An IDM like app in java

Question

I'm writing an small application like IDM in java. But this has has many Exceptions. This is the code of Downloader class which implements runnable and I want use it for multithreading.

public class Downloader implements Runnable{
private DataInputStream inputStream;
private byte[][] fileData;
private int index;
private int size;

public Downloader(DataInputStream inputStream, byte[][] fileData, int index, int size) {
    this.inputStream = inputStream;
    this.fileData = fileData;
    this.index = index;
    this.size = size;
}

public synchronized void run() {
    try{
        inputStream.skipBytes(index * size);
        for(int i= 0;i<size;i++){
            fileData[index][i] = inputStream.readByte();
            System.out.println("It works : " + index);
        }
    }
    catch(Exception e){
        System.out.println(e.getMessage());
    }
}}

and this is my main class

public class Main {

public static void main(String[] args) {
    String s;
    //Scanner input = new Scanner(System.in);
    //System.out.print("Enter file destination : ");
    //s = input.nextLine();
    s = "http://video.varzesh3.com/video/clip1/92/uclip/fun/gaf_6_borhani.mp4";
    URL url;
    URLConnection connection;
    DataInputStream inputStream;
    FileOutputStream outStream;
    byte[][] fileData;
    try{
        url = new URL(s);
        connection = url.openConnection();
        inputStream = new DataInputStream(connection.getInputStream());
        fileData = new byte[8][connection.getContentLength() / 4];
        int size = connection.getContentLength() / 4;
        Runnable d0 = new Downloader(inputStream, fileData, 0, size);
        Runnable d1 = new Downloader(inputStream, fileData, 1, size);
        Runnable d2 = new Downloader(inputStream, fileData, 2, size);
        Runnable d3 = new Downloader(inputStream, fileData, 3, size);            
        Thread thread0 = new Thread(d0);
        Thread thread1 = new Thread(d1);
        Thread thread2 = new Thread(d2);
        Thread thread3 = new Thread(d3);            
        thread0.start();
        thread1.start();
        thread2.start();
        thread3.start();            
        inputStream.close();
        String path = "C:\\Users\\NetTest\\Desktop\\test.mp4";
        outStream = new FileOutputStream(new File(path));
        outStream.write(fileData[0]);
        /*outStream.write(fileData[1]);
        outStream.write(fileData[2]);
        outStream.write(fileData[3]);
        outStream.write(fileData[4]);
        outStream.write(fileData[5]);
        outStream.write(fileData[6]);
        outStream.write(fileData[7]);*/
        outStream.close();
    }
    catch(Exception e){
        System.out.println(e.getMessage());
    }
}}

but when I run it this happens

It works: 0
null
null
null
null

What should I do now?

I think you are missing the fact that, as soon as the first Thread reads it's bytes, those bytes are removed from the byte buffer. It's a Stream, not a buffer. Just like with water streams, once something has gone by, it's gone forever. — CodeChimp, Jan 10 '14 at 21:02

score 1 · Answer 1 · edited May 23 '17 at 12:12

There are 2 Problems in your code.

At the current state you close() the InputStream from which all Thread try to read directly after you started them (-> while they are running). To solve this Problem you can call the join() Method of the Thread class. In your case your'd have to call it for all 4 Threads to make sure they are finished.
If I understand it correctly you want to seperate the download File into 4 parts downloading at the same time.

To do this you need 4 independent InputStreams. (Currently you are using ONE [See also: Java Object Copying])

So to change this your code would look something like this:

public class Downloader implements Runnable{
    private byte[][] fileData;
    private int index;
    private int size;
    private URL url;

    public Downloader(URL url, byte[][] fileData, int index, int size) {
    this.fileData = fileData;
    this.index = index;
    this.size = size;
    this.url = url;
    }

    public synchronized void run() {
    try{
        URLConnection connection = url.openConnection();
        DataInputStream inputStream = new DataInputStream(connection.getInputStream());
        inputStream.skipBytes(index * size);
        for(int i= 0;i<size;i++){
        fileData[index][i] = inputStream.readByte();
        System.out.println("It works : " + index);
        }
     }catch(Exception e){
        System.out.println(e.getMessage());
     }
     }
 }

An IDM like app in java

1 Answers1