Question

我有一个包含方法的类（DataSource.java），该方法调用MainActivity中的另一个方法，该方法接收上下文和字符串变量。

在Datasource.java中，当从MainActivity调用方法时，我已将上下文变量设置为null，即使我将其设置为“context”，我也会收到与启动应用程序时相同的错误：EXCEPTION错误这一行：context = getApplicationContext();。

有人可以建议我，我错在哪里吗？

DataSource.java：

public class ToursDataSource {

 public boolean addToFollowList(Tour tour) {
    ContentValues values = new ContentValues();
    values.put(ToursDBOpenHelper.COLUMN_IMAGE, tour.getTitle());                
    long result = database.insert(ToursDBOpenHelper.TABLE_FOLLOW, null, values);

    MainActivity orgName = new MainActivity();
    orgName.sendRegId(this, "Test string");

    return null;        
 }
}

MainActivity：

Context context;
    ...

        public void sendRegId(Context context, String organization) {

                 //PROBLEM IS AT THIS LINE
                context = getApplicationContext();

                // Check device for Play Services APK. If check succeeds, proceed with
                //  GCM registration.
                if (checkPlayServices()) {
                    gcm = GoogleCloudMessaging.getInstance(this);
                    regid = getRegistrationId(context);
                    Log.i(TAG, "Registration started!");
                    Log.d("RegisterActivity", "GCM RegId is: " + regid);           

                    if (regid.isEmpty()) {

                      registerInBackground();
                    }
                } else {
                    Log.i(TAG, "No valid Google Play Services APK found.");
                }

                Sender results = new Sender();
                results.shareRegIdWithAppServer(regid, organization);


            }


        ...

logcat的：

04-16 15:42:07.765: E/AndroidRuntime(15610): FATAL EXCEPTION: main
04-16 15:42:07.765: E/AndroidRuntime(15610): java.lang.NullPointerException
04-16 15:42:07.765: E/AndroidRuntime(15610):    at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:109)
04-16 15:42:07.765: E/AndroidRuntime(15610):    at com.exploreca.tourfinder.MainActivity.sendRegId(MainActivity.java:189)
04-16 15:42:07.765: E/AndroidRuntime(15610):    at com.exploreca.tourfinder.ToursDataSource.addToFollowList(ToursDataSource.java:205)
04-16 15:42:07.765: E/AndroidRuntime(15610):    at com.exploreca.tourfinder.TourDetailActivity.onOptionsItemSelected(TourDetailActivity.java:129)
04-16 15:42:07.765: E/AndroidRuntime(15610):    at android.app.Activity.onMenuItemSelected(Activity.java:2640)
04-16 15:42:07.765: E/AndroidRuntime(15610):    at com.android.internal.policy.impl.PhoneWindow.onMenuItemSelected(PhoneWindow.java:1171)
04-16 15:42:07.765: E/AndroidRuntime(15610):    at com.android.internal.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:735)
04-16 15:42:07.765: E/AndroidRuntime(15610):    at com.android.internal.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:152)
04-16 15:42:07.765: E/AndroidRuntime(15610):    at com.android.internal.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:874)
04-16 15:42:07.765: E/AndroidRuntime(15610):    at com.android.internal.view.menu.ActionMenuView.invokeItem(ActionMenuView.java:630)
04-16 15:42:07.765: E/AndroidRuntime(15610):    at com.android.internal.view.menu.ActionMenuItemView.onClick(ActionMenuItemView.java:200)
04-16 15:42:07.765: E/AndroidRuntime(15610):    at android.view.View.performClick(View.java:4475)
04-16 15:42:07.765: E/AndroidRuntime(15610):    at android.view.View$PerformClick.run(View.java:18786)
04-16 15:42:07.765: E/AndroidRuntime(15610):    at android.os.Handler.handleCallback(Handler.java:730)
04-16 15:42:07.765: E/AndroidRuntime(15610):    at android.os.Handler.dispatchMessage(Handler.java:92)
04-16 15:42:07.765: E/AndroidRuntime(15610):    at android.os.Looper.loop(Looper.java:137)
04-16 15:42:07.765: E/AndroidRuntime(15610):    at android.app.ActivityThread.main(ActivityThread.java:5493)
04-16 15:42:07.765: E/AndroidRuntime(15610):    at java.lang.reflect.Method.invokeNative(Native Method)
04-16 15:42:07.765: E/AndroidRuntime(15610):    at java.lang.reflect.Method.invoke(Method.java:525)
04-16 15:42:07.765: E/AndroidRuntime(15610):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1209)
04-16 15:42:07.765: E/AndroidRuntime(15610):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1025)
04-16 15:42:07.765: E/AndroidRuntime(15610):    at dalvik.system.NativeStart.main(Native Method)

Answer 1

尝试使用它：

static Context = mConext;

我认为这很有效。

Answer 2

像这样做出改变：

context = getApplicationContext();

到

this.context = context;

Answer 3

我猜MainActivity是一个Activity类。您无法实例化Activity类。您在清单文件中声明活动。每个Activity都有一个生命周期。您不能将Activity类视为普通的java类。

要知道原因阅读raghav sood的回答@

Can i Create the object of a activity in other class?

引用Raghav's Sood

通过将Activity视为普通的Java类，最终得到null 上下文。因为Activity中的大多数方法都是在其Context上调用的，所以将获得一个空指针异常，这就是你的应用程序崩溃的原因。

Android ContextWrapper的NullPointerException

3 个答案: