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一般我们直接通过递归的方式就返回一个三级联动效率很高

大家可以参考这篇文章


省市县三级数据联动 -Java


我这边呢直接用三层for循环解决的速度很慢的400条1.73秒

为什么不用上面那个呢：因为是递归的思想最后一层还有多返回一个cityVOList[]，前端这边node.js版本太低解决不了这个问题，所以就丢给后端处理了

[
    {
        "code": "1",
        "nameZh": "中国",
        "name": "中国",
        "nameEn": "China",
        "parentId": "0",
        "cityVOList": [
            {
                "code": "4",
                "nameZh": "安徽",
                "name": "安徽",
                "nameEn": "Anhui",
                "parentId": "1",
                "cityVOList": [
                    {
                        "code": "53",
                        "nameZh": "安庆市",
                        "name": "安庆市",
                        "nameEn": "Anqing",
                        "parentId": "4",
                        "cityVOList": []
                    }
                ]
            }
        ]
    }
]

		// 获取第一层
        List<Map<String, Object>> maps1 = dropdownMapper.city1(map);
        List<Map<String, Object>> listMap = new ArrayList<>();
        for (Map<String, Object> map1 : maps1) {
            Map<String,Object> list = new HashMap<>();
            String id = (String) map1.get("code");
            list.put("code",map1.get("code"));
            list.put("parentId",map1.get("parentId"));
            list.put("name",map1.get("name"));
            list.put("nameZh",map1.get("nameZh"));
            list.put("nameEn",map1.get("nameEn"));
            // 获取第二层
            List<Map<String, Object>> maps2 = dropdownMapper.city2(id);
            List<Map<String,Object>> list1 = new ArrayList<>();
            for (Map<String, Object> map2 : maps2) {
                String id2 = (String) map2.get("code");
                Map<String,Object> list4 = new HashMap<>();
                list4.put("code",map2.get("code"));
                list4.put("parentId",map2.get("parentId"));
                list4.put("name",map2.get("name"));
                list4.put("nameZh",map2.get("nameZh"));
                list4.put("nameEn",map2.get("nameEn"));
                list1.add(list4);
                // 获取第三层
                List<Map<String, Object>> maps3 = dropdownMapper.city3(id2);
                List<Map<String,Object>> list3 = new ArrayList<>();
                for (Map<String, Object> map3 : maps3) {
                    Map<String,Object> list5 = new HashMap<>();
                    list5.put("code",map3.get("code"));
                    list5.put("parentId",map3.get("parentId"));
                    list5.put("name",map3.get("name"));
                    list5.put("nameZh",map3.get("nameZh"));
                    list5.put("nameEn",map3.get("nameEn"));
                    list3.add(list5);
                }
                list4.put("cityVOList",list3);
            }
            list.put("cityVOList",list1);
            listMap.add(list);
        }

[
    {
        "code": "1",
        "nameZh": "中国",
        "name": "中国",
        "nameEn": "China",
        "parentId": "0",
        "cityVOList": [
            {
                "code": "4",
                "nameZh": "安徽",
                "name": "安徽",
                "nameEn": "Anhui",
                "parentId": "1",
                "cityVOList": [
                    {
                        "code": "53",
                        "nameZh": "安庆市",
                        "name": "安庆市",
                        "nameEn": "Anqing",
                        "parentId": "4"
                    }
                ]
            }
        ]
    }
]

处理思路

返回的为一个[name,code,city1[name,code,city2[]]]三层嵌套，进行三层for循环第一层为查询的顶级父类，拿到父类的code然后去查询该父类下面的数据，拿到后再进行查询一共三次当然这里看你是几级联动。