Mybatis深入剖析 -Executor初始化分析

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Executor的初始化

• 
      跟随SqlSession，Executory是SqlSession的对象属性
  
  
•     依赖-Transaction，执行时需要根据特定的事务，进行commit，rollback，close操作
  
•     Executory的创建过程如下
  

•     在Configuration的配置文件中执行全局的ExecutoryType，有BATCH，REUSE，SIMPLE，如果Cache开启的话还有CachingExecutor，他是前三种Executor的代理类
  

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) {
executor = new CachingExecutor(executor);
}
executor = (Executor) interceptorChain.pluginAll(executor);
return executor;
}
 
   2.     在SqlSessionFactory创建SqlSession时，根据configuration创建Executor

private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
Connection connection = null;
try {
final Environment environment = configuration.getEnvironment();
final DataSource dataSource = getDataSourceFromEnvironment(environment);
TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
connection = dataSource.getConnection();
if (level != null) {
connection.setTransactionIsolation(level.getLevel());
}
connection = wrapConnection(connection);
Transaction tx = transactionFactory.newTransaction(connection, autoCommit);
Executor executor = configuration.newExecutor(tx, execType);
return new DefaultSqlSession(configuration, executor, autoCommit);
} catch (Exception e) {
closeConnection(connection);
throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
 
   3.    各种Executor的分析

• SimpleExecutor-简单的Executor，所有的数据库操作均委托给StatementHandler处理，代码如下
  

Configuration configuration = ms.getConfiguration();
StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null);
stmt = prepareStatement(handler);
return handler.update(stmt);
 

• BatchExecutor - 批量更新的Executor，使用jdbc的batchupdate方法，使用此Executor，需要手动执行SqlSession的flushStatements，真正出发sql的执行
  
• 
  ReuseExecutor-大体上与SimpleExecutor相同，只是根据sql缓存了jdbc的statement，遇到相同的sql时，省去了statement的创建，提高效率，但需要手动执行SqlSession的flushStatements，清除缓存
  
• CachingExecutor - 在mapper中配置cache，对查询结果做缓存，其中缓存的key根据以下代码存数生成，建议最好不要用mybatis这个层次的缓存，一般业务层的缓存更实用，而且更容易控制
  

public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds)