在 java 中恢复 http 文件下载

Question

URL url = new URL("http://download.thinkbroadband.com/20MB.zip");

URLConnection connection = url.openConnection();
File fileThatExists = new File(path); 
OutputStream output = new FileOutputStream(path, true);
connection.setRequestProperty("Range", "bytes=" + fileThatExists.length() + "-");

connection.connect();

int lenghtOfFile = connection.getContentLength();

InputStream input = new BufferedInputStream(url.openStream());
byte data[] = new byte[1024];

long total = 0;

while ((count = input.read(data)) != -1) {
    total += count;

    output.write(data, 0 , count);
}

在这段代码中，我尝试恢复下载。 目标文件为 20MB。 但是当我在 10MB 上停止下载，然后继续，我得到文件大小为 30MB 的文件。 似乎它继续写入文件，但无法从服务器部分下载。 Wget -c 非常适合这个文件。 如何恢复文件下载？

Answer 1

 HttpURLConnection connection = (HttpURLConnection) url.openConnection();
    if(ISSUE_DOWNLOAD_STATUS.intValue()==ECMConstant.ECM_DOWNLOADING){
        File file=new File(DESTINATION_PATH);
        if(file.exists()){
             downloaded = (int) file.length();
             connection.setRequestProperty("Range", "bytes="+(file.length())+"-");
        }
    }else{
        connection.setRequestProperty("Range", "bytes=" + downloaded + "-");
    }
    connection.setDoInput(true);
    connection.setDoOutput(true);
    progressBar.setMax(connection.getContentLength());
     in = new BufferedInputStream(connection.getInputStream());
     fos=(downloaded==0)? new FileOutputStream(DESTINATION_PATH): new FileOutputStream(DESTINATION_PATH,true);
     bout = new BufferedOutputStream(fos, 1024);
    byte[] data = new byte[1024];
    int x = 0;
    while ((x = in.read(data, 0, 1024)) >= 0) {
        bout.write(data, 0, x);
         downloaded += x;
         progressBar.setProgress(downloaded);
    }

这不是我的代码，但它有效。

Answer 2

我猜您面临的问题是在url.openStream() url.openConnection() 。

url.openStream()等价于url.openConnection().getInputStream() 。 因此，您请求 url 两次。 特别是第二次，它没有指定范围属性。 因此下载总是从头开始。

您应该将url.openStream()替换为connection.getInputStream() 。

Answer 3

这就是我用来以块的方式下载文件的内容 使用进度更新 UI。

/*
 * @param callback = To update the UI with appropriate action
 * @param fileName = Name of the file by which downloaded file will be saved.
 * @param downloadURL = File downloading URL
 * @param filePath = Path where file will be saved
 * @param object = Any object you want in return after download is completed to do certain operations like insert in DB or show toast
 */

public void startDownload(final IDownloadCallback callback, String fileName, String downloadURL, String filePath, Object object) {
    callback.onPreExecute(); // Callback to tell that the downloading is going to start
    int count = 0;
    File outputFile = null; // Path where file will be downloaded
    try {
        File file = new File(filePath);
        file.mkdirs();
        long range = 0;
        outputFile = new File(file, fileName);
        /**
         * Check whether the file exists or not
         * If file doesn't exists then create the new file and range will be zero.
         * But if file exists then get the length of file which will be the starting range,
         * from where the file will be downloaded
         */
        if (!outputFile.exists()) {
            outputFile.createNewFile();
            range = 0;
        } else {
            range = outputFile.length();
        }
        //Open the Connection
        URL url = new URL(downloadURL);
        URLConnection con = url.openConnection();
        // Set the range parameter in header and give the range from where you want to start the downloading
        con.setRequestProperty("Range", "bytes=" + range + "-");
        /**
         * The total length of file will be the total content length given by the server + range.
         * Example: Suppose you have a file whose size is 1MB and you had already downloaded 500KB of it.
         * Then you will pass in Header as "Range":"bytes=500000".
         * Now the con.getContentLength() will be 500KB and range will be 500KB.
         * So by adding the two you will get the total length of file which will be 1 MB
         */
        final long lenghtOfFile = (int) con.getContentLength() + range;

        FileOutputStream fileOutputStream = new FileOutputStream(outputFile, true);
        InputStream inputStream = con.getInputStream();

        byte[] buffer = new byte[1024];

        long total = range;
        /**
         * Download the save the content into file
         */
        while ((count = inputStream.read(buffer)) != -1) {
            total += count;
            int progress = (int) (total * 100 / lenghtOfFile);
            EntityDownloadProgress entityDownloadProgress = new EntityDownloadProgress();
            entityDownloadProgress.setProgress(progress);
            entityDownloadProgress.setDownloadedSize(total);
            entityDownloadProgress.setFileSize(lenghtOfFile);
            callback.showProgress(entityDownloadProgress);
            fileOutputStream.write(buffer, 0, count);
        }
        //Close the outputstream
        fileOutputStream.close();
        // Disconnect the Connection
        if (con instanceof HttpsURLConnection) {
            ((HttpsURLConnection) con).disconnect();
        } else if (con instanceof HttpURLConnection) {
            ((HttpURLConnection) con).disconnect();
        }
        inputStream.close();
        /**
         * If file size is equal then return callback as success with downlaoded filepath and the object
         * else return failure
         */
        if (lenghtOfFile == outputFile.length()) {
            callback.onSuccess(outputFile.getAbsolutePath(), object);
        } else {
            callback.onFailure(object);
        }
    } catch (Exception e) {
        e.printStackTrace();
        callback.onFailure(object);
    }
}

接口 IDownloadCallback {

    void onPreExecute(); // Callback to tell that the downloading is going to start
    void onFailure(Object o); // Failed to download file
    void onSuccess(String path, Object o); // Downloaded file successfully with downloaded path
    void showProgress(EntityDownloadProgress entityDownloadProgress); // Show progress
}

公共 class EntityDownloadProgress {

    int progress; // range from 1-100
    long fileSize;// Total size of file to be downlaoded
    long downloadedSize; // Size of the downlaoded file

    public void setProgress(int progress) {this.progress = progress;}

    public void setFileSize(long fileSize) {this.fileSize = fileSize;}

    public void setDownloadedSize(long downloadedSize) {this.downloadedSize = downloadedSize;}
}

Answer 4

我有办法让你的代码工作。

1. 首先，检查文件是否存在

2. 如果文件存在，设置连接：

    connection.setRequestProperty("Range", "bytes=" + bytedownloaded + "-");

3. 如果文件不存在，请在新文件中执行相同的下载。

Answer 5

这个怎么样？

public static void download(DownloadObject object) throws IOException{
    String downloadUrl = object.getDownloadUrl();
    String downloadPath = object.getDownloadPath();
    long downloadedLength = 0;

    File file = new File(downloadPath);
    URL url = new URL(downloadUrl);

    BufferedInputStream inputStream = null;
    BufferedOutputStream outputStream = null;

    URLConnection connection = url.openConnection();

    if(file.exists()){
        downloadedLength = file.length();
        connection.setRequestProperty("Range", "bytes=" + downloadedLength + "-");
        outputStream = new BufferedOutputStream(new FileOutputStream(file, true));

    }else{
        outputStream = new BufferedOutputStream(new FileOutputStream(file));

    }

    connection.connect();

    inputStream = new BufferedInputStream(connection.getInputStream());


    byte[] buffer = new byte[1024*8];
    int byteCount;

    while ((byteCount = inputStream.read(buffer)) != -1){
        outputStream.write(buffer, 0, byteCount);
        break;

    }

    inputStream.close();
    outputStream.flush();
    outputStream.close();

}

用过的break; 测试代码.. ;)

Answer 6

查看与您的问题类似的线程。 如果 wget 工作正常，那么服务器显然支持恢复下载。 看起来您没有按照上述链接的已接受答案中所述设置If-Range header。 IE。 添加：

// Initial download.
String lastModified = connection.getHeaderField("Last-Modified");

// ...

// Resume download.
connection.setRequestProperty("If-Range", lastModified); 

Answer 7

由于问题标记为 Android：您是否尝试过使用DownloadManager 。 它可以很好地为您处理所有这些事情。