Spring Cache之Ehcache和Memcached

小鱼儿 2022-09-24 06:29 200阅读 0赞

spring框架从3.1版本开始提供了缓存支持：在spring-context.jar里的org.springframework.cache包，以及spring-context-support.jar里的org.springframework.cache包；而且提供了基于ConcurrentHashMap、JCacheCache、EhCache、GuavaCache的实现。  
这里我们先看下基于EhCache的使用，然后考虑[集成Memcached][Memcached]；版本：spring3.2和spring4，EhCache2.7，spyMemcached2.8；  
内容还涉及HashMap、LinkedHashMap、synchronizedMap、ConcurrentHashMap、ReentrantLock……   
[参考资料：spring framework 4.0.x reference][spring framework 4.0.x reference][wang chaoqun][]

## 一、EhCache配置 ##

### 1. 添加相关jar，添加ehcache.xml ###

属性意义基本明确，这里我配置了一个名称是notice\_cache的cache节点，其他的可以按此添加。

### 2. spring-cache.xml ###

配置cacheManager和cacheManagerFactory，并将ehcache.xml配入即可

### 3. @Cacheable\\@CacheEvict\\@CachePut\\@Caching… ###

注解加在相应方法上，支持spel，详细参见文档[查阅spring 4.0.x reference][spring framework 4.0.x reference]

至此配置完了，run一下，报错：没有序列化，将vo实现Serializable接口

标签：[spring][]

这样ehcache集成完了，get方法对同一条记录只从数据库查询一次，cache是成功的，但search方法却一直读库，这里没有设置cache的key，设置的话如果是固定的，那么每次结果集都一样，不会更新；文档说不设key，将使用默认key生成器DefaultKeyGenerator：

问题就在于object.hashCode()，看方法的参数string没问题，date没问题，Integer数组使用的就是Object类的hashCode是个内存地址，每次执行都变，要改用Arrays.hashCode(array)才不会变；  
当然，分页类page也要重写hashCode；顺便说下，apache的commons-lang.jar提供了EqualsBuilder、HashCodeBuilder、ToStringBuilder可用于重写各方法。还要注意：分页列表不仅要缓存list，还要缓存分页信息，这样到前端才会分页，否则是不知道这个list是多少页的，故方法的返回值（上面search方法只返回list是不行的）可采用类似`org.springframework.data.domain.Page`内部包含结果集

#### 4. 自定义key生成器 ####

解决上面问题：重写生成器（继承DefaultKeyGenerator，需要注意的是对于param是list,set,map取hashcode，其泛型类也要重写hashCode方法）并配置:

#### 5. 添加、更新、删除 ####

显然@Cacheable是缓存，@CacheEvict是擦除，@CachePut相当于擦除后再缓存，对于key是确定的很好，比如getById(id)，update(obj)，其key可以用id，obj.id；update时也可以用@CachePut，要注意update方法要返回更新后的obj，void不行。

问题又出现了：不明确的key如何更新？例如search，当新添加一条记录后，就不能使用@CacheEvict(value=“notice\_cache”, key=“?”)，因为取不到key，也不能模糊匹配；这种情况下只能使用@CacheEvict(value = “notice\_cache”, allEntries = true)，将notice\_cache所有的缓存擦除，多少有点粗糙，而memcached甚至没有某个cache的removeAll，这就要自己写个MemcachedCache

通常注解使用在service方法上，还有一个注意事项：因其使用aop的动态代理，对于内部调用无效，例如publish方法没加注解，内部调用update方法（加了@CachePut）更新状态值，但cache不会更新；controller方法（不加注解）调用service方法（加了注解）是可以；当然controller方法也可以加，需要单独处理，因为参数若有request、response之类，每次请求都变，就要在keyGenerator里做过滤了。

## 二、集成Memcached ##

背景：现在的项目使用memcached做缓存，基本上是编码式的，在需要的时候，生成key，将value转为json再set到缓存，因此打算使用注解式更优雅的处理，就需要实现spring cache的相关接口和自定义一些方法

#### 1. spring集成Memcached，使用spyMemcached ####

properties:

标签：[技术][Link 1]

#### 2. 实现MemcachedCacheManager和MemcachedCache ####

参考ehcache的源码（org.springframework.cache.ehcache包里）：EhCacheCache和EhCacheCacheManager，manager用来获取cache，重写了getCache和loadCaches方法，这样配置在ehcache.xml里的cache name都会实例化成每个EhCacheCache，当执行到@Cacheable的方法上，就会调用getCache(name)获取cache，再根据key取得value；

MemcachedCacheManager：

这样应用启动时实例化manager，在执行加缓存注解的的方法时，会调用getCache(获取或新建cache)，根据缓存的key从cache中取值（没有就读库，然后将结果加入cache，下次相同的key就能取到缓存的值了）  
要写MemcachedCache实现`org.springframework.cache.Cache`接口，先来分析EhCacheCache：

好了，来写memcachedCache，问题来了：  
1.clear方法，spy的client没有removeAll，clear之类的方法，有个flush是全部清空，服务器N多个cache都会擦掉  
2.@CacheEvict(value = “notice\_cache”, allEntries = true)就是用的clear，“添加个notice都要清掉其他非notice\_cache缓存”就很可怕，能不能根据cache名称清除呢？  
3.上面两个实际是一个问题，memcached是key-value存储，所以要对key进行分组，采用一个集合保存key，然后将实际的key-value存入；如果想模糊匹配也是可行的，需要在此基础上做修改：key就得用字符串而不是字符串的hashCode了，或者自定义注解

常用的集合数据类型如list，map，set它也支持，考虑到key的字符限制和单个value不超过1MB，使用一个set存储一个cache里所有的key能达到2万以上(看key的字节数)，使用压缩存储的更多，同时使用LRU（如LinkedHashMap，将过期的或长期不用的移除），基本满足使用  
标签：[memcached][]

MemcachedCache：

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 <td style="margin:0px; padding:0px; border:0px; font-style:inherit; font-variant:inherit; line-height:inherit; font-family:inherit; font-size:18.4px; vertical-align:middle; width:749px"> <pre style="margin-top:0px; margin-bottom:0px; padding:0px; border:none; font-style:inherit; font-variant:inherit; font-weight:inherit; line-height:1.45em; font-family:Menlo,Monaco,&quot;Andale Mono&quot;,&quot;lucida console&quot;,&quot;Courier New&quot;,monospace; font-size:13px; vertical-align:baseline; color:rgb(147,161,161); overflow:auto; background:none"><code style="margin:0px; padding:0.8em; border:0px; font-style:inherit; font-variant:inherit; font-weight:inherit; line-height:1.45em; vertical-align:baseline; overflow-y:hidden; display:block; overflow-x:auto; font-family:Menlo,Monaco,&quot;Andale Mono&quot;,&quot;lucida console&quot;,&quot;Courier New&quot;,monospace!important; background:rgb(0,0,0)">public class MemcachedCache implements Cache { private static final String PRESENT = new String(); // 单个cache存储的key最大数量 private static final int maxElements = 10000; // 默认过期时间10天 private static final int expire = 10 * 24 * 60 * 60; private String name; private MemcachedClient client; // 存储key的集合，使用LinkedHashMap实现 private KeySet keys; public MemcachedCache() {} public MemcachedCache(String name, MemcachedClient client) { this.name = name; this.client = client; this.keys = new KeySet(maxElements); } public String getName() { return this.name; } public Object getNativeCache() { return this.client; } // ckey是key+cacheName作为前缀，也是最终存入缓存的key public ValueWrapper get(Object key) { String ckey = toStringWithCacheName(key); if (keys.containsKey(ckey)) { Object value = client.get(ckey); return value != null ? new SimpleValueWrapper(value) : null; } else { return null; } } // 将ckey加入key集合并将ckey-value存入缓存 public void put(Object key, Object value) { String ckey = toStringWithCacheName(key); keys.put(ckey, PRESENT); client.set(ckey, expire, value); } // 从keys集合清除ckey，并从缓存清除 public void evict(Object key) { String ckey = toStringWithCacheName(key); keys.remove(ckey); client.delete(ckey); } private String toStringWithCacheName(Object obj) { return name + "." + String.valueOf(obj); } // 遍历清除 public void clear() { for (String ckey : keys.keySet()) { client.delete(ckey); } keys.clear(); } public MemcachedClient getClient() { return this.client; } public void setClient(MemcachedClient client) { this.client = client; } public KeySet getKeys() { return this.keys; } } </code></pre> </td> 
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这里keys也可以使用cacheName作为key存入缓存，就需要在put,evict,clear方法里使用`client.replace(name, expire, keys);`保持更新

KeySet继承LinkedHashMap，为了使用removeEldestEntry，满了移除最旧元素，保持initSize:

#### 3. 线程安全 ####

因为要存储keys，所以考虑使用哪种集合：HashSet\\HashMap都不是线程安全的，例如[Java HashMap的死循环][Java HashMap];  
安全的如Collections.synchronizedMap和ConcurrentHashMap（不允许value为null）；  
两者的区别是锁不同：synchronizedMap使用对象锁，相当于在方法上声明synchronized；ConcurrentHashMap比较复杂，在segment上加锁，将范围控制的很小，因而并发性能就高；  
这里使用LinkedHashMap，ConcurrentHashMap不好包装，synchronizedMap效率低，不如加个ReentrantLock，或者使用读写锁ReentrantReadWriteLock(但这篇文章介绍了读写锁可能存在问题：[小心LinkedHashMap的get()方法][LinkedHashMap_get])：

下面是HashMap占用cpu 100% bug的代码：

#### 4. Spring 4.0.x Cache ####

以上3.2.x使用正常，4.0版本改动了key生成器，源码也很简单：SimpleKeyGenerator和SimpleKey，toString方法将参数转为字符串，嫌长就改用hashCode

事实上，对于复杂的，类似Object数组（下面会有），那么无论是上面自定义keyGenerator还是spring4.0的都会有问题（hashCode和toString，不一致就取不到cache，会每次都读库），测试简单数组如下：

这个测试Arrays.hashCode(array)和SimpleKey.toString()多次运行是一致的，也就是自定义keyGenerator和SimpleKeyGenerator正确

复杂的：`Object[] mixed = new Object[] { 1, "11", array, list };`

结论：多次运行发现只有Arrays.deepToString和Arrays.deepHashCode是一致的，也就是对每个元素，如果是数组再递归;同理，如果是集合list,set,map之类，最好使用泛型，类型一致，不要混合

## 三、总结 ##

spring cache用好要注意很多：  
1、搞清各注解意义和使用时机，逻辑正确，更新一致  
2、缓存key的使用很重要，自定义key要考虑参数重写hashCode和toString  
3、返回结果如分页结果集，不仅要有list还要有page  
4、可虑清楚并测试加了Cacheable确实生效？  
5、效益最大化：使用多注解多缓存的情景，一次方法执行缓存多个信息（要更新时也得多个更新，才能保持一致）

原文地址： [http://wangchaoqun.com/blog/2014/04/spring-cache-zhi-ehcache-he-memcached.html/][http_wangchaoqun.com_blog_2014_04_spring-cache-zhi-ehcache-he-memcached.html]

[Memcached]: http://wangchaoqun.com/blog/2014/04/spring-cache-zhi-ehcache-he-memcached.html/#memcached
[spring framework 4.0.x reference]: http://docs.spring.io/spring/docs/4.0.x/spring-framework-reference/html/cache.html
[wang chaoqun]: http://wangchaoqun.com/
[spring]: http://wangchaoqun.com/blog/categories/spring
[Link 1]: http://wangchaoqun.com/blog/categories/tech
[memcached]: http://wangchaoqun.com/blog/categories/memcached
[Java HashMap]: http://coolshell.cn/articles/9606.html
[LinkedHashMap_get]: http://skydream.iteye.com/blog/1562880
[http_wangchaoqun.com_blog_2014_04_spring-cache-zhi-ehcache-he-memcached.html]: http://wangchaoqun.com/blog/2014/04/spring-cache-zhi-ehcache-he-memcached.html/