java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法'android.view.View.MainActivity.findViewById（int）

错误是

public MainActivity MainActivity;

永远不会初始化，因此指向null。为了使您的代码正常工作，最小的步骤是在MainActivity中

new Connection(this).execute();

在连接

public class Connection extends AsyncTask<String, Void, String> {    public MainActivity MainActivity;    public Connection(MainActivity activity) {        MainActivity = activity;    }

但是无论如何，在onCreate中创建任务并传递Activity并不是最好的主意。另外，字段名称应始终以小写字母开头。

最好的方法是将ImageView传递给AsyncTask。在启动“活动”之前不要启动任务，而且，在“活动”停止后也不要忘记取消任务。

public final class MainActivity extends Activity {    public MainActivity() {}    private Connection connection;    private ImageView imageView;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        imageView = (ImageView) findViewById(R.id.image);    }    @Override    protected void onStart() {        super.onStart();        if (connection == null || connection.getStatus() != AsyncTask.Status.RUNNING) { connection = new Connection(imageView); connection.execute();        }    }    @Override    protected void onStop() {        super.onStop();        if (connection != null && connection.getStatus() == AsyncTask.Status.RUNNING) { connection.cancel(true);        }    }}

在Connection.java中，将ImageView存储为WeakReference以避免泄漏。

public final class Connection extends AsyncTask<String, Void, String> {    private final WeakReference<ImageView> imageViewRef;    public Connection(ImageView view) {        imageViewRef = new WeakReference<ImageView>(view);    }    @Override    protected String doInBackground(String... arg0) {        // TODO Auto-generated method stub     //...        return "a string";    }    @Override    protected void onPostExecute(String result) {        super.onPostExecute(result);     //...        final ImageView imageView = imageViewRef.get();        // if the Activity is still alive, the ImageView will not be null        if (imageView != null) { // set an image or whatever you need image.setImageResource(666);        }    }