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

public void saveUrl(final String filename, final String urlString)
        throws MalformedURLException, IOException {
    BufferedInputStream in = null;
    FileOutputStream fout = null;
    try {
        in = new BufferedInputStream(new URL(urlString).openStream());
        fout = new FileOutputStream(filename);

        final byte data[] = new byte[1024];
        int count;
        while ((count = in.read(data, 0, 1024)) != -1) {
            fout.write(data, 0, count);
        }
    } finally {
        if (in != null) {
            in.close();
        }
        if (fout != null) {
            fout.close();
        }
    }
}

您将需要处理异常，可能是该方法的外部异常。

如果你使用代理，你可以在Java程序中设置代理，如下所示:

Properties systemSettings = System.getProperties();
systemSettings.put("proxySet", "true");
systemSettings.put("https.proxyHost", "HTTPS proxy of your org");
systemSettings.put("https.proxyPort", "8080");

如果您没有使用代理，请不要在代码中包含上述代码行。完整的工作代码下载文件时，你是一个代理。

public static void main(String[] args) throws IOException {
    String url = "https://raw.githubusercontent.com/bpjoshi/fxservice/master/src/test/java/com/bpjoshi/fxservice/api/TradeControllerTest.java";
    OutputStream outStream = null;
    URLConnection connection = null;
    InputStream is = null;
    File targetFile = null;
    URL server = null;

    // Setting up proxies
    Properties systemSettings = System.getProperties();
        systemSettings.put("proxySet", "true");
        systemSettings.put("https.proxyHost", "HTTPS proxy of my organisation");
        systemSettings.put("https.proxyPort", "8080");
        // The same way we could also set proxy for HTTP
        System.setProperty("java.net.useSystemProxies", "true");
        // Code to fetch file
    try {
        server = new URL(url);
        connection = server.openConnection();
        is = connection.getInputStream();
        byte[] buffer = new byte[is.available()];
        is.read(buffer);

        targetFile = new File("src/main/resources/targetFile.java");
        outStream = new FileOutputStream(targetFile);
        outStream.write(buffer);
    } catch (MalformedURLException e) {
        System.out.println("THE URL IS NOT CORRECT ");
        e.printStackTrace();
    } catch (IOException e) {
        System.out.println("I/O exception");
        e.printStackTrace();
    }
    finally{
        if(outStream != null)
            outStream.close();
    }
}

在java.net.http.HttpClient上使用授权的解决方案:

HttpClient client = HttpClient.newHttpClient();

HttpRequest request = HttpRequest.newBuilder()
        .GET()
        .header("Accept", "application/json")
        // .header("Authorization", "Basic ci5raG9kemhhZXY6NDdiYdfjlmNUM=") if you need
        .uri(URI.create("https://jira.google.ru/secure/attachment/234096/screenshot-1.png"))
        .build();

HttpResponse<InputStream> response = client.send(request, HttpResponse.BodyHandlers.ofInputStream());

try (InputStream in = response.body()) {
    Files.copy(in, Paths.get(target + "screenshot-1.png"), StandardCopyOption.REPLACE_EXISTING);
}

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)。

使用Apache Commons IO。它只是一行代码:

FileUtils.copyURLToFile(URL, File)