public abstract class AbstractDBNameInterceptor {

    /**
     * SqlSource 的裝飾器，作用是增強(qiáng)了 getBoundSql 方法，在基礎(chǔ)上增加了動(dòng)態(tài)分庫的邏輯
     */
    static class SqlSourceDecorator implements SqlSource {

        /**
         * SQL 字段名稱
         */
        private static final String SQL_FIELD_NAME = "sql";

        /**
         * 原本的 sql source
         */
        private final SqlSource sqlSource;

        /**
         * 裝飾器進(jìn)行封裝
         */
        public SqlSourceDecorator(SqlSource sqlSource) {
            this.sqlSource = sqlSource;
        }

        @Override
        public BoundSql getBoundSql(Object parameterObject) {
            try {
                // 先生成出未修改前的 SQL
                BoundSql boundSql = sqlSource.getBoundSql(parameterObject);
                // 獲取數(shù)據(jù)庫名
                String dbName = getSpecificDBName(parameterObject);
                // 有效才修改
                if (isValid(dbName)) {
                    // 生成需要修改完庫名的 SQL
                    String targetSQL = getRequiredSqlWithSpecificDBName(boundSql, dbName);
                    // 更新 SQL
                    updateSql(boundSql, targetSQL);
                }

                return boundSql;
            } catch (Exception e) {
                throw new RuntimeException(e);
            }
        }

        /**
         * 校驗(yàn)是否為有效庫名
         */
        private boolean isValid(String dbName) {
            return StringUtils.isNotEmpty(dbName) && !"null".equals(dbName);
        }

        /**
         * 獲取到我們想要的庫名的 SQL
         */
        private String getRequiredSqlWithSpecificDBName(BoundSql boundSql, String dbName) throws JSQLParserException {
            String originSql = boundSql.getSql();
            // 獲取所有的表名
            Set<String> tables = TablesNamesFinder.findTables(originSql);
            for (String table : tables) {
                originSql = originSql.replaceAll(table, dbName + "." + table);
            }
            return originSql;
        }

        /**
         * 修改 SQL
         */
        private void updateSql(BoundSql boundSql, String sql) throws NoSuchFieldException, IllegalAccessException {
            // 通過反射修改sql語句
            Field field = boundSql.getClass().getDeclaredField(SQL_FIELD_NAME);
            field.setAccessible(true);
            field.set(boundSql, sql);
        }
    }
    
    // ... 
}

public abstract class AbstractDBNameInterceptor {

    /**
     * SqlSource 字段名稱
     */
    private static final String SQL_SOURCE_FIELD_NAME = "sqlSource";

    /**
     * 執(zhí)行修改數(shù)據(jù)庫名的邏輯
     */
    protected Object updateDBName(Invocation invocation) throws Throwable {
        // 裝飾器裝飾 SqlSource
        decorateSqlSource((MappedStatement) invocation.getArgs()[0]);
        return invocation.proceed();
    }

    /**
     * 裝飾 SqlSource
     */
    private void decorateSqlSource(MappedStatement statement) throws NoSuchFieldException, IllegalAccessException {
        if (!(statement.getSqlSource() instanceof SqlSourceDecorator)) {
            Field sqlSource = statement.getClass().getDeclaredField(SQL_SOURCE_FIELD_NAME);
            sqlSource.setAccessible(true);
            sqlSource.set(statement, new SqlSourceDecorator(statement.getSqlSource()));
        }
    }
}

@Intercepts({
        @Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class}),
        @Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class, CacheKey.class, BoundSql.class})
})
public class SelectDBNameInterceptor extends AbstractDBNameInterceptor implements Interceptor {

    @Override
    public Object intercept(Invocation invocation) throws Throwable {
        return updateDBName(invocation);
    }
}