Executor介绍

1.Executor介绍：

Executor是mybatis的核心接口之一，其中定义了数据库操作的基本方法，它的子类结构图如下：这这张关系图中，涉及到了模板方法模式和装饰器模式。BaseExecutor是一个抽象父类，定义了一级缓存和事务相关的公共的固定不变的方法，同时定义了doUpdate，doQuery等抽象方法，这些抽象方法是在BaseExecutor的四个子类中实现的。这里用到了模板方法模式。对于CachingExecutor这个类，它为Executor提供了二级缓存的功能，它是一个装饰类，被装饰的目标类是BaseExecutor的四个子类，比如是SimpleExecutor，所以这里用到了装饰器模式。

2.源码分析：

2.1 Executor源码：

public interface Executor {

  ResultHandler NO_RESULT_HANDLER = null;
  // 执行update，insert，delete三种类型的SQL
  int update(MappedStatement ms, Object parameter) throws SQLException;
  // 执行select类型的SQL
  <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey cacheKey, BoundSql boundSql) throws SQLException;

  <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException;

  // 批量执行SQL
  List<BatchResult> flushStatements() throws SQLException;
  // 事务提交
  void commit(boolean required) throws SQLException;
  // 事务回滚
  void rollback(boolean required) throws SQLException;
  // 创建一级缓存中使用到的CacheKey对象
  CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql);

  boolean isCached(MappedStatement ms, CacheKey key);
  // 清空一级缓存
  void clearLocalCache();

  void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class<?> targetType);
  // 获取事务对象
  Transaction getTransaction();

  void close(boolean forceRollback);

  boolean isClosed();

  void setExecutorWrapper(Executor executor);

}

2.2 BaseExecutor

BaseExecutor是一个实现了Executor的抽象类，它实现了Executor的大部分方法，比如：一级缓存管理和事务管理的基本功能，它的四个子类只需要实现doQuery，doUpdate等抽象方法来完成数据库的相关操作。

public abstract class BaseExecutor implements Executor {

  private static final Log log = LogFactory.getLog(BaseExecutor.class);

  protected Transaction transaction;  // transaction对象
  protected Executor wrapper; // 封装的executor对象 

  protected ConcurrentLinkedQueue<DeferredLoad> deferredLoads;
  protected PerpetualCache localCache; // 一级缓存 用于缓存该Executor对象查询结果集映射得到的结果对象
  protected PerpetualCache localOutputParameterCache;
  protected Configuration configuration;

  protected int queryStack = 0;
  private boolean closed;
 
  // 获取transaction对象
  public Transaction getTransaction() {
    if (closed) throw new ExecutorException("Executor was closed.");
    return transaction;
  }

 public int update(MappedStatement ms, Object parameter) throws SQLException {
    ErrorContext.instance().resource(ms.getResource()).activity("executing an update").object(ms.getId());
    if (closed) throw new ExecutorException("Executor was closed.");
    clearLocalCache();
    return doUpdate(ms, parameter);
  }

  // 批量执行多条缓存的SQL
  public List<BatchResult> flushStatements(boolean isRollBack) throws SQLException {
    if (closed) throw new ExecutorException("Executor was closed.");
    return doFlushStatements(isRollBack);
  }

  public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    BoundSql boundSql = ms.getBoundSql(parameter);
    CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
    return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
 }
  
  // 先查询一级缓存，如果未命中，则再查询数据库
  @SuppressWarnings("unchecked")
  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;
      if (list != null) {
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
        // 此处会调用在子类中实现的doQuery方法，默认的子类是SimpleExecutor
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
      }
    } finally {
      queryStack--;
    }
    if (queryStack == 0) {
      for (DeferredLoad deferredLoad : deferredLoads) {
        deferredLoad.load();
      }
      deferredLoads.clear(); // issue #601
      if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
        clearLocalCache(); // issue #482
      }
    }
    return list;
  }

  // 事务提交
  public void commit(boolean required) throws SQLException {
    if (closed) throw new ExecutorException("Cannot commit, transaction is already closed");
    clearLocalCache();
    flushStatements();
    if (required) {
      transaction.commit();
    }
  }
  
  // 事务回滚
  public void rollback(boolean required) throws SQLException {
    if (!closed) {
      try {
        clearLocalCache();
        flushStatements(true);
      } finally {
        if (required) {
          transaction.rollback();
        }
      }
    }
  }
  
  // 需要在子类中实现的update操作
  protected abstract int doUpdate(MappedStatement ms, Object parameter)
      throws SQLException;

  protected abstract List<BatchResult> doFlushStatements(boolean isRollback)
      throws SQLException;
 
  // 需要在子类中实现的query操作
  protected abstract <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql)
      throws SQLException;
  }

}

由上面的query方法可知，BaseExecutor提供了一级缓存的查询功能，如果一级缓存未命中，则再会调用在它的子类中实现的doQuery方法，默认的子类是SimpleExecutor。

2.3 SimpleExecutor

SimpleExecutor继承了BaseExecutor抽象类，它是最简单的Executor接口的实现。这里使用到了模板方法模式，一级缓存和事务管理等固定不变的操作封装到了BaseExecutor这个父类中，而update，query等与数据库交互的基本方法，都在子类中实现的。

public class SimpleExecutor extends BaseExecutor {

  public SimpleExecutor(Configuration configuration, Transaction transaction) {
    super(configuration, transaction);
  }
  // upate，insert，delete操作
  public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
    Statement stmt = null;
    try {
      Configuration configuration = ms.getConfiguration();
      StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null);
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.update(stmt);
    } finally {
      closeStatement(stmt);
    }
  }
  // query操作
  public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Statement stmt = null;
    try {
      Configuration configuration = ms.getConfiguration();
      // 创建StatementHandler对象
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      // 完成Statement的创建和初始化
      stmt = prepareStatement(handler, ms.getStatementLog());
      // 调用StatementHandler.query方法，执行SQL语句，并通过ResultSetHandler完成结果集的映射
      return handler.<E>query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

}

对于另外三个子类这里就不做分析了。接下来介绍CachingExecutor这个类

2.4 CachingExecutor

对于CachingExecutor，通过装饰器模式，为Executor增加了二级缓存的功能，接下来就对它的属性和主要方法做个分析：

2.4.1 属性：

  private Executor delegate; // 被装饰的类，也成为目标类
  private TransactionalCacheManager tcm = new TransactionalCacheManager(); // 用于管理Cahce和TransactionCache，也就是和二级缓存相关的

2.4.2 查询方法：

  public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
      throws SQLException {
    Cache cache = ms.getCache();
    // 如果映射配置文件中配置了<cache/>,则此处cache！= null
    if (cache != null) {
      flushCacheIfRequired(ms);
      // 查询节点的useCache配置以及是否是否使用了resultHandler
      if (ms.isUseCache() && resultHandler == null) {
        ensureNoOutParams(ms, parameterObject, boundSql);
        @SuppressWarnings("unchecked")
        // 查询二级缓存
        List<E> list = (List<E>) tcm.getObject(cache, key);
        if (list == null) {
          // 如果二级缓存没有查询出结果，会调用封装的Executor对象的query方法，其中会先查询一级缓存，再执行数据库查询操作
          list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
          tcm.putObject(cache, key, list); // issue #578. Query must be not synchronized to prevent deadlocks
        }
        return list;
      }
    }
    // 没有启动二级缓存，直接调用底层Executor执行数据库的查询操作
    return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }

 2.4.3 事务提交：

  public void commit(boolean required) throws SQLException {
    delegate.commit(required); // 调用底层的Executor提交事务
    tcm.commit(); // 遍历所有的TransactionCahce对象执行commit方法
  }

关于Executor就暂时说这么多吧，其实这些内容在其他文章中都有提到，这里不过是做一个简单的分类总结。