Mybatis之工作原理

1.Mybatis的架构

1.1 Mybatis的框架分层

1.2 MyBatis的实现原理

mybatis底层还是采用原生jdbc来对数据库进行操作的，它支持定制化 SQL、存储过程以及高级映射的优秀的持久层框架，其主要就完成2件事情：

1. 封装JDBC操作
2. 利用反射打通Java类与SQL语句之间的相互转换

MyBatis的主要设计目的就是让我们对执行SQL语句时对输入输出的数据管理更加方便，所以方便地写出SQL和方便地获取SQL的执行结果才是MyBatis的核心竞争力。

它通过SqlSessionFactory，SqlSession，Executor,StatementHandler，ParameterHandler，ResultHandler和TypeHandler等几个处理器封装了这些过程

执行器：Executor(update, query, flushStatements, commit, rollback, getTransaction, close, isClosed)，是MyBatis调度的核心，负责SQL语句的生成和查询缓存的维护
参数处理器：ParameterHandler(getParameterObject, setParameters)，负责对用户传递的参数转换成JDBC Statement所对应的数据类型
结构处理器：ResultSetHandler(handleResultSets, handleOutputParameters)，负责将JDBC返回的ResultSet结果集对象转换成List类型的集合
sql查询处理器：StatementHandler(prepare, parameterize, batch, update, query)，封装了JDBC Statement操作，负责对JDBC statement 的操作，如设置参数等
Configuration：MyBatis所有的配置信息都保存在Configuration对象之中，配置文件中的大部分配置都会存储到该类中
SqlSession：作为MyBatis工作的主要顶层API，表示和数据库交互时的会话，完成必要数据库增删改查功能
TypeHandler：负责java数据类型和jdbc数据类型(也可以说是数据表列类型)之间的映射和转换
MappedStatement：MappedStatement维护一条<select|update|delete|insert>节点的封装
SqlSource：负责根据用户传递的parameterObject，动态地生成SQL语句，将信息封装到BoundSql对象中，并返回
BoundSql：表示动态生成的SQL语句以及相应的参数信
BaseBuilder：负责解析xml映射文件，生成sql语句，包括解析select|insert|update|delete|cache-ref|cache|resultMap|sql|parameterMap|namespace|where|trim|set|where|if|foreach|choose|if|when|otherwise|bind|parameterType|resultType|resultMap,该类是抽象类，它有很多子类。

其中StatementHandler用通过ParameterHandler与ResultHandler分别进行参数预编译 与结果处理。而ParameterHandler与ResultHandler都使用TypeHandler进行映射。

BaseBuilder的类图继承结构如下：

如下图：

2.Mybatis工作过程

通过读mybatis的源码进行分析mybatis的执行操作的整个过程，我们通过debug调试就可以知道Mybatis每一步做了什么事，我先把debug每一步结果 截图，然后在分析这个流程。 
第一步:读取配置文件，形成InputStream：

String resource = "mybatis.xml";

// 加载mybatis的配置文件（它也加载关联的映射文件）
InputStream inputStream = null;
try {
    inputStream = Resources.getResourceAsStream(resource);
} catch (IOException e) {
    e.printStackTrace();
}

// 构建sqlSession的工厂
sessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

2.1 解析xml文件并创建SqlSessionFacotry的过程

首先会创建SqlSessionFactoryBuilder对象，然后由它进行创建SqlSessionFactory。这里用到的是建造者模式，建造者模式最简单的理解就是不手动new对象，而是由其他类来进行对象的创建。

public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
    try {
      XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
      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对象会进行XML配置文件的解析，实际为configuration节点的解析操作。

// XMLConfigBuilder类
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 {
        //issue #117 read properties first
        propertiesElement(root.evalNode("properties"));
        Properties settings = settingsAsProperties(root.evalNode("settings"));
        loadCustomVfs(settings);
        typeAliasesElement(root.evalNode("typeAliases"));
        pluginElement(root.evalNode("plugins"));
        objectFactoryElement(root.evalNode("objectFactory"));
        objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
        reflectorFactoryElement(root.evalNode("reflectorFactory"));
        settingsElement(settings);
        // read it after objectFactory and objectWrapperFactory issue #631

        /* 处理environments节点数据 */
        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);
    }
}

在configuration节点下会依次解析properties/settings/.../mappers等节点配置。在解析environments节点时，会根据transactionManager的配置来创建事务管理器，根据dataSource的配置来创建DataSource对象，这里面包含了数据库登录的相关信息。在解析mappers节点时，会读取该节点下所有的mapper文件，然后进行解析，并将解析后的结果存到configuration对象中。

// XMLConfigBuilder类
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)) {

                /* 创建事务管理器 */
                TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager"));
                DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource"));
                DataSource dataSource = dsFactory.getDataSource();

                /* 建造者模式 设计模式 */
                Environment.Builder environmentBuilder = new Environment.Builder(id)
                        .transactionFactory(txFactory)
                        .dataSource(dataSource);
                configuration.setEnvironment(environmentBuilder.build());
            }
        }
    }
}

// 解析单独的mapper文件
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);
            InputStream inputStream = Resources.getResourceAsStream(resource);
            XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
            mapperParser.parse(); // 开始解析mapper文件了 :)
          } else if (resource == null && url != null && mapperClass == null) {
            ErrorContext.instance().resource(url);
            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.");
          }
        }
      }
    }
  }

使用类XMLMapperBuilder解析每个mapper的xml文件

public XMLMapperBuilder(InputStream inputStream, Configuration configuration, String resource, Map<String, XNode> sqlFragments) {
    this(new XPathParser(inputStream, true, configuration.getVariables(), new XMLMapperEntityResolver()),
        configuration, resource, sqlFragments);
  }

  private XMLMapperBuilder(XPathParser parser, Configuration configuration, String resource, Map<String, XNode> sqlFragments) {
    super(configuration);
    this.builderAssistant = new MapperBuilderAssistant(configuration, resource);
    this.parser = parser;
    this.sqlFragments = sqlFragments;
    this.resource = resource;
  }

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

    parsePendingResultMaps();
    parsePendingCacheRefs();
    parsePendingStatements();
  }

  public XNode getSqlFragment(String refid) {
    return sqlFragments.get(refid);
  }

  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. The XML location is '" + resource + "'. Cause: " + e, e);
    }
  }

  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);
      }
    }
  }

  private void parsePendingResultMaps() {
    Collection<ResultMapResolver> incompleteResultMaps = configuration.getIncompleteResultMaps();
    synchronized (incompleteResultMaps) {
      Iterator<ResultMapResolver> iter = incompleteResultMaps.iterator();
      while (iter.hasNext()) {
        try {
          iter.next().resolve();
          iter.remove();
        } catch (IncompleteElementException e) {
          // ResultMap is still missing a resource...
        }
      }
    }
  }

  private void parsePendingCacheRefs() {
    Collection<CacheRefResolver> incompleteCacheRefs = configuration.getIncompleteCacheRefs();
    synchronized (incompleteCacheRefs) {
      Iterator<CacheRefResolver> iter = incompleteCacheRefs.iterator();
      while (iter.hasNext()) {
        try {
          iter.next().resolveCacheRef();
          iter.remove();
        } catch (IncompleteElementException e) {
          // Cache ref is still missing a resource...
        }
      }
    }
  }

  private void parsePendingStatements() {
    Collection<XMLStatementBuilder> incompleteStatements = configuration.getIncompleteStatements();
    synchronized (incompleteStatements) {
      Iterator<XMLStatementBuilder> iter = incompleteStatements.iterator();
      while (iter.hasNext()) {
        try {
          iter.next().parseStatementNode();
          iter.remove();
        } catch (IncompleteElementException e) {
          // Statement is still missing a resource...
        }
      }
    }
  }

  private void cacheRefElement(XNode context) {
    if (context != null) {
      configuration.addCacheRef(builderAssistant.getCurrentNamespace(), context.getStringAttribute("namespace"));
      CacheRefResolver cacheRefResolver = new CacheRefResolver(builderAssistant, context.getStringAttribute("namespace"));
      try {
        cacheRefResolver.resolveCacheRef();
      } catch (IncompleteElementException e) {
        configuration.addIncompleteCacheRef(cacheRefResolver);
      }
    }
  }

  private void cacheElement(XNode context) throws Exception {
    if (context != null) {
      String type = context.getStringAttribute("type", "PERPETUAL");
      Class<? extends Cache> typeClass = typeAliasRegistry.resolveAlias(type);
      String eviction = context.getStringAttribute("eviction", "LRU");
      Class<? extends Cache> evictionClass = typeAliasRegistry.resolveAlias(eviction);
      Long flushInterval = context.getLongAttribute("flushInterval");
      Integer size = context.getIntAttribute("size");
      boolean readWrite = !context.getBooleanAttribute("readOnly", false);
      boolean blocking = context.getBooleanAttribute("blocking", false);
      Properties props = context.getChildrenAsProperties();
      builderAssistant.useNewCache(typeClass, evictionClass, flushInterval, size, readWrite, blocking, props);
    }
  }

  private void parameterMapElement(List<XNode> list) throws Exception {
    for (XNode parameterMapNode : list) {
      String id = parameterMapNode.getStringAttribute("id");
      String type = parameterMapNode.getStringAttribute("type");
      Class<?> parameterClass = resolveClass(type);
      List<XNode> parameterNodes = parameterMapNode.evalNodes("parameter");
      List<ParameterMapping> parameterMappings = new ArrayList<ParameterMapping>();
      for (XNode parameterNode : parameterNodes) {
        String property = parameterNode.getStringAttribute("property");
        String javaType = parameterNode.getStringAttribute("javaType");
        String jdbcType = parameterNode.getStringAttribute("jdbcType");
        String resultMap = parameterNode.getStringAttribute("resultMap");
        String mode = parameterNode.getStringAttribute("mode");
        String typeHandler = parameterNode.getStringAttribute("typeHandler");
        Integer numericScale = parameterNode.getIntAttribute("numericScale");
        ParameterMode modeEnum = resolveParameterMode(mode);
        Class<?> javaTypeClass = resolveClass(javaType);
        JdbcType jdbcTypeEnum = resolveJdbcType(jdbcType);
        @SuppressWarnings("unchecked")
        Class<? extends TypeHandler<?>> typeHandlerClass = (Class<? extends TypeHandler<?>>) resolveClass(typeHandler);
        ParameterMapping parameterMapping = builderAssistant.buildParameterMapping(parameterClass, property, javaTypeClass, jdbcTypeEnum, resultMap, modeEnum, typeHandlerClass, numericScale);
        parameterMappings.add(parameterMapping);
      }
      builderAssistant.addParameterMap(id, parameterClass, parameterMappings);
    }
  }

  private void resultMapElements(List<XNode> list) throws Exception {
    for (XNode resultMapNode : list) {
      try {
        resultMapElement(resultMapNode);
      } catch (IncompleteElementException e) {
        // ignore, it will be retried
      }
    }
  }

  private ResultMap resultMapElement(XNode resultMapNode) throws Exception {
    return resultMapElement(resultMapNode, Collections.<ResultMapping> emptyList());
  }

  private ResultMap resultMapElement(XNode resultMapNode, List<ResultMapping> additionalResultMappings) throws Exception {
    ErrorContext.instance().activity("processing " + resultMapNode.getValueBasedIdentifier());
    String id = resultMapNode.getStringAttribute("id",
        resultMapNode.getValueBasedIdentifier());
    String type = resultMapNode.getStringAttribute("type",
        resultMapNode.getStringAttribute("ofType",
            resultMapNode.getStringAttribute("resultType",
                resultMapNode.getStringAttribute("javaType"))));
    String extend = resultMapNode.getStringAttribute("extends");
    Boolean autoMapping = resultMapNode.getBooleanAttribute("autoMapping");
    Class<?> typeClass = resolveClass(type);
    Discriminator discriminator = null;
    List<ResultMapping> resultMappings = new ArrayList<ResultMapping>();
    resultMappings.addAll(additionalResultMappings);
    List<XNode> resultChildren = resultMapNode.getChildren();
    for (XNode resultChild : resultChildren) {
      if ("constructor".equals(resultChild.getName())) {
        processConstructorElement(resultChild, typeClass, resultMappings);
      } else if ("discriminator".equals(resultChild.getName())) {
        discriminator = processDiscriminatorElement(resultChild, typeClass, resultMappings);
      } else {
        List<ResultFlag> flags = new ArrayList<ResultFlag>();
        if ("id".equals(resultChild.getName())) {
          flags.add(ResultFlag.ID);
        }
        resultMappings.add(buildResultMappingFromContext(resultChild, typeClass, flags));
      }
    }
    ResultMapResolver resultMapResolver = new ResultMapResolver(builderAssistant, id, typeClass, extend, discriminator, resultMappings, autoMapping);
    try {
      return resultMapResolver.resolve();
    } catch (IncompleteElementException  e) {
      configuration.addIncompleteResultMap(resultMapResolver);
      throw e;
    }
  }

  private void processConstructorElement(XNode resultChild, Class<?> resultType, List<ResultMapping> resultMappings) throws Exception {
    List<XNode> argChildren = resultChild.getChildren();
    for (XNode argChild : argChildren) {
      List<ResultFlag> flags = new ArrayList<ResultFlag>();
      flags.add(ResultFlag.CONSTRUCTOR);
      if ("idArg".equals(argChild.getName())) {
        flags.add(ResultFlag.ID);
      }
      resultMappings.add(buildResultMappingFromContext(argChild, resultType, flags));
    }
  }

  private Discriminator processDiscriminatorElement(XNode context, Class<?> resultType, List<ResultMapping> resultMappings) throws Exception {
    String column = context.getStringAttribute("column");
    String javaType = context.getStringAttribute("javaType");
    String jdbcType = context.getStringAttribute("jdbcType");
    String typeHandler = context.getStringAttribute("typeHandler");
    Class<?> javaTypeClass = resolveClass(javaType);
    @SuppressWarnings("unchecked")
    Class<? extends TypeHandler<?>> typeHandlerClass = (Class<? extends TypeHandler<?>>) resolveClass(typeHandler);
    JdbcType jdbcTypeEnum = resolveJdbcType(jdbcType);
    Map<String, String> discriminatorMap = new HashMap<String, String>();
    for (XNode caseChild : context.getChildren()) {
      String value = caseChild.getStringAttribute("value");
      String resultMap = caseChild.getStringAttribute("resultMap", processNestedResultMappings(caseChild, resultMappings));
      discriminatorMap.put(value, resultMap);
    }
    return builderAssistant.buildDiscriminator(resultType, column, javaTypeClass, jdbcTypeEnum, typeHandlerClass, discriminatorMap);
  }

  private void sqlElement(List<XNode> list) throws Exception {
    if (configuration.getDatabaseId() != null) {
      sqlElement(list, configuration.getDatabaseId());
    }
    sqlElement(list, null);
  }

  private void sqlElement(List<XNode> list, String requiredDatabaseId) throws Exception {
    for (XNode context : list) {
      String databaseId = context.getStringAttribute("databaseId");
      String id = context.getStringAttribute("id");
      id = builderAssistant.applyCurrentNamespace(id, false);
      if (databaseIdMatchesCurrent(id, databaseId, requiredDatabaseId)) {
        sqlFragments.put(id, context);
      }
    }
  }

  private boolean databaseIdMatchesCurrent(String id, String databaseId, String requiredDatabaseId) {
    if (requiredDatabaseId != null) {
      if (!requiredDatabaseId.equals(databaseId)) {
        return false;
      }
    } else {
      if (databaseId != null) {
        return false;
      }
      // skip this fragment if there is a previous one with a not null databaseId
      if (this.sqlFragments.containsKey(id)) {
        XNode context = this.sqlFragments.get(id);
        if (context.getStringAttribute("databaseId") != null) {
          return false;
        }
      }
    }
    return true;
  }

  private ResultMapping buildResultMappingFromContext(XNode context, Class<?> resultType, List<ResultFlag> flags) throws Exception {
    String property;
    if (flags.contains(ResultFlag.CONSTRUCTOR)) {
      property = context.getStringAttribute("name");
    } else {
      property = context.getStringAttribute("property");
    }
    String column = context.getStringAttribute("column");
    String javaType = context.getStringAttribute("javaType");
    String jdbcType = context.getStringAttribute("jdbcType");
    String nestedSelect = context.getStringAttribute("select");
    String nestedResultMap = context.getStringAttribute("resultMap",
        processNestedResultMappings(context, Collections.<ResultMapping> emptyList()));
    String notNullColumn = context.getStringAttribute("notNullColumn");
    String columnPrefix = context.getStringAttribute("columnPrefix");
    String typeHandler = context.getStringAttribute("typeHandler");
    String resultSet = context.getStringAttribute("resultSet");
    String foreignColumn = context.getStringAttribute("foreignColumn");
    boolean lazy = "lazy".equals(context.getStringAttribute("fetchType", configuration.isLazyLoadingEnabled() ? "lazy" : "eager"));
    Class<?> javaTypeClass = resolveClass(javaType);
    @SuppressWarnings("unchecked")
    Class<? extends TypeHandler<?>> typeHandlerClass = (Class<? extends TypeHandler<?>>) resolveClass(typeHandler);
    JdbcType jdbcTypeEnum = resolveJdbcType(jdbcType);
    return builderAssistant.buildResultMapping(resultType, property, column, javaTypeClass, jdbcTypeEnum, nestedSelect, nestedResultMap, notNullColumn, columnPrefix, typeHandlerClass, flags, resultSet, foreignColumn, lazy);
  }

  private String processNestedResultMappings(XNode context, List<ResultMapping> resultMappings) throws Exception {
    if ("association".equals(context.getName())
        || "collection".equals(context.getName())
        || "case".equals(context.getName())) {
      if (context.getStringAttribute("select") == null) {
        ResultMap resultMap = resultMapElement(context, resultMappings);
        return resultMap.getId();
      }
    }
    return null;
  }

  private void bindMapperForNamespace() {
    String namespace = builderAssistant.getCurrentNamespace();
    if (namespace != null) {
      Class<?> boundType = null;
      try {
        boundType = Resources.classForName(namespace);
      } catch (ClassNotFoundException e) {
        //ignore, bound type is not required
      }
      if (boundType != null) {
        if (!configuration.hasMapper(boundType)) {
          // Spring may not know the real resource name so we set a flag
          // to prevent loading again this resource from the mapper interface
          // look at MapperAnnotationBuilder#loadXmlResource
          configuration.addLoadedResource("namespace:" + namespace);
          configuration.addMapper(boundType);
        }
      }
    }
  }

解析玩xml文件之后，使用类XMLStatementBuilder创建每个sql（包括insert,update,select,delete等）语句的MappedStatement。

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))
          ? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE;
    }

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

解析完MyBatis配置文件后，configuration就初始化完成了，然后根据configuration对象来创建SqlSessionFactory，到这里时，MyBatis的初始化的征程已经走完了。

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

从debug调试看出 返回的 sqlSessionFactory 是DefaultSesssionFactory类型的，但是configuration此时已经被初始化了。查看源码后画如下创建DefaultSessionFactory的时序图： 

2.2 创建SqlSession的过程

 
从debug调试看出SqlSessinoFactory.openSession() 返回的sqlSession是DefaultSession类型的，此SqlSession里包含一个Configuration的对象，和一个Executor对象。查看源码后画如下创建DefaultSession的时序图：

2.3 创建Mapper的过程

从debug调试可以看出，mapper是一个Mapper代理对象，而且初始化了Configuration对象，Executor的对象。查看源码后画如下创建Mapper的时序图： 

2.4 执行CRUD过程

2.4.1 以select为例查看各步执行的源码

1.mapper.selectEmployeeList()实现原理其实用的jdk动态代理，调试代码发现：

首先是MapperFactoryBean：

/**
   * {@inheritDoc}
   */
  @Override
  public T getObject() throws Exception {
    return getSqlSession().getMapper(this.mapperInterface);
  }

SqlSessionDaoSupport：它得到的是SqlSessionTemplate类，它是接口SqlSession的实现类。

public void setSqlSessionFactory(SqlSessionFactory sqlSessionFactory) {
    if (!this.externalSqlSession) {
      this.sqlSession = new SqlSessionTemplate(sqlSessionFactory);
    }
  }

  public void setSqlSessionTemplate(SqlSessionTemplate sqlSessionTemplate) {
    this.sqlSession = sqlSessionTemplate;
    this.externalSqlSession = true;
  }

  /**
   * Users should use this method to get a SqlSession to call its statement methods
   * This is SqlSession is managed by spring. Users should not commit/rollback/close it
   * because it will be automatically done.
   *
   * @return Spring managed thread safe SqlSession
   */
  public SqlSession getSqlSession() {
    return this.sqlSession;
  }

SqlSessionTemplate：

/**
   * {@inheritDoc}
   */
  @Override
  public <T> T getMapper(Class<T> type) {
    return getConfiguration().getMapper(type, this);
  }

SqlSessionTemplate：

@Override
  public Configuration getConfiguration() {
    return this.sqlSessionFactory.getConfiguration();
  }

DefaultSqlSessionFactory：

@Override
  public Configuration getConfiguration() {
    return configuration;
  }

然后是Configuration:

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
 return this.mapperRegistry.getMapper(type, sqlSession);
 }

最后是MapperRegistry:

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
 MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory)this.knownMappers.get(type);
 if(mapperProxyFactory == null) {
 throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
 } else {
 try {
 return mapperProxyFactory.newInstance(sqlSession);
 } catch (Exception var5) {
 throw new BindingException("Error getting mapper instance. Cause: " + var5, var5);
 }
 }
 }

该方法中最关键代码:mapperProxyFactory.newInstance(sqlSession);

MapperProxyFactory是一个创建MapperProxy的工厂类,调用其中的newInstance方法可以获取到一个代理对象,继续往下看

MapperProxyFactory:

protected T newInstance(MapperProxy<T> mapperProxy) {
 return Proxy.newProxyInstance(this.mapperInterface.getClassLoader(), new Class[]{this.mapperInterface}, mapperProxy);
 }
 public T newInstance(SqlSession sqlSession) {
 MapperProxy<T> mapperProxy = new MapperProxy(sqlSession, this.mapperInterface, this.methodCache);
 return this.newInstance(mapperProxy);
 }

在该类中可以看到, Proxy.newProxyInstance(this.mapperInterface.getClassLoader(), new Class[]{this.mapperInterface}, mapperProxy);

最终由JDK的动态代理,动态的为我们在内存中创建了一个代理对象。到此,我们已经看到了一部分真相,就是我们为mybatis提供Mapper接口,而mybatis使用JDK的动态代理为我们生成实现类。

下面就是InvocationHandler的实现,从上面的代码中可以看到,它叫做MapperProxy

MapperProxy:

public class MapperProxy<T> implements InvocationHandler, Serializable {
 private static final long serialVersionUID = -6424540398559729838L;
 private final SqlSession sqlSession;
 private final Class<T> mapperInterface;
 private final Map<Method, MapperMethod> methodCache;
 public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
 this.sqlSession = sqlSession;
 this.mapperInterface = mapperInterface;
 this.methodCache = methodCache;
 }
 public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
 if(Object.class.equals(method.getDeclaringClass())) {
 return method.invoke(this, args);
 } else {
 MapperMethod mapperMethod = this.cachedMapperMethod(method);
 return mapperMethod.execute(this.sqlSession, args);
 }
 }
 private MapperMethod cachedMapperMethod(Method method) {
 MapperMethod mapperMethod = (MapperMethod)this.methodCache.get(method);
 if(mapperMethod == null) {
 mapperMethod = new MapperMethod(this.mapperInterface, method, this.sqlSession.getConfiguration());
 this.methodCache.put(method, mapperMethod);
 }
 return mapperMethod;
 }
}

在invoke方法中可以看到,如果我们调用的是Object中的方法,不做任何处理,直接调用,否则执行:mapperMethod.execute(this.sqlSession, args)。

MapperMethod:经过上面的调用后进入MapperMethod里面执行

//判断sql命令类型
public Object execute(SqlSession sqlSession, Object[] args) {
        Object param;
        Object result;
        if(SqlCommandType.INSERT == this.command.getType()) {
            param = this.method.convertArgsToSqlCommandParam(args);
            result = this.rowCountResult(sqlSession.insert(this.command.getName(), param));
        } else if(SqlCommandType.UPDATE == this.command.getType()) {
            param = this.method.convertArgsToSqlCommandParam(args);
            result = this.rowCountResult(sqlSession.update(this.command.getName(), param));
        } else if(SqlCommandType.DELETE == this.command.getType()) {
            param = this.method.convertArgsToSqlCommandParam(args);
            result = this.rowCountResult(sqlSession.delete(this.command.getName(), param));
        } else if(SqlCommandType.SELECT == this.command.getType()) {
        //我们测试的是select类型，则再判断这个方法的返回类型
            if(this.method.returnsVoid() && this.method.hasResultHandler()) {
                this.executeWithResultHandler(sqlSession, args);
                result = null;
            } else if(this.method.returnsMany()) {
               //我们是查询列表，此方法执行
                result = this.executeForMany(sqlSession, args);
            } else if(this.method.returnsMap()) {
                result = this.executeForMap(sqlSession, args);
            } else {
                param = this.method.convertArgsToSqlCommandParam(args);
                result = sqlSession.selectOne(this.command.getName(), param);
            }
        } else {
            if(SqlCommandType.FLUSH != this.command.getType()) {
                throw new BindingException("Unknown execution method for: " + this.command.getName());
            }

            result = sqlSession.flushStatements();
        }

        if(result == null && this.method.getReturnType().isPrimitive() && !this.method.returnsVoid()) {
            throw new BindingException("Mapper method '" + this.command.getName() + " attempted to return null from a method with a primitive return type (" + this.method.getReturnType() + ").");
        } else {
            return result;
        }
    }

private <E> Object executeForMany(SqlSession sqlSession, Object[] args) {
//将param做处理 自动处理为param1,param2..
        Object param = this.method.convertArgsToSqlCommandParam(args);
        List result;
        if(this.method.hasRowBounds()) {
            RowBounds rowBounds = this.method.extractRowBounds(args);
            //调用该对象的DefaultSqlSession的selectList方法
            result = sqlSession.selectList(this.command.getName(), param, rowBounds);
        } else {
            result = sqlSession.selectList(this.command.getName(), param);
        }

        return !this.method.getReturnType().isAssignableFrom(result.getClass())?(this.method.getReturnType().isArray()?this.convertToArray(result):this.convertToDeclaredCollection(sqlSession.getConfiguration(), result)):result;
    }

//处理参数方法
 public Object convertArgsToSqlCommandParam(Object[] args) {
            int paramCount = this.params.size();
            if(args != null && paramCount != 0) {
                if(!this.hasNamedParameters && paramCount == 1) {
                    return args[((Integer)this.params.keySet().iterator().next()).intValue()];
                } else {
                    Map<String, Object> param = new MapperMethod.ParamMap();
                    int i = 0;

                    for(Iterator i$ = this.params.entrySet().iterator(); i$.hasNext(); ++i) {
                        Entry<Integer, String> entry = (Entry)i$.next();
                        param.put(entry.getValue(), args[((Integer)entry.getKey()).intValue()]);
                        String genericParamName = "param" + String.valueOf(i + 1);
                        if(!param.containsKey(genericParamName)) {
                            param.put(genericParamName, args[((Integer)entry.getKey()).intValue()]);
                        }
                    }

                    return param;
                }
            } else {
                return null;
            }
        }

主要将SQL分为两类执行,DML和SQL。如果是DML,调用SQLSession中对应的方法执行,并且使用rowCountResult方法根据方法的返回值和受影响的行数做处理。

如果是查询,则要根据方法的返回值的类型来执行不同的方法。如果Collection系的集合获取数组来接收,使用selectList方法执行查询。如果使用Map集合,调用selectMap方法执行查询。否则,调用selectOne执行查询。

下面是调用DefaultSqlSession的selectList的方法

public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
        List var5;
        try {
        //获取MappedStatement对象
            MappedStatement ms = this.configuration.getMappedStatement(statement);
            //调用cachingExecutor执行器的方法
            var5 = this.executor.query(ms, this.wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
        } catch (Exception var9) {
            throw ExceptionFactory.wrapException("Error querying database.  Cause: " + var9, var9);
        } finally {
            ErrorContext.instance().reset();
        }

        return var5;
    }

//CachingExector的query方法
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    //
        BoundSql boundSql = ms.getBoundSql(parameterObject);
        CacheKey key = this.createCacheKey(ms, parameterObject, rowBounds, boundSql);
        //调用下2代码
        return this.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
    }
    //2代码
 public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
        Cache cache = ms.getCache();
        if(cache != null) {
            this.flushCacheIfRequired(ms);
            if(ms.isUseCache() && resultHandler == null) {
                this.ensureNoOutParams(ms, parameterObject, boundSql);
                List<E> list = (List)this.tcm.getObject(cache, key);
                if(list == null) {
                //这里是调用Executor里的query方法 如果开启了缓存这掉CachingExecutor的 如果没有则是调用BaseExecutor的
                    list = this.delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
                    this.tcm.putObject(cache, key, list);
                }

                return list;
            }
        }

        return this.delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
    }

BaseExecutor的query方法

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(this.closed) {
            throw new ExecutorException("Executor was closed.");
        } else {
            if(this.queryStack == 0 && ms.isFlushCacheRequired()) {
                this.clearLocalCache();
            }

            List list;
            try {
                ++this.queryStack;
                list = resultHandler == null?(List)this.localCache.getObject(key):null;
                if(list != null) {
                    this.handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
                } else {
                //如果缓存中没有就从数据库中查询
                    list = this.queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
                }
            } finally {
                --this.queryStack;
            }

            if(this.queryStack == 0) {
                Iterator i$ = this.deferredLoads.iterator();

                while(i$.hasNext()) {
                    BaseExecutor.DeferredLoad deferredLoad = (BaseExecutor.DeferredLoad)i$.next();
                    deferredLoad.load();
                }

                this.deferredLoads.clear();
                if(this.configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
                    this.clearLocalCache();
                }
            }

            return list;
        }
    }

//从数据库中查询
private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
        //放入缓存
        this.localCache.putObject(key, ExecutionPlaceholder.EXECUTION_PLACEHOLDER);

        List list;
        try {
        //此处是调用子Executor的方法，ExecutorType默认是使用的SimpleExecutor
            list = this.doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
        } finally {
            this.localCache.removeObject(key);
        }

        this.localCache.putObject(key, list);
        if(ms.getStatementType() == StatementType.CALLABLE) {
            this.localOutputParameterCache.putObject(key, parameter);
        }

        return list;
    }

SimpleExecutor的doQuery方法

public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
        Statement stmt = null;

        List var9;
        try {
            Configuration configuration = ms.getConfiguration();
            //创建StateMentHandler处理器
            StatementHandler handler = configuration.newStatementHandler(this.wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
            //调用下3的方法
            stmt = this.prepareStatement(handler, ms.getStatementLog());
            //调用no4的方法
            var9 = handler.query(stmt, resultHandler);
        } finally {
            this.closeStatement(stmt);
        }

        return var9;
    }
    //下3方法
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
        Connection connection = this.getConnection(statementLog);
        Statement stmt = handler.prepare(connection);
        //SatementHanlder 采用PreparedStatementHandler来实现此方法，而PreparedStatementHandler调用的是父接口ParameterHandler的方法
        handler.parameterize(stmt);
        return stmt;
    }

ParameterHandler参数处理器的方法

public interface ParameterHandler {
    Object getParameterObject();
    //此方法是用DefaultParameterHandler实现的
    void setParameters(PreparedStatement var1) throws SQLException;
}

DefaultParameterHandler默认参数处理器的方法

public void setParameters(PreparedStatement ps) {
        ErrorContext.instance().activity("setting parameters").object(this.mappedStatement.getParameterMap().getId());
        List<ParameterMapping> parameterMappings = this.boundSql.getParameterMappings();
        if(parameterMappings != null) {
            for(int i = 0; i < parameterMappings.size(); ++i) {
                ParameterMapping parameterMapping = (ParameterMapping)parameterMappings.get(i);
                if(parameterMapping.getMode() != ParameterMode.OUT) {
                    String propertyName = parameterMapping.getProperty();
                    Object value;
                    if(this.boundSql.hasAdditionalParameter(propertyName)) {
                        value = this.boundSql.getAdditionalParameter(propertyName);
                    } else if(this.parameterObject == null) {
                        value = null;
                    } else if(this.typeHandlerRegistry.hasTypeHandler(this.parameterObject.getClass())) {
                        value = this.parameterObject;
                    } else {
                        MetaObject metaObject = this.configuration.newMetaObject(this.parameterObject);
                        value = metaObject.getValue(propertyName);
                    }
                   //这里用调用 TypeHandler类型映射处理器来映射
                    TypeHandler typeHandler = parameterMapping.getTypeHandler();
                    JdbcType jdbcType = parameterMapping.getJdbcType();
                    if(value == null && jdbcType == null) {
                        jdbcType = this.configuration.getJdbcTypeForNull();
                    }

                    try {
                    //类型处理器设置参数映射
                                               typeHandler.setParameter(ps, i + 1, value, jdbcType);
                    } catch (TypeException var10) {
                        throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + var10, var10);
                    } catch (SQLException var11) {
                        throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + var11, var11);
                    }
                }
            }
        }

    }

no4的方法

public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
       //此处调用原生sql的处理器
        PreparedStatement ps = (PreparedStatement)statement;
        //发出原生sql命令
        ps.execute();
        //采用ResultHandler结果处理器对结果集封装
        return this.resultSetHandler.handleResultSets(ps);
    }

ResultHandler代码

public interface ResultSetHandler {
    //此处调用的是DefaultResultSetHandler的方法
    <E> List<E> handleResultSets(Statement var1) throws SQLException;

    void handleOutputParameters(CallableStatement var1) throws SQLException;
}

DefaultResultSetHandler的方法

public List<Object> handleResultSets(Statement stmt) throws SQLException {
        ErrorContext.instance().activity("handling results").object(this.mappedStatement.getId());
        List<Object> multipleResults = new ArrayList();
        int resultSetCount = 0;
        ResultSetWrapper rsw = this.getFirstResultSet(stmt);
        List<ResultMap> resultMaps = this.mappedStatement.getResultMaps();
        int resultMapCount = resultMaps.size();
        this.validateResultMapsCount(rsw, resultMapCount);

        while(rsw != null && resultMapCount > resultSetCount) {
            ResultMap resultMap = (ResultMap)resultMaps.get(resultSetCount);
            this.handleResultSet(rsw, resultMap, multipleResults, (ResultMapping)null);
            rsw = this.getNextResultSet(stmt);
            this.cleanUpAfterHandlingResultSet();
            ++resultSetCount;
        }

        String[] resultSets = this.mappedStatement.getResulSets();
        if(resultSets != null) {
            while(rsw != null && resultSetCount < resultSets.length) {
                ResultMapping parentMapping = (ResultMapping)this.nextResultMaps.get(resultSets[resultSetCount]);
                if(parentMapping != null) {
                    String nestedResultMapId = parentMapping.getNestedResultMapId();
                    ResultMap resultMap = this.configuration.getResultMap(nestedResultMapId);
                    this.handleResultSet(rsw, resultMap, (List)null, parentMapping);
                }

                rsw = this.getNextResultSet(stmt);
                this.cleanUpAfterHandlingResultSet();
                ++resultSetCount;
            }
        }

        return this.collapseSingleResultList(multipleResults);
    }
//处理结果集
 private void handleResultSet(ResultSetWrapper rsw, ResultMap resultMap, List<Object> multipleResults, ResultMapping parentMapping) throws SQLException {
        try {
            if(parentMapping != null) {
                this.handleRowValues(rsw, resultMap, (ResultHandler)null, RowBounds.DEFAULT, parentMapping);
            } else if(this.resultHandler == null) {
                DefaultResultHandler defaultResultHandler = new DefaultResultHandler(this.objectFactory);
                this.handleRowValues(rsw, resultMap, defaultResultHandler, this.rowBounds, (ResultMapping)null);
                multipleResults.add(defaultResultHandler.getResultList());
            } else {
                this.handleRowValues(rsw, resultMap, this.resultHandler, this.rowBounds, (ResultMapping)null);
            }
        } finally {
            this.closeResultSet(rsw.getResultSet());
        }

    }

private void handleRowValues(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException {
        if(resultMap.hasNestedResultMaps()) {
            this.ensureNoRowBounds();
            this.checkResultHandler();
            this.handleRowValuesForNestedResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
        } else {
            this.handleRowValuesForSimpleResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
        }

    }

   private void handleRowValuesForNestedResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException {
        DefaultResultContext<Object> resultContext = new DefaultResultContext();
        this.skipRows(rsw.getResultSet(), rowBounds);
        Object rowValue = null;

        while(this.shouldProcessMoreRows(resultContext, rowBounds) && rsw.getResultSet().next()) {
            ResultMap discriminatedResultMap = this.resolveDiscriminatedResultMap(rsw.getResultSet(), resultMap, (String)null);
            CacheKey rowKey = this.createRowKey(discriminatedResultMap, rsw, (String)null);
            Object partialObject = this.nestedResultObjects.get(rowKey);
            if(this.mappedStatement.isResultOrdered()) {
                if(partialObject == null && rowValue != null) {
                    this.nestedResultObjects.clear();
                    this.storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
                }
        //获取行的值
                rowValue = this.getRowValue(rsw, discriminatedResultMap, rowKey, (String)null, partialObject);
            } else {
                rowValue = this.getRowValue(rsw, discriminatedResultMap, rowKey, (String)null, partialObject);
                if(partialObject == null) {
                    this.storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
                }
            }
        }

        if(rowValue != null && this.mappedStatement.isResultOrdered() && this.shouldProcessMoreRows(resultContext, rowBounds)) {
            this.storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
        }

    }
    String resultMapId = resultMap.getId();
        Object resultObject = partialObject;
        if(partialObject != null) {
            MetaObject metaObject = this.configuration.newMetaObject(partialObject);
            this.putAncestor(partialObject, resultMapId, columnPrefix);
            this.applyNestedResultMappings(rsw, resultMap, metaObject, columnPrefix, combinedKey, false);
            this.ancestorObjects.remove(resultMapId);
        } else {
            ResultLoaderMap lazyLoader = new ResultLoaderMap();
            resultObject = this.createResultObject(rsw, resultMap, lazyLoader, columnPrefix);
            if(resultObject != null && !this.typeHandlerRegistry.hasTypeHandler(resultMap.getType())) {
                MetaObject metaObject = this.configuration.newMetaObject(resultObject);
                boolean foundValues = !resultMap.getConstructorResultMappings().isEmpty();
                if(this.shouldApplyAutomaticMappings(resultMap, true)) {
                    foundValues = this.applyAutomaticMappings(rsw, resultMap, metaObject, columnPrefix) || foundValues;
                }

                foundValues = this.applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, columnPrefix) || foundValues;
                this.putAncestor(resultObject, resultMapId, columnPrefix);
                foundValues = this.applyNestedResultMappings(rsw, resultMap, metaObject, columnPrefix, combinedKey, true) || foundValues;
                this.ancestorObjects.remove(resultMapId);
                foundValues = lazyLoader.size() > 0 || foundValues;
                resultObject = foundValues?resultObject:null;
            }

            if(combinedKey != CacheKey.NULL_CACHE_KEY) {
                this.nestedResultObjects.put(combinedKey, resultObject);
            }
        }

        return resultObject;
    }

private boolean shouldApplyAutomaticMappings(ResultMap resultMap, boolean isNested) {
        return resultMap.getAutoMapping() != null?resultMap.getAutoMapping().booleanValue():(isNested?AutoMappingBehavior.FULL == this.configuration.getAutoMappingBehavior():AutoMappingBehavior.NONE != this.configuration.getAutoMappingBehavior());
    }
 private boolean applyAutomaticMappings(ResultSetWrapper rsw, ResultMap resultMap, MetaObject metaObject, String columnPrefix) throws SQLException {
        List<DefaultResultSetHandler.UnMappedColumAutoMapping> autoMapping = this.createAutomaticMappings(rsw, resultMap, metaObject, columnPrefix);
        boolean foundValues = false;
        if(autoMapping.size() > 0) {
            Iterator i$ = autoMapping.iterator();

            while(true) {
           //这里使用了内部类对参数和结果集进行映射
               DefaultResultSetHandler.UnMappedColumAutoMapping mapping;
                Object value;
                do {
                    if(!i$.hasNext()) {
                        return foundValues;
                    }

                    mapping = (DefaultResultSetHandler.UnMappedColumAutoMapping)i$.next();
                    value = mapping.typeHandler.getResult(rsw.getResultSet(), mapping.column);
                } while(value == null && !this.configuration.isCallSettersOnNulls());

                if(value != null || !mapping.primitive) {
                    metaObject.setValue(mapping.property, value);
                }

                foundValues = true;
            }
        } else {
            return foundValues;
        }
    }
 private static class UnMappedColumAutoMapping {
        private final String column;
        private final String property;
        private final TypeHandler<?> typeHandler;
        private final boolean primitive;

    //此处才类型器对结果进行映射
        public UnMappedColumAutoMapping(String column, String property, TypeHandler<?> typeHandler, boolean primitive) {
            this.column = column;
            this.property = property;
            this.typeHandler = typeHandler;
            this.primitive = primitive;
        }
    }

本文转自：https://blog.csdn.net/u014297148/article/details/78696096

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