实现通过互联网下载文件的简历

Question

当不为此实现resume 时，我用于下载文件的以下代码工作正常，在阅读了更多实现该解决方案并解决问题后，我知道我必须检查Last-Modified 标头并将其设置为连接，

但我不能这样做，因为我得到了诸如android Cannot set request property after connection is made 之类的错误，或者我得到null for httpURLConnection，

我正在使用这个reference

getHeaderField heaser 返回：

{
  null=[HTTP/1.1 200 OK], 
  Cache-Control=[public], 
  Connection=[keep-alive], 
  Content-Length=[8037404], 
  Content-Md5=[VEqXHCc/Off7a6D0gRFpiQ==], 
  Content-Type=[image/jpeg], 
  Date=[Tue, 19 Jan 2016 07:24:36 GMT], 
  Etag=["544a971c273f39f7fb6ba0f481116989"], 
  Expires=[Sat, 29 Jul 2017 10:07:00 GMT], 
  Last-Modified=[Thu, 18 Dec 2014 08:44:34 GMT], 
  Server=[bws], 
  X-Android-Received-Millis=[1501063623576], 
  X-Android-Response-Source=[NETWORK 200], 
  X-Android-Selected-Protocol=[http/1.1], 
  X-Android-Sent-Millis=[1501063623532]
}

现在我如何设置它以恢复下载文件？

GitHub Link

public void run() {
    final URL         url;
    HttpURLConnection httpURLConnection = null;
    try {
        try {
            url = new URL(mUrl);
            String lastModified = httpURLConnection.getHeaderField("Last-Modified");
            if (!lastModified.isEmpty()) {
                httpURLConnection.setRequestProperty("If-Range", lastModified);
            }
            httpURLConnection = (HttpURLConnection) url.openConnection();

            if (mFile.exists()) {
                downloadedLength = mFile.length();
                Log.e("downloadedLength ", downloadedLength + "");
                httpURLConnection.setRequestProperty("Range", "bytes=" + downloadedLength + "-");
                fileOutputStream = new FileOutputStream(mFile, true);
            } else {
                fileOutputStream = new FileOutputStream(mFile);
            }
            httpURLConnection.setConnectTimeout(30000);
            httpURLConnection.setReadTimeout(30000);
            httpURLConnection.setRequestMethod("GET");
        } catch (IOException e) {
        }
        final int responseCode;
        final int total;
        try {
            responseCode = httpURLConnection.getResponseCode();
            total = httpURLConnection.getContentLength();
        } catch (IOException e) {
            e.printStackTrace();
            Log.e("ER UPDATE ", e.getMessage());
        }
        if (responseCode == 200) {
            try {
                inputStream = httpURLConnection.getInputStream();
            } catch (IOException e) {
                e.printStackTrace();
                Log.e("IOException ", e.getMessage());
            }
            final byte[] buffer   = new byte[4 * 1024];
            int          length   = -1;
            int          finished = 0;
            long         start    = System.currentTimeMillis();
            try {
                while ((length = inputStream.read(buffer)) != -1) {
                    if (!isDownloading()) {
                        throw new CanceledException("canceled");
                    }
                    fileOutputStream.write(buffer, 0, length);
                    finished += length;
                    if (System.currentTimeMillis() - start > 1000) {
                        onDownloadProgressing(finished, total);
                        start = System.currentTimeMillis();
                    }
                }
                onDownloadCompleted();
            } catch (IOException e) {
                e.printStackTrace();
                Log.e("ER UPDATE ", e.getMessage());
            }
        } else {
            Log.e("responseCode ", responseCode + "");
        }
    } catch (DownloadException e) {
        e.printStackTrace();
        Log.e("ER UPDATE ", e.getMessage());
    } catch (CanceledException e) {
        e.printStackTrace();
        Log.e("ER UPDATE ", e.getMessage());
    }
}

我也得到206 响应代码而不是200

Answer 1

1- 你得到了 httpURLConnection 的空值，因为你试图在初始化之前调用它，

即这条线

httpURLConnection = (HttpURLConnection) url.openConnection();

应该在这一行之前：

String lastModified = httpURLConnection.getHeaderField("Last-Modified");

2- 你可以在httpURLConnection 上调用connect() 之前设置标题 所以你需要设置任何你想要的，然后连接（）。这样你就不应该得到错误 (android Cannot set request property after connection is made)

3- 206 是 perfectly right，这是您在使用 Range 时的 should expect，这意味着 部分内容成功，这就是您正在做的事情，您将获得部分内容，如果您获得完整内容，您将获得 200。

总而言之，您的代码如下所示： 注意：按照//***查看所需的更改。

编辑：都到了这一行

httpURLConnection.setRequestProperty("If-Range", lastModified);

设置该属性时会引发错误，

无论如何，当你看到这个时，它是没有意义的，你在问 last-modified 是否等于你刚刚从连接中获得的值！， 如果你想这样做，你需要在你的系统中存储 lastModified， 然后将其与您从 URLConn 获得的进行比较，并将其与您的文件长度（已下载）进行比较 然后继续完整下载或继续下载。

在下面找到新代码：

public void run() {
    myLastModified = getLastModified(mFile.getName()); // get last stored value for this file (use file name or other key)
    int total =0;

    final URL         url;
    HttpURLConnection httpURLConnection = null;
    try {
        try {
            url = new URL(mUrl);

            httpURLConnection = (HttpURLConnection) url.openConnection();
            httpURLConnection.setDoInput(true);

            httpURLConnection.setConnectTimeout(30000);
            httpURLConnection.setReadTimeout(30000);
            httpURLConnection.setRequestMethod("GET");

            //*** new way to handle download process
            total = httpURLConnection.getContentLength();
            if(mFile.exists()){
                if(mFile.length() == total){
                    //we are done, return.
                    return;
                }else{
                    //file was not completly donwloaded, now check lastModified:
                    long lastModified = httpURLConnection.getLastModified();//this gets the header "Last-Modified" and convert to long
                    if (lastModified == myLastModified) { //myLastModified should be retrived on each download and stored locally on ur system
                        downloadedLength = mFile.length();
                        Log.e("downloadedLength ", downloadedLength + "");
                        httpURLConnection = (HttpURLConnection) url.openConnection();
                        httpURLConnection.setDoInput(true);

                        httpURLConnection.setConnectTimeout(30000);
                        httpURLConnection.setReadTimeout(30000);
                        httpURLConnection.setRequestMethod("GET");

                        httpURLConnection.setRequestProperty("Range", "bytes=" + downloadedLength + "-"+ total); //add + total (TO)

                        //append mode
                        fileOutputStream = new FileOutputStream(mFile, true);
                    }else{
                        //file was modified after 1st uncompleted-download:
                        storeLastModified(lastModified, mFile.getName()); // use file name as key. can store in db or file ...

                        //don't set ant Range ... we want full download, with a fresh file
                        fileOutputStream = new FileOutputStream(mFile);
                    }//last mod IF

                }//Length check
            }else{
                //file not exist at all, create new file, set no Range we want full download...
                mFile.createNewFile();
                fileOutputStream = new FileOutputStream(mFile);
            }//file exists.

        } catch (IOException e) {
            e.printStackTrace();
        }
        final int responseCode;

        try {
            responseCode = httpURLConnection.getResponseCode();

        } catch (IOException e) {
            e.printStackTrace();
            Log.e("ER UPDATE ", e.getMessage());
        }

        //*****
        if (responseCode == 200 || responseCode == 206) {
            try {
                inputStream = httpURLConnection.getInputStream();
            } catch (IOException e) {
                e.printStackTrace();
                Log.e("IOException ", e.getMessage());
            }
            final byte[] buffer   = new byte[4 * 1024];
            int          length   = -1;
            int          finished = 0;
            long         start    = System.currentTimeMillis();
            try {
                while ((length = inputStream.read(buffer)) != -1) {
                    if (!isDownloading()) {
                        throw new CanceledException("canceled");
                    }
                    fileOutputStream.write(buffer, 0, length);
                    finished += length;
                    if (System.currentTimeMillis() - start > 1000) {
                        onDownloadProgressing(finished, total);
                        start = System.currentTimeMillis();
                    }
                }
                onDownloadCompleted();
            } catch (IOException e) {
                e.printStackTrace();
                Log.e("ER UPDATE ", e.getMessage());
            }
        } else {
            Log.e("responseCode ", responseCode + "");
        }
    } catch (DownloadException e) {
        e.printStackTrace();
        Log.e("ER UPDATE ", e.getMessage());
    } catch (CanceledException e) {
        e.printStackTrace();
        Log.e("ER UPDATE ", e.getMessage());
    }
}

Answer 2

看看thisPOMATu的回答。 但无论如何，如果您通过HTTP 协议下载文件，您可以使用DownloadManager - 一种系统服务（从API 级别9 开始）在后台进行长时间运行的下载。它处理HTTP 连接、连接更改、重新启动，并确保每次下载成功完成。而且它已经支持恢复以及进度通知。

你可以在this 或that 等例子中找到许多教程，在stackoverflow 上通过android-download-manager 标签找到许多解决方案。