Multithreaded File Copy takes more time 1.5 times than single thread.

Question

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.nio.channels.FileChannel;
public class TestMulti implements Runnable {
     public static Thread Th1;
     public static Thread Th2;
     String str = null;
     static int seqNumber = 1000000000;
     public static void main(String args[]) {
          Th1 = new Thread(new TestMulti("1_1"));
          Th2 = new Thread(new TestMulti("1_2"));
          Th1.start();
          Th2.start();
          try {
               Th1.join();
               Th2.join();
          } catch (Exception e) {
               e.printStackTrace();
     public TestMulti(String str) {
          this.str = str;
     public void run() {
          File f = new File("C:/Songs2/" + str);
          File files[] = f.listFiles();
          String fileName = "";
          String seqName = "";
          String seq = "";
          int sequenceNo = 0;
          try {
               for (int j = 0; j < files.length; j++) {
                    File musicFiles[] = files[j].listFiles();
                    for (int k = 0; k < musicFiles.length; k++) {
                         seq = "18072006";
                         seqName = seq + seqNumber;
                         sequenceNo = 10000 + seqNumber % 100;
                         seqNumber = seqNumber + 1;
                         fileName = musicFiles[k].getName();
                         String fileExt = fileName.substring(fileName.length() - 3,fileName.length());
                         String targetFile = "C:/Songs1/" + sequenceNo;
                         File fi = new File(targetFile);
                         if (!fi.exists()) { fi.mkdir(); }
                         targetFile = "C:/Songs1/" + sequenceNo + "/" + seqName+ "." + fileExt;
                         FileInputStream fin = new FileInputStream(musicFiles[k]);
                         FileChannel fcin = fin.getChannel();
                         FileOutputStream fout = new FileOutputStream(targetFile);
                         FileChannel fcout = fout.getChannel();
                         fcin.transferTo(0, fcin.size(), fcout);
                         fout.flush();
                         fcout.close();
                         fcin.close();
                         fout.close();
                         fin.close();
          } catch (Exception e) {
               e.printStackTrace();
Multithreaded File Copy takes more time 1.5 times than single thread.
Is there any issue with this code. Please help me.

Answer 1

If all of your threads are doing CPU-intensive work, or all are doing I/O to the same interface (for example, writing to the same physical disk), then multithreading would not be expected to help you.
Multithreading does not magically make your CPU able to do more work per unit time than it could otherwise.
Multithreading does not magically make your network interface or disk controller able to pump more bytes through than it could otherwise.
Where multithreading helps (some or all of this has already been mentioned):
* When you have multiple, independent CPU-bound tasks AND multiple CPUs available on which to execute them.
* When you have tasks that involve a mix of CPU-bound and I/O-bound work. The CPU-bound stuff can crank while the I/O-bound stuff waits for bytes to be written or read, thus making use of what would otherwise be CPU "dead time."
What you're doing does not fit either of those scenarios. Copying a file is pure I/O. If the source and destination file are on the same phsyical disk or controller, adding threads only adds overhead with no real possibility to do more work per unit time.
If your source and destination are on different disks or controllers, then it's possible that you could get some benefit from multithreading. While one thread is waiting for bytes to be written to the target disk, the other thread can be reading from the source disk.