MyBatis框架的使用及源码分析(二) 配置篇 SqlSessionFactoryBuilder，XMLConfigBuilder

在 <MyBatis框架中Mapper映射配置的使用及原理解析(一) 配置与使用> 的demo中看到了SessionFactory的创建过程：

SqlSessionFactory sessionFactory = null;
        String resource = "mybatisConfig.xml";
        try {
            sessionFactory = new SqlSessionFactoryBuilder().build(Resources
                    .getResourceAsReader(resource));
        } catch (IOException e) {
            e.printStackTrace();
        }

那么我们就从SqlSessionFactoryBuilder开始，看看Mybatis的加载过程。

SqlSessionFactoryBuilder的核心源码：

package org.apache.ibatis.session;

public class SqlSessionFactoryBuilder {

//通过Reader读取Mybatis配置

 public SqlSessionFactory build(Reader reader, String environment, Properties properties) {

    try {
      XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
      return build(parser.parse()); //parse()方法得到Configuration
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error building SqlSession.", e);
    } finally {
      ErrorContext.instance().reset();
      try {
        reader.close();
      } catch (IOException e) {
        // Intentionally ignore. Prefer previous error.
      }
    }
  }

  //通过InputStream读取Mybatis配置
  public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
    try {
      XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
      return build(parser.parse()); //parse()方法得到Configuration
    } 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.
      }
    }
  }
  
  //以上2个方法最终调用的是build(Configuration config)
  public SqlSessionFactory build(Configuration config) {
    return new DefaultSqlSessionFactory(config);
  }

}

通过源码，我们可以看到SqlSessionFactoryBuilder 通过XMLConfigBuilder 去解析我们传入的mybatis的配置文件，构造出Configuration，最终返回new DefaultSqlSessionFactory(config)的SqlSessionFactory实例。

接下来我们看看 XMLConfigBuilder 是怎样解析Mybatis的配置文件的，下面是部分源码：

package org.apache.ibatis.builder.xml;

/**
 * 解析Mybatis配置文件
 */
public class XMLConfigBuilder extends BaseBuilder {

  private boolean parsed;
  private XPathParser parser;
  private String environment;

  public XMLConfigBuilder(Reader reader) {
    this(reader, null, null);
  }

  public XMLConfigBuilder(Reader reader, String environment) {
    this(reader, environment, null);
  }

  public XMLConfigBuilder(Reader reader, String environment, Properties props) {
    this(new XPathParser(reader, true, props, new XMLMapperEntityResolver()), environment, props);
  }

  public XMLConfigBuilder(InputStream inputStream) {
    this(inputStream, null, null);
  }

  public XMLConfigBuilder(InputStream inputStream, String environment) {
    this(inputStream, environment, null);
  }

  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;
  }

  //调用此方法对mybatis配置文件进行解析,返回Configuration对象
  public Configuration parse() {
    if (parsed) {
      throw new BuilderException("Each XMLConfigBuilder can only be used once.");
    }
    parsed = true;
    //从根节点configuration，开始解析
    parseConfiguration(parser.evalNode("/configuration"));
    return configuration;
  }

  //解析configuration节点下的10个子节点。
  private void parseConfiguration(XNode root) {
    try {
      //解析子节点properties 
      propertiesElement(root.evalNode("properties")); //issue #117 read properties first
      //解析子节点typeAliases 别名
      typeAliasesElement(root.evalNode("typeAliases"));
      //解析子节点plugins 插件 
      pluginElement(root.evalNode("plugins"));
      //解析子节点objectFactory mybatis为结果创建对象时都会用到objectFactory
      objectFactoryElement(root.evalNode("objectFactory"));
      //解析子节点objectWrapperFactory
      objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
      //解析settings定义一些全局性的配置
      settingsElement(root.evalNode("settings"));
      //解析environments 可以配置多个运行环境，但是每个SqlSessionFactory 实例只能选择一个运行环境
      environmentsElement(root.evalNode("environments")); // read it after objectFactory and objectWrapperFactory issue #631
      //解析databaseIdProvider MyBatis能够执行不同的语句取决于你提供的数据库供应商。许多数据库供应商的支持是基于databaseId映射 
      databaseIdProviderElement(root.evalNode("databaseIdProvider"));
      //解析typeHandlers 当MyBatis设置参数到PreparedStatement 或者从ResultSet 结果集中取得值时，就会使用TypeHandler  来处理数据库类型与java 类型之间转换
      typeHandlerElement(root.evalNode("typeHandlers"));
      //解析mappers 主要的crud操作都是在mappers中定义的
      mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
  }

}  

从上面可以看出可以配置10个子节点， 分别为：properties、typeAliases、plugins、objectFactory、objectWrapperFactory、settings、environments、databaseIdProvider、typeHandlers、mappers。