这里有许多优雅而有效的答案。但是简洁会让我们失去一些有用的信息。特别是，人们通常不希望将连接错误视为异常，并且可能希望以不同的方式处理某些与网络相关的错误—例如，决定是否应该重试下载。

下面是一个方法，它不会为网络错误抛出异常(仅用于真正异常的问题，如url格式错误或写入文件的问题)

/**
 * Downloads from a (http/https) URL and saves to a file. 
 * Does not consider a connection error an Exception. Instead it returns:
 *  
 *    0=ok  
 *    1=connection interrupted, timeout (but something was read)
 *    2=not found (FileNotFoundException) (404) 
 *    3=server error (500...) 
 *    4=could not connect: connection timeout (no internet?) java.net.SocketTimeoutException
 *    5=could not connect: (server down?) java.net.ConnectException
 *    6=could not resolve host (bad host, or no internet - no dns)
 * 
 * @param file File to write. Parent directory will be created if necessary
 * @param url  http/https url to connect
 * @param secsConnectTimeout Seconds to wait for connection establishment
 * @param secsReadTimeout Read timeout in seconds - trasmission will abort if it freezes more than this 
 * @return See above
 * @throws IOException Only if URL is malformed or if could not create the file
 */
public static int saveUrl(final Path file, final URL url, 
  int secsConnectTimeout, int secsReadTimeout) throws IOException {
    Files.createDirectories(file.getParent()); // make sure parent dir exists , this can throw exception
    URLConnection conn = url.openConnection(); // can throw exception if bad url
    if( secsConnectTimeout > 0 ) conn.setConnectTimeout(secsConnectTimeout * 1000);
    if( secsReadTimeout > 0 ) conn.setReadTimeout(secsReadTimeout * 1000);
    int ret = 0;
    boolean somethingRead = false;
    try (InputStream is = conn.getInputStream()) {
        try (BufferedInputStream in = new BufferedInputStream(is); OutputStream fout = Files
                .newOutputStream(file)) {
            final byte data[] = new byte[8192];
            int count;
            while((count = in.read(data)) > 0) {
                somethingRead = true;
                fout.write(data, 0, count);
            }
        }
    } catch(java.io.IOException e) { 
        int httpcode = 999;
        try {
            httpcode = ((HttpURLConnection) conn).getResponseCode();
        } catch(Exception ee) {}
        if( somethingRead && e instanceof java.net.SocketTimeoutException ) ret = 1;
        else if( e instanceof FileNotFoundException && httpcode >= 400 && httpcode < 500 ) ret = 2; 
        else if( httpcode >= 400 && httpcode < 600 ) ret = 3; 
        else if( e instanceof java.net.SocketTimeoutException ) ret = 4; 
        else if( e instanceof java.net.ConnectException ) ret = 5; 
        else if( e instanceof java.net.UnknownHostException ) ret = 6;  
        else throw e;
    }
    return ret;
}

下面是一个简洁的、可读的、仅使用jdk的解决方案，其中包含适当的封闭资源:

static long download(String url, String fileName) throws IOException {
    try (InputStream in = URI.create(url).toURL().openStream()) {
        return Files.copy(in, Paths.get(fileName));
    }
}

两行代码，没有依赖关系。

下面是一个完整的文件下载示例程序，包含输出、错误检查和命令行参数检查:

package so.downloader;

import java.io.IOException;
import java.io.InputStream;
import java.net.URI;
import java.nio.file.Files;
import java.nio.file.Paths;

public class Application {
    public static void main(String[] args) throws IOException {
        if (2 != args.length) {
            System.out.println("USAGE: java -jar so-downloader.jar <source-URL> <target-filename>");
            System.exit(1);
        }

        String sourceUrl = args[0];
        String targetFilename = args[1];

        long bytesDownloaded = download(sourceUrl, targetFilename);

        System.out.println(String.format("Downloaded %d bytes from %s to %s.", bytesDownloaded, sourceUrl, targetFilename));
    }

    static long download(String url, String fileName) throws IOException {
        try (InputStream in = URI.create(url).toURL().openStream()) {
            return Files.copy(in, Paths.get(fileName));
        }
    }    
}

正如so-downloader存储库README中所指出的:

运行文件下载程序:

java -jar so-downloader.jar <source-URL> <target-filename>

例如:

java -jar so-downloader.jar https://github.com/JanStureNielsen/so-downloader/archive/main.zip so-downloader-source.zip

这可以读取互联网上的文件，并将其写入文件。

import java.net.URL;
import java.io.FileOutputStream;
import java.io.File;

public class Download {
    public static void main(String[] args) throws Exception {
         URL url = new URL("https://www.google.com/images/branding/googlelogo/1x/googlelogo_color_272x92dp.png");  // Input URL
         FileOutputStream out = new FileOutputStream(new File("out.png"));  // Output file
         out.write(url.openStream().readAllBytes());
         out.close();
    }
}

试试Java NIO:

URL website = new URL("http://www.website.com/information.asp");
ReadableByteChannel rbc = Channels.newChannel(website.openStream());
FileOutputStream fos = new FileOutputStream("information.html");
fos.getChannel().transferFrom(rbc, 0, Long.MAX_VALUE);

使用transferFrom()可能比从源通道读取并写入此通道的简单循环更有效。许多操作系统可以直接将字节从源通道传输到文件系统缓存中，而不需要实际复制它们。

点击这里查看更多信息。

注意:transferFrom中的第三个参数是传输的最大字节数。整数。MAX_VALUE将传输最多2^31字节，长。MAX_VALUE最多允许2^63字节(比现有的任何文件都大)。

下载一个文件需要你阅读它。无论哪种方式，您都必须以某种方式查看该文件。而不是逐行，你可以从流中逐字节读取:

BufferedInputStream in = new BufferedInputStream(new URL("http://www.website.com/information.asp").openStream())
byte data[] = new byte[1024];
int count;
while((count = in.read(data, 0, 1024)) != -1)
{
    out.write(data, 0, count);
}

public class DownloadManager {

    static String urls = "[WEBSITE NAME]";

    public static void main(String[] args) throws IOException{
        URL url = verify(urls);
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        InputStream in = null;
        String filename = url.getFile();
        filename = filename.substring(filename.lastIndexOf('/') + 1);
        FileOutputStream out = new FileOutputStream("C:\\Java2_programiranje/Network/DownloadTest1/Project/Output" + File.separator + filename);
        in = connection.getInputStream();
        int read = -1;
        byte[] buffer = new byte[4096];
        while((read = in.read(buffer)) != -1){
            out.write(buffer, 0, read);
            System.out.println("[SYSTEM/INFO]: Downloading file...");
        }
        in.close();
        out.close();
        System.out.println("[SYSTEM/INFO]: File Downloaded!");
    }
    private static URL verify(String url){
        if(!url.toLowerCase().startsWith("http://")) {
            return null;
        }
        URL verifyUrl = null;

        try{
            verifyUrl = new URL(url);
        }catch(Exception e){
            e.printStackTrace();
        }
        return verifyUrl;
    }
}