] [] BasicExtractor: extracted value ([id1_2_] : [BIGINT]) - [11] [11:15:04.786] [TRACE] [] BasicExtractor: extracted value ([id1_2_] : [BIGINT]) - [14] true Brett Meyer0
1.2 二级缓存（sessionFactory缓存）手动开启
开启方法： 在 hibernate.properties里配置（如果与spring结合，请搜索spring里对应的配置） 在annotation中，我们还需要在这个类上加上这样一个注解：@Cache
hibernate.cache.use_second_level_cache true
hibernate.cache.region.factory_class org.hibernate.cache.SingletonEhCacheRegionFactory
缓存的是id，和对象。（执行sql时，查出来所有的对象，按id和对象缓存。）
//直接对数据库发出语句进行查询，并生成对象 List<User> user1s= session.createQuery("from User",User.class).setFirstResult(0).setMaxResults(5).list(); session.close(); session=demo.openSession(); //使用二级缓存 User user =session.load(User.class, 2l); System.out.println(user.getName()); session.close();
输出结果：
[15:11:47.912] [DEBUG] [] SQL: select user0_.id as id1_2_, user0_.name as name2_2_ from User user0_ limit ? Hibernate: select user0_.id as id1_2_, user0_.name as name2_2_ from User user0_ limit ? [15:11:47.930] [TRACE] [] BasicExtractor: extracted value ([id1_2_] : [BIGINT]) - [2] [15:11:47.938] [TRACE] [] BasicExtractor: extracted value ([name2_2_] : [VARCHAR]) - [Brett Meyer0] [15:11:47.940] [TRACE] [] BasicExtractor: extracted value ([id1_2_] : [BIGINT]) - [5] [15:11:47.941] [TRACE] [] BasicExtractor: extracted value ([name2_2_] : [VARCHAR]) - [Brett Meyer1] [15:11:47.941] [TRACE] [] BasicExtractor: extracted value ([id1_2_] : [BIGINT]) - [8] [15:11:47.941] [TRACE] [] BasicExtractor: extracted value ([name2_2_] : [VARCHAR]) - [Brett Meyer2] [15:11:47.941] [TRACE] [] BasicExtractor: extracted value ([id1_2_] : [BIGINT]) - [11] [15:11:47.941] [TRACE] [] BasicExtractor: extracted value ([name2_2_] : [VARCHAR]) - [Brett Meyer3] [15:11:47.942] [TRACE] [] BasicExtractor: extracted value ([id1_2_] : [BIGINT]) - [14] [15:11:47.942] [TRACE] [] BasicExtractor: extracted value ([name2_2_] : [VARCHAR]) - [Brett Meyer4] [15:11:47.959] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.projects#2] [15:11:47.960] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.skills#2] [15:11:47.971] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.projects#5] [15:11:47.972] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.skills#5] [15:11:47.972] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.projects#8] [15:11:47.972] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.skills#8] [15:11:47.973] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.projects#11] [15:11:47.973] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.skills#11] [15:11:47.974] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.projects#14] [15:11:47.974] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.skills#14] [15:11:48.006] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.projects#2] [15:11:48.006
1.3 查询缓存（sessionFactory缓存）手动开启
开启方法：hibernate.properties
hibernate.cache.use_query_cache true 代码中要显示得使用 .setCacheable(true) 在annotation中，我们还需要在这个类上加上这样一个注解：@Cacheable
缓存的sql语句和对象id （所以要使用查询缓存时，二级缓存必须开启）
Session session = demo.openSession(); // 直接对数据库发出语句进行查询，并生成对象 List<User> user1s = session.createQuery("from User", User.class).setCacheable(true).setFirstResult(0).setMaxResults(5).list(); session.close(); session = demo.openSession(); // 使用二级缓存 User user = session.load(User.class, 2l); System.out.println(user.getName()); session.close(); session = demo.openSession(); // 使用查询缓存 List<User> user2s = session.createQuery("from User", User.class).setCacheable(true).setFirstResult(0) .setMaxResults(5).list(); System.out.println(user1s.get(0) == user2s.get(0)); session.close();
输出结果：
[15:14:58.165] [DEBUG] [] SQL: select user0_.id as id1_2_, user0_.name as name2_2_ from User user0_ limit ? Hibernate: select user0_.id as id1_2_, user0_.name as name2_2_ from User user0_ limit ? [15:14:58.177] [TRACE] [] BasicExtractor: extracted value ([id1_2_] : [BIGINT]) - [2] [15:14:58.183] [TRACE] [] BasicExtractor: extracted value ([name2_2_] : [VARCHAR]) - [Brett Meyer0] [15:14:58.184] [TRACE] [] BasicExtractor: extracted value ([id1_2_] : [BIGINT]) - [5] [15:14:58.184] [TRACE] [] BasicExtractor: extracted value ([name2_2_] : [VARCHAR]) - [Brett Meyer1] [15:14:58.184] [TRACE] [] BasicExtractor: extracted value ([id1_2_] : [BIGINT]) - [8] [15:14:58.185] [TRACE] [] BasicExtractor: extracted value ([name2_2_] : [VARCHAR]) - [Brett Meyer2] [15:14:58.185] [TRACE] [] BasicExtractor: extracted value ([id1_2_] : [BIGINT]) - [11] [15:14:58.185] [TRACE] [] BasicExtractor: extracted value ([name2_2_] : [VARCHAR]) - [Brett Meyer3] [15:14:58.185] [TRACE] [] BasicExtractor: extracted value ([id1_2_] : [BIGINT]) - [14] [15:14:58.185] [TRACE] [] BasicExtractor: extracted value ([name2_2_] : [VARCHAR]) - [Brett Meyer4] [15:14:58.198] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.projects#2] [15:14:58.199] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.skills#2] [15:14:58.208] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.projects#5] [15:14:58.208] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.skills#5] [15:14:58.209] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.projects#8] [15:14:58.209] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.skills#8] [15:14:58.210] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.projects#11] [15:14:58.210] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.skills#11] [15:14:58.211] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.projects#14] [15:14:58.211] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.skills#14] [15:14:58.246] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.projects#2] [15:14:58.246] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.skills#2] Brett Meyer0 [15:14:58.248] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.projects#2] [15:14:58.248] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.skills#2] [15:14:58.249] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.projects#5] [15:14:58.249] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.skills#5] [15:14:58.250] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.projects#8] [15:14:58.250] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.skills#8] [15:14:58.250] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.projects#11] [15:14:58.250] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.skills#11] [15:14:58.251] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.projects#14] [15:14:58.251] [TRACE] [] CollectionType: Created collection wrapper: [com.zhou.test.hibernate.cache.entity.User.skills#14] false
1.4 hibernate的N+1问题。
一般而言说n+1意思是，无论在一对多还是多对一当查询出n条数据之后，每条数据会关联的查询1次他的关联对象，这就叫做n+1
对需要N+1的地方（不直接查询出对象）解决方法：
1.延迟加载，当需要的时候才查询，不需要就不查询，但是感觉这种方式治标不治本，尤其是在那种报表统计查询的时候更为明显。
2.fetch="join"，默认是fetch="select"，这个其实说白了就是一个做外连接，允许外键为空的情况之下。
不需要N+1的地方则相反。
