mybatis 源码学习之getMapper过程分析

一、 简介

      这篇文章分析mybatis3.2.8中getMapper的过程，分为两个部分，一是剖析mybatis初始化的过程（这里采用加载xml配置文件的方式）

二、 mybatis初始化

1. 配置文件

首先我们看下这里使用的配置文件，在mappers标签中利用class的形式配置（如果不清楚的话，可以戳下这）。

<?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"></transactionManager>            <dataSource type="POOLED">                <property name="driver" value="com.mysql.jdbc.Driver"></property>                <property name="url" value="jdbc:mysql://localhost:3306/test2"></property>                <property name="username" value="root"></property>                <property name="password" value="root"></property>            </dataSource>        </environment>    </environments>    <mappers>        <mapper class="com.hwb.study.smybatis.mapper.EmployeesMapper"></mapper>    </mappers></configuration> 

2. 初始化

这里利用以上配置文件进行初始化

public static SqlSessionFactory buildSqlSessionFactory() throws IOException {        // 配置文件        String resource = "mybatis-config.xml";        // 讲配置文件转化为输入流        InputStream inputStream = Resources.getResourceAsStream(resource);        // 利用SqlSessionFactoryBuilder进行初始化        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);        return sqlSessionFactory;    }

简单介绍SqlSessionFactoryBuilder,来看一下它的源码

public class SqlSessionFactoryBuilder {    public SqlSessionFactory build(Reader reader) {        return build(reader, null, null);    }    ...    public SqlSessionFactory build(Reader reader, String environment, Properties properties) {        try {          // 真正生成sqlsessionfactory          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.          }        }    }    ...}

SqlSessionFactoryBuilder 采用builder设计模式，它的作用就是为了帮助生成sqlsessionfactory，关键还是利用XMLConfigBuilder类来真正解析Xml配置文件，返回sqlsessionfactory,这里的关键代码就是

      XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);      return build(parser.parse());

接下来我们可以看下XMLConfigBuilder的源码

public class XMLConfigBuilder extends BaseBuilder {    ...    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;    }    ...    public Configuration parse() {        if (parsed) {          throw new BuilderException("Each XMLConfigBuilder can only be used once.");        }        parsed = true;        parseConfiguration(parser.evalNode("/configuration"));        return configuration;      }    private void parseConfiguration(XNode root) {        try {          Properties settings = settingsAsPropertiess(root.evalNode("settings"));          //issue #117 read properties first          propertiesElement(root.evalNode("properties"));          loadCustomVfs(settings);          typeAliasesElement(root.evalNode("typeAliases"));          pluginElement(root.evalNode("plugins"));          objectFactoryElement(root.evalNode("objectFactory"));          objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));          reflectorFactoryElement(root.evalNode("reflectorFactory"));          settingsElement(settings);          // read it after objectFactory and objectWrapperFactory issue #631          environmentsElement(root.evalNode("environments"));          databaseIdProviderElement(root.evalNode("databaseIdProvider"));          typeHandlerElement(root.evalNode("typeHandlers"));          mapperElement(root.evalNode("mappers"));        } catch (Exception e) {          throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);        }    }}

其中XPathParser工具类作用是利用javax.xml中工具类对xml配置文件进行解析（这个javax.xml的类大家可以自行百度）。在完成XMLConfigBuilder的实例化之后，关键来了，在XMLConfigBuilder的parse的方法中,在这个方法中解析xml中对mapper的配置。

mapperElement(root.evalNode("mappers"));

接下来我们看这个mapperElement()方法；

private void mapperElement(XNode parent) throws Exception {    if (parent != null) {      for (XNode child : parent.getChildren()) {        if ("package".equals(child.getName())) {          String mapperPackage = child.getStringAttribute("name");          configuration.addMappers(mapperPackage);        } else {          String resource = child.getStringAttribute("resource");          String url = child.getStringAttribute("url");          String mapperClass = child.getStringAttribute("class");          if (resource != null && url == null && mapperClass == null) {            ErrorContext.instance().resource(resource);            InputStream inputStream = Resources.getResourceAsStream(resource);            XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());            mapperParser.parse();          } else if (resource == null && url != null && mapperClass == null) {            ErrorContext.instance().resource(url);            InputStream inputStream = Resources.getUrlAsStream(url);            XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());            mapperParser.parse();          } else if (resource == null && url == null && mapperClass != null) {            Class<?> mapperInterface = Resources.classForName(mapperClass);            configuration.addMapper(mapperInterface);          } else {            throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");          }      }   }}

在这段代码中，最后一个else if(resource = = null && url = = null && mapperClass != null),这里就是判断mapper是以那种方式定义，我们的配置文件中以class(interface)的方式配置，所以会进入到这个代码块中。这里的重点就是 configuration.addMapper(mapperInterface); 将这个配置加载如configuration中的mapperRegistry。至此基本的Mapper解析算是完成了。接下来我们看看sqlsession是如何获取已经注册好的mapper的。

三、getMapper 剖析

从这里开始分析sqlsession.getMapper的过程。经过第二步中的初始化之后，我们可以得到SqlSessionFactory,然后利用其创建一个sqlsession,利用这个sqlsession获得其mapper的代理类。代码如下

/**     * 研究mybatis mapper 过程     *     * @throws IOException     */    public static void testGetMapper() throws IOException {        SqlSessionFactory factory = buildSqlSessionFactory();        SqlSession sqlSession = factory.openSession();        // get mapper        EmployeesMapper mapper = sqlSession.getMapper(EmployeesMapper.class);        System.out.println(mapper.getClass());        // bulid param        Map<String,Object> paramMap = new HashMap<String, Object>();        paramMap.put("employee_id", 100);        // execute query        List<Map<String, Object>> list = mapper.selectByPrimary(paramMap);        // print result        System.out.println(list);        sqlSession.close();    }

我们进入到sqlsession.getmapper的源码中，能够发现其实它只是调用了其内部的configuration.getMapper()方法。

    return configuration.<T>getMapper(type, this);  }```  回顾一下第二步中的最后，mapperElement解析方法中对class类型的mapper的处理，

if (resource == null && url == null && mapperClass != null) {
Class

最后

关于configuration 中管理mapper的MapperRegistry流程，之后我会再写一篇博客，继续学习！！