Mybatis初始化流程（一）

Mybatis初始化流程，其实就是组装重量级All-In-One对象Configuration的过程，主要分为系统环境参数初始化和Mapper映射初始化，其中Mapper映射初始化尤为重要。

inputStream = Resources.getResourceAsStream("mybatis-config.xml");
sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
      XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
      return new DefaultSqlSessionFactory(parser.parse());
}

parser.parse()方法，已经返回了组装完毕的Configuration对象。

流程进入XMLConfigBuilder.parse()方法。

public Configuration parse() {
    parseConfiguration(parser.evalNode("/configuration"));
    return configuration;
  }

  private void parseConfiguration(XNode root) {
    try {
      Properties settings = settingsAsPropertiess(root.evalNode("settings"));
      propertiesElement(root.evalNode("properties"));
      loadCustomVfs(settings);
      typeAliasesElement(root.evalNode("typeAliases"));
      pluginElement(root.evalNode("plugins"));
      objectFactoryElement(root.evalNode("objectFactory"));
      objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
      reflectionFactoryElement(root.evalNode("reflectionFactory"));
      settingsElement(settings);
      environmentsElement(root.evalNode("environments"));
      databaseIdProviderElement(root.evalNode("databaseIdProvider"));
      typeHandlerElement(root.evalNode("typeHandlers"));
      // 重点关注
      mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
  }

以上代码，对mybatis-config.xml配置文件内的元素，使用XPath进行逐一读取。

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
	<properties resource="jdbc.properties">
		<property name="username" value="root" />
		<property name="password" value="123" />
	</properties>
	<settings>
		<setting name="localCacheScope" value="STATEMENT"/>
		<setting name="cacheEnabled" value="false" />
		<setting name="lazyLoadingEnabled" value="true" />
		<setting name="multipleResultSetsEnabled" value="true" />
		<setting name="useColumnLabel" value="true" />
		<setting name="useGeneratedKeys" value="false" />
		<setting name="defaultExecutorType" value="REUSE" />
		<setting name="defaultStatementTimeout" value="25000" />
	</settings>
	<typeAliases>
		<typeAlias alias="Student" type="com.mybatis3.domain.Student" />
		<typeAlias alias="Teacher" type="com.mybatis3.domain.Teacher" />
	</typeAliases>
	<typeHandlers>
		<typeHandler handler="com.mybatis3.typehandlers.PhoneTypeHandler" />
	</typeHandlers>
	<environments default="development">
		<environment id="development">
			<transactionManager type="JDBC" />
			<dataSource type="POOLED">
				<property name="driver" value="${driver}" />
				<property name="url" value="${url}" />
				<property name="username" value="${username}" />
				<property name="password" value="${password}" />
			</dataSource>
		</environment>
	</environments>
	<mappers>
		<mapper resource="com/mybatis3/mappers/StudentMapper.xml" />
		<mapper resource="com/mybatis3/mappers/TeacherMapper.xml" />
	</mappers>
</configuration>

Xml文件元素和Configuration属性映射表：

<properties>元素：Properties variables。

<settings>元素：Integer defaultStatementTimeout、Integer defaultFetchSize、ExecutorType defaultExecutorType……

<typeAliases>元素：TypeAliasRegistry typeAliasRegistry。

<typeHandlers>元素：TypeHandlerRegistry typeHandlerRegistry。

<environments>元素：Environment environment。配置多个<environment>元素时，Mybatis只会读取默认的那一个。

<mappers>元素：MapperRegistry mapperRegistry。

Mapper映射初始化是我们关注的重点，即mapperElement(root.evalNode("mappers"))方法。

org.apache.ibatis.builder.xml.XMLMapperBuilder.parse()方法源码。

private void configurationElement(XNode context) {
    try {
      String namespace = context.getStringAttribute("namespace");
      if (namespace == null || namespace.equals("")) {
        throw new BuilderException("Mapper's namespace cannot be empty");
      }
      builderAssistant.setCurrentNamespace(namespace);
      cacheRefElement(context.evalNode("cache-ref"));
      cacheElement(context.evalNode("cache"));
      parameterMapElement(context.evalNodes("/mapper/parameterMap"));
      resultMapElements(context.evalNodes("/mapper/resultMap"));
      sqlElement(context.evalNodes("/mapper/sql"));
      buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing Mapper XML. Cause: " + e, e);
    }
  }

逐一读取Mapper.xml文件内的各个元素。为了更为直观的了解xml元素至Mybatis的内部数据结构，我做了一个对照图。

这些Xml配置元素，Mybatis将它们分别封装成了ParameterMap、ParameterMapping、ResultMap、ResultMapping、MappedStatement、BoundSql等内部数据结构对象。

这些数据库结构对象，均放置于Configuration内部保存起来。

protected final Map<String, MappedStatement> mappedStatements = new StrictMap<MappedStatement>("Mapped Statements collection");
protected final Map<String, ResultMap> resultMaps = new StrictMap<ResultMap>("Result Maps collection");
protected final Map<String, ParameterMap> parameterMaps = new StrictMap<ParameterMap>("Parameter Maps collection");

Mybatis初始化流程，经过系统环境参数初始化和Mapper映射初始化，简单的两个步骤就完成了，过程并不复杂。