java - 低功耗蓝牙 - java.lang.NullPointerException

Question

所以在过去的几个小时里我一直在努力解决这个问题并进行搜索，然后我认为我无法找到解决问题的方法。基本上，我尝试在 Fragment 扩展内部使用 Context，但它给了我错误，我已将完整代码粘贴到此要点中

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从我在 SO 上发现的内容来看，这听起来像是当 Context 为 null 时会发生这种情况，但我不明白如何修复它。

日志猫:

> 06-18 10:20:08.972    5718-5718/com.aparosecurity.kblock W/dalvikvm﹕
> threadid=1: thread exiting with uncaught exception (group=0x41625898)
> 06-18 10:20:08.992    5718-5718/com.aparosecurity.kblock
> E/AndroidRuntime﹕ FATAL EXCEPTION: main
>     java.lang.NullPointerException
>             at com.aparosecurity.kblock.ImageFragment.onCreate(ImageFragment.java:77)
>             at android.support.v4.app.Fragment.performCreate(Fragment.java:1477)
>             at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:904)
>             at android.support.v4.app.FragmentManagerImpl.performPendingDeferredStart(FragmentManager.java:834)
>             at android.support.v4.app.Fragment.setUserVisibleHint(Fragment.java:841)
>             at android.support.v4.app.FragmentPagerAdapter.setPrimaryItem(FragmentPagerAdapter.java:130)
>             at android.support.v4.view.ViewPager.populate(ViewPager.java:1066)
>             at android.support.v4.view.ViewPager.populate(ViewPager.java:914)
>             at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1436)
>             at android.view.View.measure(View.java:15848)
>             at android.widget.LinearLayout.measureVertical(LinearLayout.java:847)
>             at android.widget.LinearLayout.onMeasure(LinearLayout.java:588)
>             at android.view.View.measure(View.java:15848)
>             at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5012)
>             at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
>             at android.view.View.measure(View.java:15848)
>             at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5012)
>             at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1404)
>             at android.widget.LinearLayout.measureVertical(LinearLayout.java:695)
>             at android.widget.LinearLayout.onMeasure(LinearLayout.java:588)
>             at android.view.View.measure(View.java:15848)
>             at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5012)
>             at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
>             at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2189)
>             at android.view.View.measure(View.java:15848)
>             at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1905)
>             at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1104)
>             at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1284)
>             at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1004)
>             at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5481)
>             at android.view.Choreographer$CallbackRecord.run(Choreographer.java:749)
>             at android.view.Choreographer.doCallbacks(Choreographer.java:562)
>             at android.view.Choreographer.doFrame(Choreographer.java:532)
>             at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:735)
>             at android.os.Handler.handleCallback(Handler.java:730)
>             at android.os.Handler.dispatchMessage(Handler.java:92)
>             at android.os.Looper.loop(Looper.java:137)
>             at android.app.ActivityThread.main(ActivityThread.java:5136)
>             at java.lang.reflect.Method.invokeNative(Native Method)
>             at java.lang.reflect.Method.invoke(Method.java:525)
>             at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
>             at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
>             at dalvik.system.NativeStart.main(Native Method)

Any help is appreciated.

The specific lines which are failing, the error occurs when mContext is used.

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    fragVal = getArguments() != null ? getArguments().getInt("val") : 1;
    mHandler = new Handler();

    // Initializes a Bluetooth adapter.  For API level 18 and above, get a reference to
    // BluetoothAdapter through BluetoothManager.
    final BluetoothManager bluetoothManager =
            (BluetoothManager) mContext.getSystemService(Context.BLUETOOTH_SERVICE);
    mBluetoothAdapter = bluetoothManager.getAdapter();

}

Answer 1

您尚未初始化mContext，它为空。在空引用上调用方法会导致 NPE。

在 fragment 中，使用getActivity() 获取对可用作Context 的托管 Activity 的引用。 (警告:仅当附加到 Activity 时。在 onCreate() 中起作用，它位于 onAttach() 之后。)

所以，替换

mContext.getSystemService()

与

getActivity().getSystemService()