动态 SQL 中插入脚本语言

public interface LanguageDriver {

    // 从 XML 节点创建 SqlSource（XML 映射文件方式）
    SqlSource createSqlSource(Configuration configuration,
        XNode script, Class<?> parameterType);

    // 从字符串创建 SqlSource（注解方式）
    SqlSource createSqlSource(Configuration configuration,
        String script, Class<?> parameterType);

    // 创建参数处理器
    ParameterHandler createParameterHandler(
        MappedStatement mappedStatement,
        Object parameterObject, BoundSql boundSql);
}

package com.feixiang.mybatis.scripting;

import org.apache.ibatis.builder.xml.XMLMapperEntityResolver;
import org.apache.ibatis.mapping.BoundSql;
import org.apache.ibatis.mapping.MappedStatement;
import org.apache.ibatis.mapping.ParameterMapping;
import org.apache.ibatis.mapping.SqlSource;
import org.apache.ibatis.parsing.XNode;
import org.apache.ibatis.scripting.LanguageDriver;
import org.apache.ibatis.scripting.defaults.DefaultParameterHandler;
import org.apache.ibatis.scripting.xmltags.XMLLanguageDriver;
import org.apache.ibatis.session.Configuration;

import java.util.ArrayList;
import java.util.List;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

/**
 * 简易模板 LanguageDriver — 演示原理
 * 支持 ${paramName} 占位符替换为 MyBatis 的 #{paramName} 预编译参数
 */
public class SimpleTemplateDriver extends XMLLanguageDriver {

    // 匹配 ${变量名} —— 非贪婪
    private static final Pattern TEMPLATE_PATTERN =
        Pattern.compile("\\$\\{(\\w+)\\}");

    @Override
    public SqlSource createSqlSource(Configuration configuration,
                                      XNode script,
                                      Class<?> parameterType) {

        // 1. 先让 XMLLanguageDriver 按标准 XML 方式处理
        //    （支持 <if> <where> <foreach> 等动态标签）
        SqlSource original = super.createSqlSource(
            configuration, script, parameterType);

        // 2. 包装一层：在 getBoundSql 时把 ${xxx} 替换为 #{xxx}
        return new SqlSource() {
            @Override
            public BoundSql getBoundSql(Object parameterObject) {
                BoundSql boundSql = original.getBoundSql(parameterObject);
                String sql = boundSql.getSql();

                // 提取 ${xxx} 中的变量名
                List<String> paramNames = new ArrayList<>();
                Matcher matcher = TEMPLATE_PATTERN.matcher(sql);
                while (matcher.find()) {
                    paramNames.add(matcher.group(1));
                }

                // 构建新的 ParameterMapping 列表
                List<ParameterMapping> newMappings =
                    new ArrayList<>(boundSql.getParameterMappings());

                for (String name : paramNames) {
                    ParameterMapping mapping =
                        new ParameterMapping.Builder(
                            configuration, name, Object.class).build();
                    newMappings.add(mapping);
                }

                // 把 ${xxx} 替换为 #{xxx} 作为 MyBatis 标准占位符
                String newSql = TEMPLATE_PATTERN.matcher(sql)
                    .replaceAll("#{$1}");

                return new BoundSql(configuration, newSql,
                    newMappings, parameterObject);
            }
        };
    }

    @Override
    public SqlSource createSqlSource(Configuration configuration,
                                      String script,
                                      Class<?> parameterType) {
        // 注解模式：直接处理字符串
        String processed = TEMPLATE_PATTERN.matcher(script)
            .replaceAll("#{$1}");
        return new XMLLanguageDriver().createSqlSource(
            configuration, processed, parameterType);
    }

    @Override
    public ParameterHandler createParameterHandler(
            MappedStatement mappedStatement,
            Object parameterObject, BoundSql boundSql) {
        // 使用默认的参数处理器即可
        return new DefaultParameterHandler(
            mappedStatement, parameterObject, boundSql);
    }
}

public interface EmployeeMapper {

    // 注解指定，优先级最高
    @Lang(SimpleTemplateDriver.class)
    @Select("SELECT * FROM t_employee WHERE name = ${name}")
    Employee findByName(@Param("name") String name);

    // 未指定，使用全局默认
    @Select("SELECT * FROM t_employee WHERE id = #{id}")
    Employee findById(int id);
}

// 即使全局默认是 velocity，这个方法也被 @Lang 锁定为 XMLLanguageDriver
@Lang(XMLLanguageDriver.class)
@Select("SELECT * FROM t WHERE name = #{name}")
Employee findByName(String name);