Mybatis的缓存

  public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
    
    List<E> list;
    try {
      queryStack++;
      	//localCache 一级缓存
      list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
        //先从一级缓存中获取，key是通过sql语句生成
      if (list != null) {
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
        // 如果缓存中没有 才从数据库查询
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
      }
    } finally {
      queryStack--;
    }
    return list;
  }

  //从数据库读取数据
  private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    List<E> list;
    localCache.putObject(key, EXECUTION_PLACEHOLDER);
    try {
      list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {
      localCache.removeObject(key);//将一级缓存清除
    }
    localCache.putObject(key, list);//返回查询结果之前，先放入一级缓存 刷新
    if (ms.getStatementType() == StatementType.CALLABLE) {
      localOutputParameterCache.putObject(key, parameter);
    }
    return list;
  }

二级缓存

二级缓存mapper中的，默认是开启的，但需要在映射文件mapper.xml中添加<cache/>标签

<mapper namespace="userMapper">
	<cache/><!-- 添加cache标签表示此mapper使用二级缓存 -->
</mapper>

mybatis:
  configuration:
     cache-enabled: false #默认值为true，表示开启

  private void configurationElement(XNode context) {
    try {
      //...
      cacheElement(context.evalNode("cache")); //解析cache标签
    } catch (Exception e) {
      throw new BuilderException("Error parsing Mapper XML. The XML location is '" + resource + "'. Cause: " + e, e);
    }
  }

  private void cacheElement(XNode context) {
    if (context != null) { // if hava cache tag 如果有cache标签才执行下面的逻辑
      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);//建立二级缓存
    }
  }

  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);//二级缓存赋值，如果cache标签为空，不会执行此方法，currentCache为空
    currentCache = cache; 
    return cache;
  }

  @Override
  public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
      throws SQLException {
    Cache cache = ms.getCache(); 
    if (cache != null) {//如果二级缓存对象不为空 尝试在二级缓存中获取（没有cache标签此对象就是空）
      flushCacheIfRequired(ms);
      if (ms.isUseCache() && resultHandler == null) {
        ensureNoOutParams(ms, boundSql);
        @SuppressWarnings("unchecked")
        List<E> list = (List<E>) tcm.getObject(cache, key); //从二级缓存中获取数据
        if (list == null) {
          list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); //如果为空，使用delegate查询（BaseExecutor）
          tcm.putObject(cache, key, list); // 查询结果保存到二级缓存
        }
        return list;
      }
    }
    return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }