如何從android中的URL下載文件的一部分？

Question

我試圖使用setRequestProperty（“Range”，“bytes =”+ startbytes +“ - ”+ endbytes）下載給定下載URL的文件的一部分; 以下代碼段顯示了我要執行的操作。

protected String doInBackground(String... aurl) {
    int count;
    Log.d(TAG,"Entered");
    try {

        URL url = new URL(aurl[0]);
        HttpURLConnection connection =(HttpURLConnection) url.openConnection();

        int lengthOfFile = connection.getContentLength();

        Log.d(TAG,"Length of file: "+ lengthOfFile);

        connection.setRequestProperty("Range", "bytes=" + 0 + "-" + 1000);

問題在於，引發了一個異常，即“在建立連接后無法設置請求屬性”。 請幫我解決這個問題。

Answer 1

選項1

如果您不需要知道內容長度：

[注意，不要調用connection.getContentLength() 。 如果你打電話，你會得到例外。 如果你需要打電話，那么檢查第二個選項]

URL url = new URL(aurl[0]);
HttpURLConnection connection =(HttpURLConnection) url.openConnection();
connection.setRequestProperty("Range", "bytes=" + 0 + "-" + 1000);
//Note that, response code will be 206 (Partial Content) instead of usual 200 (OK)
if(connection.getResponseCode() == HttpURLConnection.HTTP_PARTIAL){
    //Your code here to read response data
}

選項2

如果您需要知道內容長度：

URL url = new URL(aurl[0]);
//First make a HEAD call to get the content length  
HttpURLConnection connection =(HttpURLConnection) url.openConnection();
connection.setRequestMethod("HEAD");
if(connection.getResponseCode() == HttpURLConnection.HTTP_OK){
    int lengthOfFile = connection.getContentLength();
    Log.d("ERF","Length of file: "+ lengthOfFile);
    connection.disconnect();

    //Now that we know the content lenght, make the GET call
    connection =(HttpURLConnection) url.openConnection();
    connection.setRequestMethod("GET");
    connection.setRequestProperty("Range", "bytes=" + 0 + "-" + 1000);
    //Note that, response code will be 206 (Partial Content) instead of usual 200 (OK)
    if(connection.getResponseCode() == HttpURLConnection.HTTP_PARTIAL){
        //Your code here to read response data

    }
}

Answer 2

假設您使用HTTP進行下載，您將需要使用HEAD http動詞和RANGE http標頭。

HEAD將為您提供文件大小（如果可用），然后RANGE允許您下載字節范圍。

獲得文件大小后，將其划分為大致相等大小的塊，並為每個塊生成下載線程。 完成所有操作后，按正確的順序寫入文件塊。

如果你不知道如何使用RANGE標題，這里有另一個SO答案解釋如何： https ： //stackoverflow.com/a/6323043/1355166

[編輯]

要將文件分成塊，請使用此命令，然后開始下載過程，

private void getBytesFromFile(File file) throws IOException {
    FileInputStream is = new FileInputStream(file); //videorecorder stores video to file

    java.nio.channels.FileChannel fc = is.getChannel();
    java.nio.ByteBuffer bb = java.nio.ByteBuffer.allocate(10000);

    int chunkCount = 0;

    byte[] bytes;

    while(fc.read(bb) >= 0){
        bb.flip();
        //save the part of the file into a chunk
        bytes = bb.array();
        storeByteArrayToFile(bytes, mRecordingFile + "." + chunkCount);//mRecordingFile is the (String)path to file
        chunkCount++;
        bb.clear();
    }
}

private void storeByteArrayToFile(byte[] bytesToSave, String path) throws IOException {
    FileOutputStream fOut = new FileOutputStream(path);
    try {
        fOut.write(bytesToSave);
    }
    catch (Exception ex) {
        Log.e("ERROR", ex.getMessage());
    }
    finally {
        fOut.close();
    }
}