lingochamp/FileDownloader · error · IllegalAccessException

can't using multi-download when the output stream can't…

Error message

can't using multi-download when the output stream can't support seek

What it means

Thrown when FileDownloader attempts a multi-connection (ranged) download but the destination output stream does not support seek (random-access positioning). Multi-download requires jumping to each connection's offset, which is impossible on non-seekable streams, so the library rejects the configuration up front via FileDownloadGiveUpRetryException rather than producing corrupt output.

Solutions

  1. Download to a seekable destination such as a file path backed by RandomAccessFile instead of a streaming/wrapper output stream
  2. Set the task to single-connection download (e.g. set the download to non-multi-thread/one connection) so no seeking is required
  3. If a custom output stream is used, implement seek support so ranged writes can be positioned correctly
  4. Verify the connection count configuration is not forcing multi-download when a plain stream target is given
Defensive patterns

Strategy: validation

When it happens

Trigger: Thrown at library/src/main/java/com/liulishuo/filedownloader/download/FetchDataTask.java:120 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of lingochamp/FileDownloader@6237a8cac1 (2026-09-08). Data as JSON: /api/errors/50c86afa80cbaa66. Report an issue: GitHub.

Appendix: source

Thrown at library/src/main/java/com/liulishuo/filedownloader/download/FetchDataTask.java:120

                range = FileDownloadUtils.formatString("range[%d-%d)", currentOffset, endOffset);
            }
            throw new FileDownloadGiveUpRetryException(FileDownloadUtils.
                    formatString("require %s with contentLength(%d), but the "
                                    + "backend response contentLength is %d on "
                                    + "downloadId[%d]-connectionIndex[%d], please ask your backend "
                                    + "dev to fix such problem.",
                            range, this.contentLength, contentLength, downloadId, connectionIndex));
        }

        final long fetchBeginOffset = currentOffset;
        // start fetch
        InputStream inputStream = null;
        FileDownloadOutputStream outputStream = null;

        try {
            final boolean isSupportSeek = CustomComponentHolder.getImpl().isSupportSeek();
            if (hostRunnable != null && !isSupportSeek) {
                throw new IllegalAccessException(
                        "can't using multi-download when the output stream can't support seek");
            }

            this.outputStream = outputStream = FileDownloadUtils.createOutputStream(path);
            if (isSupportSeek) {
                outputStream.seek(currentOffset);
            }

            if (FileDownloadLog.NEED_LOG) {
                FileDownloadLog.d(this, "start fetch(%d): range [%d, %d), seek to[%d]",
                        connectionIndex, startOffset, endOffset, currentOffset);
            }

            inputStream = connection.getInputStream();

            byte[] buff = new byte[BUFFER_SIZE];

            if (paused) return;

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