Mybatis采用redis实现二级缓存

Mybatis的缓存，可以分为一级缓存与二级缓存。一级缓存以一次sqlSession为单位，保存查找的信息。每次有更新操作的时候，例如删除，添加，修改等这些操作，缓存区都会刷新。二级缓存以一个或者多个namespace为单位，保存信息。一个简述，不再细讲。
Mybatis提供了一个Cache接口，在我们要实现自己的缓存机制时，需要自己去实现这个接口。

Cache接口与方法：

下面是源码，方法都是见名析义的，自个加了些注释。

public interface Cache {

/**
* @return The identifier of this cache
*/
String getId();//返回ID，顾名思义，在自己实现接口时，也要定义一个ID属性，并且返回。

/**
* @param key Can be any object but usually it is a {@link CacheKey}
* @param value The result of a select.
*/
/**
设置key-value，就是通过键值对的方式缓存数据，mybatis的一级缓存也是通过Map这样实现的。
*/
void putObject(Object key, Object value);

/**
* @param key The key
* @return The object stored in the cache.
*/
/**
根据key查找value
*/
Object getObject(Object key);

/**
* As of 3.3.0 this method is only called during a rollback
* for any previous value that was missing in the cache.
* This lets any blocking cache to release the lock that
* may have previously put on the key.
* A blocking cache puts a lock when a value is null
* and releases it when the value is back again.
* This way other threads will wait for the value to be
* available instead of hitting the database.
*
*
* @param key The key
* @return Not used
*/
/**
按照key移除value
*/
Object removeObject(Object key);

/**
* Clears this cache instance
* 清除整个缓存空间。
*/
void clear();

/**
* Optional. This method is not called by the core.
* 返回缓存的数目
* @return The number of elements stored in the cache (not its capacity).
*/
int getSize();

/**
* Optional. As of 3.2.6 this method is no longer called by the core.
*
* Any locking needed by the cache must be provided internally by the cache provider.
* 可重入读写锁制
* @return A ReadWriteLock
*/
ReadWriteLock getReadWriteLock();

}

基本是这样子，其中，有几个方法一定要有具体的实现。分别是储值，取值，按key移除，整个清空四个方法。另外，还要实现一个带有ID参数的构造器，否则会抛异常。怎么使用jedis，网上有很多，就不再说了。自己写的代码如下：

import org.apache.ibatis.cache.Cache;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.data.redis.connection.jedis.JedisConnection;
import org.springframework.data.redis.connection.jedis.JedisConnectionFactory;
import org.springframework.data.redis.serializer.JdkSerializationRedisSerializer;
import org.springframework.data.redis.serializer.RedisSerializer;
import redis.clients.jedis.exceptions.JedisConnectionException;

import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

/**
* 测试，Mybatis采用Redis作为二级缓存
*/
public class RedisCacheTest implements Cache {

/** * logger */private static Logger logger = LoggerFactory.getLogger(RedisCacheTest.class);   private String id;private final ReadWriteLock readWriteLock = new ReentrantReadWriteLock();private static JedisConnectionFactory jedisConnectionFactory;private static String expire;public RedisCacheTest() {}public RedisCacheTest(String id) {    if (id == null) {        throw new IllegalArgumentException("Cache instances require an ID");    }    this.id = id;}public String getId() {    return id;}public void putObject(Object key, Object value) {    JedisConnection jedisConnection = null;    jedisConnectionFactory.afterPropertiesSet();    try {        jedisConnection = jedisConnectionFactory.getConnection();        RedisSerializer<Object> serializer = new JdkSerializationRedisSerializer();        jedisConnection.set(serializer.serialize(key), serializer.serialize(value));        jedisConnection.expire(serializer.serialize(key),Integer.valueOf(expire));//设置此次key的失效时间    } catch (JedisConnectionException e) {        e.printStackTrace();        logger.error( e.getMessage());    } finally {        if (jedisConnection != null) {            jedisConnection.close();        }    }}public Object getObject(Object key) {    JedisConnection jedisConnection = null;    jedisConnectionFactory.afterPropertiesSet();    byte[] result;    Object res = null;    try {        jedisConnection = jedisConnectionFactory.getConnection();        RedisSerializer<Object> serializer = new JdkSerializationRedisSerializer();        result = jedisConnection.get(serializer.serialize(key));        res = serializer.deserialize(result);    } catch (JedisConnectionException e) {        e.printStackTrace();    } finally {        if (jedisConnection != null) {            jedisConnection.close();        }    }    return res;}public Object removeObject(Object key) {    jedisConnectionFactory.afterPropertiesSet();    JedisConnection connection = null;    Object result = null;    try {        connection = jedisConnectionFactory.getConnection();        RedisSerializer<Object> serializer = new JdkSerializationRedisSerializer();        result = connection.expire(serializer.serialize(key), 0);    } catch (JedisConnectionException e) {        e.printStackTrace();    } finally {        if (connection != null) {            connection.close();        }    }    return result;}public void clear() {    JedisConnection jedisConnection = null;    jedisConnectionFactory.afterPropertiesSet();    try {        jedisConnection = jedisConnectionFactory.getConnection(); //连接清除数据        jedisConnection.flushDb();        jedisConnection.flushAll();    } catch (JedisConnectionException e) {        e.printStackTrace();    } finally {        if (jedisConnection != null) {            jedisConnection.close();        }    }}public int getSize() {    jedisConnectionFactory.afterPropertiesSet();    int result = 0;    JedisConnection jedisConnection = null;    try {        jedisConnection = jedisConnectionFactory.getConnection();        result = Integer.valueOf(jedisConnection.dbSize().toString());    } catch (JedisConnectionException e) {        e.printStackTrace();    } finally {        if (jedisConnection != null) {            jedisConnection.close();        }    }    return result;}public ReadWriteLock getReadWriteLock() {    return this.readWriteLock;}public static void setJedisConnectionFactory(JedisConnectionFactory jedisConnectionFactory) {    RedisCacheTest.jedisConnectionFactory = jedisConnectionFactory;}public static void setExpires(String expire){    RedisCacheTest.expire = expire;}

}

****依赖****==========关于Mybatis的依赖就不说了，关于redis的依赖主要有以下几个，这些依赖最好不要用最新的正式版，而是用前几个版本的，否则出了错不好找答案。当然，大神忽视。

<dependency>  <groupId>org.apache.directory.studio</groupId>  <artifactId>org.apache.commons.pool</artifactId>  <version>1.6</version></dependency><!-- https://mvnrepository.com/artifact/org.apache.commons/commons-pool2 --><dependency>  <groupId>org.apache.commons</groupId>  <artifactId>commons-pool2</artifactId>  <version>2.0</version></dependency>    <dependency>  <groupId>org.springframework.data</groupId>  <artifactId>spring-data-redis</artifactId>  <version>1.2.1.RELEASE</version></dependency>    <dependency>  <groupId>redis.clients</groupId>  <artifactId>jedis</artifactId>  <version>2.5.0</version></dependency>

**xml配置**---------xml配置主要就是mybatis的mapper文件的设置，一个spring整合mybatis的xml配置。网上有很多，照着用就好了。**spring-redis的配置主要片段如下**

<!-- redis数据源 --><bean id="poolConfig" class="redis.clients.jedis.JedisPoolConfig">    <property name="testOnBorrow" >        <value type="java.lang.Boolean">false</value>    </property></bean><!-- Spring-redis连接池管理工厂 --><bean id="jedisConnectionFactory" class="org.springframework.data.redis.connection.jedis.JedisConnectionFactory">    <property name="hostName" value="127.0.0.1" />    <property name="port">        <value type="java.lang.Integer" >6379</value>    </property>    <property name="password" value="" />    <property name="timeout">        <value type="java.lang.Integer" >10000</value>    </property>    <property name="poolConfig" ref="poolConfig" /></bean>

mapper文件里要新写一个标签，<catch />标签，这个标签主要有四个属性。

给一下官方解释的地址，> http://www.mybatis.org/mybatis-3/sqlmap-xml.html#cache。有四个属性，大概解释一下：type：要使用的缓存类，可以用官方默认的，自己实现了之后，就要换成自己的二级缓存实现类；flushInterval：缓存的刷新周期，要配合下面select标签的flushCache属性使用；size：缓存的数量，默认就是1024；eviction：缓存的刷新策略；有四种，FIFO，先入先出，LRU，最少使用的移除，SOFT与WEAK；============================****mybatis的配置文件，要开启二级缓存的配置：****

<settings>    <setting name="logImpl" value="STDOUT_LOGGING"/>    <!-- 全局映射器启用缓存 *主要将此属性设置完成即可-->    <setting name="cacheEnabled" value="true"/>    <!-- 查询时，关闭关联对象即时加载以提高性能 -->    <setting name="lazyLoadingEnabled" value="false"/>    <!-- 对于未知的SQL查询，允许返回不同的结果集以达到通用的效果 -->    <setting name="multipleResultSetsEnabled" value="true"/>    <!-- 设置关联对象加载的形态，此处为按需加载字段(加载字段由SQL指 定)，不会加载关联表的所有字段，以提高性能 -->    <setting name="aggressiveLazyLoading" value="true"/></settings>

“`

大体上spring里面用redis作mybatis的二级缓存就是这样了。有一个刷新的问题，cache标签设置了flushInterval属性后并没有起到作用，最后是通过设置redis的key失效时间，间接实现的。