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。

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);
}

总结(并以某种方式润色和更新)之前的答案。以下三种方法实际上是等效的。(我添加了明确的超时，因为我认为这是必须的。没有人希望下载在连接丢失时永远冻结。)

public static void saveUrl1(final Path file, final URL url,
    int secsConnectTimeout, int secsReadTimeout))
    throws MalformedURLException, IOException {

    // Files.createDirectories(file.getParent()); // Optional, make sure parent directory exists
    try (BufferedInputStream in = new BufferedInputStream(
         streamFromUrl(url, secsConnectTimeout,secsReadTimeout));
         OutputStream fout = Files.newOutputStream(file)) {

            final byte data[] = new byte[8192];
            int count;
            while((count = in.read(data)) > 0)
                fout.write(data, 0, count);
        }
}

public static void saveUrl2(final Path file, final URL url,
    int secsConnectTimeout, int secsReadTimeout))
    throws MalformedURLException, IOException {

    // Files.createDirectories(file.getParent()); // Optional, make sure parent directory exists
    try (ReadableByteChannel rbc = Channels.newChannel(
             streamFromUrl(url, secsConnectTimeout, secsReadTimeout)
        );
        FileChannel channel = FileChannel.open(file,
             StandardOpenOption.CREATE,
             StandardOpenOption.TRUNCATE_EXISTING,
             StandardOpenOption.WRITE)
        ) {

        channel.transferFrom(rbc, 0, Long.MAX_VALUE);
    }
}

public static void saveUrl3(final Path file, final URL url,
    int secsConnectTimeout, int secsReadTimeout))
    throws MalformedURLException, IOException {

    // Files.createDirectories(file.getParent()); // Optional, make sure parent directory exists
    try (InputStream in = streamFromUrl(url, secsConnectTimeout,secsReadTimeout) ) {
        Files.copy(in, file, StandardCopyOption.REPLACE_EXISTING);
    }
}

public static InputStream streamFromUrl(URL url,int secsConnectTimeout,int secsReadTimeout) throws IOException {
    URLConnection conn = url.openConnection();
    if(secsConnectTimeout>0)
        conn.setConnectTimeout(secsConnectTimeout*1000);
    if(secsReadTimeout>0)
        conn.setReadTimeout(secsReadTimeout*1000);
    return conn.getInputStream();
}

我没有发现明显的差异，在我看来都是对的。它们既安全又高效。(速度的差异似乎无关紧要——我从本地服务器写入180 MB到SSD磁盘的时间大约在1.2到1.5秒之间波动)。它们不需要外部库。所有这些都可以使用任意大小和(根据我的经验)HTTP重定向。

此外，如果没有找到资源(通常是404错误)，所有抛出FileNotFoundException，如果DNS解析失败则抛出java.net.UnknownHostException;其他IOException对应传输过程中的错误。

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)。