MyBatis温故而知新-底层运行原理

准备工作

public class MainClass {
  public static void main(String[] args) throws Exception {
    String resources = "mybatis-config.xml";
    InputStream inputStream = Resources.getResourceAsStream(resources);

    SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    SqlSession sqlSession = sqlSessionFactory.openSession();

    Student student = sqlSession.selectOne("org.apache.ibatis.dao.StudentMapper.getStudent",1);
    System.out.println(student.toString());
    sqlSession.close();
  }
}

MyBatis是如何获取数据源的

这是我们mybatis-config.xml中配置数据库的4个关键属性，也就是看看MyBatis是怎么来解析这个配置文件块的。

<environments default="development">
   <environment id="development">
      <transactionManager type="JDBC"/>
      <dataSource type="POOLED">
        <property name="driver" value="${jdbc.driver}"/>
        <property name="url" value="${jdbc.url}"/>
        <property name="username" value="${jdbc.username}"/>
        <property name="password" value="${jdbc.password}"/>
      </dataSource>
    </environment>
 </environments>

数据源获取，我们从上面的代码片段中开始分析，在SqlSessionFactoryBuilder().build(inputStream) 方法中入手，这里看到实例了XMLConfigBuilder类。

SqlSessionFactoryBuilder

public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
    try {
      XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
      //parser.parse()方法返回Configuration对象，然后调用build(Configuration config)
      return build(parser.parse());
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error building SqlSession.", e);
    } finally {
      ErrorContext.instance().reset();
      try {
        inputStream.close();
      } catch (IOException e) {
        // Intentionally ignore. Prefer previous error.
      }
    }
}

public SqlSessionFactory build(Configuration config) {
    return new DefaultSqlSessionFactory(config);
}

XMLConfigBuilder#parse()

public Configuration parse() {
    if (parsed) {
      throw new BuilderException("Each XMLConfigBuilder can only be used once.");
    }
    parsed = true;
    parseConfiguration(parser.evalNode("/configuration"));
    return configuration;
  }

  private void parseConfiguration(XNode root) {
    try {
      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);
      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);
    }
  }

看到parseConfiguration参数为XNode就知道这个方法里肯定是要解析xml节点了,可以debug查看下root参数的值，尝试debug后发现，其内容就是我们的配置文件mybatis-config.xml的文件内容。 接着我们在22行这里则看到了熟悉的environments节点，那就直接看下environmentsElement方法。

<configuration>
    <properties resource="db.properties"/>
    <settings>
        <setting name="logImpl" value="STDOUT_LOGGING"/>
        <setting name="cacheEnabled" value="true"/>
        <setting name="lazyLoadingEnabled" value="true"/>
        <setting name="aggressiveLazyLoading" value="false"/>
        <setting name="localCacheScope" value="SESSION"/>
    </settings>
    <typeAliases>
        <typeAlias alias="Student" type="org.apache.ibatis.domain.Student"/>
    </typeAliases>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="${jdbc.driver}"/>
                <property name="url" value="${jdbc.url}"/>
                <property name="username" value="${jdbc.username}"/>
                <property name="password" value="${jdbc.password}"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="mapper/StudentMapper.xml"/>
    </mappers>
</configuration>

XMLConfigBuilder#environmentsElement

这个方法参数XNode内容则是配置文件里的部分。这里就看到了解析dataSource节点的地方，这里使用DataSourceFactory返回一个DataSource， 那么这里是怎么拿到DataSourceFactory的，又是怎么拿到DataSource的。

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;
        }
      }
    }
  }

解析DataSourceFactory

这里看到了getDeclaredConstructor().newInstance()反射实例化一个PoolDataSourceFactory。

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

public class PooledDataSourceFactory extends UnpooledDataSourceFactory {

  public PooledDataSourceFactory() {
    this.dataSource = new PooledDataSource();
  }

}

解析DataSource

拿到PooledDataSourceFactory后则调用dsFactory.getDataSource()拿到数据源， 这里重要的代码是最后一句configuration.setEnvironment，这里最后会把解析后的Environment对象给Configuration对象赋值， Configuration对象也是MyBatis框架中相当重量级的一个对象。

​

MyBatis是如何获取SQL语句的

解析SQL语句Code Chain

文章开头我们是用sqlSession.selectOne()方法来获取SQL语句的，那么这个SQL语句是怎么解析并获取呢？上面我们分析获取数据源时提到XMLConfigBuilder.parseConfiguration方法时看到会调用mapperElement方法。

private void mapperElement(XNode parent) throws Exception {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
        if ("package".equals(child.getName())) {
          String mapperPackage = child.getStringAttribute("name");
          configuration.addMappers(mapperPackage);
        } else {
          String resource = child.getStringAttribute("resource");
          String url = child.getStringAttribute("url");
          String mapperClass = child.getStringAttribute("class");
          if (resource != null && url == null && mapperClass == null) {
            ErrorContext.instance().resource(resource);
            try(InputStream inputStream = Resources.getResourceAsStream(resource)) {
              XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
              mapperParser.parse();
            }
          } else if (resource == null && url != null && mapperClass == null) {
            ErrorContext.instance().resource(url);
            try(InputStream inputStream = Resources.getUrlAsStream(url)){
              XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());
              mapperParser.parse();
            }
          } else if (resource == null && url == null && mapperClass != null) {
            Class<?> mapperInterface = Resources.classForName(mapperClass);
            configuration.addMapper(mapperInterface);
          } else {
            throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
          }
        }
      }
    }
  }

该方法会根据配置文件依次解析package, resource, url, class4种配置mapper的方式。

<mappers>
    <mapper resource="mapper/StudentMapper.xml"/>
</mappers>

这里我们是用resource方式配置的，所以这个方法程序会在Line 12 的逻辑开始执行。

​

XMLStatementBuilder

解析SQL语句的关键逻辑都在这个类的parseStatementNode方法里，方法最后会调用MapperBuilderAssistant#.addMappedStatement(), 这个方法会很到MyBatis里很重要的一个类MappedStatement对象，这个方法最关键的一行代码是先生成MappedStatement对象，最后会把生成的MappedStatement对象放入Configuration对象的Map字典中。

看到这个方法里的代码应该很直观，就是在解析XML中select标签的一些属性，比如useCache, flushCache等，这些标签可以查看MyBatis文档，真的很详细。

public void parseStatementNode() {
    String id = context.getStringAttribute("id");
    String databaseId = context.getStringAttribute("databaseId");

    if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) {
      return;
    }

    String nodeName = context.getNode().getNodeName();
    SqlCommandType sqlCommandType = SqlCommandType.valueOf(nodeName.toUpperCase(Locale.ENGLISH));
    boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
    boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect);
    boolean useCache = context.getBooleanAttribute("useCache", isSelect);
    boolean resultOrdered = context.getBooleanAttribute("resultOrdered", false);

    XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);
    includeParser.applyIncludes(context.getNode());

    String parameterType = context.getStringAttribute("parameterType");
    Class<?> parameterTypeClass = resolveClass(parameterType);

    String lang = context.getStringAttribute("lang");
    LanguageDriver langDriver = getLanguageDriver(lang);

    // Parse selectKey after includes and remove them.
    processSelectKeyNodes(id, parameterTypeClass, langDriver);

    // Parse the SQL (pre: <selectKey> and <include> were parsed and removed)
    KeyGenerator keyGenerator;
    String keyStatementId = id + SelectKeyGenerator.SELECT_KEY_SUFFIX;
    keyStatementId = builderAssistant.applyCurrentNamespace(keyStatementId, true);
    if (configuration.hasKeyGenerator(keyStatementId)) {
      keyGenerator = configuration.getKeyGenerator(keyStatementId);
    } else {
      keyGenerator = context.getBooleanAttribute("useGeneratedKeys",
          configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType))
          ? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE;
    }

    SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
    StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString()));
    Integer fetchSize = context.getIntAttribute("fetchSize");
    Integer timeout = context.getIntAttribute("timeout");
    String parameterMap = context.getStringAttribute("parameterMap");
    String resultType = context.getStringAttribute("resultType");
    Class<?> resultTypeClass = resolveClass(resultType);
    String resultMap = context.getStringAttribute("resultMap");
    String resultSetType = context.getStringAttribute("resultSetType");
    ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType);
    if (resultSetTypeEnum == null) {
      resultSetTypeEnum = configuration.getDefaultResultSetType();
    }
    String keyProperty = context.getStringAttribute("keyProperty");
    String keyColumn = context.getStringAttribute("keyColumn");
    String resultSets = context.getStringAttribute("resultSets");

    builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
        fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
        resultSetTypeEnum, flushCache, useCache, resultOrdered,
        keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
  }

MapperBuilderAssistant

public MappedStatement addMappedStatement(
      String id,
      SqlSource sqlSource,
      StatementType statementType,
      SqlCommandType sqlCommandType,
      Integer fetchSize,
      Integer timeout,
      String parameterMap,
      Class<?> parameterType,
      String resultMap,
      Class<?> resultType,
      ResultSetType resultSetType,
      boolean flushCache,
      boolean useCache,
      boolean resultOrdered,
      KeyGenerator keyGenerator,
      String keyProperty,
      String keyColumn,
      String databaseId,
      LanguageDriver lang,
      String resultSets) {

    if (unresolvedCacheRef) {
      throw new IncompleteElementException("Cache-ref not yet resolved");
    }

    id = applyCurrentNamespace(id, false);
    boolean isSelect = sqlCommandType == SqlCommandType.SELECT;

    MappedStatement.Builder statementBuilder = new MappedStatement.Builder(configuration, id, sqlSource, sqlCommandType)
        .resource(resource)
        .fetchSize(fetchSize)
        .timeout(timeout)
        .statementType(statementType)
        .keyGenerator(keyGenerator)
        .keyProperty(keyProperty)
        .keyColumn(keyColumn)
        .databaseId(databaseId)
        .lang(lang)
        .resultOrdered(resultOrdered)
        .resultSets(resultSets)
        .resultMaps(getStatementResultMaps(resultMap, resultType, id))
        .resultSetType(resultSetType)
        .flushCacheRequired(valueOrDefault(flushCache, !isSelect))
        .useCache(valueOrDefault(useCache, isSelect))
        .cache(currentCache);

    ParameterMap statementParameterMap = getStatementParameterMap(parameterMap, parameterType, id);
    if (statementParameterMap != null) {
      statementBuilder.parameterMap(statementParameterMap);
    }

    MappedStatement statement = statementBuilder.build();
    configuration.addMappedStatement(statement);
    return statement;
}

MyBatis是如何操作数据库的

执行器(Executor)

执行器在MyBatis中是用来封装Statement执行JDBC操作，MyBatis共包含3种类型的执行器：SimpleExecutor, ReuseExecutor, BatchExecutor，默认值使用SimpleExecutor。

 @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);
    }
 }

MySQL查询的元数据和实体建立映射关系

那么我们的select语句查询出来的一条mysql元数据和java实体到底是怎样建立映射关系的？ 我们先从PreparedStatementHandler#query方法着手分析。

 @Override
 public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    //这两行代码是不是很亲切了，在JDBC中我们就是这么干的。
    //1.执行SQL
    PreparedStatement ps = (PreparedStatement) statement;
    //2.处理结果集
    ps.execute();
    return resultSetHandler.handleResultSets(ps);
 }

结果集映射

DefaultResultSetHandler#handleResultSets

@Override
 public List<Object> handleResultSets(Statement stmt) throws SQLException {
    ErrorContext.instance().activity("handling results").object(mappedStatement.getId());
    //1.<select>标签的resultMap属性，用来装映射后的实体对象数据
    final List<Object> multipleResults = new ArrayList<>();

    int resultSetCount = 0;
    //2.获取第一个结果集数据
    ResultSetWrapper rsw = getFirstResultSet(stmt);
    //3.这里就是取出需要映射的ResultMap
    List<ResultMap> resultMaps = mappedStatement.getResultMaps();
    //4.这里就是需要映射的ResultMap的数量
    int resultMapCount = resultMaps.size();
    validateResultMapsCount(rsw, resultMapCount);
    //5.循环处理每个ResultMap
    while (rsw != null && resultMapCount > resultSetCount) {
      //6.循环取出需要映射的ResultMap（id,type）实体名称
      ResultMap resultMap = resultMaps.get(resultSetCount);
      //7.从rsw结果集参数中获取查询结果，再根据resultMap映射信息，将查询结果装到到multipleResults中
      handleResultSet(rsw, resultMap, multipleResults, null);
      rsw = getNextResultSet(stmt);
      cleanUpAfterHandlingResultSet();
      resultSetCount++;
    }

    //这里会获取select标签的resultSet属性
    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++;
      }
    }

    return collapseSingleResultList(multipleResults);
}

​

DefaultResultSetHandler#handleResultSet

private void handleResultSet(ResultSetWrapper rsw, ResultMap resultMap, List<Object> multipleResults, ResultMapping parentMapping) throws SQLException {
    try {
      if (parentMapping != null) {
        handleRowValues(rsw, resultMap, null, RowBounds.DEFAULT, parentMapping);
      } else {
        if (resultHandler == null) {
          //1.创建DefaultResultHandler 来处理结果集
          DefaultResultHandler defaultResultHandler = new DefaultResultHandler(objectFactory);
          //2.这里就是处理映射结果集的最终方法
          handleRowValues(rsw, resultMap, defaultResultHandler, rowBounds, null);
          multipleResults.add(defaultResultHandler.getResultList());
        } else {
          handleRowValues(rsw, resultMap, resultHandler, rowBounds, null);
        }
      }
    } finally {
      closeResultSet(rsw.getResultSet());
    }
  }

DefaultResultSetHandler#handleRowValues

public void handleRowValues(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException {
    if (resultMap.hasNestedResultMaps()) {
      ensureNoRowBounds();
      checkResultHandler();
      //嵌套结果映射
      handleRowValuesForNestedResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
    } else {
      //简单结果映射
      handleRowValuesForSimpleResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
    }
  }

ResultSetWrapper

这里还有一个关键的类在上述方法调用链里没有提到就是ResultSetWrapper， 它的构造器里有3个集合分别是columnNames，jdbcTypes， classNames， 当你看到这3个数组时脑海里是不是浮现出MyBatis框架的意图呢，代替你管理JDBC，并从结果集建立到java对象的映射。

public ResultSetWrapper(ResultSet rs, Configuration configuration) throws SQLException {
    super();
    this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();
    this.resultSet = rs;
    final ResultSetMetaData metaData = rs.getMetaData();
    final int columnCount = metaData.getColumnCount();
    for (int i = 1; i <= columnCount; i++) {
      columnNames.add(configuration.isUseColumnLabel() ? metaData.getColumnLabel(i) : metaData.getColumnName(i));
      jdbcTypes.add(JdbcType.forCode(metaData.getColumnType(i)));
      classNames.add(metaData.getColumnClassName(i));
    }
  }

DefaultResultSetHandler#handleRowValuesForSimpleResultMap

private void handleRowValuesForSimpleResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping)
      throws SQLException {
    DefaultResultContext<Object> resultContext = new DefaultResultContext<>();
    //1.因为在创建ResultSetWrapper的时候 我们将结果集封装进去了 现在将结果集取出来
    ResultSet resultSet = rsw.getResultSet();
    //2.分页信息
    skipRows(resultSet, rowBounds);
    while (shouldProcessMoreRows(resultContext, rowBounds) && !resultSet.isClosed() && resultSet.next()) {
      ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(resultSet, resultMap, null);
      ///3.将查询结果封装到POJO中(这一行代码重点关注)
      Object rowValue = getRowValue(rsw, discriminatedResultMap, null);
      storeObject(resultHandler, resultContext, rowValue, parentMapping, resultSet);
    }
  }

DefaultResultSetHandler#getRowValue

private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap, String columnPrefix) throws SQLException {
    final ResultLoaderMap lazyLoader = new ResultLoaderMap();
    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)) {
        //封装结果集 将sql的结果与实体类封装对应起来
        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;
  }

DefaultResultSetHandler#applyAutomaticMappings

这个方法里会循环处理行列字段并映射实体属性。

private boolean applyAutomaticMappings(ResultSetWrapper rsw, ResultMap resultMap, MetaObject metaObject, String columnPrefix) throws SQLException {
    //1.封装你的返回结果集对象
    List<UnMappedColumnAutoMapping> autoMapping = createAutomaticMappings(rsw, resultMap, metaObject, columnPrefix);
    boolean foundValues = false;
    if (!autoMapping.isEmpty()) {
      //2.遍历 给实体类对象赋值
      for (UnMappedColumnAutoMapping mapping : autoMapping) {
        //3.根据实体属性 去sql中取值，拿到SQL的结果值和实体类的属性值，并封装在metaObject对象中。
        final Object value = mapping.typeHandler.getResult(rsw.getResultSet(), mapping.column);
        if (value != null) {
          foundValues = true;
        }
        if (value != null || (configuration.isCallSettersOnNulls() && !mapping.primitive)) {
          metaObject.setValue(mapping.property, value);
        }
      }
    }
    //返回值为boolean值
    return foundValues;
  }

这里autoMapping集合里能看到就是存放三个字段的元数据。OK，执行到这一步，外层的getRowValue的返回值就拿到了返回的实体对象。