mybatis 缓存实现

mybatis查询缓存分为一级缓存(相同会话SqlSession中Executor的PerpetualCache=localCache)和二级缓存(相同MappedStatement的id共用缓存)

1. 一级缓存的创建:BaseExecutor.PerpetualCache (PerpetualCache内部为HashMap)

  protected PerpetualCache localCache = new PerpetualCache("LocalCache");

2. 二级缓存的使用:解析@CacheNamespace或者使用<cache />
   注解org.apache.ibatis.annotations.CacheNamespace 使用org.apache.ibatis.builder.annotation.MapperAnnotationBuilder解析

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface CacheNamespace {
  Class<? extends org.apache.ibatis.cache.Cache> implementation() default PerpetualCache.class;

  Class<? extends org.apache.ibatis.cache.Cache> eviction() default LruCache.class;

  long flushInterval() default 0;

  int size() default 1024;

  boolean readWrite() default true;
  
  boolean blocking() default false;
  
}


private void parseCache() {
    CacheNamespace cacheDomain = type.getAnnotation(CacheNamespace.class);
    if (cacheDomain != null) {
      Integer size = cacheDomain.size() == 0 ? null : cacheDomain.size();
      Long flushInterval = cacheDomain.flushInterval() == 0 ? null : cacheDomain.flushInterval();
      assistant.useNewCache(cacheDomain.implementation(), cacheDomain.eviction(), flushInterval, size, cacheDomain.readWrite(), cacheDomain.blocking(), null);
    }
  }

xml解析:org.apache.ibatis.builder.xml.XMLMapperBuilder

private void cacheElement(XNode context) throws Exception {
    if (context != null) {
      String type = context.getStringAttribute("type", "PERPETUAL");
      Class<? extends Cache> typeClass = typeAliasRegistry.resolveAlias(type);
      String eviction = context.getStringAttribute("eviction", "LRU");
      Class<? extends Cache> evictionClass = typeAliasRegistry.resolveAlias(eviction);
      Long flushInterval = context.getLongAttribute("flushInterval");
      Integer size = context.getIntAttribute("size");
      boolean readWrite = !context.getBooleanAttribute("readOnly", false);
      boolean blocking = context.getBooleanAttribute("blocking", false);
      Properties props = context.getChildrenAsProperties();
      builderAssistant.useNewCache(typeClass, evictionClass, flushInterval, size, readWrite, blocking, props);
    }
  }

3. 二级缓存的创建及包装:build通过参数flushInterval size readWrite blocking进行包装

public Cache useNewCache(Class<? extends Cache> typeClass,
      Class<? extends Cache> evictionClass,
      Long flushInterval,
      Integer size,
      boolean readWrite,
      boolean blocking,
      Properties props) {
    Cache cache = new CacheBuilder(currentNamespace)
        .implementation(valueOrDefault(typeClass, PerpetualCache.class))
        .addDecorator(valueOrDefault(evictionClass, LruCache.class))
        .clearInterval(flushInterval)
        .size(size)
        .readWrite(readWrite)
        .blocking(blocking)
        .properties(props)
        .build();
    configuration.addCache(cache);
    currentCache = cache;
    return cache;
  }

4. 内置的cache实现类和包装类（cache实现细节自己慢慢品味吧尤其是LruCache SoftCache WeakCache）