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
<mapper namespace="com.example.UserMapper">
<select id="selectAllUsers" resultType="com.example.User">
SELECT * FROM user
</select>
</mapper>
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
@Bean
public 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<InnerInterceptor> 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
