Java中无法从URL下载文件

Question

我在制作一个程序，可以从URL下载文件。下载总是开始了，但没有完成。例如，如果文件大小为3 MB，程序只下载一半，所以我无法打开已下载的文件。但程序显示文件已成功下载。

public class FileDownloader {

    public static void main (String [] args) throws IOException {

        InputStream fileIn;
        FileOutputStream fileOut;
        Scanner s = new Scanner(System.in);

        System.out.println("Enter URL: ");
        String urlStr = s.nextLine();

        URL url = new URL(urlStr);
        URLConnection urlConnect = url.openConnection();
        fileIn = urlConnect.getInputStream();

        System.out.println("Enter file name: ");
        String fileStr = s.nextLine();
        fileOut = new FileOutputStream(fileStr);

        while (fileIn.read() != -1) {   
            fileOut.write(fileIn.read());
        }
        System.out.println("File is downloaded");
    }
}

那我该怎么解决呢？需要使用其他方式下载吗？

- Azamat Salamat

---

可能是重复的问题：如何使用Java从互联网下载并保存文件？ - Robin Green

Answer 1

由于某些原因，您正在丢失每个交替的字节

    while (fileIn.read() != -1) {     //1st read
        fileOut.write(fileIn.read());     //2nd read - 1st write
    }

你需要做的是阅读两次，只写一次。

您需要做的是

    int x;
    while ((x = fileIn.read()) != -1) {   //1st read
        fileOut.write(x);     //1st write
    }

这是您的完整代码

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.URL;
import java.net.URLConnection;
import java.util.Scanner;

public class FileDownloader {

    public static void main(String[] args) throws IOException {

        InputStream fileIn;
        FileOutputStream fileOut;
        Scanner s = new Scanner(System.in);

        System.out.println("Enter URL: ");
        String urlStr = s.nextLine();

        URL url = new URL(urlStr);
        URLConnection urlConnect = url.openConnection();
        fileIn = urlConnect.getInputStream();

        System.out.println("Enter file name: ");
        String fileStr = s.nextLine();
        fileOut = new FileOutputStream(fileStr);

        int x;
        while ((x = fileIn.read()) != -1) {
            fileOut.write(x);
        }
        System.out.println("File is downloaded");

}

- Shrinivas Shukla

Answer 2

您可以使用以下代码高效地下载大文件。

 public static void main(String[] args) throws IOException {
    InputStream in = null;
    FileOutputStream out = null;
    try {
        System.out.println("Starting download");
        long t1 = System.currentTimeMillis();
        URL url = new URL(args[0]);// or you can hard code the URL
        // Open the input and out files for the streams
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        in = conn.getInputStream();
        out = new FileOutputStream(args[1]);//// or you can hard code the filename
        // Read data into buffer and then write to the output file
        byte[] buffer = new byte[8192];
        int bytesRead;
        while ((bytesRead = in.read(buffer)) != -1) {
            out.write(buffer, 0, bytesRead);
        }
        long t2 = System.currentTimeMillis();
        System.out.println("Time for download & save file in millis:"+(t2-t1));
    } catch (Exception e) {
        // Display or throw the error
        System.out.println("Erorr while execting the program: "
                + e.getMessage());
    } finally {
        // Close the resources 
        if (in != null) {
            in.close();
        }
        if (out != null) {
            out.close();
        }
    }

}

- Ravindra babu

Answer 3

只需简单地使用这个：

import org.apache.commons.io.FileUtils;
import java.net.URL;
String path =  "F:/"
String fileName =  "song"
FileUtils.copyURLToFile(myUrl, new File(path + fileName + ".mp3"));

- Jayhello