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mybatis 源码分析(二)mapper 初始化

mybatis 的初始化还是相对比较复杂,但是作者在初始化过程中使用了多种设计模式,包括建造者、动态代理、策略、外观等,使得代码的逻辑仍然非常清晰,这一点非常值得我们学习;

一、mapper 初始化主要流程

mybatis 初始化的过程中,主要是 XML 配置的解析,不同的部分又分别委托给了不同的解析器;

mybatis 源码分析(二)mapper 初始化

解析流程为:

XMLConfigBuilder -> XMLMapperBuilder -> XMLStatementBuilder -> XMLScriptBuilder -> SqlSourceBuilder

  • XMLConfigBuilder:负责全局的 mybatis-conf.xml 配置解析;
  • XMLMapperBuilder:负责 sql 配置的 mapper 配置解析;
  • XMLStatementBuilder:负责 mapper 配置文件中 select|insert|update|delete 节点解析;
  • XMLScriptBuilder:负责各 sql 节点解析,主要是动态 sql 解析;
  • SqlSourceBuilder:负责构建 SqlSource;

源码分析:

首先在 XMLConfigBuilder 确定了主要的解析流程:

private void parseConfiguration(XNode root) { // 解析的代码和xml的配置一一对应
  try {
    //issue #117 read properties first
    propertiesElement(root.evalNode("properties"));
    Properties settings = settingsAsProperties(root.evalNode("settings"));
    loadCustomVfs(settings);
    loadCustomLogImpl(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
    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);
  }
}

private void mapperElement(XNode parent) throws Exception {
  if (parent != null) {
    for (XNode child : parent.getChildren()) {
      if ("package".equals(child.getName())) {  // package 方式,mapper 必须和 xml 配置文件在同一目录下
        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();
        } 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.");
        }
      }
    }
  }
}

然后在 XMLMapperBuilder 中解析 mapper

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

  // 下面的三个方式是继续之前未完成的节点解析;比如在 cache-ref 解析的时候,依赖的 cache namespace 还未创建的时候,就需要暂停
  parsePendingResultMaps();
  parsePendingCacheRefs();
  parsePendingStatements();
}

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();  // 未找到依赖的 cache 时,暂停解析
    } catch (IncompleteElementException e) {
      configuration.addIncompleteCacheRef(cacheRefResolver);
    }
  }
}

二、动态 sql 解析

此外在 mapper 各节点的解析过程中 resultMap 和 sql 节点的解析最为复杂,resultMap 解析主要是 xml 和 反射的处理,有一点繁琐有兴趣可以自己看一下;这里主要讲一下 sql 节点的解析要点;

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

private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) {
  for (XNode context : list) {
    final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);
    try {
      statementParser.parseStatementNode();  // 主要的解析过程放到了XMLStatementBuilder中
    } catch (IncompleteElementException e) {
      configuration.addIncompleteStatement(statementParser);
    }
  }
}
// XMLStatementBuilder
public void parseStatementNode() {
  ...
  String parameterType = context.getStringAttribute("parameterType");
  Class<?> parameterTypeClass = resolveClass(parameterType);

  String lang = context.getStringAttribute("lang");
  LanguageDriver langDriver = getLanguageDriver(lang);

  // Parse selectKey after includes and remove them.
  processSelectKeyNodes(id, parameterTypeClass, langDriver);

  // Parse the SQL (pre: <selectKey> and <include> were parsed and removed)
  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;
  }

  SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
  ...

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

代码中的 LanguageDriver 就封装了动态 sql 的解析规则,通过这个接口也可以使用其他的模版引擎或者解析规则(可以通过配置或者注解指定);

mybatis 源码分析(二)mapper 初始化

其中 XMLLanguageDriver 主要处理动态 sql,RawLanguageDriver 主要处理静态 sql;

从代码中可以看到最后 LanguageDriver 将 xml 配置解析成了 SqlSource,其结构如下:

mybatis 源码分析(二)mapper 初始化

其中:

  • RawSqlSource:处理静态sql,去掉xml标签;
  • DynamicSqlSource:处理动态sql,去掉xml标签;
  • ProviderSqlSource:处理注解形式的sql;
  • StaticSqlSource:最终将上面 SqlSource 处理结果中的占位符,替换为 "?",构成真正可执行的sql;

其解析的整体流程如下:

mybatis 源码分析(二)mapper 初始化

从图中可以看到 sql 节点的主要解析逻辑就在于 parseDynamicTagsMixedSqlNode rootSqlNode = parseDynamicTags(context);

在看源码之前先看一下 SqlNode 的结构;

mybatis 源码分析(二)mapper 初始化

这里的每个 node 和 sql 节点下的子节点一一对应;

protected MixedSqlNode parseDynamicTags(XNode node) {
  List<SqlNode> contents = new ArrayList<>();
  NodeList children = node.getNode().getChildNodes();
  for (int i = 0; i < children.getLength(); i++) {
    XNode child = node.newXNode(children.item(i));
    if (child.getNode().getNodeType() == Node.CDATA_SECTION_NODE || child.getNode().getNodeType() == Node.TEXT_NODE) {
      String data = child.getStringBody("");
      TextSqlNode textSqlNode = new TextSqlNode(data);
      if (textSqlNode.isDynamic()) {
        contents.add(textSqlNode);
        isDynamic = true;
      } else {
        contents.add(new StaticTextSqlNode(data));
      }
    } else if (child.getNode().getNodeType() == Node.ELEMENT_NODE) { // issue #628
      String nodeName = child.getNode().getNodeName();
      NodeHandler handler = nodeHandlerMap.get(nodeName);
      if (handler == null) {
        throw new BuilderException("Unknown element <" + nodeName + "> in SQL statement.");
      }
      handler.handleNode(child, contents);
      isDynamic = true;
    }
  }
  return new MixedSqlNode(contents);
}

这里主要逻辑是首先通过子标签的名字,获取对应的处理器,然后将所有的子标签生成的 SqlNode 合成 MixedSqlNode;

mybatis 源码分析(二)mapper 初始化

private void initNodeHandlerMap() {
  nodeHandlerMap.put("trim", new TrimHandler());
  nodeHandlerMap.put("where", new WhereHandler());
  nodeHandlerMap.put("set", new SetHandler());
  nodeHandlerMap.put("foreach", new ForEachHandler());
  nodeHandlerMap.put("if", new IfHandler());
  nodeHandlerMap.put("choose", new ChooseHandler());
  nodeHandlerMap.put("when", new IfHandler());
  nodeHandlerMap.put("otherwise", new OtherwiseHandler());
  nodeHandlerMap.put("bind", new BindHandler());
}

到这里就已经比较清楚了,这个 sql 节点的解析过程使用的是策略模式,整个 sql 节点被封装成 SqlSource,其子节点封装为 SqlNode,每个 Node 的解析行为又封装到 NodeHandler 中;整个流程虽然比较长,但是每个模块都非常的清晰,这里非常值得我们学习;

三、mapper 动态代理

首先简单看一个动态代理的 demo

interface Car { void run(String name); }

@Test
public void testDynamic() {
  Car car = (Car) Proxy.newProxyInstance(
    Car.class.getClassLoader(), // 代理目标的类加载器
    new Class[]{Car.class},     // 代理的接口数组,因为可以实现多个接口
    new InvocationHandler() {   // 动态代理的逻辑代码
      @Override
      public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        System.out.println("----动态代理开始----");
        // 目标逻辑代码
        System.out.println("----动态代理结束----");
        return null;
      }
  });

  car.run("sdf");
}

从上面的代码可以看到,我们只定义一个接口并没有实现类,但是通过动态代理就可以动态生成实现类;在使用 mapper 的时候也是一样的,每次调用mapper方法的时候,都会动态生成一个实现类;

初始化:

// MapperRegistry
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 {
      knownMappers.put(type, new MapperProxyFactory<>(type));  // 为每一个接口添加一个动态代理工厂
      MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);  // 解析注解配置
      parser.parse();
      loadCompleted = true;
    } finally {
      if (!loadCompleted) {
        knownMappers.remove(type);
      }
    }
  }
}

使用:

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);
  }
}

其主要流程大致如下:

mybatis 源码分析(二)mapper 初始化

原文  http://www.cnblogs.com/sanzao/p/11382189.html
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