使用的版本1.2.12

Map<String, String> map = new HashMap<>();
        map.put("a", null);
        map.put("b", "1");

        String x = JSON.toJSONString(map, SerializerFeature.WriteMapNullValue);
        System.out.println(x);

        JSONObject jsonObject = JSON.parseObject(x);
        System.out.println(JSONPath.contains(jsonObject, "$.a") + "\t" + jsonObject.containsKey("a"));
        System.out.println(JSONPath.contains(jsonObject, "$.b") + "\t" + jsonObject.containsKey("b"));

Output:
{"b":"1","a":null}
false true
true true