深入Mybatis源码——配置解析

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前言

上一篇分析了Mybatis的基础组件,Mybatis的运行调用就是建立在这些基础组件之上的,那它的执行原理又是怎样的呢?在往下之前不妨先思考下如果是你会怎么实现。

正文

熟悉Mybatis的都知道,在使用Mybatis时需要配置一个mybatis-config.xml文件,另外还需要定义Mapper接口和Mapper.xml文件,在config文件中引入或扫描对应的包才能被加载解析(现在由于大多是SpringBoot工程,基本上都不会配置config文件,而是通过注解进行扫描就行了,但本质上的实现和xml配置没有太大区别,所以本篇仍以xml配置方式进行分析。),所以Mybatis的第一个阶段必然是要去加载并解析配置文件,这个阶段在项目启动时就应该完成,后面直接调用即可。加载完成之后,自然就是等待调用,但是我们在项目中只会定义Mapper接口和Mapper.xml文件,那具体的实现类在哪呢?Mybatis是通过动态代理实现的,所以第二个阶段应该是生成Mapper接口的代理实现类。通过调用代理类,最终会生成对应的sql访问数据库并获取结果,所以最后一个阶段就是SQL解析(参数映射、SQL映射、结果映射)。本文主要分析配置解析阶段。

配置解析

Mybatis可以通过下面的方式解析配置文件:

    final String resource = "org/apache/ibatis/builder/MapperConfig.xml";
    final Reader reader = Resources.getResourceAsReader(resource);
    SqlSessionFactory sqlMapper = new SqlSessionFactoryBuilder().build(reader);

所以入口就是build方法(从名字可以看出使用了建造者模式,它和工厂模式一样,也是解用于创建对象的一种模式,不过与工厂模式不一样的是,前者需要我们自己参与构建的细节,而后者则不需要):

  public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
    try {
      //读取配置文件
      XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
      return build(parser.parse());//解析配置文件得到configuration对象,并返回SqlSessionFactory
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error building SqlSession.", e);
    } finally {
      ErrorContext.instance().reset();
      try {
        reader.close();
      } catch (IOException e) {
        // Intentionally ignore. Prefer previous error.
      }
    }
  }

这里先是创建了一个XMLConfigBuilder对象,这个对象就是用来加载解析config文件的,先看看它的构造方法中做了些什么事情:

  public XMLConfigBuilder(Reader reader, String environment, Properties props) {
    this(new XPathParser(reader, true, props, new XMLMapperEntityResolver()), environment, props);
  }

  private XMLConfigBuilder(XPathParser parser, String environment, Properties props) {
    super(new Configuration());
    ErrorContext.instance().resource("SQL Mapper Configuration");
    this.configuration.setVariables(props);
    this.parsed = false;
    this.environment = environment;
    this.parser = parser;
  }

需要注意的是这里创建了一个Configuration对象,他就是Mybatis的核心CPU,保存了所有的配置信息,在后面的执行阶段所需要的信息都是从这个类取的,因为这个类比较大,这里就不贴详细代码了,读者请务必阅读源码熟悉该类。因为这个类对象保存了所有的配置信息,那么必然这个类是全局单例的,事实上这个对象的创建也只有这里一个入口,保证了全局唯一。
在该类的构造方法中,首先就注册了核心组件的别名和对应的类映射关系:

  public Configuration() {
    typeAliasRegistry.registerAlias("JDBC", JdbcTransactionFactory.class);
    typeAliasRegistry.registerAlias("MANAGED", ManagedTransactionFactory.class);

    typeAliasRegistry.registerAlias("JNDI", JndiDataSourceFactory.class);
    typeAliasRegistry.registerAlias("POOLED", PooledDataSourceFactory.class);
    typeAliasRegistry.registerAlias("UNPOOLED", UnpooledDataSourceFactory.class);

	......省略

    languageRegistry.setDefaultDriverClass(XMLLanguageDriver.class);
    languageRegistry.register(RawLanguageDriver.class);
  }

而注册类在实例化时同样也注册了一些基础类型的别名映射:

  public TypeAliasRegistry() {
    registerAlias("string", String.class);

    registerAlias("byte", Byte.class);
    registerAlias("long", Long.class);
    registerAlias("short", Short.class);
    registerAlias("int", Integer.class);
    registerAlias("integer", Integer.class);
    registerAlias("double", Double.class);
    registerAlias("float", Float.class);
    registerAlias("boolean", Boolean.class);

	......省略
	
    registerAlias("ResultSet", ResultSet.class);
  }

看到这相信你就知道parameterType和resultType属性的简写是怎么实现的了。回到主流程,进入到parser.parse方法中:

  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
     //解析<properties>节点
      propertiesElement(root.evalNode("properties"));
      //解析<settings>节点
      Properties settings = settingsAsProperties(root.evalNode("settings"));
      loadCustomVfs(settings);
      //解析<typeAliases>节点
      typeAliasesElement(root.evalNode("typeAliases"));
      //解析<plugins>节点
      pluginElement(root.evalNode("plugins"));
      //解析<objectFactory>节点
      objectFactoryElement(root.evalNode("objectFactory"));
      //解析<objectWrapperFactory>节点
      objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
      //解析<reflectorFactory>节点
      reflectorFactoryElement(root.evalNode("reflectorFactory"));
      settingsElement(settings);//将settings填充到configuration
      // read it after objectFactory and objectWrapperFactory issue #631
      //解析<environments>节点
      environmentsElement(root.evalNode("environments"));
      //解析<databaseIdProvider>节点
      databaseIdProviderElement(root.evalNode("databaseIdProvider"));
      //解析<typeHandlers>节点
      typeHandlerElement(root.evalNode("typeHandlers"));
      //解析<mappers>节点
      mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
  }

这个方法就是去解析配置文件中的各个节点,并将其封装到Configuration对象中去,前面的节点解析没啥好说的,自己看一下就明白了,重点看一下最后一个对mapper节点的解析,这个就是加载我们的Mapper.xml文件:

  <mappers>
    <mapper resource="org/apache/ibatis/builder/AuthorMapper.xml"/>
    <mapper resource="org/apache/ibatis/builder/BlogMapper.xml"/>
    <mapper resource="org/apache/ibatis/builder/CachedAuthorMapper.xml"/>
    <mapper resource="org/apache/ibatis/builder/PostMapper.xml"/>
    <mapper resource="org/apache/ibatis/builder/NestedBlogMapper.xml"/>
  </mappers>
 private void mapperElement(XNode parent) throws Exception {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {//处理mapper子节点
        if ("package".equals(child.getName())) {//package子节点
          String mapperPackage = child.getStringAttribute("name");
          configuration.addMappers(mapperPackage);
        } else {//获取<mapper>节点的resource、url或mClass属性这三个属性互斥
          String resource = child.getStringAttribute("resource");
          String url = child.getStringAttribute("url");
          String mapperClass = child.getStringAttribute("class");
          if (resource != null && url == null && mapperClass == null) {//如果resource不为空
            ErrorContext.instance().resource(resource);
            InputStream inputStream = Resources.getResourceAsStream(resource);//加载mapper文件
            //实例化XMLMapperBuilder解析mapper映射文件
            XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
            mapperParser.parse();
          } else if (resource == null && url != null && mapperClass == null) {//如果url不为空
            ErrorContext.instance().resource(url);
            InputStream inputStream = Resources.getUrlAsStream(url);//加载mapper文件
            //实例化XMLMapperBuilder解析mapper映射文件
            XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());
            mapperParser.parse();
          } else if (resource == null && url == null && mapperClass != null) {//如果class不为空
            Class<?> mapperInterface = Resources.classForName(mapperClass);//加载class对象
            configuration.addMapper(mapperInterface);//向代理中心注册mapper
          } else {
            throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
          }
        }
      }
    }
  }

从上面的代码中我们可以看到这里有两种配置方式,一种是配置package子节点,即扫描并批量加载指定的包中的文件;另一种则是使用mapper子节点引入单个文件,而mapper节点又可以配置三种属性:resource、url、class。而解析XML的核心类是XMLMapperBuilder,进入parse方法:

  public void parse() {
	//判断是否已经加载该配置文件
    if (!configuration.isResourceLoaded(resource)) {
      configurationElement(parser.evalNode("/mapper"));//处理mapper节点
      configuration.addLoadedResource(resource);//将mapper文件添加到configuration.loadedResources中
      bindMapperForNamespace();//注册mapper接口
    }
    //处理解析失败的ResultMap节点
    parsePendingResultMaps();
    //处理解析失败的CacheRef节点
    parsePendingCacheRefs();
    //处理解析失败的Sql语句节点
    parsePendingStatements();
  }

  private void configurationElement(XNode context) {
    try {
    	//获取mapper节点的namespace属性
      String namespace = context.getStringAttribute("namespace");
      if (namespace == null || namespace.equals("")) {
        throw new BuilderException("Mapper's namespace cannot be empty");
      }
      //设置builderAssistant的namespace属性
      builderAssistant.setCurrentNamespace(namespace);
      //解析cache-ref节点
      cacheRefElement(context.evalNode("cache-ref"));
      //重点分析 :解析cache节点----------------1-------------------
      cacheElement(context.evalNode("cache"));
      //解析parameterMap节点(已废弃)
      parameterMapElement(context.evalNodes("/mapper/parameterMap"));
      //重点分析 :解析resultMap节点(基于数据结果去理解)----------------2-------------------
      resultMapElements(context.evalNodes("/mapper/resultMap"));
      //解析sql节点
      sqlElement(context.evalNodes("/mapper/sql"));
      //重点分析 :解析select、insert、update、delete节点 ----------------3-------------------
      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);
    }
  }

核心的处理逻辑又是通过configurationElement实现的,接下来挨个分析几个重要节点解析过程。

1. cacheRefElement/cacheElement

这两个节点都是解析二级缓存配置的,前者是引用其它namespace的二级缓存,后者则是直接开启当前namespace的二级缓存,所以重点看后者:

  private void cacheElement(XNode context) throws Exception {
    if (context != null) {
      //获取cache节点的type属性,默认为PERPETUAL
      String type = context.getStringAttribute("type", "PERPETUAL");
      //找到type对应的cache接口的实现
      Class<? extends Cache> typeClass = typeAliasRegistry.resolveAlias(type);
      //读取eviction属性,既缓存的淘汰策略,默认LRU
      String eviction = context.getStringAttribute("eviction", "LRU");
      //根据eviction属性,找到装饰器
      Class<? extends Cache> evictionClass = typeAliasRegistry.resolveAlias(eviction);
      //读取flushInterval属性,既缓存的刷新周期
      Long flushInterval = context.getLongAttribute("flushInterval");
      //读取size属性,既缓存的容量大小
      Integer size = context.getIntAttribute("size");
     //读取readOnly属性,既缓存的是否只读
      boolean readWrite = !context.getBooleanAttribute("readOnly", false);
      //读取blocking属性,既缓存的是否阻塞
      boolean blocking = context.getBooleanAttribute("blocking", false);
      Properties props = context.getChildrenAsProperties();
      //通过builderAssistant创建缓存对象,并添加至configuration
      builderAssistant.useNewCache(typeClass, evictionClass, flushInterval, size, readWrite, blocking, props);
    }
  }

  public Cache useNewCache(Class<? extends Cache> typeClass,
      Class<? extends Cache> evictionClass,
      Long flushInterval,
      Integer size,
      boolean readWrite,
      boolean blocking,
      Properties props) {
	//经典的建造起模式,创建一个cache对象
    Cache cache = new CacheBuilder(currentNamespace)
        .implementation(valueOrDefault(typeClass, PerpetualCache.class))
        .addDecorator(valueOrDefault(evictionClass, LruCache.class))
        .clearInterval(flushInterval)
        .size(size)
        .readWrite(readWrite)
        .blocking(blocking)
        .properties(props)
        .build();
    //将缓存添加至configuration,注意二级缓存以命名空间为单位进行划分
    configuration.addCache(cache);
    currentCache = cache;
    return cache;
  }

  public void addCache(Cache cache) {
    caches.put(cache.getId(), cache);
  }

从这里我们可以看到默认创建了PerpetualCache对象,这个是缓存的基本实现类,然后根据配置给缓存加上装饰器,默认会装饰LRU。配置解析完成后,才会通过MapperBuilderAssistant类真正创建缓存对象并添加到Configuration对象中。为什么这里要通过MapperBuilderAssistant对象创建缓存对象呢?从名字可以看出它是XMLMapperBuilder的协助者,因为XML的解析和配置对象的装填是非常繁琐的一个过程,如果全部由一个类来完成,会非常的臃肿难看,并且耦合性较高,所以这里又雇佣了一个“协助者”。

2. resultMapElements

  private void resultMapElements(List<XNode> list) throws Exception {
	//遍历所有的resultmap节点
    for (XNode resultMapNode : list) {
      try {
    	 //解析具体某一个resultMap节点
        resultMapElement(resultMapNode);
      } catch (IncompleteElementException e) {
        // ignore, it will be retried
      }
    }
  }

  private ResultMap resultMapElement(XNode resultMapNode, List<ResultMapping> additionalResultMappings) throws Exception {
    ErrorContext.instance().activity("processing " + resultMapNode.getValueBasedIdentifier());
    //获取resultmap节点的id属性
    String id = resultMapNode.getStringAttribute("id",
        resultMapNode.getValueBasedIdentifier());
    //获取resultmap节点的type属性
    String type = resultMapNode.getStringAttribute("type",
        resultMapNode.getStringAttribute("ofType",
            resultMapNode.getStringAttribute("resultType",
                resultMapNode.getStringAttribute("javaType"))));
    //获取resultmap节点的extends属性,描述继承关系
    String extend = resultMapNode.getStringAttribute("extends");
    //获取resultmap节点的autoMapping属性,是否开启自动映射
    Boolean autoMapping = resultMapNode.getBooleanAttribute("autoMapping");
    //从别名注册中心获取entity的class对象
    Class<?> typeClass = resolveClass(type);
    Discriminator discriminator = null;
    //记录子节点中的映射结果集合
    List<ResultMapping> resultMappings = new ArrayList<>();
    resultMappings.addAll(additionalResultMappings);
    //从xml文件中获取当前resultmap中的所有子节点,并开始遍历
    List<XNode> resultChildren = resultMapNode.getChildren();
    for (XNode resultChild : resultChildren) {
      if ("constructor".equals(resultChild.getName())) {//处理<constructor>节点
        processConstructorElement(resultChild, typeClass, resultMappings);
      } else if ("discriminator".equals(resultChild.getName())) {//处理<discriminator>节点
        discriminator = processDiscriminatorElement(resultChild, typeClass, resultMappings);
      } else {//处理<id> <result> <association> <collection>节点
        List<ResultFlag> flags = new ArrayList<>();
        if ("id".equals(resultChild.getName())) {
          flags.add(ResultFlag.ID);//如果是id节点,向flags中添加元素
        }
        //创建ResultMapping对象并加入resultMappings集合中
        resultMappings.add(buildResultMappingFromContext(resultChild, typeClass, flags));
      }
    }
    //实例化resultMap解析器
    ResultMapResolver resultMapResolver = new ResultMapResolver(builderAssistant, id, typeClass, extend, discriminator, resultMappings, autoMapping);
    try {
      //通过resultMap解析器实例化resultMap并将其注册到configuration对象
      return resultMapResolver.resolve();
    } catch (IncompleteElementException  e) {
      configuration.addIncompleteResultMap(resultMapResolver);
      throw e;
    }
  }

这个方法同样先是解析节点的属性,然后通过buildResultMappingFromContext方法创建ResultMapping对象并封装到ResultMapResolver中去,最后还是通过MapperBuilderAssistant实例化ResultMap对象并添加到ConfigurationresultMaps属性中:

  public ResultMap addResultMap(
      String id,
      Class<?> type,
      String extend,
      Discriminator discriminator,
      List<ResultMapping> resultMappings,
      Boolean autoMapping) {
	 //完善id,id的完整格式是"namespace.id"
    id = applyCurrentNamespace(id, false);
    //获得父类resultMap的完整id
    extend = applyCurrentNamespace(extend, true);

    //针对extend属性的处理
    if (extend != null) {
      if (!configuration.hasResultMap(extend)) {
        throw new IncompleteElementException("Could not find a parent resultmap with id '" + extend + "'");
      }
      ResultMap resultMap = configuration.getResultMap(extend);
      List<ResultMapping> extendedResultMappings = new ArrayList<>(resultMap.getResultMappings());
      extendedResultMappings.removeAll(resultMappings);
      // Remove parent constructor if this resultMap declares a constructor.
      boolean declaresConstructor = false;
      for (ResultMapping resultMapping : resultMappings) {
        if (resultMapping.getFlags().contains(ResultFlag.CONSTRUCTOR)) {
          declaresConstructor = true;
          break;
        }
      }
      if (declaresConstructor) {
        Iterator<ResultMapping> extendedResultMappingsIter = extendedResultMappings.iterator();
        while (extendedResultMappingsIter.hasNext()) {
          if (extendedResultMappingsIter.next().getFlags().contains(ResultFlag.CONSTRUCTOR)) {
            extendedResultMappingsIter.remove();
          }
        }
      }
      //添加需要被继承下来的resultMapping对象结合
      resultMappings.addAll(extendedResultMappings);
    }
    //通过建造者模式实例化resultMap,并注册到configuration.resultMaps中
    ResultMap resultMap = new ResultMap.Builder(configuration, id, type, resultMappings, autoMapping)
        .discriminator(discriminator)
        .build();
    configuration.addResultMap(resultMap);
    return resultMap;
  }

3. sqlElement

解析SQL节点,只是将其缓存到XMLMapperBuildersqlFragments属性中。

4. buildStatementFromContext

  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) {
      //创建XMLStatementBuilder 专门用于解析sql语句节点
      final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);
      try {
    	//解析sql语句节点
        statementParser.parseStatementNode();
      } catch (IncompleteElementException e) {
        configuration.addIncompleteStatement(statementParser);
      }
    }
  }

这个方法是重点,通过XMLStatementBuilder对象解析select、update、insert、delete节点:

  public void parseStatementNode() {
	//获取sql节点的id
    String id = context.getStringAttribute("id");
    String databaseId = context.getStringAttribute("databaseId");

    if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) {
      return;
    }
    /*获取sql节点的各种属性*/
    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);

    
    
    //根据sql节点的名称获取SqlCommandType(INSERT, UPDATE, DELETE, SELECT)
    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
    //在解析sql语句之前先解析<include>节点
    XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);
    includeParser.applyIncludes(context.getNode());

    // Parse selectKey after includes and remove them.
    //在解析sql语句之前,处理<selectKey>子节点,并在xml节点中删除
    processSelectKeyNodes(id, parameterTypeClass, langDriver);
    
    // Parse the SQL (pre: <selectKey> and <include> were parsed and removed)
    //解析sql语句是解析mapper.xml的核心,实例化sqlSource,使用sqlSource封装sql语句
    SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
    String resultSets = context.getStringAttribute("resultSets");//获取resultSets属性
    String keyProperty = context.getStringAttribute("keyProperty");//获取主键信息keyProperty
    String keyColumn = context.getStringAttribute("keyColumn");///获取主键信息keyColumn
    
    //根据<selectKey>获取对应的SelectKeyGenerator的id
    KeyGenerator keyGenerator;
    String keyStatementId = id + SelectKeyGenerator.SELECT_KEY_SUFFIX;
    keyStatementId = builderAssistant.applyCurrentNamespace(keyStatementId, true);
    
    
    //获取keyGenerator对象,如果是insert类型的sql语句,会使用KeyGenerator接口获取数据库生产的id;
    if (configuration.hasKeyGenerator(keyStatementId)) {
      keyGenerator = configuration.getKeyGenerator(keyStatementId);
    } else {
      keyGenerator = context.getBooleanAttribute("useGeneratedKeys",
          configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType))
          ? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE;
    }

    //通过builderAssistant实例化MappedStatement,并注册至configuration对象
    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)
        .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;
  }

同样的,最后根据sql语句和属性实例化MappedStatement对象,并添加到Configuration对象的mappedStatements属性中。

回到XMLMapperBuilder.parse方法中,在解析完xml之后又调用了bindMapperForNamespace方法:

  private void bindMapperForNamespace() {
	//获取命名空间
    String namespace = builderAssistant.getCurrentNamespace();
    if (namespace != null) {
      Class<?> boundType = null;
      try {
    	//通过命名空间获取mapper接口的class对象
        boundType = Resources.classForName(namespace);
      } catch (ClassNotFoundException e) {
        //ignore, bound type is not required
      }
      if (boundType != null) {
        if (!configuration.hasMapper(boundType)) {//是否已经注册过该mapper接口?
          // 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
          //将命名空间添加至configuration.loadedResource集合中
          configuration.addLoadedResource("namespace:" + namespace);
          //将mapper接口添加到mapper注册中心
          configuration.addMapper(boundType);
        }
      }
    }
  }

这个方法中首先通过namespace拿到xml对应的Mapper接口类型,然后委托给Configuration类中的mapperRegistry注册动态的代理的工厂MapperProxyFactory

  public <T> void addMapper(Class<T> type) {
    if (type.isInterface()) {
        if (hasMapper(type)) {
          throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
        }
      boolean loadCompleted = false;
      try {
    	//实例化Mapper接口的代理工程类,并将信息添加至knownMappers
        knownMappers.put(type, new MapperProxyFactory<T>(type));
        // It's important that the type is added before the parser is run
        // otherwise the binding may automatically be attempted by the
        // mapper parser. If the type is already known, it won't try.
        //解析接口上的注解信息,并添加至configuration对象
        MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
        parser.parse();
        loadCompleted = true;
      } finally {
        if (!loadCompleted) {
          knownMappers.remove(type);
        }
      }
    }
  }

这个就是创建Mapper接口的动态代理对象的工厂类,所以Mapper的代理对象实际上并不是在启动的时候就创建好了,而是在方法调用时才会创建,为什么会这么设计呢?因为代理对象和SqlSession是一一对应的,而我们每一次调用Mapper的方法都是创建一个新的SqlSession,所以这里只是缓存了代理工厂对象。
代理工厂注册之后还通过MapperAnnotationBuilder类提供了对注解方式的支持,这里就不阐述了,结果就是将注解的值添加到Configuration中去。

总结

解析配置文件的流程虽然比较长,但逻辑一点都不复杂,主要就是获取xml配置的属性值,实例化不同的配置对象,并将这些配置都丢到Configuration对象中去,我们只需要重点关注哪些对象被注册到了Configuration中去,最后根据Configuration对象实例化DefaultSqlSessionFactory对象并返回,而DefaultSqlSessionFactory就是用来创建SqlSession对象的,这个对象就是上一篇架构图中的接口层,它提供了所有访问数据库的操作并屏蔽了底层复杂实现细节,具体的实现原理将在下一篇进行分析。

posted @ 2020-07-07 12:13  夜勿语  阅读(171)  评论(0编辑  收藏