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自定义Mybatis-plus插件(限制最大查询数量)需求背景

​一次查询如果结果返回太多（1万或更多），往往会导致系统性能下降，有时更会内存不足，影响系统稳定性，故需要做限制。

解决思路

1.经分析最后决定，应限制一次查询返回的最大结果数量不应该超出1万，对于一次返回结果大于限制的时候应该抛出异常，而不应该截取(limit 10000)最大结果(结果需求不匹配)。

2.利用mybatis拦截器技术，统一拦截sql,并真对大结果的查询先做一次count查询。

步骤一1.1 定义拦截器PreCheckBigQueryInnerInterceptor

public class PreCheckBigQueryInnerInterceptor implements InnerInterceptor {}

1.2 重写willDoQuery方法

public boolean willDoQuery(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {        // 解析sql        Statement stmt = CCJSqlParserUtil.parse(boundSql.getSql());        if (stmt instanceof Select) {            PlainSelect selectStmt = (PlainSelect) ((Select) stmt).getSelectBody();            if (Objects.nonNull(selectStmt.getLimit())) {                //包含limit查询                return true;            }            for (SelectItem selectItem : selectStmt.getSelectItems()) {                //计数查询 count();                SelectExpressionItem selectExpressionItem = (SelectExpressionItem) selectItem;                if (selectExpressionItem.getExpression() instanceof Function) {                    //包含function查询                    return true;                }            }            Long aLong = doQueryCount(executor, ms, parameter, rowBounds, resultHandler, boundSql);            if (aLong == 0L) {                return false;            }            if (aLong > 20) {                throw new RuntimeException("单个查询结果大于20条!!!");            }        }        return true;    }

1.3 代码解析1.3.1 利用CCJSqlParserUtil解析sql，并判断sql类型，只对Select的SQL拦击.1.3.2 对于已有limit的sql查询,直接放行.1.3.3 对于包含function查询（例如count（1）计算，max（）...）,直接放行.1.3.4 否则判断为大结果查询，执行（doQueryCount）与查询数量.1.3.5 对于大于指定数量的结果，抛出异常.1.4 定义doQueryCount方法

private Long doQueryCount(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {        MappedStatement countMs = buildAutoCountMappedStatement(ms);        String countSqlStr = autoCountSql(true, boundSql.getSql());        PluginUtils.MPBoundSql mpBoundSql = PluginUtils.mpBoundSql(boundSql);        BoundSql countSql = new BoundSql(countMs.getConfiguration(), countSqlStr, mpBoundSql.parameterMappings(), parameter);        PluginUtils.setAdditionalParameter(countSql, mpBoundSql.additionalParameters());        CacheKey cacheKey = executor.createCacheKey(countMs, parameter, rowBounds, countSql);        Object result = executor.query(countMs, parameter, rowBounds, resultHandler, cacheKey, countSql).get(0);        System.out.println(result);        return (result == null ? 0L : Long.parseLong(result.toString()));    }

代码解读：参考PaginationInnerInterceptor（mybatis-plus）分页插件1.4.1：构造MappedStatement对象buildAutoCountMappedStatement(ms)，MappedStatement相当于一个存储 SQL 语句、输入参数和输出结果映射等信息的封装体，它对应一条 SQL 语句，并包含了该 SQL 语句执行所需的所有信息。如下代码

          SELECT * FROM user   

注意：必须重新构造，不能直接使用入参中的ms

1.4.2：autoCountSql(true, boundSql.getSql()) 定义并优化计数查询语句

String.format("SELECT COUNT(1) FROM (%s) TOTAL", originalSql);

1.4.3: 执行查询executor.query步骤二1.1 注册拦截器PreCheckBigQueryInnerInterceptor

@Beanpublic MybatisPlusInterceptor mybatisPlusInterceptor() {    MybatisPlusInterceptor interceptor = new MybatisPlusInterceptor();    interceptor.addInnerInterceptor(new PaginationInnerInterceptor(DbType.MYSQL));//分页插件(Mybatis-plus)    interceptor.addInnerInterceptor(new BlockAttackInnerInterceptor());//防止全表更新(Mybatis-plus)    interceptor.addInnerInterceptor(new PreCheckBigQueryInnerInterceptor());//防止全表查询(自定义插件)    return interceptor;}

知识小结:MybatisPlusInterceptor

public class MybatisPlusInterceptor implements Interceptor {    @Setter    private List interceptors = new ArrayList<>();}

​ 他是基于mybatis的Interceptor接口做的拦截器,上文中我们 注册拦截器PreCheckBigQueryInnerInterceptor的拦截器其实添加到MybatisPlusInterceptor.interceptors集合中。

为啥重写willDoQuery见代码而不是beforeQuery

public Object intercept(Invocation invocation) throws Throwable {       ......                for (InnerInterceptor query : interceptors) {                    if (!query.willDoQuery(executor, ms, parameter, rowBounds, resultHandler, boundSql)) {                        return Collections.emptyList();                    }                    query.beforeQuery(executor, ms, parameter, rowBounds, resultHandler, boundSql);                }     ......        return invocation.proceed(); }

2.1 willDoQuery先于beforeQuery方法，且一定会执行

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