Android 如何解决空对象引用上的服务 我有服务类来下载一些数据（在我使用Asynctask下载数据之前，但现在我需要使用服务） 当我初始化服务类，所以我得到这个错误

java.lang.NullPointerException：尝试对空对象引用调用虚拟方法“android.content.ComponentName android.content.Context.startService（android.content.Intent）” 位于android.content.ContextWrapper.startService（ContextWrapper.java:606） 在com.monnfamily.libraryapp.Utility.DownloadManager.downloadBookData（DownloadManager.java:55） 在com.monnffamily.libraryapp.Contentful.ContentfulAdapter$1$2.onClick（ContentfulAdapter.java:99） 在com.android.internal.app.AlertController$ButtonHandler.handleMessage上（AlertController.java:173）

所以请给我一些建议来克服这个错误

public class Services extends Service {

    private DownloadManager.DownloadCompletedListner mListner;
    private BookProperties mBookDetails;
    private Context context;


    public Services(DownloadManager.DownloadCompletedListner mListner, BookProperties mBookDetails) {
       // super("Downloader");
        this.mListner = mListner;
        this.mBookDetails = mBookDetails;
    }

    @Override
    public IBinder onBind(Intent intent) {

        return null;
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        try {
        Log.i("onStartCommand","service sudddddd");
        String tMainFolder = String.valueOf(BaseApplication.getInstance().getAppContext().getDir("MonnFamily", Context.MODE_PRIVATE));
        tMainFolder += "/Book" + mBookDetails.getBookId();
        Log.i(TAG, "book id and book name" + mBookDetails.getBookId() + mBookDetails.getBookName());
        downloadBookDetails(tMainFolder, ContentfulConstants.BOOK_MAIN_IMAGE + ".png", mBookDetails.getBookMainImage());
        downloadBookDetails(tMainFolder, ContentfulConstants.BOOK_MAIN_AUDIO + ".mp3", mBookDetails.getBookSound());

        for (PageDetailProperties pageDetails : mBookDetails.getPageDetail()) {
            String tPageNumber = pageDetails.getPageNumber().toString();
            downloadBookDetails(tMainFolder, ContentfulConstants.PAGE_IMAGE + tPageNumber + ".png", pageDetails.getPageImage());
            downloadBookDetails(tMainFolder, ContentfulConstants.PAGE_AUDIO + tPageNumber + ".mp3", pageDetails.getPageAudio());
            downloadBookDetails(tMainFolder, ContentfulConstants.PAGE_TEXT + tPageNumber + ".txt", pageDetails.getPageText());
        }

        // myddownloadmethod();// calling my download method
    } catch (Exception e1) {
        // TODO Auto-generated catch block
        Log.d("Error",e1.getMessage());
    }
        return super.onStartCommand(intent, flags, startId);
    }



    private void downloadBookDetails(String pMainFolder, String pFileName, String pDownloadURL) {
        Log.i(TAG, "Coming to this downloadBookDetails ");
        try {
            URL url = new URL(pDownloadURL);
            URLConnection ucon = url.openConnection();
            Log.i("Service" , "run in " + url);
            ucon.setReadTimeout(5000);
            ucon.setConnectTimeout(10000);

            InputStream is = ucon.getInputStream();
            BufferedInputStream inStream = new BufferedInputStream(is, 1024 * 5);

            File directory = new File(pMainFolder, pFileName);
            FileOutputStream outStream = new FileOutputStream(directory);
            byte[] buff = new byte[5 * 1024];

            int len;
            while ((len = inStream.read(buff)) != -1) {
                outStream.write(buff, 0, len);
            }

            outStream.flush();
            outStream.close();
            inStream.close();
        } catch (Exception e) {
            //Add Network Error.
            Log.e(TAG, "Download Error Exception " + e.getMessage());

            e.printStackTrace();
        }
    }

---

---

您正在从这一行的非受火服务获取上下文，这就是问题所在

new Services(mListner,mBookDetails).getApplicationContext().startService(new Intent(BaseApplication.getInstance().getAppContext(),Services.class)); // My service call

如果您在

活动中调用它，请将其更改为：

startService(new Intent(BaseApplication.getInstance().getAppContext(),Services.class));

或来自
片段

getActivity().startService(new Intent(BaseApplication.getInstance().getAppContext(),Services.class));

否则，我将返回一个
上下文
，以便可以像这样使用它：

BaseApplication.getInstance().getAppContext().startService(new Intent(BaseApplication.getInstance().getAppContext(),Services.class));

您是否尝试传递此信息而不是上下文？我想这会解决你的问题

new Services(mListner,mBookDetails).getApplicationContext().startService(new Intent(BaseApplication.getInstance().getAppContext(),Services.class));

与

您好，如果像上面这样调用服务，我收到错误java.lang.RuntimeException:无法实例化服务com.monnfamily.libraryapp.UIActivity.Services:java.lang.InstanceException:java.lang.Class没有零参数构造函数因为您使用了错误的方法将数据传递给服务，您需要使用
Intent.putExtra（）
不在构造函数中传递它们。
new Services(mListner,mBookDetails).getApplicationContext().startService(new Intent(BaseApplication.getInstance().getAppContext(),Services.class));

new Services(mListner,mBookDetails).ClassEx.this.startService(new Intent(ClassEx.this,Services.class));