【Spring源码】SpringBoot_MyBatis_starter源码（二）

发表于 更新于 分类于 ， ， 本文字数： 3.4k 阅读时长 ≈ 12 分钟

SqlSession

好了，继上一节的 SqlSession 继续，一气呵成。

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private class SqlSessionInterceptor implements InvocationHandler {
  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    SqlSession sqlSession = getSqlSession(SqlSessionTemplate.this.sqlSessionFactory,
        SqlSessionTemplate.this.executorType, SqlSessionTemplate.this.exceptionTranslator);
    try {
      // 来到这里调用SqlSession的方法
      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) {
        closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
      }
    }
  }
}

OK，来到 DefaultSqlSession#selectOne：

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@Override
public <T> T selectOne(String statement, Object parameter) {
// Popular vote was to return null on 0 results and throw exception on too many.
  List<T> list = this.selectList(statement, parameter);
  if (list.size() == 1) {
    return list.get(0);
  } else if (list.size() > 1) {
    throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());
  } else {
    return null;
  }
}

看得到，有对象封装的情况下，无论查询一个还是多个，都是使用 List 来接收。

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@Override
public <E> List<E> selectList(String statement, Object parameter) {
  return this.selectList(statement, parameter, RowBounds.DEFAULT);
}

@Override
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
  try {
    MappedStatement ms = configuration.getMappedStatement(statement);
    return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
  } catch (Exception e) {
    throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
  } finally {
    ErrorContext.instance().reset();
  }
}

应该不难理解他是干嘛的，就是存储 SQL 的执行情况。

Executor执行查询

官网上是这么写的：

ExecutorType.SIMPLE：这个执行器类型不做特殊的事情。它为每个语句的执行创建一个新的预处理语句。 ExecutorType.REUSE：这个执行器类型会复用预处理语句。 ExecutorType.BATCH：这个执行器会批量执行所有更新语句，如果 SELECT 在它们中间执行，必要时请把它们区分开来以保证行为的易读性。

那现在默认是启用 Cache 的（装配的时候，注入了 CachingExecutor，他可以说是个装饰类，装饰了 SimpleExecutor，必要的时候缓存数据）：

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public class CachingExecutor implements Executor {
  @Override
  public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    BoundSql boundSql = ms.getBoundSql(parameterObject);
    CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
    return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }
}

BoundSql 可以说是一个执行 jdbc 所需要数据的最小子集，剥离了 MyBatis 的配置，那下一步就是一个 CacheKey，我们来看看怎么获取 CacheKey 的：

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@Override
public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {
  return delegate.createCacheKey(ms, parameterObject, rowBounds, boundSql);
}
// delegate=SimpleExecutor，下面这个方法是在 BaseExecutor 中的
@Override
public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {
  if (closed) {
    throw new ExecutorException("Executor was closed.");
  }
  // 根据一些必要的参数取名
  CacheKey cacheKey = new CacheKey();
  cacheKey.update(ms.getId());
  cacheKey.update(rowBounds.getOffset());
  cacheKey.update(rowBounds.getLimit());
  cacheKey.update(boundSql.getSql());
  List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
  TypeHandlerRegistry typeHandlerRegistry = ms.getConfiguration().getTypeHandlerRegistry();
  // 处理参数
  for (ParameterMapping parameterMapping : parameterMappings) {
    if (parameterMapping.getMode() != ParameterMode.OUT) {
      Object value;
      String propertyName = parameterMapping.getProperty();
      if (boundSql.hasAdditionalParameter(propertyName)) {
        value = boundSql.getAdditionalParameter(propertyName);
      } else if (parameterObject == null) {
        value = null;
      } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
        value = parameterObject;
      } else {
        MetaObject metaObject = configuration.newMetaObject(parameterObject);
        value = metaObject.getValue(propertyName);
      }
      cacheKey.update(value);
    }
  }
  if (configuration.getEnvironment() != null) {
    // issue #176
    cacheKey.update(configuration.getEnvironment().getId());
  }
  return cacheKey;
}

再看 query 方法：

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@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
    throws SQLException {
  Cache cache = ms.getCache();
  // 没有缓存，直接调用下一个Executor查询数据
  if (cache != null) {
    flushCacheIfRequired(ms);
    if (ms.isUseCache() && resultHandler == null) {
      ensureNoOutParams(ms, boundSql);
      @SuppressWarnings("unchecked")
      List<E> list = (List<E>) tcm.getObject(cache, key);
      if (list == null) {
        list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
        tcm.putObject(cache, key, list); // issue #578 and #116
      }
      return list;
    }
  }
  return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
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// BaseExecutor
@SuppressWarnings("unchecked")
@Override
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
  ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
  if (closed) {
    throw new ExecutorException("Executor was closed.");
  }
  if (queryStack == 0 && ms.isFlushCacheRequired()) {
    clearLocalCache();
  }
  List<E> list;
  try {
    queryStack++;
    list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
    if (list != null) {
      handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
    } else {
      // 没有缓存，从数据库查询
      list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
    }
  } finally {
    queryStack--;
  }
  if (queryStack == 0) {
    for (DeferredLoad deferredLoad : deferredLoads) {
      deferredLoad.load();
    }
    // issue #601
    deferredLoads.clear();
    if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
      // issue #482
      clearLocalCache();
    }
  }
  return list;
}

queryFromDatabase 函数：

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private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
  List<E> list;
  localCache.putObject(key, EXECUTION_PLACEHOLDER);
  try {
    // 真正查询出来，然后存入缓存
    list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
  } finally {
    localCache.removeObject(key);
  }
  localCache.putObject(key, list);
  if (ms.getStatementType() == StatementType.CALLABLE) {
    localOutputParameterCache.putObject(key, parameter);
  }
  return list;
}

准备Statement

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@Override
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
  Statement stmt = null;
  try {
    Configuration configuration = ms.getConfiguration();
    StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
    stmt = prepareStatement(handler, ms.getStatementLog());
    return handler.query(stmt, resultHandler);
  } finally {
    closeStatement(stmt);
  }
}

// Configuration
public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
  StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
  // 使用拦截器的方式，修改StatementHandler
  // 目前拦截器链为空，没有配置任何东西
  statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
  return statementHandler;
}

那 StatementHandler 类型的 RoutingStatementHandler 是做什么用的，就是指路用什么方式来实现 jdbc 的调用：

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public class RoutingStatementHandler implements StatementHandler {

  private final StatementHandler delegate;

  public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {

    switch (ms.getStatementType()) {
      case STATEMENT:
        delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case PREPARED:
        delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case CALLABLE:
        delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      default:
        throw new ExecutorException("Unknown statement type: " + ms.getStatementType());
    }

  }
}

那上面拿到了 StatementHandler 以后就是准备跟 Connection 连接在一起了 stmt = prepareStatement(handler, ms.getStatementLog())：

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private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
  Statement stmt;
  Connection connection = getConnection(statementLog);
  stmt = handler.prepare(connection, transaction.getTimeout());
  // 然后设置预编译的查询参数
  handler.parameterize(stmt);
  return stmt;
}
// BaseStatementHandler
@Override
public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
  ErrorContext.instance().sql(boundSql.getSql());
  Statement statement = null;
  try {
    // 初始化Statement
    statement = instantiateStatement(connection);
    // 设置超时时间
    setStatementTimeout(statement, transactionTimeout);
    setFetchSize(statement);
    return statement;
  } catch (SQLException e) {
    closeStatement(statement);
    throw e;
  } catch (Exception e) {
    closeStatement(statement);
    throw new ExecutorException("Error preparing statement.  Cause: " + e, e);
  }
}
// PreparedStatementHandler
@Override
protected Statement instantiateStatement(Connection connection) throws SQLException {
  String sql = boundSql.getSql();
  if (mappedStatement.getKeyGenerator() instanceof Jdbc3KeyGenerator) {
    String[] keyColumnNames = mappedStatement.getKeyColumns();
    if (keyColumnNames == null) {
      return connection.prepareStatement(sql, PreparedStatement.RETURN_GENERATED_KEYS);
    } else {
      return connection.prepareStatement(sql, keyColumnNames);
    }
  } else if (mappedStatement.getResultSetType() == ResultSetType.DEFAULT) {
    // 默认ResultSetType方式直接像我们用 jdbc 一样调用
    return connection.prepareStatement(sql);
  } else {
    return connection.prepareStatement(sql, mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
  }
}

参数处理器

也是一样交给了 PreparedStatementHandler 来处理：

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@Override
public void parameterize(Statement statement) throws SQLException {
  parameterHandler.setParameters((PreparedStatement) statement);
}

使用 ParameterHandler 来处理参数，ParameterHandler 是在装配的时候，根据 mapper.xml 来注入参数处理器，像我们使用 xml 就是被装配了个默认的参数处理器（DefaultParameterHandler）。

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public class DefaultParameterHandler implements ParameterHandler {
  @Override
  public void setParameters(PreparedStatement ps) {
    ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
    // 获取需要的参数mapping
    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
    if (parameterMappings != null) {
      for (int i = 0; i < parameterMappings.size(); i++) {
        ParameterMapping parameterMapping = parameterMappings.get(i);
        if (parameterMapping.getMode() != ParameterMode.OUT) {
          Object value;
          String propertyName = parameterMapping.getProperty();
          if (boundSql.hasAdditionalParameter(propertyName)) { // issue #448 ask first for additional params
            value = boundSql.getAdditionalParameter(propertyName);
          } else if (parameterObject == null) {
            value = null;
          } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
            // 在MyBatis初始化时已经注册了一系列jdk原生类型的处理器
            // 所以这里不用进行转换，直接注入的值就是参数值
            value = parameterObject;
          } else {
            MetaObject metaObject = configuration.newMetaObject(parameterObject);
            value = metaObject.getValue(propertyName);
          }
          // 获取类型处理器进行处理
          TypeHandler typeHandler = parameterMapping.getTypeHandler();
          JdbcType jdbcType = parameterMapping.getJdbcType();
          if (value == null && jdbcType == null) {
            jdbcType = configuration.getJdbcTypeForNull();
          }
          try {
            typeHandler.setParameter(ps, i + 1, value, jdbcType);
          } catch (TypeException  SQLException e) {
            throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
          }
        }
      }
    }
  }
}

类型解析并处理

首先使用了一个 UnknownTypeHandler：

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// BaseTypeHandler.java
@Override
public void setParameter(PreparedStatement ps, int i, T parameter, JdbcType jdbcType) throws SQLException {
  if (parameter == null) {
    if (jdbcType == null) {
      throw new TypeException("JDBC requires that the JdbcType must be specified for all nullable parameters.");
    }
    try {
      ps.setNull(i, jdbcType.TYPE_CODE);
    } catch (SQLException e) {
      throw new TypeException("Error setting null for parameter #" + i + " with JdbcType " + jdbcType + " . "
            + "Try setting a different JdbcType for this parameter or a different jdbcTypeForNull configuration property. "
            + "Cause: " + e, e);
    }
  } else {
    try {
      setNonNullParameter(ps, i, parameter, jdbcType);
    } catch (Exception e) {
      throw new TypeException("Error setting non null for parameter #" + i + " with JdbcType " + jdbcType + " . "
            + "Try setting a different JdbcType for this parameter or a different configuration property. "
            + "Cause: " + e, e);
    }
  }
}

然后，父级模板方法调用了 setNonNullParameter 来设置参数，那既然是 UnknownTypeHandler 就需要先判断类型再做事情了。

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// UnknownTypeHandler.java
@Override
public void setNonNullParameter(PreparedStatement ps, int i, Object parameter, JdbcType jdbcType)
    throws SQLException {
  // 第一步：寻找对应的类型处理器
  TypeHandler handler = resolveTypeHandler(parameter, jdbcType);
  // 然后就继续调用对应处理器的方法，也就是上面那个
  handler.setParameter(ps, i, parameter, jdbcType);
}
private TypeHandler<?> resolveTypeHandler(Object parameter, JdbcType jdbcType) {
  TypeHandler<?> handler;
  if (parameter == null) {
    handler = OBJECT_TYPE_HANDLER;
  } else {
    // 第一步：从默认的注册一系列的jdk类型寻找处理器
    handler = typeHandlerRegistry.getTypeHandler(parameter.getClass(), jdbcType);
    // check if handler is null (issue #270)
    if (handler == null  handler instanceof UnknownTypeHandler) {
      handler = OBJECT_TYPE_HANDLER;
    }
  }
  return handler;
}

那寻找到处理器以后，就是 LongTypeHandler 了，继续调用模板方法，这时候就是调用到 LongTypeHandler 的 setNonNullParameter 方法了：

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// LongTypeHandler.java
@Override
public void setNonNullParameter(PreparedStatement ps, int i, Long parameter, JdbcType jdbcType)
    throws SQLException {
  // jdbc老套路
  ps.setLong(i, parameter);
}

查询数据

回到我们上面查询数据的方法里面来：

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@Override
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
  Statement stmt = null;
  try {
    Configuration configuration = ms.getConfiguration();
    StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
    stmt = prepareStatement(handler, ms.getStatementLog());
    // 接下来处理完成就是调用数据库进行查询了
    return handler.query(stmt, resultHandler);
  } finally {
    closeStatement(stmt);
  }
}

RoutingStatementHandler#query 会路由到 PreparedStatementHandler 里面来，所以现在我们就直接进入 PreparedStatementHandler 看就好了：

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@Override
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
  // 拿到PreparedStatement
  PreparedStatement ps = (PreparedStatement) statement;
  // 执行
  ps.execute();
  // 开始处理结果
  return resultSetHandler.handleResultSets(ps);
}

处理结果封装

resultSetHandler 是一个 DefaultResultSetHandler：

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// DefaultResultSetHandler.java
@Override
public List<Object> handleResultSets(Statement stmt) throws SQLException {
  ErrorContext.instance().activity("handling results").object(mappedStatement.getId());

  // 用于存储多个ResultMap处理结果
  final List<Object> multipleResults = new ArrayList<>();

  int resultSetCount = 0;
  // 获取ResultSet封装对象数据
  ResultSetWrapper rsw = getFirstResultSet(stmt);

  // 获取配置的ResultMap
  List<ResultMap> resultMaps = mappedStatement.getResultMaps();
  /*
  private ResultSetWrapper getFirstResultSet(Statement stmt) throws SQLException {
    // 获取jdbc的ResultSet
    ResultSet rs = stmt.getResultSet();
    while (rs == null) {
      // move forward to get the first resultset in case the driver
      // doesn't return the resultset as the first result (HSQLDB 2.1)
      if (stmt.getMoreResults()) {
        rs = stmt.getResultSet();
      } else {
        if (stmt.getUpdateCount() == -1) {
          // no more results. Must be no resultset
          break;
        }
      }
    }
    // 包装起来
    return rs != null ? new ResultSetWrapper(rs, configuration) : null;
  }
  */

  // 结果集
  int resultMapCount = resultMaps.size();
  // 验证ResultSet是否为空
  validateResultMapsCount(rsw, resultMapCount);
  // 循环resultMap列表来封装对象
  while (rsw != null && resultMapCount > resultSetCount) {
    // 取出ResultMap，根据ResultMap读取ResultSet
    ResultMap resultMap = resultMaps.get(resultSetCount);
    // 处理封装结果集的地方
    handleResultSet(rsw, resultMap, multipleResults, null);
    // 通过stmt.getConnection().getMetaData().supportsMultipleResultSets()判断是否支持多ResultSet，但是MySQL不支持
    rsw = getNextResultSet(stmt);
    cleanUpAfterHandlingResultSet();
    resultSetCount++;
  }

  // 为Null
  String[] resultSets = mappedStatement.getResultSets();
  if (resultSets != null) {
    while (rsw != null && resultSetCount < resultSets.length) {
      ResultMapping parentMapping = nextResultMaps.get(resultSets[resultSetCount]);
      if (parentMapping != null) {
        String nestedResultMapId = parentMapping.getNestedResultMapId();
        ResultMap resultMap = configuration.getResultMap(nestedResultMapId);
        handleResultSet(rsw, resultMap, null, parentMapping);
      }
      rsw = getNextResultSet(stmt);
      cleanUpAfterHandlingResultSet();
      resultSetCount++;
    }
  }

  // 收集ResultList
  return collapseSingleResultList(multipleResults);
}

OK，那接下来就是怎么封装对象的问题：

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// DefaultResultSetHandler.java
private void handleResultSet(ResultSetWrapper rsw, ResultMap resultMap, List<Object> multipleResults, ResultMapping parentMapping) throws SQLException {
  try {
    // 继承ResultMapping的处理
    if (parentMapping != null) {
      handleRowValues(rsw, resultMap, null, RowBounds.DEFAULT, parentMapping);
    } else {
      if (resultHandler == null) {
        // 没有处理器走了这里
        DefaultResultHandler defaultResultHandler = new DefaultResultHandler(objectFactory);
        // 使用默认的结果处理器处理ResultSet数据
        handleRowValues(rsw, resultMap, defaultResultHandler, rowBounds, null);
        multipleResults.add(defaultResultHandler.getResultList());
      } else {
        handleRowValues(rsw, resultMap, resultHandler, rowBounds, null);
      }
    }
  } finally {
    // issue #228 (close resultsets)
    closeResultSet(rsw.getResultSet());
  }
}

public void handleRowValues(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException {
  // 判断是否有嵌套结果集
  // 比如我们查询的时候在xml的ResultMap中查询子集合的数据
  if (resultMap.hasNestedResultMaps()) {
    ensureNoRowBounds();
    checkResultHandler();
    handleRowValuesForNestedResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
  } else {
    // 普通的数据走这里
    handleRowValuesForSimpleResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
  }
}

private void handleRowValuesForSimpleResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping)
  throws SQLException {
  DefaultResultContext<Object> resultContext = new DefaultResultContext<>();
  ResultSet resultSet = rsw.getResultSet();
  skipRows(resultSet, rowBounds);
  while (shouldProcessMoreRows(resultContext, rowBounds) && !resultSet.isClosed() && resultSet.next()) {
    // 判断配置需要使用到的ResultMap
    ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(resultSet, resultMap, null);
    // 转换成对象
    Object rowValue = getRowValue(rsw, discriminatedResultMap, null);
    // 存储到resultContext中
    storeObject(resultHandler, resultContext, rowValue, parentMapping, resultSet);
  }
}

private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap, String columnPrefix) throws SQLException {
  final ResultLoaderMap lazyLoader = new ResultLoaderMap();
  // 使用无参构造器通过反射构造一个空值的对象
  Object rowValue = createResultObject(rsw, resultMap, lazyLoader, columnPrefix);
  if (rowValue != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) {
    final MetaObject metaObject = configuration.newMetaObject(rowValue);
    boolean foundValues = this.useConstructorMappings;
    if (shouldApplyAutomaticMappings(resultMap, false)) {
      // 自动映射设置值
      foundValues = applyAutomaticMappings(rsw, resultMap, metaObject, columnPrefix)  foundValues;
    }
    foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, columnPrefix)  foundValues;
    foundValues = lazyLoader.size() > 0  foundValues;
    rowValue = foundValues  configuration.isReturnInstanceForEmptyRow() ? rowValue : null;
  }
  return rowValue;
}

通过属性setter设置结果

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// DefaultResultSetHandler.java
private boolean applyAutomaticMappings(ResultSetWrapper rsw, ResultMap resultMap, MetaObject metaObject, String columnPrefix) throws SQLException {
  // 建立了自动映射对象
  List<UnMappedColumnAutoMapping> autoMapping = createAutomaticMappings(rsw, resultMap, metaObject, columnPrefix);
  boolean foundValues = false;
  if (!autoMapping.isEmpty()) {
    // 开始使用自动映射对象对之前反射的空对象进行设值
    for (UnMappedColumnAutoMapping mapping : autoMapping) {
      final Object value = mapping.typeHandler.getResult(rsw.getResultSet(), mapping.column);
      if (value != null) {
        foundValues = true;
      }
      if (value != null  (configuration.isCallSettersOnNulls() && !mapping.primitive)) {
        // gcode issue #377, call setter on nulls (value is not 'found')
        metaObject.setValue(mapping.property, value);
      }
    }
  }
  return foundValues;
}

查找属性自动映射器

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// DefaultResultSetHandler.java
private List<UnMappedColumnAutoMapping> createAutomaticMappings(ResultSetWrapper rsw, ResultMap resultMap, MetaObject metaObject, String columnPrefix) throws SQLException {
  final String mapKey = resultMap.getId() + ":" + columnPrefix;
  // 从缓存中命中
  List<UnMappedColumnAutoMapping> autoMapping = autoMappingsCache.get(mapKey);
  if (autoMapping == null) {
    // 没有命中，开始处理，后存入缓存
    autoMapping = new ArrayList<>();
    // 取出所有数据库列名
    final List<String> unmappedColumnNames = rsw.getUnmappedColumnNames(resultMap, columnPrefix);
    for (String columnName : unmappedColumnNames) {
      String propertyName = columnName;
      if (columnPrefix != null && !columnPrefix.isEmpty()) {
        // 判断前缀是否与列名匹配，不匹配则跳过处理
        if (columnName.toUpperCase(Locale.ENGLISH).startsWith(columnPrefix)) {
          propertyName = columnName.substring(columnPrefix.length());
        } else {
          continue;
        }
      }
      // 从对象名中查找属性名，isMapUnderscoreToCamelCase是转换下划线到驼峰的配置，这里是false
      final String property = metaObject.findProperty(propertyName, configuration.isMapUnderscoreToCamelCase());
      // 如果没有setter，跳过，继续下个列名处理
      if (property != null && metaObject.hasSetter(property)) {
        if (resultMap.getMappedProperties().contains(property)) {
          continue;
        }
        // 需要设置的值类型
        final Class<?> propertyType = metaObject.getSetterType(property);
        // 预置的处理器进行处理
        if (typeHandlerRegistry.hasTypeHandler(propertyType, rsw.getJdbcType(columnName))) {
          final TypeHandler<?> typeHandler = rsw.getTypeHandler(propertyType, columnName);
          autoMapping.add(new UnMappedColumnAutoMapping(columnName, property, typeHandler, propertyType.isPrimitive()));
        } else {
          configuration.getAutoMappingUnknownColumnBehavior()
              .doAction(mappedStatement, columnName, property, propertyType);
        }
      } else {
        configuration.getAutoMappingUnknownColumnBehavior()
            .doAction(mappedStatement, columnName, (property != null) ? property : propertyName, null);
      }
    }
    // 缓存起来
    autoMappingsCache.put(mapKey, autoMapping);
  }
  return autoMapping;
}

设值

metaObject.setValue(mapping.property, value)

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// MetaObject.java
public void setValue(String name, Object value) {
  PropertyTokenizer prop = new PropertyTokenizer(name);
  // 下一级属性，比如配置了<result property="addr.name" column="addr_name"/>property带有小数点的属性
  if (prop.hasNext()) {
    MetaObject metaValue = metaObjectForProperty(prop.getIndexedName());
    if (metaValue == SystemMetaObject.NULL_META_OBJECT) {
      if (value == null) {
        // don't instantiate child path if value is null
        return;
      } else {
        metaValue = objectWrapper.instantiatePropertyValue(name, prop, objectFactory);
      }
    }
    metaValue.setValue(prop.getChildren(), value);
  } else {
    // 没有直接设置值到对象的属性
    objectWrapper.set(prop, value);
  }
}

BeanWrapper，一个包装实际对象的封装对象，封装操作这个对象的函数。

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// BeanWrapper.java
@Override
public void set(PropertyTokenizer prop, Object value) {
  if (prop.getIndex() != null) {
    // 集合属性的封装
    Object collection = resolveCollection(prop, object);
    setCollectionValue(prop, collection, value);
  } else {
    // 单个普通属性的封装
    setBeanProperty(prop, object, value);
  }
}
// 通过反射获取setter设置对象的数据库值
private void setBeanProperty(PropertyTokenizer prop, Object object, Object value) {
  try {
    Invoker method = metaClass.getSetInvoker(prop.getName());
    Object[] params = {value};
    try {
      method.invoke(object, params);
    } catch (Throwable t) {
      throw ExceptionUtil.unwrapThrowable(t);
    }
  } catch (Throwable t) {
    throw new ReflectionException("Could not set property '" + prop.getName() + "' of '" + object.getClass() + "' with value '" + value + "' Cause: " + t.toString(), t);
  }
}

好了，所有属性值循环完成，开始出栈，出去到哪里，到 SqlSession 出去。

返回结果对象集

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@Override
public <T> T selectOne(String statement, Object parameter) {
  // 都是查询集合，根据查询个数进行判断
  List<T> list = this.selectList(statement, parameter);
  if (list.size() == 1) {
    return list.get(0);
  } else if (list.size() > 1) {
    // 超过一个结果就会报这个错误，应该不陌生吧
    throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());
  } else {
    return null;
  }
}

完