private long getContentLength(HttpURLConnection connection) {
        String contentLengthValue = connection.getHeaderField("Content-Length");
        return contentLengthValue == null ? -1 : Long.parseLong(contentLengthValue);
    }
{@link Source} that uses http resource as source for {@link ProxyCache}.
VideoCache/src/main/java/com/yc/videocache/HttpUrlSource.java