MyBatis源码阅读——MyBatis初始化流程解析

╰半夏微凉° 2022-05-23 11:11 293阅读 0赞

### 前言 ###

在之前的几篇文章中，我们在源码中看到到了很多类，比如mapperRegistry、mappedStatements等，虽然我们知道它们都是在MyBatis初始化的时候完成加载的，那么我们还是有必要去了解一下其加载过程。  
还是跟之前一样，写一个demo，去边debug 边阅读源码。

public static void main(String[] args) throws IOException {
            String resource = "mybatis/conf/mybatis-config.xml";
            InputStream inputStream = Resources.getResourceAsStream(resource);
            //从 XML 中构建 SqlSessionFactory
            SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
            SqlSession session = sqlSessionFactory.openSession();
            try {
                BlogMapper mapper = session.getMapper(BlogMapper.class);
                Blog blog = mapper.selectBlog(1L);
                System.out.println(blog);
                blog = mapper.selectBlog(1L);
                System.out.println(blog);
            } finally {
                session.close();
            }
        }

### SqlSessionFactory的产生 ###

我们直接debug进入org.apache.ibatis.session.SqlSessionFactoryBuilder

public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
        try {
           //获取配置信息 mybatis-config.xml
          XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
          //解析配置并初始化建立SqlSessionFactory 
          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.
          }
        }
      }

进入parser.parse() -> parseConfiguration(parser.evalNode(“/configuration”)); 这里是分析配置并加载初始化类的入口。

private void parseConfiguration(XNode root) {
        try {
          //先加载properties。这些属性都是可外部配置且可动态替换的，既可以在典型的 Java 属性文件中配置，亦可通过 properties 元素的子元素来传递。
          propertiesElement(root.evalNode("properties"));
          //加载别名。类型别名是为 Java 类型设置一个短的名字。它只和 XML 配置有关，存在的意义仅在于用来减少类完全限定名的冗余
          typeAliasesElement(root.evalNode("typeAliases"));
          //加载插件plugin
          pluginElement(root.evalNode("plugins"));
          //MyBatis 每次在创建结果对象的新实例时， 是使用 ObjectFactory （对象工厂）实例来完成的。如果有自定义，则读取配置中的自定义的类
          objectFactoryElement(root.evalNode("objectFactory"));
          //结果对象转换（驼峰法等）
          objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
          //ReflectorFactory
          reflectionFactoryElement(root.evalNode("reflectionFactory"));
          //一些settings
          settingsElement(root.evalNode("settings"));
          // read it after objectFactory and objectWrapperFactory issue #631
          //数据库连接、事务管理器的配置
          environmentsElement(root.evalNode("environments"));
          //databaseIdProvider的配置，可以根据不用的id产生不同的sql语句
          databaseIdProviderElement(root.evalNode("databaseIdProvider"));
          //加载负责java数据类型和jdbc数据类型之间的映射和转换 javaType\jdbcType 这些
          typeHandlerElement(root.evalNode("typeHandlers"));
          //加载映射文件Mapper（划重点）
          mapperElement(root.evalNode("mappers"));
        } catch (Exception e) {
          throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
        }
      }

### 加载映射文件Mapper的过程解析->mapperElement() ###

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();
              } 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.");
              }
            }
          }
        }
      }

这段代码的核心，Mapper的注册等操作都在此

XMLMapperBuilder mapperParser = new XMLMapperBuilder();
    mapperParser.parse();

### 二级缓存相关 ###

XMLMapperBuilder中关联了 MapperBuilderAssistant 对象，而MapperBuilderAssistant 中关联了**Cache**对象，Cache是二级缓存的核心，而它在映射文件Mapper初始化的时候附带，那说明它的生命周期是基于Mapper的。