无法从java中的URL下载文件 [英] Cannot download file from URL in java

问题描述

我正在制作一个从URL下载文件的程序。下载始终启动，但尚未完成。例如，如果文件的大小是3 MB，程序只下载一半，所以我无法打开下载的文件。但是程序表示该文件已成功下载。

  public class FileDownloader {
 
 public static void main String [] args）throws IOException {
 
 InputStream fileIn; 
 FileOutputStream fileOut; 
扫描仪s =新扫描仪（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（输入文件名：）; 
 String fileStr = s.nextLine（）; 
 fileOut = new FileOutputStream（fileStr）; 
 
 while（fileIn.read（）！= -1）{
 fileOut.write（fileIn.read（））; 
} 
 System.out.println（File is downloaded）; 
} 
} 
  

那么我该如何解决呢？应该使用另一种方式下载？

解决方案

您正在丢失每个备用的字节

  while（fileIn.read（）！= -1）{//第一次读
 fileOut。写（fileIn.read（））; //第二读 - 第一写
} 
  

你正在阅读两次，只写一次。

您需要做的是

  int x; 
 while（（x = fileIn.read（））！= -1）{//第一次读取
 fileOut.write（x）; //第一个写
} 
  

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; 
扫描仪s =新扫描仪（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（输入文件名：）; 
 String fileStr = s.nextLine（）; 
 fileOut = new FileOutputStream（fileStr）; 
 
 int x; 
 while（（x = fileIn.read（））！= -1）{
 fileOut.write（x）; 
} 
 System.out.println（File is downloaded）; 
 
} 
  

I'm making a program that will download files from URL. The downloading always starts, but it is not completed. For example, if file's size is 3 MB, program download only half of that so I cannot open the downloaded file. But program says that file is downloaded succesfully.

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");
    }
}

So how can I solve it? Should use another way to download?

解决方案

You are losing every alternate bytedue to

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

You are reading twice and writing only once.

What you need to do is

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

Here is your complete code

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");

}

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