Java to download a file from internet

    There is an Online file (such as https://i-blog.csdnimg.cn/blog_migrate/d58c352ff5383460ed180137ba09132b.gif)，as below:

    

If you wan to download this picture,how can we grap and save to a directory?

ok,here have some ways as below:

The first way

 using the Java NIO,here is the main code snippet:

import java.net.*;
import java.nio.channels.*;
import java.io.*;
public class download {
	 public static void main(String[] args) {
	       
	        
	        try {
				URL website=new URL("https://i-blog.csdnimg.cn/blog_migrate/d58c352ff5383460ed180137ba09132b.gif");
				ReadableByteChannel rbc=Channels.newChannel(website.openStream());
				FileOutputStream fos = new FileOutputStream("C://information.html");
				fos.getChannel().transferFrom(rbc, 0, Long.MAX_VALUE);
				
			} catch (MalformedURLException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			} catch (IOException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
	    }
}

Using transferFrom() is potentially much more efficient than a simple loop that reads from the source channel and writes to this channel. Many operating systems can transfer bytes directly from the source channel into the filesystem cache without actually copying them.

Check more about it here.

Note: The third parameter in transferFrom is the maximum number of bytes to transfer. Integer.Max_VALUE will transfer at most 2^31 bytes, Long.MAX_VALUE will allow at most 2^63 bytes (larger than any file in existence).

Build and run! Very cool, the result as below:

The seconed way:

use apache commons-io,just one line code,as below:

org.apache.commons.io.FileUtils.copyURLToFile(URL, File)

Take note that  copyURLToFile  with timeout parameter is only available since version 2.0 of Commons IO library. See  Java docs

The Third way:

Downloading a file requires you to read it, either way you will have to go through the file in some way. Instead of line by line, you can just read it by bytes from the stream:

import java.io.*;
import java.net.*;

public class filedown {
    public static void download(String address, String localFileName) {
        OutputStream out = null;
        URLConnection conn = null;
        InputStream in = null;

        try {
            URL url = new URL(address);
            out = new BufferedOutputStream(new FileOutputStream(localFileName));
            conn = url.openConnection();
            in = conn.getInputStream();
            byte[] buffer = new byte[1024];

            int numRead;
            long numWritten = 0;

            while ((numRead = in.read(buffer)) != -1) {
                out.write(buffer, 0, numRead);
                numWritten += numRead;
            }

            System.out.println(localFileName + "\t" + numWritten);
        } 
        catch (Exception exception) { 
            exception.printStackTrace();
        } 
        finally {
            try {
                if (in != null) {
                    in.close();
                }
                if (out != null) {
                    out.close();
                }
            } 
            catch (IOException ioe) {
            }
        }
    }

    public static void download(String address) {
        int lastSlashIndex = address.lastIndexOf('/');
        if (lastSlashIndex >= 0 &&
        lastSlashIndex < address.length() - 1) {
            download(address, address.substring(lastSlashIndex + 1));
        } 
        else {
            System.err.println("Could not figure out local file name for "+address);
        }
    }

    public static void main(String[] args) {
        for (int i = 0; i < args.length; i++) {
            download(args[i]);
        }
    }
}

If in.close throws an exception, out.close is not called.You'll need to handle exceptions, probably external to this method.  Using a BufferedInputStream can sometimes lead to SocketTimeoutException being thrown. Replace it byInputStream and you should be ok. Here are some background details:stackoverflow.com/questions/2964044/…

Finally:

Personally, I've found Apache's HttpClient to be more than capable of everything I've needed to do with regards to this. Here is a great tutorial on using HttpClient. Hope this post can help you! 

Last But no Least:if you have any issues,you can  check my blog,thinks your attention !