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MyBatis源码分析-SQL语句执行的完整流程

　　MyBatis 是支持定制化 SQL、存储过程以及高级映射的优秀的持久层框架。MyBatis 避免了几乎所有的 JDBC 代码和手动设置参数以及获取结果集。MyBatis 可以对配置和原生 Map 使用简单的 XML 或注解，将接口和 Java 的 POJOs(Plain Old Java Objects, 普通的 Java 对象) 映射成数据库中的记录。如何新建MyBatis 源码工程请点击MyBatis 源码分析 -IDEA 新建 MyBatis 源码工程。

　　MyBatis 框架主要完成的是以下 2 件事情：

  1. 根据 JDBC 规范建立与数据库的连接。
  2. 通过反射打通 Java 对象与数据库参数交互之间相互转换的关系。

　　MyBatis 框架是一种典型的交互式框架，先准备好交互的必要条件，然后构建一个交互的环境，在交互环境中划分会话，在会话中与数据库进行交互数据。

1 MyBatis 主要的类

  • Configuration        MyBatis 所有的配置信息都维持在 Configuration 对象之中。
  • SqlSession            作为 MyBatis 工作的主要顶层 API，表示和数据库交互的会话，完成必要数据库增删改查功能
  • Executor               MyBatis 执行器，是 MyBatis 调度的核心，负责 SQL 语句的生成和查询缓存的维护
  • StatementHandler 封装了 JDBC Statement 操作，负责对 JDBC statement 的操作，如设置参数、将 Statement 结果集转换成 List 集合。
  • ParameterHandler  负责对用户传递的参数转换成 JDBC Statement 所需要的参数，
  • ResultSetHandler   负责将 JDBC 返回的 ResultSet 结果集对象转换成 List 类型的集合；
  • TypeHandler          负责 java 数据类型和 jdbc 数据类型之间的映射和转换
  • MappedStatement  MappedStatement 维护了一条 <select|update|delete|insert> 节点的封装，
  • SqlSource              负责根据用户传递的 parameterObject，动态地生成 SQL 语句，将信息封装到 BoundSql 对象中，并返回
  • BoundSql              表示动态生成的 SQL 语句以及相应的参数信息

　　以上几个类在 SQL 操作中都会涉及，在 SQL 操作中重点关注下 SQL 参数什么时候写入和结果集怎么转换为 Java 对象，这两个过程正好对应的类是 PreparedStatementHandler 和 ResultSetHandler 类。

( 图片来自《深入理解 mybatis 原理》 MyBatis 的架构设计以及实例分析)

2 SQL 执行流程

　　MyBatis 主要设计目的还是为了让我们在执行 SQL 时对输入输出的数据的管理更加方便，所以方便的让我们写出 SQL 和方便的获取 SQL 的执行结果是 MyBatis 的核心竞争力。下面就用一个例子来从源码角度看一下 SQL 的完整执行流程。

新建配置文件 conf.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>


<span style="color: rgba(0, 0, 255, 1)">&lt;</span><span style="color: rgba(128, 0, 0, 1)">settings</span><span style="color: rgba(0, 0, 255, 1)">&gt;</span>
    <span style="color: rgba(0, 0, 255, 1)">&lt;</span><span style="color: rgba(128, 0, 0, 1)">setting </span><span style="color: rgba(255, 0, 0, 1)">name</span><span style="color: rgba(0, 0, 255, 1)">="cacheEnabled"</span><span style="color: rgba(255, 0, 0, 1)"> value</span><span style="color: rgba(0, 0, 255, 1)">="true"</span><span style="color: rgba(0, 0, 255, 1)">/&gt;</span>
    <span style="color: rgba(0, 0, 255, 1)">&lt;</span><span style="color: rgba(128, 0, 0, 1)">setting </span><span style="color: rgba(255, 0, 0, 1)">name</span><span style="color: rgba(0, 0, 255, 1)">="lazyLoadingEnabled"</span><span style="color: rgba(255, 0, 0, 1)"> value</span><span style="color: rgba(0, 0, 255, 1)">="false"</span><span style="color: rgba(0, 0, 255, 1)">/&gt;</span>
    <span style="color: rgba(0, 128, 0, 1)">&lt;!--</span><span style="color: rgba(0, 128, 0, 1)">setting name="logImpl" value="STDOUT_LOGGING"/</span><span style="color: rgba(0, 128, 0, 1)">--&gt;</span> <span style="color: rgba(0, 128, 0, 1)">&lt;!--</span><span style="color: rgba(0, 128, 0, 1)"> 日志 </span><span style="color: rgba(0, 128, 0, 1)">--&gt;</span>
<span style="color: rgba(0, 0, 255, 1)">&lt;/</span><span style="color: rgba(128, 0, 0, 1)">settings</span><span style="color: rgba(0, 0, 255, 1)">&gt;</span>

<span style="color: rgba(0, 0, 255, 1)">&lt;</span><span style="color: rgba(128, 0, 0, 1)">typeAliases</span><span style="color: rgba(0, 0, 255, 1)">&gt;</span>
    <span style="color: rgba(0, 0, 255, 1)">&lt;</span><span style="color: rgba(128, 0, 0, 1)">typeAlias </span><span style="color: rgba(255, 0, 0, 1)">type</span><span style="color: rgba(0, 0, 255, 1)">="com.luoxn28.dao.User"</span><span style="color: rgba(255, 0, 0, 1)"> alias</span><span style="color: rgba(0, 0, 255, 1)">="User"</span><span style="color: rgba(0, 0, 255, 1)">/&gt;</span>
<span style="color: rgba(0, 0, 255, 1)">&lt;/</span><span style="color: rgba(128, 0, 0, 1)">typeAliases</span><span style="color: rgba(0, 0, 255, 1)">&gt;</span>

<span style="color: rgba(0, 0, 255, 1)">&lt;</span><span style="color: rgba(128, 0, 0, 1)">environments </span><span style="color: rgba(255, 0, 0, 1)">default</span><span style="color: rgba(0, 0, 255, 1)">="development"</span><span style="color: rgba(0, 0, 255, 1)">&gt;</span>
    <span style="color: rgba(0, 0, 255, 1)">&lt;</span><span style="color: rgba(128, 0, 0, 1)">environment </span><span style="color: rgba(255, 0, 0, 1)">id</span><span style="color: rgba(0, 0, 255, 1)">="development"</span><span style="color: rgba(0, 0, 255, 1)">&gt;</span>
        <span style="color: rgba(0, 0, 255, 1)">&lt;</span><span style="color: rgba(128, 0, 0, 1)">transactionManager </span><span style="color: rgba(255, 0, 0, 1)">type</span><span style="color: rgba(0, 0, 255, 1)">="JDBC"</span> <span style="color: rgba(0, 0, 255, 1)">/&gt;</span> <span style="color: rgba(0, 128, 0, 1)">&lt;!--</span><span style="color: rgba(0, 128, 0, 1)"> 声明使用那种事务管理机制 JDBC/MANAGED </span><span style="color: rgba(0, 128, 0, 1)">--&gt;</span>
        <span style="color: rgba(0, 128, 0, 1)">&lt;!--</span><span style="color: rgba(0, 128, 0, 1)"> 配置数据库连接信息 </span><span style="color: rgba(0, 128, 0, 1)">--&gt;</span>
        <span style="color: rgba(0, 0, 255, 1)">&lt;</span><span style="color: rgba(128, 0, 0, 1)">dataSource </span><span style="color: rgba(255, 0, 0, 1)">type</span><span style="color: rgba(0, 0, 255, 1)">="POOLED"</span><span style="color: rgba(0, 0, 255, 1)">&gt;</span>
            <span style="color: rgba(0, 0, 255, 1)">&lt;</span><span style="color: rgba(128, 0, 0, 1)">property </span><span style="color: rgba(255, 0, 0, 1)">name</span><span style="color: rgba(0, 0, 255, 1)">="driver"</span><span style="color: rgba(255, 0, 0, 1)"> value</span><span style="color: rgba(0, 0, 255, 1)">="com.mysql.jdbc.Driver"</span> <span style="color: rgba(0, 0, 255, 1)">/&gt;</span>
            <span style="color: rgba(0, 0, 255, 1)">&lt;</span><span style="color: rgba(128, 0, 0, 1)">property </span><span style="color: rgba(255, 0, 0, 1)">name</span><span style="color: rgba(0, 0, 255, 1)">="url"</span><span style="color: rgba(255, 0, 0, 1)"> value</span><span style="color: rgba(0, 0, 255, 1)">="jdbc:mysql://192.168.1.150:3306/xxx"</span> <span style="color: rgba(0, 0, 255, 1)">/&gt;</span>
            <span style="color: rgba(0, 0, 255, 1)">&lt;</span><span style="color: rgba(128, 0, 0, 1)">property </span><span style="color: rgba(255, 0, 0, 1)">name</span><span style="color: rgba(0, 0, 255, 1)">="username"</span><span style="color: rgba(255, 0, 0, 1)"> value</span><span style="color: rgba(0, 0, 255, 1)">="xxx"</span> <span style="color: rgba(0, 0, 255, 1)">/&gt;</span>
            <span style="color: rgba(0, 0, 255, 1)">&lt;</span><span style="color: rgba(128, 0, 0, 1)">property </span><span style="color: rgba(255, 0, 0, 1)">name</span><span style="color: rgba(0, 0, 255, 1)">="password"</span><span style="color: rgba(255, 0, 0, 1)"> value</span><span style="color: rgba(0, 0, 255, 1)">="xxx"</span> <span style="color: rgba(0, 0, 255, 1)">/&gt;</span>
        <span style="color: rgba(0, 0, 255, 1)">&lt;/</span><span style="color: rgba(128, 0, 0, 1)">dataSource</span><span style="color: rgba(0, 0, 255, 1)">&gt;</span>
    <span style="color: rgba(0, 0, 255, 1)">&lt;/</span><span style="color: rgba(128, 0, 0, 1)">environment</span><span style="color: rgba(0, 0, 255, 1)">&gt;</span>
<span style="color: rgba(0, 0, 255, 1)">&lt;/</span><span style="color: rgba(128, 0, 0, 1)">environments</span><span style="color: rgba(0, 0, 255, 1)">&gt;</span>

<span style="color: rgba(0, 0, 255, 1)">&lt;</span><span style="color: rgba(128, 0, 0, 1)">mappers</span><span style="color: rgba(0, 0, 255, 1)">&gt;</span>
    <span style="color: rgba(0, 0, 255, 1)">&lt;</span><span style="color: rgba(128, 0, 0, 1)">mapper </span><span style="color: rgba(255, 0, 0, 1)">resource</span><span style="color: rgba(0, 0, 255, 1)">="userMapper.xml"</span><span style="color: rgba(0, 0, 255, 1)">/&gt;</span>
<span style="color: rgba(0, 0, 255, 1)">&lt;/</span><span style="color: rgba(128, 0, 0, 1)">mappers</span><span style="color: rgba(0, 0, 255, 1)">&gt;</span>



</configuration>

conf.xml

首先建立数据表，这里就以 user 表为例 ：

DROP TABLE IF EXISTS user;
CREATE TABLE user (
  id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
  name VARCHAR(32) NOT NULL,
  password VARCHAR(32) NOT NULL,
  sex int,
  email VARCHAR(32),
  phone VARCHAR(16),
  admin VARCHAR(16)
);

然后新建与数据表对应的类 User：

/**
 * User - 用户类
 */
public class User {

</span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">static</span> <span style="color: rgba(0, 0, 255, 1)">final</span> <span style="color: rgba(0, 0, 255, 1)">int</span> MAN  = 0;   <span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)"> 男生</span>
<span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">static</span> <span style="color: rgba(0, 0, 255, 1)">final</span> <span style="color: rgba(0, 0, 255, 1)">int</span> WOMAN = 1;  <span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)"> 女生</span>
<span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">static</span> <span style="color: rgba(0, 0, 255, 1)">final</span> <span style="color: rgba(0, 0, 255, 1)">int</span> OTHER = 2;  <span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)"> 其他</span>

<span style="color: rgba(0, 0, 255, 1)">private</span> <span style="color: rgba(0, 0, 255, 1)">int</span> id;             <span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)"> 用户id</span>
<span style="color: rgba(0, 0, 255, 1)">private</span> String name;        <span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)"> 用户名</span>
<span style="color: rgba(0, 0, 255, 1)">private</span> String password;    <span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)"> 用户密码</span>
<span style="color: rgba(0, 0, 255, 1)">private</span> <span style="color: rgba(0, 0, 255, 1)">int</span> sex;            <span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)"> 用户性别</span>
<span style="color: rgba(0, 0, 255, 1)">private</span> String email;       <span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)"> 用户邮箱</span>
<span style="color: rgba(0, 0, 255, 1)">private</span> String phone;       <span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)"> 用户手机</span>
<span style="color: rgba(0, 0, 255, 1)">private</span> String admin;       <span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)"> 用户是否是管理员，"admin"表示是管理员，其他为普通用户</span>

<span style="color: rgba(0, 0, 255, 1)">public</span><span style="color: rgba(0, 0, 0, 1)"> User() { }

</span><span style="color: rgba(0, 0, 255, 1)">public</span> User(String name, String password, <span style="color: rgba(0, 0, 255, 1)">int</span><span style="color: rgba(0, 0, 0, 1)"> sex, String email, String phone) {
    </span><span style="color: rgba(0, 0, 255, 1)">this</span>.name =<span style="color: rgba(0, 0, 0, 1)"> name;
    </span><span style="color: rgba(0, 0, 255, 1)">this</span>.password =<span style="color: rgba(0, 0, 0, 1)"> password;
    </span><span style="color: rgba(0, 0, 255, 1)">this</span>.sex =<span style="color: rgba(0, 0, 0, 1)"> sex;
    </span><span style="color: rgba(0, 0, 255, 1)">this</span>.email =<span style="color: rgba(0, 0, 0, 1)"> email;
    </span><span style="color: rgba(0, 0, 255, 1)">this</span>.phone =<span style="color: rgba(0, 0, 0, 1)"> phone;
    </span><span style="color: rgba(0, 0, 255, 1)">this</span>.admin = ""<span style="color: rgba(0, 0, 0, 1)">;
}
</span><span style="color: rgba(0, 0, 255, 1)">public</span><span style="color: rgba(0, 0, 0, 1)"> User(String name, String password, String sex, String email, String phone) {
    </span><span style="color: rgba(0, 0, 255, 1)">this</span>.name =<span style="color: rgba(0, 0, 0, 1)"> name;
    </span><span style="color: rgba(0, 0, 255, 1)">this</span>.password =<span style="color: rgba(0, 0, 0, 1)"> password;
    setSex(sex); </span><span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)"> this.sex = sex;</span>
    <span style="color: rgba(0, 0, 255, 1)">this</span>.email =<span style="color: rgba(0, 0, 0, 1)"> email;
    </span><span style="color: rgba(0, 0, 255, 1)">this</span>.phone =<span style="color: rgba(0, 0, 0, 1)"> phone;
    </span><span style="color: rgba(0, 0, 255, 1)">this</span>.admin = ""<span style="color: rgba(0, 0, 0, 1)">;
}

</span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">int</span><span style="color: rgba(0, 0, 0, 1)"> getId() {
    </span><span style="color: rgba(0, 0, 255, 1)">return</span><span style="color: rgba(0, 0, 0, 1)"> id;
}

</span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">void</span> setId(<span style="color: rgba(0, 0, 255, 1)">int</span><span style="color: rgba(0, 0, 0, 1)"> id) {
    </span><span style="color: rgba(0, 0, 255, 1)">this</span>.id =<span style="color: rgba(0, 0, 0, 1)"> id;
}

</span><span style="color: rgba(0, 0, 255, 1)">public</span><span style="color: rgba(0, 0, 0, 1)"> String getName() {
    </span><span style="color: rgba(0, 0, 255, 1)">return</span><span style="color: rgba(0, 0, 0, 1)"> name;
}

</span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">void</span><span style="color: rgba(0, 0, 0, 1)"> setName(String name) {
    </span><span style="color: rgba(0, 0, 255, 1)">this</span>.name =<span style="color: rgba(0, 0, 0, 1)"> name;
}

</span><span style="color: rgba(0, 0, 255, 1)">public</span><span style="color: rgba(0, 0, 0, 1)"> String getPassword() {
    </span><span style="color: rgba(0, 0, 255, 1)">return</span><span style="color: rgba(0, 0, 0, 1)"> password;
}

</span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">void</span><span style="color: rgba(0, 0, 0, 1)"> setPassword(String password) {
    </span><span style="color: rgba(0, 0, 255, 1)">this</span>.password =<span style="color: rgba(0, 0, 0, 1)"> password;
}

</span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">int</span><span style="color: rgba(0, 0, 0, 1)"> getSex() {
    </span><span style="color: rgba(0, 0, 255, 1)">return</span><span style="color: rgba(0, 0, 0, 1)"> sex;
}

</span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">void</span> setSex(<span style="color: rgba(0, 0, 255, 1)">int</span><span style="color: rgba(0, 0, 0, 1)"> sex) {
    </span><span style="color: rgba(0, 0, 255, 1)">this</span>.sex =<span style="color: rgba(0, 0, 0, 1)"> sex;
}

</span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">void</span><span style="color: rgba(0, 0, 0, 1)"> setSex(String sexStr) {
    </span><span style="color: rgba(0, 0, 255, 1)">int</span> sex =<span style="color: rgba(0, 0, 0, 1)"> Integer.valueOf(sexStr);
    </span><span style="color: rgba(0, 0, 255, 1)">switch</span><span style="color: rgba(0, 0, 0, 1)"> (Integer.valueOf(sexStr)) {
        </span><span style="color: rgba(0, 0, 255, 1)">case</span> 0<span style="color: rgba(0, 0, 0, 1)">: {
            </span><span style="color: rgba(0, 0, 255, 1)">this</span>.sex =<span style="color: rgba(0, 0, 0, 1)"> MAN;
            </span><span style="color: rgba(0, 0, 255, 1)">break</span><span style="color: rgba(0, 0, 0, 1)">;
        }
        </span><span style="color: rgba(0, 0, 255, 1)">case</span> 1<span style="color: rgba(0, 0, 0, 1)">: {
            </span><span style="color: rgba(0, 0, 255, 1)">this</span>.sex =<span style="color: rgba(0, 0, 0, 1)"> WOMAN;
            </span><span style="color: rgba(0, 0, 255, 1)">break</span><span style="color: rgba(0, 0, 0, 1)">;
        }
        </span><span style="color: rgba(0, 0, 255, 1)">default</span><span style="color: rgba(0, 0, 0, 1)">: {
            </span><span style="color: rgba(0, 0, 255, 1)">this</span>.sex =<span style="color: rgba(0, 0, 0, 1)"> OTHER;
            </span><span style="color: rgba(0, 0, 255, 1)">break</span><span style="color: rgba(0, 0, 0, 1)">;
        }
    }
}

</span><span style="color: rgba(0, 0, 255, 1)">public</span><span style="color: rgba(0, 0, 0, 1)"> String getEmail() {
    </span><span style="color: rgba(0, 0, 255, 1)">return</span><span style="color: rgba(0, 0, 0, 1)"> email;
}

</span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">void</span><span style="color: rgba(0, 0, 0, 1)"> setEmail(String email) {
    </span><span style="color: rgba(0, 0, 255, 1)">this</span>.email =<span style="color: rgba(0, 0, 0, 1)"> email;
}

</span><span style="color: rgba(0, 0, 255, 1)">public</span><span style="color: rgba(0, 0, 0, 1)"> String getPhone() {
    </span><span style="color: rgba(0, 0, 255, 1)">return</span><span style="color: rgba(0, 0, 0, 1)"> phone;
}

</span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">void</span><span style="color: rgba(0, 0, 0, 1)"> setPhone(String phone) {
    </span><span style="color: rgba(0, 0, 255, 1)">this</span>.phone =<span style="color: rgba(0, 0, 0, 1)"> phone;
}

</span><span style="color: rgba(0, 0, 255, 1)">public</span><span style="color: rgba(0, 0, 0, 1)"> String getAdmin() {
    </span><span style="color: rgba(0, 0, 255, 1)">return</span><span style="color: rgba(0, 0, 0, 1)"> admin;
}

</span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">void</span><span style="color: rgba(0, 0, 0, 1)"> setAdmin(String admin) {
    </span><span style="color: rgba(0, 0, 255, 1)">this</span>.admin =<span style="color: rgba(0, 0, 0, 1)"> admin;
}

@Override
</span><span style="color: rgba(0, 0, 255, 1)">public</span><span style="color: rgba(0, 0, 0, 1)"> String toString() {
    </span><span style="color: rgba(0, 0, 255, 1)">return</span> "User{" +
            "id=" + id +
            ", name='" + name + '\'' +
            ", password='" + password + '\'' +
            ", sex=" + sex +
            ", email='" + email + '\'' +
            ", phone='" + phone + '\'' +
            ", admin='" + admin + '\'' +
            '}'<span style="color: rgba(0, 0, 0, 1)">;
}



}

User

再新建 usre 表的配置文件：

<?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.luoxn28.dao.UserDao">


<span style="color: rgba(0, 0, 255, 1)">&lt;</span><span style="color: rgba(128, 0, 0, 1)">select </span><span style="color: rgba(255, 0, 0, 1)">id</span><span style="color: rgba(0, 0, 255, 1)">="getById"</span><span style="color: rgba(255, 0, 0, 1)"> parameterType</span><span style="color: rgba(0, 0, 255, 1)">="int"</span><span style="color: rgba(255, 0, 0, 1)"> resultType</span><span style="color: rgba(0, 0, 255, 1)">="User"</span><span style="color: rgba(0, 0, 255, 1)">&gt;</span><span style="color: rgba(0, 0, 0, 1)">
    SELECT * FROM user WHERE id=#{id}; </span><span style="color: rgba(0, 128, 0, 1)">&lt;!--</span><span style="color: rgba(0, 128, 0, 1)"> #{xxx} xxx为类中的数据域名称 </span><span style="color: rgba(0, 128, 0, 1)">--&gt;</span>
<span style="color: rgba(0, 0, 255, 1)">&lt;/</span><span style="color: rgba(128, 0, 0, 1)">select</span><span style="color: rgba(0, 0, 255, 1)">&gt;</span>

<span style="color: rgba(0, 0, 255, 1)">&lt;</span><span style="color: rgba(128, 0, 0, 1)">select </span><span style="color: rgba(255, 0, 0, 1)">id</span><span style="color: rgba(0, 0, 255, 1)">="getAll"</span><span style="color: rgba(255, 0, 0, 1)"> resultType</span><span style="color: rgba(0, 0, 255, 1)">="com.luoxn28.dao.User"</span><span style="color: rgba(0, 0, 255, 1)">&gt;</span><span style="color: rgba(0, 0, 0, 1)">
    SELECT * FROM user;
</span><span style="color: rgba(0, 0, 255, 1)">&lt;/</span><span style="color: rgba(128, 0, 0, 1)">select</span><span style="color: rgba(0, 0, 255, 1)">&gt;</span>



</mapper>

userMapper.xml

最后新建测试类：

/**
 * MyBatis 测试类
 */
public class TestMain {

</span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">static</span> <span style="color: rgba(0, 0, 255, 1)">void</span> main(String[] args) <span style="color: rgba(0, 0, 255, 1)">throws</span><span style="color: rgba(0, 0, 0, 1)"> IOException {
    String resouce </span>= "conf.xml"<span style="color: rgba(0, 0, 0, 1)">;
    InputStream is </span>=<span style="color: rgba(0, 0, 0, 1)"> Resources.getResourceAsStream(resouce);

    </span><span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)"> 构建sqlSession工厂</span>
    SqlSessionFactory sqlSessionFactory = <span style="color: rgba(0, 0, 255, 1)">new</span><span style="color: rgba(0, 0, 0, 1)"> SqlSessionFactoryBuilder().build(is);
            </span><span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)"> 获取sqlSession</span>
    SqlSession session =<span style="color: rgba(0, 0, 0, 1)"> sqlSessionFactory.openSession();

    User user;

    </span><span style="color: rgba(0, 0, 255, 1)">try</span><span style="color: rgba(0, 0, 0, 1)"> {
        </span><span style="color: rgba(0, 128, 0, 1)">/**</span><span style="color: rgba(0, 128, 0, 1)">
         * 第一种方式: 直接执行已映射的 SQL 语句
         </span><span style="color: rgba(0, 128, 0, 1)">*/</span><span style="color: rgba(0, 0, 0, 1)">
        String statement </span>= "com.luoxn28.dao.UserDao.getById"<span style="color: rgba(0, 0, 0, 1)">;
        user </span>= session.selectOne(statement, 1<span style="color: rgba(0, 0, 0, 1)">);
        System.out.println(user);
    }
    </span><span style="color: rgba(0, 0, 255, 1)">finally</span><span style="color: rgba(0, 0, 0, 1)"> {
        session.close();
    }

    </span><span style="color: rgba(0, 128, 0, 1)">/**</span><span style="color: rgba(0, 128, 0, 1)">
     * 第二种方式: 执行更清晰和类型安全的代码
     </span><span style="color: rgba(0, 128, 0, 1)">*/</span>



//        UserDao userDao = session.getMapper(UserDao.class);
//        user = userDao.getById(1);
//        System.out.println(user);
    }


}

　　由于我们分析的是 SQL 的执行流程，那就重点关注下 user = session.selectOne(statement, 1); 这行代码 ~ 注意，传进去的参数是 1。

　　session 是 DefaultSqlSession 类型的，因为 sqlSessionFactory 默认生成的 SqlSession 是 DefaultSqlSession 类型。selectOne()会调用 selectList()。

// DefaultSqlSession 类
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
        MappedStatement ms = configuration.getMappedStatement(statement);
        // CURD 操作是交给 Excetor 去处理的
        return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);} catch (Exception e) {
        throw ExceptionFactory.wrapException("Error querying database.  Cause:" + e, e);} finally {ErrorContext.instance().reset();}
}

　　在 DefaultSqlSession.selectList 中的各种 CURD 操作都是通多 Executor 进行的，这里 executor 的类型是 CachingExecutor，接着跳转到其中的 query 方法中。

// CachingExecutor 类
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    BoundSql boundSql = ms.getBoundSql(parameterObject);  // 获取绑定的 sql 命令，比如 "SELECT * FROM xxx"
    CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
    return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

　　getBoundSql 为了获取绑定的 sql 命令，在创建完 cacheKey 之后，就进入到 CachingExecutor 类中的另一个 query 方法中。

// CachingExecutor 类
@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
        throws SQLException {
    Cache cache = ms.getCache();
    if (cache != null) {flushCacheIfRequired(ms);
        if (ms.isUseCache() && resultHandler == null) {ensureNoOutParams(ms, parameterObject, boundSql);
            @SuppressWarnings("unchecked")
            List<E> list = (List<E>)tcm.getObject(cache, key);
            if (list == null) {
                list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
                tcm.putObject(cache, key, list); // issue #578 and #116
            }
            return list;
        }
    }
    return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

　　这里真正执行 query 操作的是 SimplyExecutor 代理来完成的，接着就进入到了 SimplyExecutor 的父类 BaseExecutor 的 query 方法中。

// SimplyExecutor 的父类 BaseExecutor 类
@SuppressWarnings("unchecked")
@Override
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
    if (closed) {
        throw new ExecutorException("Executor was closed.");}
    if (queryStack == 0 && ms.isFlushCacheRequired()) {clearLocalCache();
    }
    List<E> list;
    try {
        queryStack++;
        /**
         * localCache 是一级缓存，如果找不到就调用 queryFromDatabase 从数据库中查找
         */
        list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
        if (list != null) {handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
        } else {
            list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
        }
    } finally {
        queryStack--;
    }
    if (queryStack == 0) {
        for (DeferredLoad deferredLoad : deferredLoads) {deferredLoad.load();
        }
        // issue #601
        deferredLoads.clear();
        if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
            // issue #482
            clearLocalCache();}
    }
    return list;
}

　　因为是第一次 SQL 查询操作，所以会调用 queryFromDatabase 方法来执行查询。

// SimplyExecutor 的父类 BaseExecutor 类
private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    List<E> list;
    localCache.putObject(key, EXECUTION_PLACEHOLDER);
    try {
        list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {localCache.removeObject(key);
    }
    localCache.putObject(key, list);
    if (ms.getStatementType() == StatementType.CALLABLE) {localOutputParameterCache.putObject(key, parameter);
    }
    return list;
}

　　从数据库中查询数据，进入到 SimplyExecutor 中进行操作。

// SimplyExecutor 类
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Statement stmt = null;
    try {
        Configuration configuration = ms.getConfiguration();
        StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
        // 子流程 1：SQL 查询参数的设置
        stmt = prepareStatement(handler, ms.getStatementLog());
        // StatementHandler 封装了 Statement
        // 子流程 2：SQL 查询操作和结果集的封装
        return handler.<E>query(stmt);
    } finally {closeStatement(stmt);
    }
}

　　注意，在 prepareStatement 方法中会进行 SQL 查询参数的设置，也就是咱们最开始传递进来的参数，其值为 1。handler.<E>query(stmt) 方法中会进行实际的 SQL 查询操作和结果集的封装（封装成 Java 对象）。当流程走到这里时，程序已经压栈有一定深度了，因为接下来程序分析会兵分两路，一方面深入到 SQL查询及结果集的设置子流程 1 中，然后再深入到 SQL 查询操作和结果集的封装子流程 2，因为还会回到这里，所以就来一张调用栈的特写吧：

子流程 1：SQL 查询参数的设置

// SimplyExecutor 类
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    // 获取一个 Connection
    Connection connection = getConnection(statementLog);
    stmt = handler.prepare(connection, transaction.getTimeout());
    handler.parameterize(stmt); // 设置 SQL 查询中的参数值
    return stmt;
}

　　通过 getConnection 方法来获取一个 Connection，调用 prepare 方法来获取一个 Statement（这里的 handler 类型是 RoutingStatementHandler，RoutingStatementHandler 的 prepare 方法调用的是 PrepareStatementHandler 的 prepare 方法，因为 PrepareStatementHandler 并没有覆盖其父类的 prepare 方法，其实最后调用的是 BaseStatementHandler 中的 prepare 方法。是不是绕晕了，那就再看一遍吧 :) ）。调用 parameterize 方法来设置 SQL 的参数值（这里最后调用的是 PrepareStatementHandler 中的 parameterize 方法，而 PrepareStatementHandler.parameterize 方法调用的是 DefaultParameterHandler 中的 setParameters 方法）。

// PrepareStatementHandler 类
@Override
public void parameterize(Statement statement) throws SQLException {parameterHandler.setParameters((PreparedStatement) statement);}
// DefaultParameterHandler 类
@Override
public void setParameters(PreparedStatement ps) {ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
    if (parameterMappings != null) {
        for (int i = 0; i < parameterMappings.size(); i++) {
            ParameterMapping parameterMapping = parameterMappings.get(i);
            if (parameterMapping.getMode() != ParameterMode.OUT) {
                Object value;
                String propertyName = parameterMapping.getProperty();
                if (boundSql.hasAdditionalParameter(propertyName)) { // issue #448 ask first for additional params
                    value = boundSql.getAdditionalParameter(propertyName);
                } else if (parameterObject == null) {
                    value = null;
                } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
                    value = parameterObject;
                } else {
                    MetaObject metaObject = configuration.newMetaObject(parameterObject);
                    value = metaObject.getValue(propertyName);
                }
                TypeHandler typeHandler = parameterMapping.getTypeHandler();
                JdbcType jdbcType = parameterMapping.getJdbcType();
                if (value == null && jdbcType == null) {
                    jdbcType = configuration.getJdbcTypeForNull();}
                try {
                    typeHandler.setParameter(ps, i + 1, value, jdbcType);} catch (TypeException e) {
                    throw new TypeException("Could not set parameters for mapping:" + parameterMapping + ". Cause:" + e, e);} catch (SQLException e) {
                    throw new TypeException("Could not set parameters for mapping:" + parameterMapping + ". Cause:" + e, e);}
            }
        }
    }
}

　　到这里为止，已经给 Statement 设置了最初传递进去的参数（值为 1）了，那么接着分析流程 2：

流程 2：SQL 查询及结果集的设置

// RoutingStatementHandler 类
@Override
public <E> List<E> query(Statement statement) throws SQLException {
    return delegate.<E>query(statement);
}
// RoutingStatementHandler 类
@Override
public <E> List<E> query(Statement statement) throws SQLException {
    // 这里就到了熟悉的 PreparedStatement 了
    PreparedStatement ps = (PreparedStatement) statement;
    // 执行 SQL 查询操作
    ps.execute();
    // 结果交给 ResultHandler 来处理
    return resultSetHandler.<E> handleResultSets(ps);
}
// DefaultResultSetHandler 类（封装返回值，将查询结果封装成 Object 对象）
@Override
public List<Object> handleResultSets(Statement stmt) throws SQLException {ErrorContext.instance().activity("handling results").object(mappedStatement.getId());

</span><span style="color: rgba(0, 0, 255, 1)">final</span> List&lt;Object&gt; multipleResults = <span style="color: rgba(0, 0, 255, 1)">new</span> ArrayList&lt;Object&gt;<span style="color: rgba(0, 0, 0, 1)">();

</span><span style="color: rgba(0, 0, 255, 1)">int</span> resultSetCount = 0<span style="color: rgba(0, 0, 0, 1)">;
ResultSetWrapper rsw </span>=<span style="color: rgba(0, 0, 0, 1)"> getFirstResultSet(stmt);

List</span>&lt;ResultMap&gt; resultMaps =<span style="color: rgba(0, 0, 0, 1)"> mappedStatement.getResultMaps();
</span><span style="color: rgba(0, 0, 255, 1)">int</span> resultMapCount =<span style="color: rgba(0, 0, 0, 1)"> resultMaps.size();
validateResultMapsCount(rsw, resultMapCount);
</span><span style="color: rgba(0, 0, 255, 1)">while</span> (rsw != <span style="color: rgba(0, 0, 255, 1)">null</span> &amp;&amp; resultMapCount &gt;<span style="color: rgba(0, 0, 0, 1)"> resultSetCount) {
    ResultMap resultMap </span>=<span style="color: rgba(0, 0, 0, 1)"> resultMaps.get(resultSetCount);
    handleResultSet(rsw, resultMap, multipleResults, </span><span style="color: rgba(0, 0, 255, 1)">null</span><span style="color: rgba(0, 0, 0, 1)">);
    rsw </span>=<span style="color: rgba(0, 0, 0, 1)"> getNextResultSet(stmt);
    cleanUpAfterHandlingResultSet();
    resultSetCount</span>++<span style="color: rgba(0, 0, 0, 1)">;
}

String[] resultSets </span>=<span style="color: rgba(0, 0, 0, 1)"> mappedStatement.getResultSets();
</span><span style="color: rgba(0, 0, 255, 1)">if</span> (resultSets != <span style="color: rgba(0, 0, 255, 1)">null</span><span style="color: rgba(0, 0, 0, 1)">) {
    </span><span style="color: rgba(0, 0, 255, 1)">while</span> (rsw != <span style="color: rgba(0, 0, 255, 1)">null</span> &amp;&amp; resultSetCount &lt;<span style="color: rgba(0, 0, 0, 1)"> resultSets.length) {
        ResultMapping parentMapping </span>=<span style="color: rgba(0, 0, 0, 1)"> nextResultMaps.get(resultSets[resultSetCount]);
        </span><span style="color: rgba(0, 0, 255, 1)">if</span> (parentMapping != <span style="color: rgba(0, 0, 255, 1)">null</span><span style="color: rgba(0, 0, 0, 1)">) {
            String nestedResultMapId </span>=<span style="color: rgba(0, 0, 0, 1)"> parentMapping.getNestedResultMapId();
            ResultMap resultMap </span>=<span style="color: rgba(0, 0, 0, 1)"> configuration.getResultMap(nestedResultMapId);
            handleResultSet(rsw, resultMap, </span><span style="color: rgba(0, 0, 255, 1)">null</span><span style="color: rgba(0, 0, 0, 1)">, parentMapping);
        }
        rsw </span>=<span style="color: rgba(0, 0, 0, 1)"> getNextResultSet(stmt);
        cleanUpAfterHandlingResultSet();
        resultSetCount</span>++<span style="color: rgba(0, 0, 0, 1)">;
    }
}

</span><span style="color: rgba(0, 0, 255, 1)">return</span><span style="color: rgba(0, 0, 0, 1)"> collapseSingleResultList(multipleResults);



}

　　ResultSetWrapper 是 ResultSet 的包装类，调用 getFirstResultSet 方法获取第一个 ResultSet，同时获取数据库的 MetaData 数据，包括数据表列名、列的类型、类序号等，这些信息都存储在 ResultSetWrapper 类中了。然后调用 handleResultSet 方法来来进行结果集的封装。

// DefaultResultSetHandler 类
private void handleResultSet(ResultSetWrapper rsw, ResultMap resultMap, List<Object> multipleResults, ResultMapping parentMapping) throws SQLException {
    try {
        if (parentMapping != null) {
            handleRowValues(rsw, resultMap, null, RowBounds.DEFAULT, parentMapping);} else {
            if (resultHandler == null) {
                DefaultResultHandler defaultResultHandler = new DefaultResultHandler(objectFactory);
                handleRowValues(rsw, resultMap, defaultResultHandler, rowBounds, null);
                multipleResults.add(defaultResultHandler.getResultList());
            } else {
                handleRowValues(rsw, resultMap, resultHandler, rowBounds, null);}
        }
    } finally {
        // issue #228 (close resultsets)
        closeResultSet(rsw.getResultSet());
    }
}

　　这里调用 handleRowValues 方法来进行值的设置：

// DefaultResultSetHandler 类
public void handleRowValues(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException {
    if (resultMap.hasNestedResultMaps()) {ensureNoRowBounds();
        checkResultHandler();
        handleRowValuesForNestedResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
    } else {
        // 封装数据
        handleRowValuesForSimpleResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
    }
}
// DefaultResultSetHandler 类
// 封装数据
private void handleRowValuesForSimpleResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping)
        throws SQLException {
    DefaultResultContext<Object> resultContext = new DefaultResultContext<Object>();
    skipRows(rsw.getResultSet(), rowBounds);
    while (shouldProcessMoreRows(resultContext, rowBounds) && rsw.getResultSet().next()) {
        ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(rsw.getResultSet(), resultMap, null);
        Object rowValue = getRowValue(rsw, discriminatedResultMap);
        storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
    }
}
// DefaultResultSetHandler 类
private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap) throws SQLException {
    final ResultLoaderMap lazyLoader = new ResultLoaderMap();
    // createResultObject 为新创建的对象，数据表对应的类
    Object resultObject = createResultObject(rsw, resultMap, lazyLoader, null);
    if (resultObject != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) {
        final MetaObject metaObject = configuration.newMetaObject(resultObject);
        boolean foundValues = !resultMap.getConstructorResultMappings().isEmpty();
        if (shouldApplyAutomaticMappings(resultMap, false)) {
            // 这里把数据填充进去，metaObject 中包含了 resultObject 信息
            foundValues = applyAutomaticMappings(rsw, resultMap, metaObject, null) || foundValues;
        }
        foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, null) || foundValues;
        foundValues = lazyLoader.size() > 0 || foundValues;
        resultObject = foundValues ? resultObject : null;
        return resultObject;
    }
    return resultObject;
}
// DefaultResultSetHandler 类（把 ResultSet 中查询结果填充到 JavaBean 中）
private boolean applyAutomaticMappings(ResultSetWrapper rsw, ResultMap resultMap, MetaObject metaObject, String columnPrefix) throws SQLException {
    List<UnMappedColumnAutoMapping> autoMapping = createAutomaticMappings(rsw, resultMap, metaObject, columnPrefix);
    boolean foundValues = false;
    if (autoMapping.size() > 0) {
                // 这里进行 for 循环调用，因为 user 表中总共有 7 项，所以也就调用 7 次
        for (UnMappedColumnAutoMapping mapping : autoMapping) {
            // 这里将 esultSet 中查询结果转换为对应的实际类型
            final Object value = mapping.typeHandler.getResult(rsw.getResultSet(), mapping.column);
            if (value != null) {
                foundValues = true;
            }
            if (value != null || (configuration.isCallSettersOnNulls() && !mapping.primitive)) {
                // gcode issue #377, call setter on nulls (value is not 'found')
                metaObject.setValue(mapping.property, value);
            }
        }
    }
    return foundValues;
}

　　mapping.typeHandler.getResult 会获取查询结果值的实际类型，比如我们 user 表中 id 字段为 int 类型，那么它就对应 Java 中的 Integer 类型，然后通过调用 statement.getInt("id") 来获取其 int 值，其类型为 Integer。metaObject.setValue 方法会把获取到的 Integer 值设置到 Java 类中的对应字段。

// MetaObject 类
public void setValue(String name, Object value) {
    PropertyTokenizer prop = new PropertyTokenizer(name);
    if (prop.hasNext()) {
        MetaObject metaValue = metaObjectForProperty(prop.getIndexedName());
        if (metaValue == SystemMetaObject.NULL_META_OBJECT) {
            if (value == null && prop.getChildren() != null) {
                // don't instantiate child path if value is null
                return;
            } else {
                metaValue = objectWrapper.instantiatePropertyValue(name, prop, objectFactory);
            }
        }
        metaValue.setValue(prop.getChildren(), value);} else {objectWrapper.set(prop, value);
    }
}

　　metaValue.setValue 方法最后会调用到 Java 类中对应数据域的 set 方法，这样也就完成了 SQL 查询结果集的 Java 类封装过程。最后贴一张调用栈到达 Java 类的 set 方法中的快照：

 

参考：

　　1、MyBatis 源码

　　2、《深入分析 Java Web 技术内幕》的 iBatis 章节

　　3、《深入理解 mybatis 原理》 MyBatis 的架构设计以及实例分析

　　4、luoxn28/tuiku

　　5、Java JDBC 基础学习小结