# 第14天：完善MyBatis插件

## 📚 今日目标

1. 完善DynamicMybatisPlugin插件
2. 优化SQL替换逻辑
3. 处理表别名和JOIN语句
4. 处理各种SQL场景

---

## 🛠️ 实践任务：完善DynamicMybatisPlugin

### 完整实现代码

在 `src/main/java/cn/bugstack/middleware/db/router/dynamic/` 目录下完善 `DynamicMybatisPlugin.java`：

```java
package cn.bugstack.middleware.db.router.dynamic;

import cn.bugstack.middleware.db.router.DBContextHolder;
import org.apache.ibatis.executor.statement.StatementHandler;
import org.apache.ibatis.mapping.BoundSql;
import org.apache.ibatis.mapping.MappedStatement;
import org.apache.ibatis.plugin.*;
import org.apache.ibatis.reflection.DefaultReflectorFactory;
import org.apache.ibatis.reflection.MetaObject;
import org.apache.ibatis.reflection.SystemMetaObject;

import java.lang.reflect.Field;
import java.sql.Connection;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

/**
 * MyBatis动态表名插件
 * 
 * @author 小傅哥
 */
@Intercepts({
    @Signature(
        type = StatementHandler.class,
        method = "prepare",
        args = {Connection.class, Integer.class}
    )
})
public class DynamicMybatisPlugin implements Interceptor {

    private Pattern pattern = Pattern.compile("(from|into|update|FROM|INTO|UPDATE)\\s+(\\w+)\\s+", Pattern.CASE_INSENSITIVE);

    @Override
    public Object intercept(Invocation invocation) throws Throwable {
        // 获取StatementHandler
        StatementHandler statementHandler = (StatementHandler) invocation.getTarget();
        
        // 获取BoundSql
        BoundSql boundSql = statementHandler.getBoundSql();
        String sql = boundSql.getSql();
        
        // 获取表索引
        String tbKey = DBContextHolder.getTBKey();
        if (null != tbKey && !tbKey.isEmpty()) {
            // 修改SQL中的表名
            sql = sql.replaceAll("(from|into|update|FROM|INTO|UPDATE)\\s+(\\w+)\\s+", 
                "$1 $2_" + tbKey + " ");
        }
        
        // 使用反射修改BoundSql的sql字段
        MetaObject metaObject = SystemMetaObject.forObject(boundSql);
        metaObject.setValue("sql", sql);
        
        return invocation.proceed();
    }

    @Override
    public Object plugin(Object target) {
        return Plugin.wrap(target, this);
    }

    @Override
    public void setProperties(Properties properties) {
        // 可以读取配置
    }
}
```

### 优化：处理表别名

```java
// 更精确的正则表达式
private Pattern pattern = Pattern.compile(
    "(from|into|update|FROM|INTO|UPDATE)\\s+(\\w+)(\\s+\\w+)?\\s+", 
    Pattern.CASE_INSENSITIVE
);

// 替换逻辑
Matcher matcher = pattern.matcher(sql);
StringBuffer sb = new StringBuffer();
while (matcher.find()) {
    String tableName = matcher.group(2);
    String alias = matcher.group(3);
    matcher.appendReplacement(sb, matcher.group(1) + " " + tableName + "_" + tbKey + 
        (alias != null ? alias : "") + " ");
}
matcher.appendTail(sb);
sql = sb.toString();
```

### 优化：处理JOIN语句

```java
// 处理JOIN语句中的表名
sql = sql.replaceAll("(join|JOIN)\\s+(\\w+)\\s+", 
    "$1 $2_" + tbKey + " ");
```

---

## 🎓 知识点拓展

### 拓展1：SQL替换的精确性

**问题场景**：
```sql
-- 原始SQL
SELECT * FROM user WHERE name = 'from table'

-- 简单replace会误替换
SELECT * FROM user_01 WHERE name = 'from_01 table'  -- 错误！
```

**解决方案**：使用正则表达式精确匹配

### 拓展2：处理各种SQL场景

**SELECT语句**：
```sql
SELECT * FROM user WHERE id = ?
-- 替换为
SELECT * FROM user_01 WHERE id = ?
```

**INSERT语句**：
```sql
INSERT INTO user (name) VALUES (?)
-- 替换为
INSERT INTO user_01 (name) VALUES (?)
```

**UPDATE语句**：
```sql
UPDATE user SET name = ? WHERE id = ?
-- 替换为
UPDATE user_01 SET name = ? WHERE id = ?
```

**DELETE语句**：
```sql
DELETE FROM user WHERE id = ?
-- 替换为
DELETE FROM user_01 WHERE id = ?
```

### 拓展3：性能优化

**缓存Pattern对象**：
```java
private static final Pattern PATTERN = Pattern.compile("...");
```

**避免重复编译正则表达式**

---

## ✅ 今日检查清单

- [ ] 完善了DynamicMybatisPlugin插件
- [ ] 优化了SQL替换逻辑
- [ ] 处理了表别名和JOIN语句
- [ ] 处理了各种SQL场景
- [ ] 完成了拓展阅读

---

## 🎯 明日预告

明天我们将完善自动配置类：
- 完善DataSourceAutoConfig
- 处理数据源创建
- 处理配置读取

---

## 💡 思考题

1. 如何精确匹配SQL中的表名？
2. 如何处理表别名？
3. 如何优化SQL替换的性能？

---

## 📚 参考资源

- [MyBatis插件开发](https://mybatis.org/mybatis-3/zh/configuration.html#plugins)
- [正则表达式教程](https://www.runoob.com/regexp/regexp-tutorial.html)
- [SQL解析](https://github.com/alibaba/druid/wiki/SQL-Parser)