疑问：为什么mybatis-plus可以根据dao中的参数是否包含page对象，给sql拼上对应的limit语句
==ps.这里研究的是最新版本(3.5.11)代码==

1.配置

当执行sql的时候mybatis-plus会对所有SQL语句进行拦截并做各种判断与附加操作(这里只添加分页拦截器),会进入到Mybatis-Plus全局拦截器.

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@Bean  
public MybatisPlusInterceptor mybatisPlusInterceptor() {  
    MybatisPlusInterceptor interceptor = new MybatisPlusInterceptor();  
    interceptor.addInnerInterceptor(new PaginationInnerInterceptor(DbType.MYSQL)); // 如果配置多个插件, 切记分页最后添加  
    // 如果有多数据源可以不配具体类型, 否则都建议配上具体的 DbType    return interceptor;  
}

2.全局拦截器

主要关注下面这段代码
可以看出,当前sql执行时,被拦截器拦截,发现是查询语句,就会先执行winllDoQuery方法,其次做完在执行 beforeQuery
这两个方法的实现在具体的插件拦截器中，如下面的分页拦截器

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for (InnerInterceptor query : interceptors) {  
    if (!query.willDoQuery(executor, ms, parameter, rowBounds, resultHandler, boundSql)) {  
        return Collections.emptyList();  
    }  
    query.beforeQuery(executor, ms, parameter, rowBounds, resultHandler, boundSql);  
}

3.分页拦截器

3.1 willDoQuery

先count所有数据，将总数存入分页参数中

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@Override  
public boolean willDoQuery(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {  
    IPage<?> page = ParameterUtils.findPage(parameter).orElse(null);  
    if (page == null || page.getSize() < 0 || !page.searchCount() || resultHandler != Executor.NO_RESULT_HANDLER) {  
        return true;  
    }  
  
    BoundSql countSql;  
    MappedStatement countMs = buildCountMappedStatement(ms, page.countId());  
    if (countMs != null) {  
        countSql = countMs.getBoundSql(parameter);  
    } else {  
        countMs = buildAutoCountMappedStatement(ms);  
        String countSqlStr = autoCountSql(page, boundSql.getSql());  
        PluginUtils.MPBoundSql mpBoundSql = PluginUtils.mpBoundSql(boundSql);  
        countSql = new BoundSql(countMs.getConfiguration(), countSqlStr, mpBoundSql.parameterMappings(), parameter);  
        PluginUtils.setAdditionalParameter(countSql, mpBoundSql.additionalParameters());  
    }  
  
    CacheKey cacheKey = executor.createCacheKey(countMs, parameter, rowBounds, countSql);  
    List<Object> result = executor.query(countMs, parameter, rowBounds, resultHandler, cacheKey, countSql);  
    long total = 0;  
    if (CollectionUtils.isNotEmpty(result)) {  
        // 个别数据库 count 没数据不会返回 0        Object o = result.get(0);  
        if (o != null) {  
            total = Long.parseLong(o.toString());  
        }  
    }  
    page.setTotal(total);  
    return continuePage(page);  
}

3.2 beforeQuery

判断数据操作是否有分页参数

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IPage<?> page = ParameterUtils.findPage(parameter).orElse(null);

处理order拼接

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List<OrderItem> orders = page.orders();  
if (CollectionUtils.isNotEmpty(orders)) {  
    addOrdered = true;  
    buildSql = this.concatOrderBy(buildSql, orders);  
}

处理分页操作

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handlerLimit(page, _limit);  
IDialect dialect = findIDialect(executor);  
  
final Configuration configuration = ms.getConfiguration();  
DialectModel model = dialect.buildPaginationSql(buildSql, page.offset(), page.getSize());  
PluginUtils.MPBoundSql mpBoundSql = PluginUtils.mpBoundSql(boundSql);  
  
List<ParameterMapping> mappings = mpBoundSql.parameterMappings();  
Map<String, Object> additionalParameter = mpBoundSql.additionalParameters();  
model.consumers(mappings, configuration, additionalParameter);  
mpBoundSql.sql(model.getDialectSql());  
mpBoundSql.parameterMappings(mappings);

未完待续。。。。