mybatis一級緩存與二級緩存的原理

1.mybatis中的緩存是在mybatis框架中的Executor中來實現的，我們來看一下Executor的繼承圖

2.通過以上類圖我們可以發現Executor接口下有兩大實現類BaseExecutor與CachingExecutor

（1）BaseExecutor（用來存儲我們的一級緩存）

@Override
  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());
    if (closed) {
      throw new ExecutorException("Executor was closed.");
    }
    if (queryStack == 0 && ms.isFlushCacheRequired()) {
      clearLocalCache();
    }
    List<E> list;
    try {
      queryStack++;
      list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;  //會先去localCache中去查找我們的數據 if (list != null) {
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);  //如不存在則會去數據庫中查找
      }
    } finally {
      queryStack--;
    }
    if (queryStack == 0) {
      for (DeferredLoad deferredLoad : deferredLoads) {
        deferredLoad.load();
      }
      // issue #601
      deferredLoads.clear();
      if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
        // issue #482
        clearLocalCache();
      }
    }
    return list;
  }

 

（2）CachingExecutor（是裝飾器模式的實現，用來查詢我們的二級緩存）

@Override
  public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
      throws SQLException {
    Cache cache = ms.getCache();    //在MappedStatement中去獲取二級緩存的類型。 if (cache != null) {
      flushCacheIfRequired(ms);
      if (ms.isUseCache() && resultHandler == null) {
        ensureNoOutParams(ms, boundSql);
        @SuppressWarnings("unchecked")
        List<E> list = (List<E>) tcm.getObject(cache, key); //通過tcm來查詢緩存，而tcm是CachingExecutor中的變量TransactionalCacheManager if (list == null) {
          list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); //delegate為CachingExecutor中保存的BaseExecutor的引用，若二級緩存不存在回去調用BaseExecutor的方法。
          tcm.putObject(cache, key, list); // issue #578 and #116
        }
        return list;
      }
    }

3.當我們開啟二級緩存后Mybatis會使用CachingExecutor去裝飾我們的BaseExecutor,所以會先查詢二級緩存后再去查詢一級緩存。

Configuration中的newExecutor方法
public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
    executorType = executorType == null ? defaultExecutorType : executorType;
    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
    Executor executor;
    if (ExecutorType.BATCH == executorType) {    //判斷Executor的類型
      executor = new BatchExecutor(this, transaction);
    } else if (ExecutorType.REUSE == executorType) {
      executor = new ReuseExecutor(this, transaction);
    } else {
      executor = new SimpleExecutor(this, transaction);
    }
    if (cacheEnabled) {                          //如果二級緩存開啟則使用CachingExecutor去裝飾我們的executor;
      executor = new CachingExecutor(executor);
    }
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }

4.一級緩存與二級緩存的作用范圍

　　（1）一級緩存(由於一級緩存是存在BaseExecutor中的，而Executor又作為創建SqlSession的參數，因此一級緩存具有和sqlsession一樣的生命周期)

這是在SqlSessionFactory中調用openSession（）中調用的方法
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
      final Environment environment = configuration.getEnvironment();
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
      final Executor executor = configuration.newExecutor(tx, execType);  //新建一個Executor作為參數傳給DefaultSqlSession return new DefaultSqlSession(configuration, executor, autoCommit);
    } catch (Exception e) {
      closeTransaction(tx); // may have fetched a connection so lets call close()
      throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

　　（2）二級緩存(使用CachingExecutor來裝飾)

//CachingExecutor中的query方法
@Override
  public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
      throws SQLException {
    Cache cache = ms.getCache();        //由此可見我們的二級緩存並不是儲存在CachingExecutor中的，而是從MappedStatement中去獲取。因此mybatis的二級緩存的生命周期為mapper級別的 if (cache != null) {
      flushCacheIfRequired(ms);
      if (ms.isUseCache() && resultHandler == null) {
        ensureNoOutParams(ms, boundSql);
        @SuppressWarnings("unchecked")
        List<E> list = (List<E>) tcm.getObject(cache, key);
        if (list == null) {
          list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
          tcm.putObject(cache, key, list); // issue #578 and #116
        }
        return list;
      }
    }
    return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }