1.Mybatis中的一级缓存

下面提几个问题来说明：
(1).为什么Mybatis中是默认开启一级缓存的，而且只能在单独的sqlSession中有效？

在创建SqlSession的时候，有如下代码：

 SqlSession sqlSession=sqlSessionFactory.openSession();
 --> return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
     --> final Executor executor = configuration.newExecutor(tx, execType);

newExecutor定义如下：

  public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
    executorType = executorType == null ? defaultExecutorType : executorType;
    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
    Executor executor;
    if (ExecutorType.BATCH == executorType) {
      executor = new BatchExecutor(this, transaction);
    } else if (ExecutorType.REUSE == executorType) {
      executor = new ReuseExecutor(this, transaction);
    } else {
      executor = new SimpleExecutor(this, transaction);
    }
    if (cacheEnabled) {    //这里cacheEnabled默认是true,即默认executor是CachingExecutor
      executor = new CachingExecutor(executor);
    }
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }

下面对查询代码进行代码跟踪

return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER); //selectList方法
-->return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); //CachingExector
  --> list = resultHandler == null ? (List<E>) localCache.getObject(key) : null; //这的localCache就是一级缓存
  --> list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);   
      --> localCache.putObject(key, list); //添加一级缓存

所以一级缓存默认是开启的的，而且只能在单独的sqlSession中有效。

(2)为什么在一个sesion中进行增删改会使一级缓存失效？
下面跟踪DefaultSqlSession中的update方法：

 return executor.update(ms, wrapCollection(parameter));
 -->return delegate.update(ms, parameterObject);
    --> clearLocalCache(); //这里会清除一级缓存
        --> localCache.clear();

2.Mybatis中的二级缓存

1.二级缓存的作用域，为什么只有在commit后才有效？
Mybatis中的二级级缓存是需要手动配置的，默认是不开启的。
下面看一下配置的过程

parseConfiguration(parser.evalNode("/configuration"));
-->mapperElement(root.evalNode("mappers"));
   -->XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource,        configuration.getSqlFragments());
   -->mapperParser.parse();
      --> configurationElement(parser.evalNode("/mapper"));
          -->cacheElement(context.evalNode("cache"));

cacheElement定义如下：

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

关注这一句

builderAssistant.useNewCache(typeClass, evictionClass, flushInterval, size, readWrite, blocking, props);
--> configuration.addCache(cache);
--> currentCache = cache; //这个currentCache最后也被添加到statementBuilder中去

可以看出，configuartion中的cache属性就是二级缓存的相关配置。
下面再跟踪一下sqlSession下的selectList方法

  return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  --> Cache cache = ms.getCache(); //此处就是获取二级缓存
  --> List<E> list = (List<E>) tcm.getObject(cache, key);
      -->return getTransactionalCache(cache).getObject(key);
         --> Object object = delegate.getObject(key); //注意这里拿的是delegate的key

  --> tcm.putObject(cache, key, list); // issue #578 and #116
      --> getTransactionalCache(cache).putObject(key, value);
          -->entriesToAddOnCommit.put(key, object); //注意这里加的是entriesToAddOnCommit的key，value

看一下commit的方法：

executor.commit(isCommitOrRollbackRequired(force));
--> tcm.commit();
    -->txCache.commit();
       --> flushPendingEntries();

  //flushPendingEntries方法如下      
  private void flushPendingEntries() {
    for (Map.Entry<Object, Object> entry : entriesToAddOnCommit.entrySet()) {
      delegate.putObject(entry.getKey(), entry.getValue()); //关键这里：将entriesToAddOnCommit中的k-v添加到delegate，这里就解释了为什么在commit后二级缓存才有效。
    }
    for (Object entry : entriesMissedInCache) {
      if (!entriesToAddOnCommit.containsKey(entry)) {
        delegate.putObject(entry, null);
      }
    }
  }

又因为tcm是被final修饰的，所以作用域是整个namespace.