Mybatis的执行过程

1、Mybatis的作用

Mybatis的主要作用可以用下面的一段代码解释

Class.forName("com.mysql.jdbc.Driver");
Connection connection = DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/test", "root", "root");
String sql = "select * from tab_user where id= ?";
// prepare sql
PreparedStatement pstmt = connection.prepareStatement(sql);
// set parameter
pstmt.setInt(1, 18);
ResultSet rs = pstmt.executeQuery();
User user = null;
// extract resultset
while (rs.next()) {
    user = new User(rs.getInt("id"), rs.getString("name"));
    System.out.println(user);
}
// release resource
rs.close();
pstmt.close();
connection.close();

2、Mybatis的demo

Mysql数据库创建表tab_user

CREATE TABLE `tab_user` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(100) NOT NULL,
  `birthday` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  PRIMARY KEY (`id`)
);
INSERT INTO `tab_user` VALUES ('18', 'jack', '2017-01-24 22:15:03');

新建demo工程在classpath下新建config/mybatis-config.xml配置文件和config/mapper/UserMapper.xml映射文件

mybatis-config.xml文件内容

<?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/test" />
                <property name="username" value="root" />
                <property name="password" value="root" />
            </dataSource>
        </environment>
    </environments>

    <mappers>
        <mapper resource="config/mapper/UserMapper.xml" />
    </mappers>

</configuration>

UserMapper.xml文件内容

<?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="UserMapper">

    <select id="selectUser" parameterType="int" resultType="com.fit.bean.User">
        select * from tab_user where id = #{id}
    </select>

</mapper>

Java代码(引入依赖jar包:mysql-connector.jar和mybatis-3.3.x.jar)

SqlSession sqlSession = null;
try {
    String resource = "config/mybatis-config.xml";
    InputStream inputStream = Resources.getResourceAsStream(resource);
    SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream); //start
    sqlSession = sqlSessionFactory.openSession();
    User user = sqlSession.selectOne("UserMapper.selectUser", 18); //query db
    System.out.println(user);
    user = sqlSession.selectOne("UserMapper.selectUser", 18);
    System.out.println(user);
} catch (Exception e) {
    e.printStackTrace();
} finally {
    if (sqlSession != null)
        sqlSession.close(); //release resource
}

实体类User.java

import java.io.Serializable;

public class User implements Serializable {

    private static final long serialVersionUID = 1L;

    private int id;

    private String name;

    public User() {
        super();
    }

    public User(int id, String name) {
        super();
        this.id = id;
        this.name = name;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "{id=" + id + ", name=" + name + "}";
    }

}

demo工程结构

正常情况下就可以输出一个user对象信息

3、Mybatis执行过程分析

3.1启动

String resource = "config/mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

上面的demo可以看出实现过程是先读取mybatis配置文件，通过SqlSessionFactoryBuilder基于文件输入流创建SqlSessionFactory。

具体实现过程：

SqlSessionFactoryBuiler的build(inputStream)最终调用了下面的方法

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.
      }
    }
  }

XMLConfigBuilder的parse()方法主要就是解析mybatis-config.xml,通过configuraton根节点具体解析的属性如下

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"));
      reflectionFactoryElement(root.evalNode("reflectionFactory"));
      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);
    }
  }

根据配置文件生成Configuration对象，这个对象非常重要，是全局的配置信息，其中比较重要的包括根据plugin标签配置生成用户配置的自定义插件（比如用户可以实现mysql分页插件）、全局的setting属性配置（二级缓存配置等）、mapper映射文件信息读取。

最后用生成的configuration对象生成DefaultSqlSessionFactory对象

public SqlSessionFactory build(Configuration config) {
    return new DefaultSqlSessionFactory(config);
  }

3.2执行增删改查

sqlSession = sqlSessionFactory.openSession();
User user = sqlSession.selectOne("UserMapper.selectUser", 18);

通过3.1已经获取了SqlSessionFactory，通过SqlSessionFactory创建SqlSession，Mybatis对数据库的操作主要也就是由SqlSession接口定义，由DefaultSqlSession实现。

首先看一下SqlSession接口的定义（定义了增删改查等基本数据库操作和一些事务的操作）

//TODO：解析sql文件、根据入参赋值预编译sql、处理结果集

3.3释放资源

sqlSession.close();