之前介绍过，Executor接口定义了对象操作库操作的基本方法：select/update/insert/delete/commit/rollbak/close。Mybatis对Executor接口的实现主要采用了模板模式和装饰模式两种设计模式。

Executor类关系

其中CloseExecutor为一个类的内部了，而且是private的，先不讨论！

BaseExecutor为模板模式中的模板类。这个类在Executor接口实现中非常重要，其实现了Executor的大部分方法。他的子类只要实现三个方法即可，其中两个是doUpdate和doSelect方法，子类在实现这两个方法时直接操作数据库即可，其余的工作交由BaseExecutor完成。
CachingExecutor是一个Executor的装饰器，给一个Executor增加了缓存的功能。

模板模式和装饰模式前面分析加载Configuration中也用到，说明这个设计模式的重要性，这两种设计模式也在Spring中很多地方采用。作为java程序员应该详细解读并应用这两种设计模式！

Executor接口主要方法

public interface Executor {
   //执行update/insert/delete
  int update(MappedStatement ms, Object parameter) throws SQLException;
  //执行查询
  <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey cacheKey, BoundSql boundSql) throws SQLException;
  //执行查询
  <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException;
  //以后有机会在分析 
  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;
  //生成缓存的key
  CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql);
  
  boolean isCached(MappedStatement ms, CacheKey key);

  void clearLocalCache();

  void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class<?> targetType);

  Transaction getTransaction();

  void close(boolean forceRollback);

  boolean isClosed();
  
  void setExecutorWrapper(Executor executor);

}

BaseExecutor

再来看看BaseExcutor模板是怎么实现Executor的

BaseExecutor有两个主要的属性:事务及本地缓存

protected BaseExecutor(Configuration configuration, Transaction transaction) {
    //transaction，实现commit/rollback/close
    this.transaction = transaction;
    this.deferredLoads = new ConcurrentLinkedQueue<DeferredLoad>();
    //本地缓存，也就是一级缓存
    this.localCache = new PerpetualCache("LocalCache");
    this.localOutputParameterCache = new PerpetualCache("LocalOutputParameterCache");
    this.closed = false;
    this.configuration = configuration;
    this.wrapper = this;
  }

再来看看BaseExecutor是怎么实现查询和更新的

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();
    //调用了doUpdate方法完成更新，这个方法是抽象的，由子类实现
    return doUpdate(ms, parameter);
  }

 protected abstract int doUpdate(MappedStatement ms, Object parameter)
      throws SQLException;

   //查询
  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 {
     //调用doQuery方法，这个方法也是抽象的
      list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {
      localCache.removeObject(key);
    }
    localCache.putObject(key, list);
    if (ms.getStatementType() == StatementType.CALLABLE) {
      localOutputParameterCache.putObject(key, parameter);
    }
    return list;
  }

protected abstract <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql)
      throws SQLException;

CachingExecutor实现缓存

 public CachingExecutor(Executor delegate) {
    //操作数据库的动作都是由这个Executor来完成的。
    this.delegate = delegate;
    delegate.setExecutorWrapper(this);
  }

对于Mybatis的缓存以后再详细分析。

Mybatis3源码分析(10)-Executor接口实现方式

Executor类关系

Executor接口主要方法

BaseExecutor

CachingExecutor实现缓存

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