android - 在内部存储android上下载文件时出错

Question

因为我认为下载非常简单，所以当我尝试从 Internet 下载 xml 文件并将其保存在内部存储器时遇到错误。我使用内存是因为我正在三星 galaxy s3 上测试我的应用程序。

这是下载器的代码。它实际上是一个从 Activity 中调用的线程

public class DownloaderThread extends Thread
{
    private static final int DOWNLOAD_BUFFER_SIZE = 4096;
    Context context;
    private String downloadUrl = "some address";

    public void run()
    {
            URL url;
            URLConnection conn;
            String fileName;
            BufferedInputStream inStream;
            BufferedOutputStream outStream;
            File outFile;
            FileOutputStream fileStream;

            try
            {
                    url = new URL(downloadUrl);
                    conn = url.openConnection();
                    conn.setUseCaches(false);
                    fileName = "file.xml";

                    Log.v("XML", "Download Started");

                    // start download
                    inStream = new BufferedInputStream(conn.getInputStream());
                    outFile = new File(context.getFilesDir() + "/" + fileName);
                    fileStream = new FileOutputStream(outFile);
                    outStream = new BufferedOutputStream(fileStream, DOWNLOAD_BUFFER_SIZE);
                    byte[] data = new byte[DOWNLOAD_BUFFER_SIZE];
                    int bytesRead = 0;
                    while(!isInterrupted() && (bytesRead = inStream.read(data, 0, data.length)) >= 0)
                    {
                            outStream.write(data, 0, bytesRead);
                    }

                    outStream.close();
                    fileStream.close();
                    inStream.close();

                    if(isInterrupted())
                    {
                            outFile.delete();
                    }
                    else
                    {
                            Log.v("XML","Download Finished");
                            Log.v("PATH","File is stored in direcotry:" + outFile.getAbsolutePath().toString());
                    }
            }
            catch(MalformedURLException e)
            {
                e.printStackTrace();
                Log.v("Error",e.toString());

            }
            catch(FileNotFoundException e)
            {
                e.printStackTrace();
                Log.v("Error",e.toString());
            }
            catch(Exception e)
            {
                e.printStackTrace();
                Log.v("Error",e.toString());
            }
    }

这里是调用线程的代码:

public boolean onOptionsItemSelected(MenuItem item) {
     ConnectivityManager cm = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
     android.net.NetworkInfo wifi = cm.getNetworkInfo(ConnectivityManager.TYPE_WIFI);
     android.net.NetworkInfo datac = cm.getNetworkInfo(ConnectivityManager.TYPE_MOBILE);
     if ( item.getItemId() == DOWNLOAD_XML_MENU_ITEM ) {
            if ((wifi != null & datac != null) && (wifi.isConnected() | datac.isConnected())) 
            {
                downloaderThread = new DownloaderThread();
                downloaderThread.start();
            }
            else 
            {
                displayConnectionAlert();
            }
        }
        return true;
    }

我收到的错误是: 标记跟踪 - 没有这样的文件和目录在日志下载开始后，我在 outFile 的路径上得到 NullPointerException。任何想法为什么？

Answer 1

您正在使用从未分配过值的 Context 引用，因此是 NPE。

Context context; // never initialised anywhere, thus null

由于所有变量都用于解析 context.getFilesDir()，我建议您只需为您的线程类创建一个构造函数，将路径作为参数；即:

public DownloaderThread(String targetDir) { /* implement here */ }

或者，您实际上可以传入 Context 实例，但只需确保它是应用程序上下文(而不是 Activity)以避免潜在的内存泄漏。