为什么写在xml文件中的sql，却可以执行，中间经过了怎样的流程？今天我们就来看一下这个过程。所有的开始源于一个Bean的定义

@Value(value = "classpath:mybatis/sqlmap/*.xml")
private Resource[] mapperLocations;

@Value(value = "classpath:mybatis/mybatis-config.xml")
private Resource configLocation;


@Bean(autowire = Autowire.BY_NAME)
public SqlSessionFactoryBean sqlSessionFactory() {
    SqlSessionFactoryBean ssfb = new SqlSessionFactoryBean();
    ssfb.setMapperLocations(mapperLocations);
    ssfb.setConfigLocation(configLocation);
    return ssfb;
}

我们的流程始于这个SqlSessionFactoryBean，可以设置多个属性值，在这里我们只设置了两个，第一个就是configLocation，配置文件的位置，另外一个就mapperLocations，就是mapper xml的文件的位置。老规矩，碰到FactoryBean，就可以看一下getObject准没错

@Override
public SqlSessionFactory getObject() throws Exception {
  if (this.sqlSessionFactory == null) {
    afterPropertiesSet();
  }

  return this.sqlSessionFactory;
}

我们看一下afterPropertiesSet，看这个方法名就是InitializingBean的生命周期方法，所以表明在Bean初始化之后就会调用，不说闲话了，继续看吧

/**
 * Build a {@code SqlSessionFactory} instance.
 *
 * The default implementation uses the standard MyBatis {@code XMLConfigBuilder} API to build a
 * {@code SqlSessionFactory} instance based on an Reader.
 *
 * @return SqlSessionFactory
 * @throws IOException if loading the config file failed
 */
protected SqlSessionFactory buildSqlSessionFactory() throws IOException {

  Configuration configuration;

  XMLConfigBuilder xmlConfigBuilder = null;
  if (this.configLocation != null) {
    xmlConfigBuilder = new XMLConfigBuilder(this.configLocation.getInputStream(), null, this.configurationProperties);
    configuration = xmlConfigBuilder.getConfiguration();
  } else {
    if (LOGGER.isDebugEnabled()) {
      LOGGER.debug("Property 'configLocation' not specified, using default MyBatis Configuration");
    }
    configuration = new Configuration();
    configuration.setVariables(this.configurationProperties);
  }

  if (this.objectFactory != null) {
    configuration.setObjectFactory(this.objectFactory);
  }

  if (this.objectWrapperFactory != null) {
    configuration.setObjectWrapperFactory(this.objectWrapperFactory);
  }

  if (hasLength(this.typeAliasesPackage)) {
    String[] typeAliasPackageArray = tokenizeToStringArray(this.typeAliasesPackage,
        ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
    for (String packageToScan : typeAliasPackageArray) {
      configuration.getTypeAliasRegistry().registerAliases(packageToScan,
              typeAliasesSuperType == null ? Object.class : typeAliasesSuperType);
      if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("Scanned package: '" + packageToScan + "' for aliases");
      }
    }
  }

  if (!isEmpty(this.typeAliases)) {
    for (Class<?> typeAlias : this.typeAliases) {
      configuration.getTypeAliasRegistry().registerAlias(typeAlias);
      if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("Registered type alias: '" + typeAlias + "'");
      }
    }
  }

  if (!isEmpty(this.plugins)) {
    for (Interceptor plugin : this.plugins) {
      configuration.addInterceptor(plugin);
      if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("Registered plugin: '" + plugin + "'");
      }
    }
  }

  if (hasLength(this.typeHandlersPackage)) {
    String[] typeHandlersPackageArray = tokenizeToStringArray(this.typeHandlersPackage,
        ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
    for (String packageToScan : typeHandlersPackageArray) {
      configuration.getTypeHandlerRegistry().register(packageToScan);
      if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("Scanned package: '" + packageToScan + "' for type handlers");
      }
    }
  }

  if (!isEmpty(this.typeHandlers)) {
    for (TypeHandler<?> typeHandler : this.typeHandlers) {
      configuration.getTypeHandlerRegistry().register(typeHandler);
      if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("Registered type handler: '" + typeHandler + "'");
      }
    }
  }

  if (xmlConfigBuilder != null) {
    try {
      xmlConfigBuilder.parse();

      if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("Parsed configuration file: '" + this.configLocation + "'");
      }
    } catch (Exception ex) {
      throw new NestedIOException("Failed to parse config resource: " + this.configLocation, ex);
    } finally {
      ErrorContext.instance().reset();
    }
  }

  if (this.transactionFactory == null) {
    this.transactionFactory = new SpringManagedTransactionFactory();
  }

  configuration.setEnvironment(new Environment(this.environment, this.transactionFactory, this.dataSource));

  if (this.databaseIdProvider != null) {
    try {
      configuration.setDatabaseId(this.databaseIdProvider.getDatabaseId(this.dataSource));
    } catch (SQLException e) {
      throw new NestedIOException("Failed getting a databaseId", e);
    }
  }

  if (!isEmpty(this.mapperLocations)) {
    for (Resource mapperLocation : this.mapperLocations) {
      if (mapperLocation == null) {
        continue;
      }

      try {
        XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(mapperLocation.getInputStream(),
            configuration, mapperLocation.toString(), configuration.getSqlFragments());
        xmlMapperBuilder.parse();
      } catch (Exception e) {
        throw new NestedIOException("Failed to parse mapping resource: '" + mapperLocation + "'", e);
      } finally {
        ErrorContext.instance().reset();
      }

      if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("Parsed mapper file: '" + mapperLocation + "'");
      }
    }
  } else {
    if (LOGGER.isDebugEnabled()) {
      LOGGER.debug("Property 'mapperLocations' was not specified or no matching resources found");
    }
  }

  return this.sqlSessionFactoryBuilder.build(configuration);
}

首先映入眼帘的就是XMLConfigBuilder，主要就是用来解析配置文件的。从这个类里面可以得到一个Configuration的事例，贯穿我们执行流程全部，我们解析到的所有的信息都会放到这里面，调用了
xmlConfigBuilder.parse()后就开始了配置文件的解析的过程。解析完毕后就开始解析我们的mapper了，根据传入的mapperLocation,进行遍历处理，封装成
XMLMapperBuilder后，调用他的parse方法

public void parse() {
  if (!configuration.isResourceLoaded(resource)) {
    configurationElement(parser.evalNode("/mapper"));
    configuration.addLoadedResource(resource);
    bindMapperForNamespace();
  }

  parsePendingResultMaps();
  parsePendingChacheRefs();
  parsePendingStatements();
}
 
private void configurationElement(XNode context) {
  try {
    String namespace = context.getStringAttribute("namespace");
    if (namespace == null || namespace.equals("")) {
      throw new BuilderException("Mapper's namespace cannot be empty");
    }
    builderAssistant.setCurrentNamespace(namespace);
    cacheRefElement(context.evalNode("cache-ref"));
    cacheElement(context.evalNode("cache"));
    parameterMapElement(context.evalNodes("/mapper/parameterMap"));
    resultMapElements(context.evalNodes("/mapper/resultMap"));
    sqlElement(context.evalNodes("/mapper/sql"));
    buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
  } catch (Exception e) {
    throw new BuilderException("Error parsing Mapper XML. Cause: " + e, e);
  }
}
 
### org.apache.ibatis.builder.xml.XMLMapperBuilder#buildStatementFromContext(java.util.List<org.apache.ibatis.parsing.XNode>)
private void buildStatementFromContext(List<XNode> list) {
  if (configuration.getDatabaseId() != null) {
    buildStatementFromContext(list, configuration.getDatabaseId());
  }
  buildStatementFromContext(list, null);
}
 
private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) {
  for (XNode context : list) {
    final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);
    try {
      statementParser.parseStatementNode();
    } catch (IncompleteElementException e) {
      configuration.addIncompleteStatement(statementParser);
    }
  }
}

我们最为关心的是怎么处理mapper文件，我们看到了mapper文件的根节点/mapper，就是他了。在解析的过程中，我们关心的是如果处理对应的方法也就是

buildStatementFromContext(context.evalNodes("select|insert|update|delete"))

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

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

  Integer fetchSize = context.getIntAttribute("fetchSize");
  Integer timeout = context.getIntAttribute("timeout");
  String parameterMap = context.getStringAttribute("parameterMap");
  String parameterType = context.getStringAttribute("parameterType");
  Class<?> parameterTypeClass = resolveClass(parameterType);
  String resultMap = context.getStringAttribute("resultMap");
  String resultType = context.getStringAttribute("resultType");
  String lang = context.getStringAttribute("lang");
  LanguageDriver langDriver = getLanguageDriver(lang);

  Class<?> resultTypeClass = resolveClass(resultType);
  String resultSetType = context.getStringAttribute("resultSetType");
  StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString()));
  ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType);

  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);

  // Include Fragments before parsing
  XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);
  includeParser.applyIncludes(context.getNode());

  // Parse selectKey after includes and remove them.
  processSelectKeyNodes(id, parameterTypeClass, langDriver);
  
  // Parse the SQL (pre: <selectKey> and <include> were parsed and removed)
  SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
  String resultSets = context.getStringAttribute("resultSets");
  String keyProperty = context.getStringAttribute("keyProperty");
  String keyColumn = context.getStringAttribute("keyColumn");
  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))
        ? new Jdbc3KeyGenerator() : new NoKeyGenerator();
  }

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

  setStatementParameterMap(parameterMap, parameterType, statementBuilder);
  setStatementResultMap(resultMap, resultType, resultSetType, statementBuilder);
  setStatementCache(isSelect, flushCache, useCache, currentCache, statementBuilder);

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

解析这个数据节点后，最终成为了configuration中的MappedStatement，所以我们的sql的源信息都在MappedStatement中，再次印证了所有的信息都在configuration中。解析的流程基本上就是这样了。