如何使用Java从Internet下载和保存文件？


How to download and save a file from Internet using Java?

有一个在线文件(如http://www.example.com/information.asp)我需要抓取并保存到一个目录中。我知道有几种方法可以逐行地抓取和读取在线文件(URL)，但是有没有办法用Java下载和保存文件呢？

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尝试Java NIO：

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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);

号

使用transferFrom()可能比从源通道读取并写入该通道的简单循环更有效。许多操作系统可以将字节直接从源通道传输到文件系统缓存，而无需实际复制它们。

在这里查看更多信息。

注意：transferFrom中的第三个参数是要传输的最大字节数。Integer.MAX_VALUE最多传输2^31字节，Long.MAX_VALUE最多允许2^63字节(大于现有的任何文件)。

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使用ApacheCommons IO，只需一行代码：

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FileUtils.copyURLToFile(URL, File)

。

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更简单的NIO用法：

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URL website = new URL("http://www.website.com/information.asp");
try (InputStream in = website.openStream()) {
    Files.copy(in, target, StandardCopyOption.REPLACE_EXISTING);
}

。

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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();
        }
    }
}

您需要处理异常，可能是这个方法的外部异常。

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下载一个文件需要你阅读它，不管是哪种方式，你都必须以某种方式浏览该文件。您可以从流中按字节读取它，而不是逐行读取：

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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);
    }

这是一个古老的问题，但这里有一个简洁、易读、仅限JDK的解决方案，它具有适当的封闭资源：

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

两行代码，没有依赖项。

当使用Java 7+时，使用以下方法从Internet下载文件并将其保存到某个目录：

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private static Path download(String sourceURL, String targetDirectory) throws IOException
{
    URL url = new URL(sourceURL);
    String fileName = sourceURL.substring(sourceURL.lastIndexOf('/') + 1, sourceURL.length());
    Path targetPath = new File(targetDirectory + File.separator + fileName).toPath();
    Files.copy(url.openStream(), targetPath, StandardCopyOption.REPLACE_EXISTING);

    return targetPath;
}

此处为文档。

此答案几乎与所选答案完全相同，但有两个增强：它是一个方法，它关闭了fileoutputstream对象：

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    public static void downloadFileFromURL(String urlString, File destination) {    
        try {
            URL website = new URL(urlString);
            ReadableByteChannel rbc;
            rbc = Channels.newChannel(website.openStream());
            FileOutputStream fos = new FileOutputStream(destination);
            fos.getChannel().transferFrom(rbc, 0, Long.MAX_VALUE);
            fos.close();
            rbc.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

。

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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]);
        }
    }
}
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就我个人而言，我发现Apache的httpclient能够胜任与此相关的所有工作。下面是一个关于使用httpclient的很好的教程

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这是另一个Java7变体，基于Brian Risk的答案，使用Try With语句：

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public static void downloadFileFromURL(String urlString, File destination) throws Throwable {

      URL website = new URL(urlString);
      try(
              ReadableByteChannel rbc = Channels.newChannel(website.openStream());
              FileOutputStream fos = new FileOutputStream(destination);  
              ){
          fos.getChannel().transferFrom(rbc, 0, Long.MAX_VALUE);
      }

  }

。

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这里有许多优雅而有效的答案。但是简洁性会使我们丢失一些有用的信息。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，决定是否应重试下载。

这里有一个方法，它不会因网络错误而引发异常(仅适用于真正的异常问题，如格式错误的URL或写入文件的问题)

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/**
 * 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;
}

号

可以用Apache的HttpComponents而不是Commons-IO下载文件。此代码允许您根据Java的URL下载文件，并将其保存在特定的目的地。

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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;
}

号

与单行代码相比：

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

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此代码将为您提供对进程的更多控制，并允许您不仅指定超时值，还指定对许多网站至关重要的User-Agent和Referer值。

在下划线Java库中有方法U.fetch(URL)。

pom.xml文件：

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  <groupId>com.github.javadev</groupId>
  underscore</artifactId>
  <version>1.45</version>

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代码示例：

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import com.github.underscore.lodash.U;

public class Download {
    public static void main(String ... args) {
        String text = U.fetch("https://stackoverflow.com/questions"
        +"/921262/how-to-download-and-save-a-file-from-internet-using-java").text();
    }
}

号

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总结(并以某种方式润色和更新)以前的答案。以下三种方法实际上是等效的。(我添加了显式超时，因为我认为它们是必须的，没有人希望在连接断开时下载永久冻结。)

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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 dir 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 dir 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 dir 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();
}

号

我没有发现显著的差异，对我来说都是正确的。它们安全高效。(速度上的差异似乎无关紧要——我将180MB从本地服务器写入到一个SSD磁盘的时间波动在1.2到1.5段左右)。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，如果找不到资源(通常是错误404)，则全部抛出FileNotFoundException，如果DNS解析失败，则抛出java.net.UnknownHostException；其他ioexception对应于传输过程中的错误。

(标记为社区维基，请随意添加信息或更正)

简单用法存在问题：

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

。

如果需要下载和保存非常大的文件，或者在连接中断的情况下，通常需要自动重试。

在这种情况下，我建议使用Apachehttpclient和org.apache.commons.io.fileutils。例如：

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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();
}

下面是用Java代码从Internet下载电影的示例代码：

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URL url = new
URL("http://103.66.178.220/ftp/HDD2/Hindi%20Movies/2018/Hichki%202018.mkv");
    BufferedInputStream bufferedInputStream = new  BufferedInputStream(url.openStream());
    FileOutputStream stream = new FileOutputStream("/home/sachin/Desktop/test.mkv");


    int count=0;
    byte[] b1 = new byte[100];

    while((count = bufferedInputStream.read(b1)) != -1) {
        System.out.println("b1:"+b1+">>"+count+">> KB downloaded:"+new File("/home/sachin/Desktop/test.mkv").length()/1024);
        stream.write(b1, 0, count);
    }

。

相关讨论

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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;
    }
}

号

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您可以在1行中使用Java的NETLoad进行操作：

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new NetFile(new File("my/zips/1.zip"),"https://example.com/example.zip", -1).load(); //returns true if succeed, otherwise false.

号

如果你在代理后面，你可以在Java程序中设置代理如下：

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

如果您不在代理之后，请不要在代码中包含上面的行。完整的工作代码，用于在代理之后下载文件。

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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("Io exception");
            e.printStackTrace();
        }
        finally{
            if(outStream!=null) outStream.close();
        }
    }

号