关于mybatis里面的Executor--转载

原文地址：http://blog.csdn.net/w_intercool/article/details/7893344

使用mybatis查寻数据，跟踪其执行流程

最开始执行的语句

this.getSqlSession().selectList("QUERY-QUESTION", data, rowBounds);

这里需要找到sqlsession是从哪里来的

getSqlSession是SqlSessionDaoSupport类里面的方法，该类通过spring的自动注入可以把sqlSessionTemplate注入进来，当然这里的sqlSessionTemplate是需要spring配置的

@Autowired(required = false)
public final void setSqlSessionTemplate(SqlSessionTemplate sqlSessionTemplate) {
  this.sqlSession = sqlSessionTemplate;
  this.externalSqlSession = true;
}

<bean id="sqlSession" class="org.mybatis.spring.SqlSessionTemplate">
        <constructor-arg index="0" ref="sqlSessionFactory"/>
        <constructor-arg index="1" value="BATCH"/>
    </bean>

@Autowired(required = false)是通过类型匹配来注入的，如果没有找到相应对，就不用注入

所以selectList方法为SqlSessionTemlate里面的，再看SqlSessionTemplage，里面的都是通过sqlSessionProxy来执行selectList方法的，也就是通过代理方式来的

public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory, ExecutorType executorType,
    PersistenceExceptionTranslator exceptionTranslator) {

  notNull(sqlSessionFactory, "Property 'sqlSessionFactory' is required");
  notNull(executorType, "Property 'executorType' is required");

  this.sqlSessionFactory = sqlSessionFactory;
  this.executorType = executorType;
  this.exceptionTranslator = exceptionTranslator;
  this.sqlSessionProxy = (SqlSession) newProxyInstance(
      SqlSessionFactory.class.getClassLoader(),
      new Class[] { SqlSession.class },
      new SqlSessionInterceptor());
}

这里用到了java的动态代理，详细可以见java api，有详细的说明

SqlSessionInterceptor实现了InvocationHandler，在invoke方法里面的开始有这样代码，那里是真正的sqlsession

final SqlSession sqlSession = getSqlSession(
         SqlSessionTemplate.this.sqlSessionFactory,
         SqlSessionTemplate.this.executorType,
         SqlSessionTemplate.this.exceptionTranslator);

跟踪geteSqlSession可以找到他的创建来源，见

SqlSession session = sessionFactory.openSession(executorType);

继续跟踪可以找到DefaultSqlSessionFactory里面的该方法

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, autoCommit);
    return new DefaultSqlSession(configuration, executor);
  } 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();
  }
}

通过

final Executor executor = configuration.newExecutor(tx, execType, autoCommit);

你就知道executor是怎么回来的了

mybatis默认使用了cache，在创建exector时，外面就包了一层CacheExecutor，详细见

public Executor newExecutor(Transaction transaction, ExecutorType executorType, boolean autoCommit) {
  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, autoCommit);
  }
  executor = (Executor) interceptorChain.pluginAll(executor);
  return executor;
}

CachingExecutor可以使mybatis先从缓存中提取数据，数据缓存中没有数据时才从数据库里面提取数据。