Parallel File Downloader (Java)

Project Structure

• Main.java: entry point that triggers a download.
• FileDownloader.java: orchestrates the download (HEAD request, chunk calculation, scheduling parallel downloads, combining results).
• ChunkDownloader.java: Callable<byte[]> that downloads one specific byte range.
• FileDownloaderTest.java: unit tests for file size detection, chunk calculation, and chunk combination.

How the Downloader Works

1. Discover file size (HEAD request)
   FileDownloader#getFileSize sends an HTTP HEAD request to the provided URL and reads the Content-Length header to determine the total size of the file in bytes. The server should return Accept-Ranges: bytes, indicating support for range requests.

2. Split into chunks
   Based on the file size and configured number of threads, FileDownloader#calculateChunks divides the file into contiguous ranges [start, end] so that every byte in the file is covered exactly once and there are no gaps or overlaps.

3. Download chunks in parallel
   FileDownloader#download creates a fixed-size thread pool and submits a ChunkDownloader task for each range. Each ChunkDownloader:

   • sends an HTTP GET request to the same URL
   • adds the header Range: bytes=startByte-endByte
   • returns the response body as a byte[]

4. Combine chunks into the final file
   Once all futures complete, FileDownloader#combineChunks concatenates the byte[] results in the correct order and writes the final byte array to the output file on disk.

Running Unit Tests

The project includes unit tests that validate:

• reading Content-Length from a HEAD response (getFileSize)
• splitting a file size into correct chunks (calculateChunks)
• correct concatenation of byte arrays (combineChunks)
• error handling when the server returns a non-200 status