上篇文章《深入浅出Mybatis系列（一）---Mybatis入门》， 写了一个Demo简单体现了一下Mybatis的流程。本次，将简单介绍一下Mybatis的配置文件：
上次例子中，我们以 SqlSessionFactoryBuilder 去创建 SqlSessionFactory, 那么，我们就先从SqlSessionFactoryBuilder入手， 咱们先看看源码是怎么实现的：
SqlSessionFactoryBuilder源码片段：
1 public class SqlSessionFactoryBuilder { 2 3 //Reader读取mybatis配置文件，传入构造方法 4 //除了Reader外，其实还有对应的inputStream作为参数的构造方法， 5 //这也体现了mybatis配置的灵活性 6 public SqlSessionFactory build(Reader reader) { 7 return build(reader, null, null); 8 } 9 10 public SqlSessionFactory build(Reader reader, String environment) { 11 return build(reader, environment, null); 12 } 13 14 //mybatis配置文件 + properties, 此时mybatis配置文件中可以不配置properties，也能使用${}形式 15 public SqlSessionFactory build(Reader reader, Properties properties) { 16 return build(reader, null, properties); 17 } 18 19 //通过XMLConfigBuilder解析mybatis配置，然后创建SqlSessionFactory对象 20 public SqlSessionFactory build(Reader reader, String environment, Properties properties) { 21 try { 22 XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties); 23 //下面看看这个方法的源码 24 return build(parser.parse()); 25 } catch (Exception e) { 26 throw ExceptionFactory.wrapException("Error building SqlSession.", e); 27 } finally { 28 ErrorContext.instance().reset(); 29 try { 30 reader.close(); 31 } catch (IOException e) { 32 // Intentionally ignore. Prefer previous error. 33 } 34 } 35 } 36 37 public SqlSessionFactory build(Configuration config) { 38 return new DefaultSqlSessionFactory(config); 39 } 40 41 }
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， 下面就接着看看 XMLConfigBuilder 部分源码：
1 /** 2 * mybatis 配置文件解析 3 */ 4 public class XMLConfigBuilder extends BaseBuilder { 5 public XMLConfigBuilder(InputStream inputStream, String environment, Properties props) { 6 this(new XPathParser(inputStream, true, props, new XMLMapperEntityResolver()), environment, props); 7 } 8 9 private XMLConfigBuilder(XPathParser parser, String environment, Properties props) { 10 super(new Configuration()); 11 ErrorContext.instance().resource("SQL Mapper Configuration"); 12 this.configuration.setVariables(props); 13 this.parsed = false; 14 this.environment = environment; 15 this.parser = parser; 16 } 17 18 //外部调用此方法对mybatis配置文件进行解析 19 public Configuration parse() { 20 if (parsed) { 21 throw new BuilderException("Each XMLConfigBuilder can only be used once."); 22 } 23 parsed = true; 24 //从根节点configuration 25 parseConfiguration(parser.evalNode("/configuration")); 26 return configuration; 27 } 28 29 //此方法就是解析configuration节点下的子节点 30 //由此也可看出，我们在configuration下面能配置的节点为以下10个节点 31 private void parseConfiguration(XNode root) { 32 try { 33 propertiesElement(root.evalNode("properties")); //issue #117 read properties first 34 typeAliasesElement(root.evalNode("typeAliases")); 35 pluginElement(root.evalNode("plugins")); 36 objectFactoryElement(root.evalNode("objectFactory")); 37 objectWrapperFactoryElement(root.evalNode("objectWrapperFactory")); 38 settingsElement(root.evalNode("settings")); 39 environmentsElement(root.evalNode("environments")); // read it after objectFactory and objectWrapperFactory issue #631 40 databaseIdProviderElement(root.evalNode("databaseIdProvider")); 41 typeHandlerElement(root.evalNode("typeHandlers")); 42 mapperElement(root.evalNode("mappers")); 43 } catch (Exception e) { 44 throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e); 45 } 46 } 47 }
通过以上源码，我们就能看出，在mybatis的配置文件中：
1. configuration节点为根节点。
2. 在configuration节点之下，我们可以配置10个子节点， 分别为：properties、typeAliases、plugins、objectFactory、objectWrapperFactory、settings、environments、databaseIdProvider、typeHandlers、mappers。
