mybatis源码分析（2）-----SqlSession创建

1. 在创建好sqlSessionFactory之后，接着就要配置sqlSession的创建。

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

• 构造参数，包括sqlSessionFactory对象，以及ExecutorType（simple）

2. sqlSession接口

• 我们的应用程序，是直接注入sqlSessionTemplate ，操作数据库

simpleTempalte.delete(Statement.getStatement(CxCaseMapper.class, "deleteById"), id);

• 实现类sqlSessionTemplate

public class SqlSessionTemplate implements SqlSession {
  //session 工场的引用
  private final SqlSessionFactory sqlSessionFactory;
  // 对于数据库的操作类型
  private final ExecutorType executorType;
  //sqlSession代理
  private final SqlSession sqlSessionProxy;
  private final PersistenceExceptionTranslator exceptionTranslator;
}

• sqlSession对每一个数据库的操作，实际上是引用代理对象sqlSessionProxy 对于目标方法的执行。

  @Override
  public int delete(String statement) {
    return this.sqlSessionProxy.delete(statement);
  }

• 在构造方法中，给代理对象已经其他属性赋予的默认值

    public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory, ExecutorType executorType,
      PersistenceExceptionTranslator exceptionTranslator) {
    this.sqlSessionFactory = sqlSessionFactory;
    this.executorType = executorType;
    this.exceptionTranslator = exceptionTranslator;
    //构造代理对象
    this.sqlSessionProxy = (SqlSession) newProxyInstance(
        SqlSessionFactory.class.getClassLoader(),
        new Class[] { SqlSession.class },
        new SqlSessionInterceptor());
  }

  private class SqlSessionInterceptor implements InvocationHandler {
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
　　　 //这里获取的sqlSession 其实是DefaultSqlSession
      SqlSession sqlSession = getSqlSession(
          SqlSessionTemplate.this.sqlSessionFactory,
          SqlSessionTemplate.this.executorType,
          SqlSessionTemplate.this.exceptionTranslator);
      try {
        // 代理对象对于目标对象的调用，其实是defaultSqlSession 对于目标方法的调用
        Object result = method.invoke(sqlSession, args);
        if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
          // force commit even on non-dirty sessions because some databases require
          // a commit/rollback before calling close()
          sqlSession.commit(true);
        }
        return result;
      } catch (Throwable t) {
        Throwable unwrapped = unwrapThrowable(t);
        if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {
          // release the connection to avoid a deadlock if the translator is no loaded. See issue #22
          closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
          sqlSession = null;
          Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException) unwrapped);
          if (translated != null) {
            unwrapped = translated;
          }
        }
        throw unwrapped;
      } finally {
        if (sqlSession != null) {
          //调用目标方法后，关闭Session。 后续会重点讲解
          closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
        }
      }
    }
  }

• defaultSqlSession 对于目标方法的执行

public class DefaultSqlSession implements SqlSession {

  private Configuration configuration;
  private Executor executor;

  private boolean autoCommit;
  private boolean dirty;

  @Override
  public void select(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler) {
    try {
      MappedStatement ms = configuration.getMappedStatement(statement);
      executor.query(ms, wrapCollection(parameter), rowBounds, handler);
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

}

3.   代理的优点

　　无论是多个dao使用一个SqlSessionTemplate，还是一个dao使用一个SqlSessionTemplate，SqlSessionTemplate都是对应一个sqlSession，当多个web线程调用同一个dao时，它们使用的是同一个SqlSessionTemplate，也就是同一个SqlSession，保证线程安全，

4 .总结