Mybatis是怎么工作的（二）

目标：

1. 理清mybatis加载解析mapper文件的过程；
2. 理清mybatis执行SQL的过程。

上一篇文章分析mybatis加载配置的源码时提到了org.apache.ibatis.builder.xml.XMLConfigBuilder#parseConfiguration方法，现在继续分析其中的mapperElement方法。先看源码：

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);
            // 生成XMLMapperBuilder
            XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
            // 解析配置文件
            mapperParser.parse();
          } 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.");
          }
        }
      }
    }
  }

考虑到项目的配置，看下生成XMLMapperBuilder和mapperParser.parse()的代码。
在生成XMLMapperBuilder的过程中，使用了MapperBuilderAssistant，这个类继承了BaseBuilder。在该类的构造器中加载了TypeAliasRegistry和TypeHandlerRegistry。
下面重点看mapperParserparse()

public void parse() {
    // 判断是否已经加载资源
    if (!configuration.isResourceLoaded(resource)) {
      // 配置/mapper节点下的子节点
      configurationElement(parser.evalNode("/mapper"));
      // 加载resource资源
      configuration.addLoadedResource(resource);
      // 绑定命名空间
      bindMapperForNamespace();
    }
    // 加载未加载完成的资源
    parsePendingResultMaps();
    parsePendingCacheRefs();
    parsePendingStatements();
  }

下面主要看下configurationElement的代码：

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);
    }
  }

可以看出主要是解析不同的节点，并放进builderAssistant里面去。
下面看下执行SQL的过程。

ClipsDAO clipsDAO = session.getMapper(ClipsDAO.class);
            ClipsEntity clipsEntity = clipsDAO.selectById(1);

查看session.getMapper()的实现：

// org.apache.ibatis.session.defaults.DefaultSqlSession#getMapper
  @Override
  public <T> T getMapper(Class<T> type) {
    return configuration.<T>getMapper(type, this);
  }
  
  // org.apache.ibatis.session.Configuration#getMapper
    public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    return mapperRegistry.getMapper(type, sqlSession);
  }
  
  // org.apache.ibatis.binding.MapperRegistry#getMapper
    public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null) {
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }

可以看出，mybatis通过动态代理为接口生成了代理类，我们知道在加载配置时，bindMapperForNamespace方法调用了configuration.addMapper()方法把Class映射到org.apache.ibatis.binding.MapperRegistry#knownMappers中去的。
下面看一下MapperProxy代码：

@Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      if (Object.class.equals(method.getDeclaringClass())) {
        return method.invoke(this, args);
      } else if (isDefaultMethod(method)) {
        return invokeDefaultMethod(proxy, method, args);
      }
    } catch (Throwable t) {
      throw ExceptionUtil.unwrapThrowable(t);
    }
    // 从缓存中获取MapperMethod对象
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    // 执行SQL
    return mapperMethod.execute(sqlSession, args);
  }

下面是MapperMethod.execute()方法：

public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    switch (command.getType()) {
      case INSERT: {
      Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.insert(command.getName(), param));
        break;
      }
      case UPDATE: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.update(command.getName(), param));
        break;
      }
      case DELETE: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.delete(command.getName(), param));
        break;
      }
      case SELECT:
        if (method.returnsVoid() && method.hasResultHandler()) {
          executeWithResultHandler(sqlSession, args);
          result = null;
        } else if (method.returnsMany()) {
          result = executeForMany(sqlSession, args);
        } else if (method.returnsMap()) {
          result = executeForMap(sqlSession, args);
        } else if (method.returnsCursor()) {
          result = executeForCursor(sqlSession, args);
        } else {
          Object param = method.convertArgsToSqlCommandParam(args);
          result = sqlSession.selectOne(command.getName(), param);
        }
        break;
      case FLUSH:
        result = sqlSession.flushStatements();
        break;
      default:
        throw new BindingException("Unknown execution method for: " + command.getName());
    }
    if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
      throw new BindingException("Mapper method '" + command.getName() 
          + " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
    }
    return result;
  }

至此，SQL执行完成。