ibatis源码学习(一)整体设计和核心流程

	Class.forName("com.mysql.jdbc.Driver");
	String url = "jdbc:mysql://localhost:3306/learnworld";
	Connection con = DriverManager.getConnection(url, "root","learnworld");			
	String sql = "select * from test";
	PreparedStatement ps = con.prepareStatement(sql);			
	ResultSet rs = ps.executeQuery();
	while(rs.next()){
		System.out.println("id=" + rs.getInt(1)+". score=" + rs.getInt(2));
	}

    <bean id="sqlMapClient" class="org.springframework.orm.ibatis.SqlMapClientFactoryBean">
        <property name="dataSource" ref="dataSource" />
        <property name="configLocation" value="classpath/sqlmap/sqlmap-ibatis.xml" />
    </bean>

	public void afterPropertiesSet() throws Exception {
		...

		this.sqlMapClient = buildSqlMapClient(this.configLocations, this.mappingLocations, this.sqlMapClientProperties);   //初始化核心方法，构建sqlMapClient对象

		...
	}

	protected SqlMapClient buildSqlMapClient(
			Resource[] configLocations, Resource[] mappingLocations, Properties properties)
			throws IOException {
                ...
		SqlMapClient client = null;
		SqlMapConfigParser configParser = new SqlMapConfigParser();
		for (int i = 0; i < configLocations.length; i++) {
			InputStream is = configLocations[i].getInputStream();
			try {
				client = configParser.parse(is, properties); //通过SqlMapConfigParser解析配置文件，生成SQLMapClientImpl对象
			}
			catch (RuntimeException ex) {
				throw new NestedIOException("Failed to parse config resource: " + configLocations[i], ex.getCause());
			}
		}
		...
		return client;
	}

	public Object queryForObject(final String statementName, final Object parameterObject)
			throws DataAccessException {

		return execute(new SqlMapClientCallback() {
			public Object doInSqlMapClient(SqlMapExecutor executor) throws SQLException {
				return executor.queryForObject(statementName, parameterObject);
			}
		});
	}

public Object execute(SqlMapClientCallback action) throws DataAccessException {
                ...
		SqlMapSession session = this.sqlMapClient.openSession(); //获取session信息
		
		Connection ibatisCon = null;     //获取Connection

		try {
			Connection springCon = null;
			DataSource dataSource = getDataSource();
			boolean transactionAware = (dataSource instanceof TransactionAwareDataSourceProxy);

			try {
				ibatisCon = session.getCurrentConnection();
				if (ibatisCon == null) {
					springCon = (transactionAware ?
							dataSource.getConnection() : DataSourceUtils.doGetConnection(dataSource));
					session.setUserConnection(springCon);
				...
				}
			}
			...

			// Execute given callback...
			try {
				return action.doInSqlMapClient(session); //执行SQL
			}
			...
			finally {
                        // 关闭Connection
				try {
					if (springCon != null) {
						if (transactionAware) {
							springCon.close(); 
						}
						else {
							DataSourceUtils.doReleaseConnection(springCon, dataSource);
						}
					}
				}
				
			if (ibatisCon == null) {
				session.close(); //关闭session
			}
		}

	public Object queryForObject(final String statementName, final Object parameterObject)
			throws DataAccessException {

		return execute(new SqlMapClientCallback() {
        //这里的SqlMapExecutor对象类型为SqlMapSessionImpl
			public Object doInSqlMapClient(SqlMapExecutor executor) throws SQLException {
				return executor.queryForObject(statementName, parameterObject);
			}
		});
	}

  public Object queryForObject(String id, Object paramObject) throws SQLException {
    return delegate.queryForObject(session, id, paramObject);
  }

  public Object queryForObject(SessionScope session, String id, Object paramObject, Object resultObject) throws SQLException {
    Object object = null;

    MappedStatement ms = getMappedStatement(id); //MappedStatement对象集是上文中提及的初始化方法SqlMapClientFactoryBean.afterPropertiesSet()中，由配置文件构建而成
    Transaction trans = getTransaction(session); // 用于事务执行
    boolean autoStart = trans == null;

    try {
      trans = autoStartTransaction(session, autoStart, trans);

      RequestScope request = popRequest(session, ms);  // 从RequestScope池中获取该次sql执行中的上下文环境RequestScope 
      try {
        object = ms.executeQueryForObject(request, trans, paramObject, resultObject);   // 执行sql
      } finally {
        pushRequest(request);  //归还RequestScope
      }

      autoCommitTransaction(session, autoStart);
    } finally {
      autoEndTransaction(session, autoStart);
    }

    return object;
  }

  public Object executeQueryForObject(RequestScope request, Transaction trans, Object parameterObject, Object resultObject)
      throws SQLException {
    try {
      Object object = null;

      DefaultRowHandler rowHandler = new DefaultRowHandler();
      executeQueryWithCallback(request, trans.getConnection(), parameterObject, resultObject, rowHandler, SqlExecutor.NO_SKIPPED_RESULTS, SqlExecutor.NO_MAXIMUM_RESULTS); //执行sql语句
      
      //结果处理，返回结果
      List list = rowHandler.getList(); 
      if (list.size() > 1) {
        throw new SQLException("Error: executeQueryForObject returned too many results.");
      } else if (list.size() > 0) {
        object = list.get(0);
      }

      return object;
    } 
    ....
  }

  protected void executeQueryWithCallback(RequestScope request, Connection conn, Object parameterObject, Object resultObject, RowHandler rowHandler, int skipResults, int maxResults)
      throws SQLException {
      ...
    try {
      parameterObject = validateParameter(parameterObject);  //验证入参

      Sql sql = getSql();  //获取SQL对象

      errorContext.setMoreInfo("Check the parameter map.");
      ParameterMap parameterMap = sql.getParameterMap(request, parameterObject);// 入参映射

      errorContext.setMoreInfo("Check the result map.");
      ResultMap resultMap = sql.getResultMap(request, parameterObject); //结果映射

      request.setResultMap(resultMap);
      request.setParameterMap(parameterMap);

      errorContext.setMoreInfo("Check the parameter map.");
      Object[] parameters = parameterMap.getParameterObjectValues(request, parameterObject);  //获取参数值

      errorContext.setMoreInfo("Check the SQL statement.");
      String sqlString = sql.getSql(request, parameterObject);  //获取拼装后的sql语句

      errorContext.setActivity("executing mapped statement");
      errorContext.setMoreInfo("Check the SQL statement or the result map.");
      RowHandlerCallback callback = new RowHandlerCallback(resultMap, resultObject, rowHandler);
      sqlExecuteQuery(request, conn, sqlString, parameters, skipResults, maxResults, callback);  //sql执行

      errorContext.setMoreInfo("Check the output parameters.");
      if (parameterObject != null) {
        postProcessParameterObject(request, parameterObject, parameters);
      }

      errorContext.reset();
      sql.cleanup(request);
      notifyListeners();
      ....
  }

  public void executeQuery(RequestScope request, Connection conn, String sql, Object[] parameters, int skipResults, int maxResults, RowHandlerCallback callback) throws SQLException {
    ...
    PreparedStatement ps = null;
    ResultSet rs = null;
    setupResultObjectFactory(request);
    try {
      errorContext.setMoreInfo("Check the SQL Statement (preparation failed).");
      Integer rsType = request.getStatement().getResultSetType();
      //初始化PreparedStatement，设置sql、参数值等
      if (rsType != null) {
        ps = prepareStatement(request.getSession(), conn, sql, rsType);
      } else {
        ps = prepareStatement(request.getSession(), conn, sql);
      }
      setStatementTimeout(request.getStatement(), ps);
      Integer fetchSize = request.getStatement().getFetchSize();
      if (fetchSize != null) {
        ps.setFetchSize(fetchSize.intValue());
      }
      errorContext.setMoreInfo("Check the parameters (set parameters failed).");
      request.getParameterMap().setParameters(request, ps, parameters);
      errorContext.setMoreInfo("Check the statement (query failed).");
      ps.execute(); //执行
      errorContext.setMoreInfo("Check the results (failed to retrieve results).");

      // ResultSet处理
      rs = handleMultipleResults(ps, request, skipResults, maxResults, callback);
    } finally {
      try {
        closeResultSet(rs);
      } finally {
        closeStatement(request.getSession(), ps);
      }
    }

  }

为什么spring的实现

每次都重新生成一个SqlMapSessionImpl，为何不调用SqlMapClientImpl类的方法，它的方法是线程安全的

SqlMapClientImpl实现线程安全的方式是，通过内部维护一个ThreadLocal localSqlMapSession，在每个线程中都有一份SqlMapSessionImpl副本，SqlMapClientImpl中主要方法处理都是转交给SqlMapSessionImpl处理。如果调用SqlMapClientImpl，效果是一样的，可以看一下SqlMapClientImpl.getLocalSqlMapSession()的实现:

  protected SqlMapSessionImpl getLocalSqlMapSession() {
    SqlMapSessionImpl sqlMapSession = (SqlMapSessionImpl) localSqlMapSession.get();  //从本地ThreadLocal获取
    if (sqlMapSession == null || sqlMapSession.isClosed()) {
      sqlMapSession = new SqlMapSessionImpl(this);  //新建对象
      localSqlMapSession.set(sqlMapSession);
    }
    return sqlMapSession;
  }

  protected SqlMapSessionImpl getLocalSqlMapSession() {
    SqlMapSessionImpl sqlMapSession = (SqlMapSessionImpl) localSqlMapSession.get();  //从本地ThreadLocal获取
    if (sqlMapSession == null || sqlMapSession.isClosed()) {
      sqlMapSession = new SqlMapSessionImpl(this);  //新建对象
      localSqlMapSession.set(sqlMapSession);
    }
    return sqlMapSession;
  }

   调用SqlMapClientImpl最起码一个线程了共享一个SqlMapSessionImpl,但是现在是每次请求都生成一个新的SqlMapSessionImpl，相对线程ThreadLocal一个线程里多个请求就需要多个SqlMapSessionImpl对象，虽然SqlMapSessionImpl对象实际上只是一个空壳而已

  protected SqlMapSessionImpl getLocalSqlMapSession() {
    SqlMapSessionImpl sqlMapSession = (SqlMapSessionImpl) localSqlMapSession.get();  //从本地ThreadLocal获取
    if (sqlMapSession == null || sqlMapSession.isClosed()) {
      sqlMapSession = new SqlMapSessionImpl(this);  //新建对象
      localSqlMapSession.set(sqlMapSession);
    }
    return sqlMapSession;
  }

使用SqlMapClientImpl是可以实现线程安全的，我没有清晰理解spring为何这样实现(每次重新创建)，或许spring本身的实现就不是最好的，呵呵，求达人解释~ 一个可能的原因是，同一个容器处理线程在处理不同请求时，如果使用ThreadLocal，需要在每一个请求处理时完成对，对ThreadLocal中的Session状态进行清理。 而每次重新创建SqlMapSessionImpl，会简化这些操作。