library/src/main/java/com/liulishuo/filedownloader/util/FileDownloadUtils.java
public static long findInstanceLengthForTrial(FileDownloadConnection connection) {
        long length = findInstanceLengthFromContentRange(connection);
        if (length < 0) {
            length = TOTAL_VALUE_IN_CHUNKED_RESOURCE;
            FileDownloadLog.w(FileDownloadUtils.class, "don't get instance length from"
                    + "Content-Range header");
        }
        // the response of HEAD method is not very canonical sometimes(it depends on server
        // implementation)
        // so that it's uncertain the content-length is the same as the response of GET method if
        // content-length=0, so we have to filter this case in here.
        if (length == 0 && FileDownloadProperties.getImpl().trialConnectionHeadMethod) {
            length = TOTAL_VALUE_IN_CHUNKED_RESOURCE;
        }

        return length;
    }
public static long findInstanceLengthForTrial(FileDownloadConnection connection) {
        long length = findInstanceLengthFromContentRange(connection);
        if (length < 0) {
            length = TOTAL_VALUE_IN_CHUNKED_RESOURCE;
            FileDownloadLog.w(FileDownloadUtils.class, "don't get instance length from"
                    + "Content-Range header");
        }
        // the response of HEAD method is not very canonical sometimes(it depends on server
        // implementation)
        // so that it's uncertain the content-length is the same as the response of GET method if
        // content-length=0, so we have to filter this case in here.
        if (length == 0 && FileDownloadProperties.getImpl().trialConnectionHeadMethod) {
            length = TOTAL_VALUE_IN_CHUNKED_RESOURCE;
        }

        return length;
    }
content-length.