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Hibernate性能优化

Jun 07, 2016 pm 02:50 PM
hibernateoptimizationOriginalperformance

本文为博主原创,允许转载,但请声明原文地址:http://www.coselding.cn/blog/8/8-141.html (1)大量分页查询时,及时session.clear(),可能导致内存溢出; 大集合遍历:如寻找敏感词汇; 注:内存泄漏,Java本身不会内存泄漏,但是Java调用的底层C会内存泄

本文为博主原创,允许转载,但请声明原文地址:http://www.coselding.cn/blog/8/8-141.html

(1)大量分页查询时,及时session.clear(),可能导致内存溢出;
大集合遍历:如寻找敏感词汇;
 注:内存泄漏,Java本身不会内存泄漏,但是Java调用的底层C会内存泄漏。
(2)1+N问题:(典型面试题)
 一个实体A关联着另一个实体B,查询所有的实体A时,原本一条sql语句就可完成,结果后面却多了N条查询各个A关联的实体B的查询语句的问题,影响性能。
 解决办法:A. 实体B设置为LAZY;
           B.BatchSize多次单条查询合并成少次的多条查询,减少sql次数;
           C.join fetch:设置外连接,把后面要查询的也一起查出来,减少查询次数。
(3)BatchSize多次单条查询合并成少次的多条查询,减少sql次数。
(4)List的Iterator:
   A.List:Query查询到的List是直接一条sql查询所有结果;
   B.Iterator:只查询表的最小主键,接下来每访问一条才发送一条sql语句查询具体的记录,但是Iterator使用了Hibernate的缓存,同一个session查询两次,底层sql只发送一次而已,它会自己去缓存中查询结果是否已经有了,减少sql查询次数。
(5)一级缓存、二级缓存、查询缓存(面试题)
   A.一级缓存:session中的缓存,查询过一次之后的同一个session下再次查询会从缓存中查询;
   B.二级缓存:SessionFactory级别的缓存,可以跨越session,Hibernate没有实现,需要使用第三方二级缓存实现,默认EHCache,需要EHCache和Logging的jar;
     a.打开二级缓存:Hibernate主配置文件设置打开缓存并指定二级缓存实现类;
        @Cache注解:标注需要缓存到二级缓存的实体Entity;
        Ehcache.xml:EHCache二级缓存的配置文件,可设置多个缓存配置,根据
        name选择设置的属性配置;
     b.适用二级缓存:经常访问、改动不大、数量有限;
     c.load、iterator默认使用二级缓存,list有往二级缓存加数据但是查询时不使用;
     d.query若要使用二级缓存,需要打开查询缓存,
        true
        setCachable(true);指明Query打开二级缓存;
     e.缓存算法:LRU最近最少使用、FIFO(先进先出)、LFU(最近最不常使用)
   C.查询缓存:特殊的二级缓存,专门用于查询操作;
(6)二级缓存使用步骤:
   A.导入ehcache的jar和commons-logging的jar;
   B.类路径加入ehcache的配置文件,里面配置缓存配置;
   C.hibernate的全局配置文件配置开启二级缓存;
   D.标签设置要开启二级缓存的实体类;
   E.:usage设置缓存策略,region设置要使用的EHCache配置选项,include设置是否缓存该实体所关联的关联实体。
(7)事务并发处理(面试):
事务基础知识见JDBC笔记;
  A. 悲观锁:解决不可重复读,load添加第三个参数设置;
悲观地认为别人总想着影响自己,直接加锁,防止别人的影响;
  B. 乐观锁:解决多人同时修改,程序中实现;
乐观地认为别人不会来影响自己,加一个记录版本号的属性,@Version,事务开始时检查版本号,到真正要修改时再检查版本号,若版本号不变,则正确执行,版本号变了说明被影响了,报错;
(8)一定要设置连接池!!!
(9)一定注意数据库的全表查询是禁止的,关联关系的多方直接不延迟也是禁止的,Hibernate会把整个表加入内存和一级缓存,很容易导致内存溢出。

本文为博主原创,允许转载,但请声明原文地址:http://www.coselding.cn/blog/8/8-141.html

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