mybatis 3.4.2 DefaultSqlSession的配置

无论在 DefaultSqlSessionFactory 还是在 SqlSessionManager 中，session 都是那个 DefaultSqlSession。

SqlSessionManager解决的只是缓存当前线程中的session，实现对session的复用；减少DefaultSqlSessionFactory提供的、每次SQL操作都要重新创建一个新的session实例。

前面几篇文章，从配置文件分析到了session。

一、来看看session是如何配置的：

1、获取session的入口

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

2、配置session

  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();
    }
  }

3、相关的参数

 ExecutorType execType = configuration.getDefaultExecutorType();

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



TransactionIsolationLevel level= null;

boolean autoCommit = false;

TransactionFactory transactionFactory= getTransactionFactoryFromEnvironment(environment);

  Transaction tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);

二、参数的获得

1、ExecutorType

ExecutorType是一个枚举类

public enum ExecutorType {
  SIMPLE, REUSE, BATCH
}

在Configuration类中，设定的defaultExecutorType默认值为ExecutorType.SIMPLE

public class Configuration {
...
protected ExecutorType defaultExecutorType = ExecutorType.SIMPLE;
...
}

ExecutorType的设定，决定session的执行器Executor使用不同的实现类。

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

2、session执行器的设定和封装
ExecutorType决定session执行器的封装策略。

  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;
  }

session执行器executor只能是SimpleExecutor、ReuseExecutor和BatchExecutor三者之一。然后，如果开启了cache，则再装饰CachingExecutor，然后在包装所有插件链的执行器。

executor = (Executor) interceptorChain.pluginAll(executor)

执行器的关系后面单独一篇文章再详细的介绍。

3、TransactionFactory

这个TransactionFactory对应配置文件mybatis-config.xml中environments中的子元素 transactionManager。

   <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC" />
            <dataSource type="POOLED">
                <property name="driver" value="${db.driver}" />
                <property name="url" value="${db.url}" />
                <property name="username" value="${db.username}" />
                <property name="password" value="${db.password}" />
                <property name="poolPingQuery" value="SELECT NOW()" />
                <property name="poolPingEnabled" value="true" />
            </dataSource>
        </environment>
    </environments>

transactionManager 的取值只有两个：JDBC和MANAGED
如果没有设置，mybatis会使用MANAGED设定session。

从Configuration的构造函数中可以看到这些参数的可选值：

  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);
  }

JdbcTransactionFactory 创建的是 JdbcTransaction；
ManagedTransactionFactory创建的是ManagedTransaction。

实际上ManagedTransaction很多方法都是空的，通常使用JdbcTransaction。

Transaction tx 被赋值给执行器 executor：

Transaction tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);

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

执行器executor和configuration一起生成 DefaultSqlSession。

new DefaultSqlSession(configuration, executor, autoCommit);