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, (new URL(address)).getFile());
        } 
        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]);
        }
    }
}

使用Apache Commons IO。它只是一行代码:

FileUtils.copyURLToFile(URL, File)

简单使用有一个问题:

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

如果你需要下载和保存非常大的文件，或者在一般情况下，如果你需要自动重试以防连接断开。

在这种情况下，我建议使用Apache HttpClient以及org.apache.commons.io.FileUtils。例如:

GetMethod method = new GetMethod(resource_url);
try {
    int statusCode = client.executeMethod(method);
    if (statusCode != HttpStatus.SC_OK) {
        logger.error("Get method failed: " + method.getStatusLine());
    }
    org.apache.commons.io.FileUtils.copyInputStreamToFile(
        method.getResponseBodyAsStream(), new File(resource_file));
    } catch (HttpException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    } finally {
    method.releaseConnection();
}

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;
}

可以使用Apache的HttpComponents而不是Commons IO来下载文件。这段代码允许您根据URL在Java中下载文件，并将其保存到特定的目的地。

public static boolean saveFile(URL fileURL, String fileSavePath) {

    boolean isSucceed = true;

    CloseableHttpClient httpClient = HttpClients.createDefault();

    HttpGet httpGet = new HttpGet(fileURL.toString());
    httpGet.addHeader("User-Agent", "Mozilla/5.0 (Windows NT 6.3; WOW64; rv:34.0) Gecko/20100101 Firefox/34.0");
    httpGet.addHeader("Referer", "https://www.google.com");

    try {
        CloseableHttpResponse httpResponse = httpClient.execute(httpGet);
        HttpEntity fileEntity = httpResponse.getEntity();

        if (fileEntity != null) {
            FileUtils.copyInputStreamToFile(fileEntity.getContent(), new File(fileSavePath));
        }

    } catch (IOException e) {
        isSucceed = false;
    }

    httpGet.releaseConnection();

    return isSucceed;
}

与单行代码相比:

FileUtils.copyURLToFile(fileURL, new File(fileSavePath),
                        URLS_FETCH_TIMEOUT, URLS_FETCH_TIMEOUT);

这段代码将使您对进程有更多的控制，不仅可以指定超时，还可以指定User-Agent和Referer值，这对许多网站来说都是至关重要的。

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();
    }
}