LocalDateTime 在秒为0时不显示

LocalDateTime 重写了toString方法，在注释中说明了这种情况，当为0时会省略。

/**
     * Outputs this time as a {@code String}, such as {@code 10:15}.
     * <p>
     * The output will be one of the following ISO-8601 formats:
     * <ul>
     * <li>{@code HH:mm}</li>
     * <li>{@code HH:mm:ss}</li>
     * <li>{@code HH:mm:ss.SSS}</li>
     * <li>{@code HH:mm:ss.SSSSSS}</li>
     * <li>{@code HH:mm:ss.SSSSSSSSS}</li>
     * </ul>
     * The format used will be the shortest that outputs the full value of
     * the time where the omitted parts are implied to be zero.
     *
     * @return a string representation of this time, not null
     */
    @Override
    public String toString() {
        StringBuilder buf = new StringBuilder(18);
        int hourValue = hour;
        int minuteValue = minute;
        int secondValue = second;
        int nanoValue = nano;
        buf.append(hourValue < 10 ? "0" : "").append(hourValue)
            .append(minuteValue < 10 ? ":0" : ":").append(minuteValue);
        if (secondValue > 0 || nanoValue > 0) {
            buf.append(secondValue < 10 ? ":0" : ":").append(secondValue);
            if (nanoValue > 0) {
                buf.append('.');
                if (nanoValue % 1000_000 == 0) {
                    buf.append(Integer.toString((nanoValue / 1000_000) + 1000).substring(1));
                } else if (nanoValue % 1000 == 0) {
                    buf.append(Integer.toString((nanoValue / 1000) + 1000_000).substring(1));
                } else {
                    buf.append(Integer.toString((nanoValue) + 1000_000_000).substring(1));
                }
            }
        }
        return buf.toString();
    }

解决的办法：

String UTC_FORMATTER_PATTERN = "yyyy-MM-dd'T'HH:mm:ss";
DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern(UTC_FORMATTER_PATTERN);
System.out.println(dateTimeFormatter.format(LocalDateTime.parse("2022-01-01T00:00:00")));