(Java) HTTP Download in Parallel with Simultaneous Range Requests

Demonstrates how to download a large file with parallel simultaneous requests, where each request downloads a segment (range) of the remote file.

Chilkat Java Downloads Java Libs for Windows, MacOS, Linux, Alpine Linux, Solaris Java Libs for Android

import com.chilkatsoft.*;

public class ChilkatExample {

  static {
    try {
        System.loadLibrary("chilkat");
    } catch (UnsatisfiedLinkError e) {
      System.err.println("Native code library failed to load.\n" + e);
      System.exit(1);
    }
  }

  public static void main(String argv[])
  {
    // This requires the Chilkat API to have been previously unlocked.
    // See Global Unlock Sample for sample code.

    CkHttp http = new CkHttp();
    boolean success;

    // First get the size of the file to be downloaded.
    String url = "https://www.chilkatsoft.com/hamlet.xml";

    CkHttpResponse resp = http.GetHead(url);
    if (http.get_LastMethodSuccess() == false) {
        System.out.println(http.lastErrorText());
        return;
        }

    int remoteFileSize = resp.get_ContentLength();

    System.out.println("Downloading " + remoteFileSize + " bytes...");

    // Let's download in 4 chunks.
    // (the last chunk will be whatever remains after the 1st 3 equal sized chunks)
    int chunkSize = remoteFileSize / 4;

    // The Range header is used to download a range from a resource
    // Range: bytes=<range-start>-<range-end>
    // or
    // Range: bytes=<range-start>-

    // We're writing code this way for clarity..
    CkHttp http1 = new CkHttp();
    CkHttp http2 = new CkHttp();
    CkHttp http3 = new CkHttp();
    CkHttp http4 = new CkHttp();

    CkStringBuilder sbRange = new CkStringBuilder();
    sbRange.SetString("bytes=<range-start>-<range-end>");
    int numReplaced = sbRange.ReplaceI("<range-start>",0);
    numReplaced = sbRange.ReplaceI("<range-end>",chunkSize - 1);
    System.out.println(sbRange.getAsString());
    http1.SetRequestHeader("Range",sbRange.getAsString());

    sbRange.SetString("bytes=<range-start>-<range-end>");
    numReplaced = sbRange.ReplaceI("<range-start>",chunkSize);
    numReplaced = sbRange.ReplaceI("<range-end>",2 * chunkSize - 1);
    System.out.println(sbRange.getAsString());
    http2.SetRequestHeader("Range",sbRange.getAsString());

    sbRange.SetString("bytes=<range-start>-<range-end>");
    numReplaced = sbRange.ReplaceI("<range-start>",2 * chunkSize);
    numReplaced = sbRange.ReplaceI("<range-end>",3 * chunkSize - 1);
    System.out.println(sbRange.getAsString());
    http3.SetRequestHeader("Range",sbRange.getAsString());

    sbRange.SetString("bytes=<range-start>-");
    numReplaced = sbRange.ReplaceI("<range-start>",3 * chunkSize);
    System.out.println(sbRange.getAsString());
    http4.SetRequestHeader("Range",sbRange.getAsString());

    // Start each range download
    CkTask task1 = http1.DownloadAsync(url,"qa_output/chunk1.dat");
    task1.Run();

    CkTask task2 = http2.DownloadAsync(url,"qa_output/chunk2.dat");
    task2.Run();

    CkTask task3 = http3.DownloadAsync(url,"qa_output/chunk3.dat");
    task3.Run();

    CkTask task4 = http4.DownloadAsync(url,"qa_output/chunk4.dat");
    task4.Run();

    // Wait for the downloads to complete.
    int numLive = 4;
    while (numLive > 0) {
        numLive = 0;
        if (task1.get_Live() == true) {
            numLive = numLive+1;
            }

        if (task2.get_Live() == true) {
            numLive = numLive+1;
            }

        if (task3.get_Live() == true) {
            numLive = numLive+1;
            }

        if (task4.get_Live() == true) {
            numLive = numLive+1;
            }

        if (numLive > 0) {
            // SleepMs is a convenience method to cause the caller to sleep for N millisec.
            // It does not cause the given task to sleep..
            task1.SleepMs(10);
            }

        }

    // All should be downloaded now..
    // Examine the result of each Download.
    int numErrors = 0;
    if (task1.GetResultBool() == false) {
        System.out.println(task1.resultErrorText());
        numErrors = numErrors+1;
        }

    if (task2.GetResultBool() == false) {
        System.out.println(task2.resultErrorText());
        numErrors = numErrors+1;
        }

    if (task3.GetResultBool() == false) {
        System.out.println(task3.resultErrorText());
        numErrors = numErrors+1;
        }

    if (task4.GetResultBool() == false) {
        System.out.println(task4.resultErrorText());
        numErrors = numErrors+1;
        }

    if (numErrors > 0) {

        return;
        }

    // All downloads were successful.
    // Compose the file from the parts.
    CkFileAccess fac = new CkFileAccess();
    success = fac.ReassembleFile("qa_output","chunk","dat","qa_output/hamlet.xml");
    if (success == false) {
        System.out.println(fac.lastErrorText());
        }
    else {
        System.out.println("Success.");
        }

    // Let's download in the regular way, and then compare files..
    success = http.Download(url,"qa_output/hamletRegular.xml");

    // Compare files.
    boolean bSame = fac.FileContentsEqual("qa_output/hamlet.xml","qa_output/hamletRegular.xml");
    System.out.println("bSame = " + bSame);
  }
}