1. 相关代码

package com.boge.mapper;

import com.boge.pojo.User;

import java.util.List;

public interface UserMapper {

    List<User> selectUserList(User user);

    User selectUserById(Integer id);

    int updateById(User user);
}

<?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.boge.mapper.UserMapper">

    <cache/>

    <resultMap id="BaseResultMap" type="com.boge.pojo.User">
        <id property="id" column="id" jdbcType="INTEGER"/>
        <result property="userName" column="user_name" jdbcType="VARCHAR"/>
        <result property="realName" column="real_name" jdbcType="VARCHAR"/>
        <result property="password" column="password" jdbcType="VARCHAR"/>
        <result property="age" column="age" jdbcType="INTEGER"/>
        <result property="dId" column="d_id" jdbcType="INTEGER"/>
    </resultMap>

    <sql id="baseSQL">
        id,user_name,real_name,password,age,d_id
    </sql>

    <select id="selectUserById" resultType="com.boge.pojo.User">
        select
          <include refid="baseSQL"></include>
        from
            t_user
        where
            id = #{id}
    </select>

    <select id="selectUserList" resultMap="BaseResultMap">
        select
            <include refid="baseSQL"></include>
        from
            t_user t
        <where>
            <if test="userName != null and userName.trim() != ''">
                and t.user_name like concat('%', #{userName},'%')
            </if>
            <if test="age != null">
                and t.age = {age}
            </if>
        </where>
    </select>

    <update id="updateById">
        update t_user set user_name = #{userName} where id = #{id}
    </update>

</mapper>

2. SQL 语句解析全流程

2.1 涉及到的重要类

XMLStatementBuilder。映射文件由<select>、<insert>、<delete>、<update>等标签是由XMLStatementBuilder.parseStatementNode()进行解析，不在由XMLMapperBuilder解析。
SqlSource。用来表示解析之后的sql语句。
MappedStatement 。用来表示解析之后的sql标签，包含sqlSource和sqlCommandType，分别记录了SQL 标签中定义的 SQL 语句和 SQL 语句的类型（INSERT、UPDATE、DELETE、SELECT 或 FLUSH 类型）。

public void parseStatementNode() {
	// 获取SQL标签的id以及databaseId属性
    String id = context.getStringAttribute("id");
    String databaseId = context.getStringAttribute("databaseId");
 	// 若databaseId属性值与当前使用的数据库不匹配，则不加载该SQL标签
    // 若存在相同id且databaseId不为空的SQL标签，则不再加载该SQL标签
    if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) {
      return;
    }
    // 获取节点的名称  select insert delete update
    String nodeName = context.getNode().getNodeName();
    // 获取到具体的 sql 命令 类型
    SqlCommandType sqlCommandType = SqlCommandType.valueOf(nodeName.toUpperCase(Locale.ENGLISH));
    boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
    boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect);
    boolean useCache = context.getBooleanAttribute("useCache", isSelect);
    boolean resultOrdered = context.getBooleanAttribute("resultOrdered", false);

    // Include Fragments before parsing  解析 include标签  替换include 标签  完成 ${} 解析
    XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);
    includeParser.applyIncludes(context.getNode());

    String parameterType = context.getStringAttribute("parameterType");
    Class<?> parameterTypeClass = resolveClass(parameterType);

    String lang = context.getStringAttribute("lang");
    LanguageDriver langDriver = getLanguageDriver(lang);

    // Parse selectKey after includes and remove them.
    processSelectKeyNodes(id, parameterTypeClass, langDriver);

    // Parse the SQL (pre: <selectKey> and <include> were parsed and removed)
    KeyGenerator keyGenerator;
    String keyStatementId = id + SelectKeyGenerator.SELECT_KEY_SUFFIX;
    keyStatementId = builderAssistant.applyCurrentNamespace(keyStatementId, true);
    if (configuration.hasKeyGenerator(keyStatementId)) {
      keyGenerator = configuration.getKeyGenerator(keyStatementId);
    } else {
      keyGenerator = context.getBooleanAttribute("useGeneratedKeys",
          configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType))
          ? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE;
    }
    // 动态SQL的加载解析 同时记录了 sql中的占位符 ParameterMapping
    SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
    StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString()));
    Integer fetchSize = context.getIntAttribute("fetchSize");
    Integer timeout = context.getIntAttribute("timeout");
    String parameterMap = context.getStringAttribute("parameterMap");
    String resultType = context.getStringAttribute("resultType");
    Class<?> resultTypeClass = resolveClass(resultType);
    String resultMap = context.getStringAttribute("resultMap");
    String resultSetType = context.getStringAttribute("resultSetType");
    ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType);
    if (resultSetTypeEnum == null) {
      resultSetTypeEnum = configuration.getDefaultResultSetType();
    }
    String keyProperty = context.getStringAttribute("keyProperty");
    String keyColumn = context.getStringAttribute("keyColumn");
    String resultSets = context.getStringAttribute("resultSets");

    // >> 关键的一步： MappedStatement 的创建
    builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
        fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
        resultSetTypeEnum, flushCache, useCache, resultOrdered,
        keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
  }

2.2 解析标签

2.2.1 <include>

<include>作用是引入由<sql>标签定义的sql片段，<sql>实际在XMLMapperBuilder.sqlElement()解析。
在这里插入图片描述
<include> 标签由XMLIncludeTransformer.applyIncludes(）处理，同时还会处理<include> 标签下的<property>标签和“${}"占位符。

2.2.2 <selectKey>

略。

2.2.3 处理 SQL 语句

  // 动态SQL的加载解析 同时记录了 sql中的占位符 ParameterMapping
    SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);

在这里插入图片描述

3. 获取真正执行的sql

在CachingExecutor.query()生成真正待执行的sql。

 @Override
  public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    // 获取SQL
    BoundSql boundSql = ms.getBoundSql(parameterObject);
    // 创建CacheKey：什么样的SQL是同一条SQL？ >>
    // select * from t_user   select id,username,password form t_user
    // 根据特定的规则生成一个key 保证不冲突
    CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
    return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }

进入getBoundSql(),

 public BoundSql getBoundSql(Object parameterObject) {
 	// 拼接sql
    BoundSql boundSql = sqlSource.getBoundSql(parameterObject);
    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
    if (parameterMappings == null || parameterMappings.isEmpty()) {
      boundSql = new BoundSql(configuration, boundSql.getSql(), parameterMap.getParameterMappings(), parameterObject);
    }

    // check for nested result maps in parameter mappings (issue #30)
    for (ParameterMapping pm : boundSql.getParameterMappings()) {
      String rmId = pm.getResultMapId();
      if (rmId != null) {
        ResultMap rm = configuration.getResultMap(rmId);
        if (rm != null) {
          hasNestedResultMaps |= rm.hasNestedResultMaps();
        }
      }
    }

    return boundSql;
  }