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在以前文章中，我们把Mybatis源码阅读的整个流程梳理了一遍。今天，我们来详细聊聊，Mybatis是如何解析配置文件的。

这是今天分析的流程图：

还是从案例开始。

demo案例

   public static void main(String[] args) {String resource = "mybatis-config.xml";InputStream inputStream = null;SqlSession sqlSession = null;try {inputStream = Resources.getResourceAsStream(resource);SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);sqlSession = sqlSessionFactory.openSession();UserMapper userMapper = sqlSession.getMapper(UserMapper.class);System.out.println(userMapper.selectById(1));} catch (Exception e) {e.printStackTrace();} finally {try {inputStream.close();} catch (IOException e) {e.printStackTrace();}sqlSession.close();}}

见证奇迹

从SqlSessionFactoryBuilder开始。

SqlSessionFactoryBuilder类

org.apache.ibatis.session.SqlSessionFactoryBuilder

该类里全是build方法各种重载。

//这个方法啥也没干
public SqlSessionFactory build(InputStream inputStream) {return build(inputStream, null, null);}

最终来到另外一个build方法里：

  public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {try {//创建一个XMLConfigBuilder对象  XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);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.}}}

XMLConfigBuilder类

该类的构造方法重载：

首先进入：

public XMLConfigBuilder(InputStream inputStream, String environment, Properties props) {this(new XPathParser(inputStream, 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;
}

build(parser.parse());中的parser.parse()；

mybatis-config.xml在哪里解析的呢？

请看下面这个方法：

//该方法返回一个Configuration对象
public Configuration parse() {if (parsed) {throw new BuilderException("Each XMLConfigBuilder can only be used once.");}parsed = true;//关键点parseConfiguration(parser.evalNode("/configuration"));return configuration;
}

parseConfiguration(parser.evalNode("/configuration"));

终于看到开始解析配置文件了:

进入方法parseConfiguration。

  private void parseConfiguration(XNode root) {try {//issue #117 read properties firstpropertiesElement(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 #631environmentsElement(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);}}

这里就是把mybatis-config.xml内容解析，然后设置到Configuration对象中。

那么我们定义的Mapper.xml是在哪里解析的呢？

我们的Mapper.xml在mybatis-config.xml中的配置是这样的：

<mapper>使用方式有以下四种：

//1使用类路径
<mappers><mapper resource="org/mybatis/builder/AuthorMapper.xml"/><mapper resource="org/mybatis/builder/BlogMapper.xml"/><mapper resource="org/mybatis/builder/PostMapper.xml"/>
</mappers>
//2使用绝对url路径
<mappers><mapper url="file:///var/mappers/AuthorMapper.xml"/><mapper url="file:///var/mappers/BlogMapper.xml"/><mapper url="file:///var/mappers/PostMapper.xml"/>
</mappers>
//3使用java类名
<mappers><mapper class="org.mybatis.builder.AuthorMapper"/><mapper class="org.mybatis.builder.BlogMapper"/><mapper class="org.mybatis.builder.PostMapper"/>
</mappers>//4自动扫描包下所有映射器
<mappers><package name="org.mybatis.builder"/>
</mappers>

继续源码分析，我们在上面mybatis-config.xml解析中可以看到：

我们不妨进入这个方法看看：

 private void mapperElement(XNode parent) throws Exception {if (parent != null) {for (XNode child : parent.getChildren()) {//自动扫描包下所有映射器if ("package".equals(child.getName())) {String mapperPackage = child.getStringAttribute("name");//放  configuration.addMappers(mapperPackage);} else {String resource = child.getStringAttribute("resource");String url = child.getStringAttribute("url");String mapperClass = child.getStringAttribute("class");//使用java类名if (resource != null && url == null && mapperClass == null) {ErrorContext.instance().resource(resource);//根据文件存放目录，读取XxxMapper.xmlInputStream inputStream = Resources.getResourceAsStream(resource);//映射器比较复杂，调用XMLMapperBuilder//注意在for循环里每个mapper都重新new一个XMLMapperBuilder，来解析XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());mapperParser.parse();//使用绝对url路径} else if (resource == null && url != null && mapperClass == null) {ErrorContext.instance().resource(url);InputStream inputStream = Resources.getUrlAsStream(url);//映射器比较复杂，调用XMLMapperBuilderXMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());mapperParser.parse();//使用类路径    } else if (resource == null && url == null && mapperClass != null) {Class<?> mapperInterface = Resources.classForName(mapperClass);//直接把这个映射加入配置configuration.addMapper(mapperInterface);} else {throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");}}}}}

这里刚刚和我们的上面说的<mapper>使用的方式完全是一模一样的。

到这里，配置文件mybatis-config.xml和我们定义映射文件XxxMapper.xml就全部解析完成。

回到SqlSessionFactoryBuilder类

前面讲到了XMLConfigBuilder中的parse方法，并返回了一个Configuration对象。

build(parser.parse());

这个build方法就是传入一个Configuration对象，然后构建一个DefaultSqlSession对象。

  public SqlSessionFactory build(Configuration config) {return new DefaultSqlSessionFactory(config);}

继续回到我们的demo代码中这一行代码里：

SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

这一行代码就相当于：

SqlSessionFactory sqlSessionFactory = new new DefaultSqlSessionFactory();

到这里，我们的整个流程就搞定了。

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