mybatis底层源码分析之－－配置文件读取和解析

现在企业级开发中ssm是很常见的技术标配，mybatis比hibernate轻量了很多，而且学习成本相对较低，简单易上手。

那么，问题来了，简单好用的mybatis底层到底是如何实现的呢？都使用了什么原理呢？

为了深入的了解底层原理呢，写了几段简单的代码结合debug，开始学习底层咯，直接先上代码。

public class XmlReaderTest {
    @Test
    public void mybatisXmlReaderTest() {
        String resourceXml = "mybatis-test-config.xml";
        SqlSessionFactory factory = null;
        SqlSession sqlSession = null;
        try {
            Reader reader = Resources.getResourceAsReader(resourceXml);
            factory = new SqlSessionFactoryBuilder().build(reader);
            sqlSession = factory.openSession();
            List<Student> list = sqlSession.selectList("getStudentInfo","lily");
            System.out.println(list.size()+">>>"+list.get(0).getAddress());
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            sqlSession.close();
        }
    }
}

<?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>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://127.0.0.1:3306/logistics?useUnicode=true&amp;characterEncoding=UTF-8"/>
                <property name="username" value="root"/>
                <property name="password" value="root"/>
            </dataSource>
        </environment>
    </environments>

    <mappers>
        <mapper resource="com/springapp/mybatis/StudentMapper.xml"/>
        <mapper resource="com/springapp/mybatis/TeacherMapper.xml"/>
    </mappers>

</configuration>

首先是读取mybatis的配置文件，创建工程类，获得session,调用SqlSession里面的方法进行数据库的相关操作。

factory = new SqlSessionFactoryBuilder().build(reader);主要看后面的build()方法，返回DefaultSqlSessionFactory对象：其实通过build()方法最终调的是这个方法：

public SqlSessionFactory build(Reader reader, String environment, Properties properties) {  try {    XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);    return build(parser.parse());  } 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.    }  }}在这个方法中使用了配置文件的解析类xmlconfigbuilder,重点看这一句：

 return build(parser.parse());  进入xmlconfigbuilder的parse()方法进一步查看：

public Configuration parse() {  if (parsed) {    throw new BuilderException("Each XMLConfigBuilder can only be used once.");  }  parsed = true;  parseConfiguration(parser.evalNode("/configuration"));  return configuration;}返回的是一个Configuration对象，其中

parser.evalNode("/configuration")，解析配置文件，返回指定节点的内容。内容如下：

<configuration>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC"/>
<dataSource type="POOLED">
<property name="driver" value="com.mysql.jdbc.Driver"/>
<property name="url" value="jdbc:mysql://127.0.0.1:3306/logistics?useUnicode=true&characterEncoding=UTF-8"/>
<property name="username" value="root"/>
<property name="password" value="root"/>
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="com/springapp/mybatis/StudentMapper.xml"/>
<mapper resource="com/springapp/mybatis/TeacherMapper.xml"/>
</mappers>
</configuration>

它的外层方法parseConfiguration(),进行具体的解析configuration下的子节点，并给configuration的对应变量赋值。

重点关注其中的：

environmentsElement(root.evalNode("environments")); ／／相当于jdbc配置数据源，以及事务的配置mapperElement(root.evalNode("mappers"));／／解析sql映射文件

最后附上流程图：