@Test
    public void test_json_bigint_2() throws Exception {
        String str = "12345678901234567890";
        BigInteger bigInt = new BigInteger(str);
        bigInt.longValue();
        String json = "{\"id\":" + str + "}";
        Object id = JSONReader.of(json
                        .toCharArray())
                .readObject()
                .get("id");

        assertEquals("12345678901234567890", id.toString());
    }
This mask is used to obtain the value of an int as if it were unsigned.
core/src/test/java/com/alibaba/fastjson2/read/BigIntTest.java