（源码解析）mybatis调用链之XMLMapperBuilder解析Mapper

创建XMLMapperBuilder对象

XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());

XMLMapperBuilder 继承于BaseBuilder

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

初始化一个XPathParser对象，该对象使用dom和xpath技术解析xml文件

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;
  }

进入XMLMapperBuilder另外构造函数，首先调用父类构造函数，为configuration、typeAliasRegistry、typeHandlerRegistry对象赋值。

其次创建MapperBuilderAssistant对象，用于辅助XMLMapperBuilder和XMLStatementBuilder解析mapper.xml文件，完善属性信息，并注册到configuration对象。

MapperBuilderAssistant 同样继承于BaseBuilder。 在创建对象时，调用父类构造函数，为configuration、typeAliasRegistry、typeHandlerRegistry对象赋值。

最终给XMLMapperBuilder对象的parser、sqlFragments、resource属性赋值，XMLMapperBuilder对象构建完成。

mapperParser.parse();

调用XMLMapperBuilder的parse方法，方法如下：

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();
  }

parse方法中，首先判断判断是否已经加载该配置文件，若未加载，贼进入configurationElement方法，开始处理mapper节点。

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);
    }
  }

configurationElement方法主要作用就是解析mapper中的各个节点，此处一一说明。

1、获取mapper节点的namespace属性

String namespace = context.getStringAttribute("namespace");

2、设置builderAssistant的namespace属性

builderAssistant.setCurrentNamespace(namespace);

3、cacheRefElement解析cache-ref节点

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);
      }
    }
  }

4、cacheElement解析cache节点

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);
    }
  }

上述cache解析即为解析二级缓存，若是配置了二级缓存，则会通过builderAssistant创建缓存对象，并添加至configuration

//通过builderAssistant创建缓存对象，并添加至configuration
  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;
  }

经典的建造者模式，创建一个cache对象，

public Cache build() {
      //设置缓存的主实现类为PerpetualCache
    setDefaultImplementations();
    //通过反射实例化PerpetualCache对象
    Cache cache = newBaseCacheInstance(implementation, id);
    setCacheProperties(cache);//根据cache节点下的<property>信息，初始化cache
    // issue #352, do not apply decorators to custom caches
    
    if (PerpetualCache.class.equals(cache.getClass())) {//如果cache是PerpetualCache的实现，则为其添加标准的装饰器
      for (Class<? extends Cache> decorator : decorators) {//为cache对象添加装饰器，这里主要处理缓存清空策略的装饰器
        cache = newCacheDecoratorInstance(decorator, cache);
        setCacheProperties(cache);
      }
      //通过一些属性为cache对象添加装饰器
      cache = setStandardDecorators(cache);
    } else if (!LoggingCache.class.isAssignableFrom(cache.getClass())) {
      //如果cache不是PerpetualCache的实现，则为其添加日志的能力
      cache = new LoggingCache(cache);
    }
    return cache;
  }

观察下图可以看到，整体的Cache使用了装饰器模式进行层层嵌套。

可以看到，最终将二级缓存添加到configuration中，二级缓存以命名空间为单位进行划分。以及将该cache设置为MapperBuilderAssistant对象的currentCache属性值。在后续XMLStatementBuilder解析sql语句生成MappedStatement对象时会使用到。

5、parameterMapElement解析parameterMap节点

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<>();
      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);
    }
  }

6、resultMapElements解析resultMap节点

//解析resultMap节点,实际就是解析sql查询的字段与pojo属性之间的转化规则
  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) throws Exception {
    return resultMapElement(resultMapNode, Collections.<ResultMapping> emptyList());
  }
  
  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;
    }
  }

7、解析sql节点

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);
      }
    }
  }

8、解析select、insert、update、delete节点

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

具体详情请查看mybatis调用链之XMLStatementBuilder解析解析sql语句节点

处理完mapper节点后，将将mapper文件添加到configuration.loadedResources属性中，

configuration.addLoadedResource(resource);

然后需要注册mapper接口，

bindMapperForNamespace();

//注册mapper接口
  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);
        }
      }
    }
  }

上述方法，具体步骤：

1、获取mapper接口的命名空间

2、将命名空间添加至configuration.loadedResources集合中

3、将mapper接口添加到mapper注册中心MapperRegistry的knownMappers中

//记录了mapper接口与对应MapperProxyFactory之间的关系
  private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<>();
  
......

//将mapper接口的工厂类添加到mapper注册中心
  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);
        }
      }
    }
  }

最后，

处理解析失败的ResultMap节点 parsePendingResultMaps();

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...
        }
      }
    }
  }

处理解析失败的CacheRef节点 parsePendingCacheRefs();

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...
        }
      }
    }
  }

处理解析失败的Sql语句节点 parsePendingStatements();

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...
        }
      }
    }
  }