java - 类似 IDM 的 java 应用程序

Question

我正在用java编写一个像IDM这样的小型应用程序。但这有很多异常(exception)。这是Downloader类的代码，它实现了runnable，我想将它用于多线程。

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());
    }
}}

这是我的主课

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());
    }
}}

但是当我运行它时，就会发生这种情况

It works: 0
null
null
null
null

我现在应该做什么？

Answer 1

您的代码中有 2 个问题。

1. 在当前状态下，您close() InputStream，所有Thread在启动它们后尝试直接读取它们( -> 当他们运行时)。为了解决这个问题，你可以调用Thread类的join()方法。在您的情况下，您必须为所有 4 个线程调用它以确保它们完成。

2. 如果我理解正确的话，您希望将下载的文件分成4个部分同时下载。

为此，您需要 4 个独立的 InputStreams。 (目前您正在使用 ONE [另请参阅:Java Object Copying ])

因此，要更改此设置，您的代码将如下所示:

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());
     }
     }
 }