public DefaultSqlSession(Configuration configuration, Executor executor, boolean autoCommit) {
  this.configuration = configuration;
  this.executor = executor;
  this.dirty = false;
  this.autoCommit = autoCommit;
 }

 protected BaseExecutor(Configuration configuration, Transaction transaction) {
  this.transaction = transaction;
  this.deferredLoads = new ConcurrentLinkedQueue<>();
  this.localCache = new PerpetualCache("LocalCache"); //默認(rèn)一級緩存
  this.localOutputParameterCache = new PerpetualCache("LocalOutputParameterCache");
  this.closed = false;
  this.configuration = configuration;
  this.wrapper = this;
 }

 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 一級緩存
   list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
    //先從一級緩存中獲取，key是通過sql語句生成
   if (list != null) {
    handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
   } else {
    // 如果緩存中沒有 才從數(shù)據(jù)庫查詢
    list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
   }
  } finally {
   queryStack--;
  }
  return list;
 }

 //從數(shù)據(jù)庫讀取數(shù)據(jù)
 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);//將一級緩存清除
  }
  localCache.putObject(key, list);//返回查詢結(jié)果之前，先放入一級緩存 刷新
  if (ms.getStatementType() == StatementType.CALLABLE) {
   localOutputParameterCache.putObject(key, parameter);
  }
  return list;
 }

<mapper namespace="userMapper">
	<cache/><!-- 添加cache標(biāo)簽表示此mapper使用二級緩存 -->
</mapper>

 private void configurationElement(XNode context) {
  try {
   //...
   cacheElement(context.evalNode("cache")); //解析cache標(biāo)簽
  } 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標(biāo)簽才執(zhí)行下面的邏輯
   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);//建立二級緩存
  }
 }

 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);//二級緩存賦值，如果cache標(biāo)簽為空，不會執(zhí)行此方法，currentCache為空
  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) {//如果二級緩存對象不為空 嘗試在二級緩存中獲?。]有cache標(biāo)簽此對象就是空）
   flushCacheIfRequired(ms);
   if (ms.isUseCache() && resultHandler == null) {
    ensureNoOutParams(ms, boundSql);
    @SuppressWarnings("unchecked")
    List<E> list = (List<E>) tcm.getObject(cache, key); //從二級緩存中獲取數(shù)據(jù)
    if (list == null) {
     list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); //如果為空，使用delegate查詢（BaseExecutor）
     tcm.putObject(cache, key, list); // 查詢結(jié)果保存到二級緩存
    }
    return list;
   }
  }
  return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
 }