mybatis的启动过程

准备工作

• 建一个springboot的项目目录结构如下

• 相应的代码展示

// UserDao.java
public interface UserDao {
    List<User> getUserList();
}

// User.java
@Data
public class User {
    private String username;
    private String password;
}

// MybatisApplication.java
public class MybatisApplication {
    public static void main(String[] args) throws Exception{
        String resource = "mybatis.xml";
        InputStream resourceAsStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(resourceAsStream);
        SqlSession sqlSession = factory.openSession();
        try{
            UserDao userDao = sqlSession.getMapper(UserDao.class);
            List<User> userList = userDao.getUserList();
            System.out.println("**********"+JSON.toJSON(userList));
        }finally {
            sqlSession.close();
        }
    }
}

// UserMapper.xml
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd" >

<mapper namespace="com.wyl.mybatis.dao.UserDao">
    <select id="getUserList" resultType="user">
        select * from user;
    </select>
</mapper>

// db.properties
driver=com.mysql.cj.jdbc.Driver
url=jdbc:mysql://127.0.0.1:3306/umi?useUnicode=true&characterEncoding=utf8
username=root
password=mysqlpwd

// mybatis.xml
?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="db.properties"> </properties>
    <typeAliases>
        <package name="com.wyl.mybatis.model"/>
    </typeAliases>
    <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="mapper/UserMapper.xml"/>
    </mappers>
</configuration>

mybatis到底如何正确的读取配置文件？

1. 把mybatis.xml文件变成流

String resource = "mybatis.xml";
        InputStream resourceAsStream = Resources.getResourceAsStream(resource);

2. 通过SqlSessionFactoryBuilder创建SqlSessionFactory

SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(resourceAsStream);

2.1 解析SqlSessionFactoryBuilder

**
 * Builds {@link SqlSession} instances.
 *
 * @author Clinton Begin
 */
public class SqlSessionFactoryBuilder {

  public SqlSessionFactory build(Reader reader) {
    return build(reader, null, null);
  }

  public SqlSessionFactory build(Reader reader, String environment) {
    return build(reader, environment, null);
  }

  public SqlSessionFactory build(Reader reader, Properties properties) {
    return build(reader, null, properties);
  }
...
public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
    try {
      XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
      return build(parser.parse());
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error building SqlSession.", e);
    } finally {
      ErrorContext.instance().reset();
      try {
        inputStream.close();
      } catch (IOException e) {
        // Intentionally ignore. Prefer previous error.
      }
    }
  }

通过查看源代码可以看到SqlSessionFactoryBuilder就是用来创建SqlSessionFactory的，并且通过构建XMLConfigBuilder对象解析配置文件。

2.2 查看XMLConfigBuilder对象的parse()

public Configuration parse() {
    if (parsed) {
      throw new BuilderException("Each XMLConfigBuilder can only be used once.");
    }
    parsed = true;
    parseConfiguration(parser.evalNode("/configuration"));
    return configuration;
  }
​```
在这里我们已经可以看到通过解析器解析xml文件的**configuration**节点了，再往下看parseConfiguration的方法
​```java
private void parseConfiguration(XNode root) {
    try {
      //issue #117 read properties first
      propertiesElement(root.evalNode("properties"));
      Properties settings = settingsAsProperties(root.evalNode("settings"));
      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);
    }
  }

很明显我们可以看到这里就是通过调用不同的方法解析不同的标签，到这里已经解析成了mybatis。xml了，下一步就是看在哪里解析UserMapper.xml 进入上面的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.");
          }
        }
      }
    }
  }

从这里我们可以看到mapper是根据不同属性进行适配的，我们用的是第一种通过构建XMLMapperBuilder去映射mapper.xml文件，这时点击查看mapperParser.parse();

public void parse() {
    if (!configuration.isResourceLoaded(resource)) {
      configurationElement(parser.evalNode("/mapper"));
      configuration.addLoadedResource(resource);
      bindMapperForNamespace();
    }

    parsePendingResultMaps();
    parsePendingCacheRefs();
    parsePendingStatements();
  }

在这里我们可以看到是解析mapper的地，继续点击configurationElement方法， ·

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. The XML location is '" + resource + "'. Cause: " + e, e);
    }
  }

这里就是对UserMapper.xml的各部分的解析，包括sql的增删改查。

全部映射成功后返回一个DefaultSqlSessionFactory对象，

这图片是SqlSessionFactory的继承结构图yo

最后

后面就是通过SqlSessionFactory创建一个sql会话，开启事务，进行一系列的数据库操作，操作完毕后，提交事务，关闭会话。