深入浅出Mybatis系列（二）---配置简介（mybatis源码篇）-白红宇

上篇文章《》，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configuration25     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 first34       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 #63140       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。