Android：新 Intent 在 AsyncTask 中导致 NullPointerException

Question

我有一个在后台运行的异步任务，它通过网络不断更新我的本地 sqLite 数据库。一切正常，除了我暂停我的应用程序后的一些随机时间。这是我收到错误的部分代码：

public class AsyncHardwareDBUpdate extends AsyncTask {
ImageCellAdapter mImageCellAdapter;
GridView dynGrid;
Context mContext;
Boolean mFinished = false;

HardwareSyncReceiver myHardwareReceiver;
private Handler handler;
public AsyncHardwareDBUpdate(Context mContext) {
        this.mContext = mContext;
    }

@Override
protected Object doInBackground(Object... params) {
    if (mContext != null){

错误转到以下行：

        Intent intentHardwareSync = new Intent(mContext,SyncHardwareStateService.class);

我无法理解，如果我已经检查了 mContext 是否为 null，那么这里有什么可以为 null 的！= null。 其余代码：

        mContext.startService(intentHardwareSync);
      myHardwareReceiver = new HardwareSyncReceiver();
      //register BroadcastReceiver

      IntentFilter intentHardwareSyncFilter = new IntentFilter(SyncHardwareStateService.ACTION_SyncHardwareStateService);

      intentHardwareSyncFilter.addCategory(Intent.CATEGORY_DEFAULT);

      //Handler for the separate Thread


      HandlerThread handlerThread = new HandlerThread("MyNewThread");
      handlerThread.start();

      Looper looper = handlerThread.getLooper();
      // Create a handler for the service
      handler = new Handler(looper);
      // Register the broadcast receiver to run on the separate Thread
      mContext.registerReceiver (myHardwareReceiver, intentHardwareSyncFilter, null, handler);
    }
      return null;
}


05-11 18:07:37.830: E/AndroidRuntime(15063): FATAL EXCEPTION: AsyncTask #1
05-11 18:07:37.830: E/AndroidRuntime(15063): java.lang.RuntimeException: An error occured while executing doInBackground()
05-11 18:07:37.830: E/AndroidRuntime(15063):    at android.os.AsyncTask$3.done(AsyncTask.java:299)
05-11 18:07:37.830: E/AndroidRuntime(15063):    at java.util.concurrent.FutureTask$Sync.innerSetException(FutureTask.java:273)
05-11 18:07:37.830: E/AndroidRuntime(15063):    at java.util.concurrent.FutureTask.setException(FutureTask.java:124)
05-11 18:07:37.830: E/AndroidRuntime(15063):    at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:307)
05-11 18:07:37.830: E/AndroidRuntime(15063):    at java.util.concurrent.FutureTask.run(FutureTask.java:137)
05-11 18:07:37.830: E/AndroidRuntime(15063):    at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:230)
05-11 18:07:37.830: E/AndroidRuntime(15063):    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1076)
05-11 18:07:37.830: E/AndroidRuntime(15063):    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:569)
05-11 18:07:37.830: E/AndroidRuntime(15063):    at java.lang.Thread.run(Thread.java:856)
05-11 18:07:37.830: E/AndroidRuntime(15063): Caused by: java.lang.NullPointerException
05-11 18:07:37.830: E/AndroidRuntime(15063):    at android.content.ContextWrapper.getPackageName(ContextWrapper.java:127)
05-11 18:07:37.830: E/AndroidRuntime(15063):    at android.content.ComponentName.<init>(ComponentName.java:75)
05-11 18:07:37.830: E/AndroidRuntime(15063):    at android.content.Intent.<init>(Intent.java:3350)
05-11 18:07:37.830: E/AndroidRuntime(15063):    at com.smarthomev5.AsyncUpdateThread.AsyncHardwareDBUpdate.doInBackground(AsyncHardwareDBUpdate.java:38)
05-11 18:07:37.830: E/AndroidRuntime(15063):    at android.os.AsyncTask$2.call(AsyncTask.java:287)
05-11 18:07:37.830: E/AndroidRuntime(15063):    at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:305)
05-11 18:07:37.830: E/AndroidRuntime(15063):    ... 5 more

Answer 1

对我来说听起来像是内存泄漏。在将Context 或Activity 或其他此类对象传递给它时，始终在AsyncTasks 中使用WeakReferences。试试这样的：

public class ExampleAsyncTask extends AsyncTask<Void, Void, Boolean> {

    private final WeakReference<Context> contextReference;

    public ExampleAsyncTask(Context context) {
        this.contextReference = new WeakReference<Context>(context);
    }

    @Override
    protected Boolean doInBackground(Void... params) {

        Context context = this.contextReference.get();
        if(context != null) {
            // Do your work
        }
        return false;
    }

    @Override
    protected void onPostExecute(Boolean aBoolean) {
        super.onPostExecute(aBoolean);

        Context context = this.contextReference.get();
        if(context != null) {
            // Do your work
        }
    }
}

WeakReference 所做的是允许对其引用的对象进行垃圾回收。如果您直接持有引用，则无法对对象进行垃圾回收，这可能导致Threads 和AsyncTasks 中的内存泄漏。例如，当您直接持有 Context 引用并重新创建 Context 对象所在的 Activity 时，例如当您旋转设备或 Activity 在后台一段时间时，就会发生这种情况。

在上面的示例中，您将Context 对象保存在WeakReference 中，并且每次要使用Context 时，您首先必须从WeakReference 中获取它。从WeakReference 获得它后，您可以简单地检查它是否为空。如果它不为空，那么使用Context 是安全的。如果它为空，则Context 或相应的Activity 已被垃圾回收并且不再可用。