2.MyBatis源码解读——容器的加载与初始化

SqlSessionFactory是通过SqlSessionFactoryBuilder工厂类创建的，而不是直接使用构造器。如下图：

SqlSessionFactoryBuilder 源码分析

主要代码部分为：

/*
 * Builds {@link SqlSession} instances.
 * 构建SqlSessionFactory的工厂.工厂模式
 *
 */
public class SqlSessionFactoryBuilder {
   //以下3个方法都是调用下面第4种方法
   // 使用java.io.Reader构建
  public SqlSessionFactory build(Reader reader) {
    return build(reader, null, null);
  }

  public SqlSessionFactory build(Reader reader, String environment) {
    return build(reader, environment, null);
  }

  public SqlSessionFactory build(Reader reader, Properties properties) {
    return build(reader, null, properties);
  }

  //第4种方法是最常用的，它使用了一个参照了XML文档或更特定的SqlMapConfig.xml文件的Reader实例。
  //可选的参数是environment和properties。Environment决定加载哪种环境(开发环境/生产环境)，包括数据源和事务管理器。
  //如果使用properties，那么就会加载那些properties（属性配置文件），那些属性可以用${propName}语法形式多次用在配置文件中。和Spring很像，一个思想？
  public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
    try {
        //委托XMLConfigBuilder来解析xml文件，并构建
      XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
      return build(parser.parse());
    } catch (Exception e) {
        //这里是捕获异常，包装成自己的异常并抛出的idiom？，最后还要reset ErrorContext
      throw ExceptionFactory.wrapException("Error building SqlSession.", e);
    } finally {
      ErrorContext.instance().reset();
      try {
        reader.close();
      } catch (IOException e) {
        // Intentionally ignore. Prefer previous error.
      }
    }
  }
//最后一个build方法使用了一个Configuration作为参数,并返回DefaultSqlSessionFactory
  public SqlSessionFactory build(Configuration config) {
    return new DefaultSqlSessionFactory(config);
  }
}

根据上述接口可知，先将XML配置文件构建为Configuration配置类，然后将Configuration设置到SQLSessionFactory默认实现DefaultSqlSessionFactory中并返回。

解析xml委托给了XMLConfigBuilder

public class XMLConfigBuilder extends BaseBuilder {
	//是否已解析，XPath解析器,环境
  private boolean parsed;
  private XPathParser parser;
  private String environment;
//...
public XMLConfigBuilder(InputStream inputStream, String environment, Properties props) {
    this(new XPathParser(inputStream, true, props, new XMLMapperEntityResolver()), environment, props);
  }
//上面6个构造函数最后都合流到这个函数，传入XPathParser
  private XMLConfigBuilder(XPathParser parser, String environment, Properties props) {
    //首先调用父类初始化Configuration
    super(new Configuration());
    //错误上下文设置成SQL Mapper Configuration(XML文件配置),以便后面出错了报错用吧
    ErrorContext.instance().resource("SQL Mapper Configuration");
    //将Properties全部设置到Configuration里面去
    this.configuration.setVariables(props);
    this.parsed = false;
    this.environment = environment;
    this.parser = parser;
  }
//解析配置
  private void parseConfiguration(XNode root) {
    try {
      //分步骤解析
      //issue #117 read properties first
      //1.properties
      propertiesElement(root.evalNode("properties"));
      //2.类型别名
      typeAliasesElement(root.evalNode("typeAliases"));
      //3.插件
      pluginElement(root.evalNode("plugins"));
      //4.对象工厂
      objectFactoryElement(root.evalNode("objectFactory"));
      //5.对象包装工厂
      objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
      //6.设置
      settingsElement(root.evalNode("settings"));
      // read it after objectFactory and objectWrapperFactory issue #631
      //7.环境
      environmentsElement(root.evalNode("environments"));
      //8.databaseIdProvider
      databaseIdProviderElement(root.evalNode("databaseIdProvider"));
      //9.类型处理器
      typeHandlerElement(root.evalNode("typeHandlers"));
      //10.映射器
      mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
  }
}