mybatis随笔二之SqlSessionFactory

在上一篇文章我们已经得到了DefaultSqlSessionFactory

@Override
  public SqlSession openSession() {
    return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
  }

在configuration内部默认指定了protected ExecutorType defaultExecutorType = ExecutorType.SIMPLE;

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);
      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();
    }
  }

从代码中我们可知开启了一个jdbc事务，并调用newExecutor得到一个执行期

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;
  }

如果开启了cache，那么会得到一个CachingExecutor，这采用的是装饰设计模式，它的语句执行实际是交给内部的delegate来执行的。
里面值得注意的是使用了(Executor) interceptorChain.pluginAll(executor)将所有的拦截器注入进去形成一个拦截链。

public Object pluginAll(Object target) {
    for (Interceptor interceptor : interceptors) {
      target = interceptor.plugin(target);
    }
    return target;
  }

@Override
    public Object plugin(Object target) {
        return Plugin.wrap(target, this);
    }

其实Plugin是mybatis的一个工具类，用来方便生成代理对象的，注意这个地方只会生成拦截目标是Executor的拦截类的代理对象，如下所示这种就不会调用。

@Intercepts({
        @Signature(type = StatementHandler.class,
                method = "prepare",
                args = {Connection.class})
})

然后返回DefaultSqlSession的一个实例对象

public DefaultSqlSession(Configuration configuration, Executor executor, boolean autoCommit) {
    this.configuration = configuration;
    this.executor = executor;
    this.dirty = false;
    this.autoCommit = autoCommit;
  }

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至此DefaultSqlSessionFactory创建DefaultSqlSession的过程完成。