hibernate--一级缓存_二级缓存_查询缓存（面试）

一级缓存：session级别的缓存

二级缓存：sessionFactory级别的缓存，可以跨session存在（一般在对项目要求的效率很高的情况下考虑使用）

二级缓存适合经常访问、数据量有限、改动不大的情况下。

关于使用二级缓存的相关配置：

1）hibernate文档中的ehcache.xml:

<ehcache>    <!-- Sets the path to the directory where cache .data files are created.         If the path is a Java System Property it is replaced by         its value in the running VM.         The following properties are translated:         user.home - User's home directory         user.dir - User's current working directory         java.io.tmpdir - Default temp file path -->    <diskStore path="java.io.tmpdir"/>    <!--Default Cache configuration. These will applied to caches programmatically created through        the CacheManager.        The following attributes are required for defaultCache:        maxInMemory       - Sets the maximum number of objects that will be created in memory        eternal           - Sets whether elements are eternal. If eternal,  timeouts are ignored and the element                            is never expired.        timeToIdleSeconds - Sets the time to idle for an element before it expires. Is only used                            if the element is not eternal. Idle time is now - last accessed time        timeToLiveSeconds - Sets the time to live for an element before it expires. Is only used                            if the element is not eternal. TTL is now - creation time        overflowToDisk    - Sets whether elements can overflow to disk when the in-memory cache                            has reached the maxInMemory limit.        -->    <defaultCache        maxElementsInMemory="10000"        eternal="false"        timeToIdleSeconds="120"        timeToLiveSeconds="120"        overflowToDisk="true"        />    <!--Predefined caches.  Add your cache configuration settings here.        If you do not have a configuration for your cache a WARNING will be issued when the        CacheManager starts        The following attributes are required for defaultCache:        name              - Sets the name of the cache. This is used to identify the cache. It must be unique.        maxInMemory       - Sets the maximum number of objects that will be created in memory        eternal           - Sets whether elements are eternal. If eternal,  timeouts are ignored and the element                            is never expired.        timeToIdleSeconds - Sets the time to idle for an element before it expires. Is only used                            if the element is not eternal. Idle time is now - last accessed time        timeToLiveSeconds - Sets the time to live for an element before it expires. Is only used                            if the element is not eternal. TTL is now - creation time        overflowToDisk    - Sets whether elements can overflow to disk when the in-memory cache                            has reached the maxInMemory limit.        -->    <!-- Sample cache named sampleCache1        This cache contains a maximum in memory of 10000 elements, and will expire        an element if it is idle for more than 5 minutes and lives for more than        10 minutes.        If there are more than 10000 elements it will overflow to the        disk cache, which in this configuration will go to wherever java.io.tmp is        defined on your system. On a standard Linux system this will be /tmp"        -->    <cache name="sampleCache1"        maxElementsInMemory="10000"        eternal="false"        timeToIdleSeconds="300"        timeToLiveSeconds="600"        overflowToDisk="true"        />    <!-- Sample cache named sampleCache2        This cache contains 1000 elements. Elements will always be held in memory.        They are not expired. -->    <cache name="sampleCache2"        maxElementsInMemory="1000"        eternal="true"        timeToIdleSeconds="0"        timeToLiveSeconds="0"        overflowToDisk="false"        /> -->    <!-- Place configuration for your caches following --></ehcache>

2）cfg.xml中的配置：

 <!-- Disable the second-level cache  -->        <property name="cache.provider_class">org.hibernate.cache.internal.NoCacheProvider</property>  //表示没有使用二级缓存使用二级缓存

   <property name="hibernate.cache.use_second_level_cache">true</property>      <property name="cache.provider_class">org.hibernate.cache.ehcache.EhCacheRegionFactory</property>

3）@cache在类申明之前注解

Load默认使用二级缓存、iterator默认使用二级缓存

list默认使用二级缓存加载数据，但是查询的时候不使用，可以通过setCache()使用

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三、查询缓存（对同一条sql语句重复使用才起作用）：

配合二级缓存使用：

      <property name="hibernate.cache.use_second_level_cache">true</property>      <property name="cache.provider_class">org.hibernate.cache.ehcache.EhCacheRegionFactory</property>      <property name="hibernate.cache.use_query_cache">true</property>

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缓存算法（纯为了面试）：对于内存中的缓存空间已经满了之后，再往缓存中加入对象时，如何拿出一些对象？
LRU、LFU、FIFO:
1、:Least Recently Used

2、Least Frequently Used

3、First In First Out