public static void main(String[] args) {
    String str = "[ {\"123\":\"321\"} ]";
    String str2 = "[{\"123\":\"321\"}]";
    JSONArray array = JSON.parseArray(str);
    System.out.println(JSON.isValidArray(str));
    System.out.println(JSON.isValidArray(str2));
}

输出

false
true