如何使用Java从Internet下载和保存文件？-Java 学习之路

382

有一个在线文件（例如 http://www.example.com/information.asp ）我需要抓取并保存到目录中 . 我知道有几种方法可以逐行获取和读取在线文件（URL），但有没有办法只使用Java下载和保存文件？

20 回答

这是另一个基于Brian Risk's answer的java7变体，使用了try-with语句：

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);
      }

  }

回复于 2024-04-27T11:01:26+08:00

1
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() 是 potentially 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从源通道将字节传输到文件系统缓存中，而无需实际复制它们 .

查看更多相关信息here .

Note ：transferFrom中的第三个参数是要传输的最大字节数 . Integer.MAX_VALUE 最多将传输2 ^ 31个字节， Long.MAX_VALUE 最多允许2 ^ 63个字节（大于现有的任何文件） .
回复于 2024-04-27T11:01:26+08:00
9
使用apache commons-io，只需一行代码：
```
FileUtils.copyURLToFile(URL, File)
```
回复于 2024-04-27T11:01:26+08:00

更简单的nio用法：

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

回复于 2024-04-27T11:01:26+08:00

112

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，可能是此方法的外部异常 .

回复于 2024-04-27T11:01:26+08:00

下载文件需要您阅读它，无论哪种方式，您都必须以某种方式浏览文件 . 您可以直接从流中读取字节，而不是逐行读取：

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);
    }

回复于 2024-04-27T11:01:26+08:00

这个答案几乎就像选择的答案，但有两个增强功能：它是一个方法，它关闭了FileOutputStream对象：

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();
        }
    }

回复于 2024-04-27T11:01:26+08:00

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

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;
}

文档here .

回复于 2024-04-27T11:01:26+08:00

这是一个老问题，但这是一个优雅的JDK解决方案：

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

简洁，可读，适当封闭的资源，除了核心JDK和语言功能之外什么都不用 .

回复于 2024-04-27T11:01:26+08:00

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]);
        }
    }
}

回复于 2024-04-27T11:01:26+08:00

528

就个人而言，我发现Apache's HttpClient能够满足我对此需要做的所有事情 . Here是一个使用HttpClient的好教程

回复于 2024-04-27T11:01:26+08:00

这里有许多优雅而有效的答案 . 但简洁可能会让我们失去一些有用的信息 . 特别是，一个人可能想要区别对待某种与网络相关的错误 - 例如，决定是否应该重试下载 .

这是一个不会为网络错误抛出异常的方法（仅针对真正异常的问题，因为格式错误的URL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;
}

回复于 2024-04-27T11:01:26+08:00

简单使用有一个问题：

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

如果您需要下载并保存非常大的文件，或者通常如果您需要自动重试，以防连接断开 .

在这种情况下我建议的是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();
}

回复于 2024-04-27T11:01:26+08:00

它's possible to download the file with with Apache' s HttpComponents 而不是 Commons-IO . 此代码允许您根据URL下载Java文件并将其保存在特定目标位置 .

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;
}

与单行代码形成对比：

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

此代码将为您提供对进程的更多控制，并且不仅可以指定超时，还可以指定 User-Agent 和 Referer 值，这对许多网站都很重要 .

回复于 2024-04-27T11:01:26+08:00

总结（并以某种方式抛光和更新）以前的答案 . 以下三种方法实际上是等同的 . （我添加了显式超时，因为我认为它们是必须的，当连接丢失时，没有人希望下载永远冻结 . ）

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 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();
}

我没有发现显着的差异，对我来说都是正确的 . 它们安全有效 . （速度上的差异似乎不太相关 - 我在本地服务器上写入180Mb到SSD磁盘的时间波动在1.2到1.5左右） . 它们不需要外部库 . 所有工作都具有任意大小和（根据我的经验）HTTP重定向 .

此外，如果找不到资源，则全部抛出 FileNotFoundException （通常是错误404），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，随时添加信息或更正）

回复于 2024-04-27T11:01:26+08:00

underscore-java库中有方法U.fetch（url） .

pom.xml中：

<groupId>com.github.javadev</groupId>
  <artifactId>underscore</artifactId>
  <version>1.36</version>

代码示例：

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();
    }
}

回复于 2024-04-27T11:01:26+08:00

452

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;
    }
}

回复于 2024-04-27T11:01:26+08:00

您可以使用netloader for Java在1行中执行此操作：

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

回复于 2024-04-27T11:01:26+08:00

如果您在代理后面，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");

如果您不在代理服务器后面，请不要在代码中包含上面的行 . 当您在代理后面时下载文件的完整工作代码 .

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();
        }
    }

回复于 2024-04-27T11:01:26+08:00

以下是使用java代码从Internet下载电影的示例代码：

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);
    }

回复于 2024-04-27T11:01:26+08:00

如何使用Java从Internet下载和保存文件？

20 回答

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