总览

InputStream is = Resources.getResourceAsStream(“mybatis.xml”);
SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(is);
session = factory.openSession();
mapper = session.getMapper(StudentDao.class);

正文

本文只关注第三步，看源码的方式就是按住ctrl一阵点点点。

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

public ExecutorType getDefaultExecutorType() {
  return defaultExecutorType;
}
protected ExecutorType defaultExecutorType = ExecutorType.SIMPLE;

传入了ExecutorType为SIMPLE

直接调

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();
  }
}

东西很多，由返回值可以知道：
SqlSession的默认实现是DefaultSqlSession

番外：Executor

由上文可以知道：
DefaultSqlSession管理一个执行器Executor
Executor管理一个事务Transaction。

新建

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

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;
}

这里有很多个Executor：
BatchExecutor
ReuseExecutor
SimpleExecutor：初始的
CachingExecutor：带缓存的

最后为执行器加入了一系列拦截器

public class InterceptorChain {

  private final List<Interceptor> interceptors = new ArrayList<>();

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

番外：Transaction

管理事务用的

public interface Transaction {
  Connection getConnection() throws SQLException;
  void commit() throws SQLException;
  void rollback() throws SQLException;
  void close() throws SQLException;
  Integer getTimeout() throws SQLException;
}

回到之前的代码

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();
  }
}

工厂模式

private TransactionFactory getTransactionFactoryFromEnvironment(Environment environment) {
  if (environment == null || environment.getTransactionFactory() == null) {
    return new ManagedTransactionFactory();
  }
  return environment.getTransactionFactory();
}

配置了就用，没有就是Managed

public TransactionFactory getTransactionFactory() {
  return this.transactionFactory;
}
private final TransactionFactory transactionFactory;

没找到，应该是解析的时候创建好了。

回到解析的时候：

private void parseConfiguration(XNode root) {
  try {
    // issue #117 read properties first
    propertiesElement(root.evalNode("properties"));
    Properties settings = settingsAsProperties(root.evalNode("settings"));
    loadCustomVfs(settings);
    loadCustomLogImpl(settings);
    typeAliasesElement(root.evalNode("typeAliases"));
    pluginElement(root.evalNode("plugins"));
    objectFactoryElement(root.evalNode("objectFactory"));
    objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
    reflectorFactoryElement(root.evalNode("reflectorFactory"));
    settingsElement(settings);
    // read it after objectFactory and objectWrapperFactory issue #631
    environmentsElement(root.evalNode("environments"));
    databaseIdProviderElement(root.evalNode("databaseIdProvider"));
    typeHandlerElement(root.evalNode("typeHandlers"));
    mapperElement(root.evalNode("mappers"));
  } catch (Exception e) {
    throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
  }
}

关键看environments

private void environmentsElement(XNode context) throws Exception {
  if (context != null) {
    if (environment == null) {
      environment = context.getStringAttribute("default");
    }
    for (XNode child : context.getChildren()) {
      String id = child.getStringAttribute("id");
      if (isSpecifiedEnvironment(id)) {
        TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager"));
        DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource"));
        DataSource dataSource = dsFactory.getDataSource();
        Environment.Builder environmentBuilder = new Environment.Builder(id)
          .transactionFactory(txFactory)
          .dataSource(dataSource);
        configuration.setEnvironment(environmentBuilder.build());
        break;
      }
    }
  }
}

只看TransactionFactory

private TransactionFactory transactionManagerElement(XNode context) throws Exception {
  if (context != null) {
    String type = context.getStringAttribute("type");
    Properties props = context.getChildrenAsProperties();
    TransactionFactory factory = (TransactionFactory) resolveClass(type).getDeclaredConstructor().newInstance();
    factory.setProperties(props);
    return factory;
  }
  throw new BuilderException("Environment declaration requires a TransactionFactory.");
}

这是配置文件应有的样子

<transactionManager type="JDBC"/>

应该是对JDBC做了什么处理

protected <T> Class<? extends T> resolveClass(String alias) {
  if (alias == null) {
    return null;
  }
  try {
    return resolveAlias(alias);
  } catch (Exception e) {
    throw new BuilderException("Error resolving class. Cause: " + e, e);
  }
}

非空，走下面

protected <T> Class<? extends T> resolveAlias(String alias) {
  return typeAliasRegistry.resolveAlias(alias);
}

这是解决缩写的地方
那缩写在哪呢，配置对象的构造里面就有：

public Configuration() {
  typeAliasRegistry.registerAlias("JDBC", JdbcTransactionFactory.class);
  typeAliasRegistry.registerAlias("MANAGED", ManagedTransactionFactory.class);
  typeAliasRegistry.registerAlias("JNDI", JndiDataSourceFactory.class);
  typeAliasRegistry.registerAlias("POOLED", PooledDataSourceFactory.class);
  typeAliasRegistry.registerAlias("UNPOOLED", UnpooledDataSourceFactory.class);
  typeAliasRegistry.registerAlias("PERPETUAL", PerpetualCache.class);
  typeAliasRegistry.registerAlias("FIFO", FifoCache.class);
  typeAliasRegistry.registerAlias("LRU", LruCache.class);
  typeAliasRegistry.registerAlias("SOFT", SoftCache.class);
  typeAliasRegistry.registerAlias("WEAK", WeakCache.class);
  typeAliasRegistry.registerAlias("DB_VENDOR", VendorDatabaseIdProvider.class);
  typeAliasRegistry.registerAlias("XML", XMLLanguageDriver.class);
  typeAliasRegistry.registerAlias("RAW", RawLanguageDriver.class);
  typeAliasRegistry.registerAlias("SLF4J", Slf4jImpl.class);
  typeAliasRegistry.registerAlias("COMMONS_LOGGING", JakartaCommonsLoggingImpl.class);
  typeAliasRegistry.registerAlias("LOG4J", Log4jImpl.class);
  typeAliasRegistry.registerAlias("LOG4J2", Log4j2Impl.class);
  typeAliasRegistry.registerAlias("JDK_LOGGING", Jdk14LoggingImpl.class);
  typeAliasRegistry.registerAlias("STDOUT_LOGGING", StdOutImpl.class);
  typeAliasRegistry.registerAlias("NO_LOGGING", NoLoggingImpl.class);
  typeAliasRegistry.registerAlias("CGLIB", CglibProxyFactory.class);
  typeAliasRegistry.registerAlias("JAVASSIST", JavassistProxyFactory.class);
  languageRegistry.setDefaultDriverClass(XMLLanguageDriver.class);
  languageRegistry.register(RawLanguageDriver.class);
}

此时就找到了JdbcTransactionFactory这个类

回到之前的代码：

TransactionFactory factory = (TransactionFactory) resolveClass(type).getDeclaredConstructor().newInstance();

这是反射创建新JdbcTransactionFactory实例。

public class JdbcTransactionFactory implements TransactionFactory {

  @Override
  public Transaction newTransaction(Connection conn) {
    return new JdbcTransaction(conn);
  }

  @Override
  public Transaction newTransaction(DataSource ds, TransactionIsolationLevel level, boolean autoCommit) {
    return new JdbcTransaction(ds, level, autoCommit);
  }
}

此时，得到了JdbcTransaction为默认的事务管理器

我们只看些简单的，比如提交事务

@Override
public void commit() throws SQLException {
  if (connection != null && !connection.getAutoCommit()) {
    if (log.isDebugEnabled()) {
      log.debug("Committing JDBC Connection [" + connection + "]");
    }
    connection.commit();
  }
}

Mybatis源码——获取SqlSession的过程

总览

正文

番外：Executor

番外：Transaction