java - Android Intent/IntentService 的空指针异常

Question

我的应用程序崩溃，返回以下堆栈跟踪:

java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.**REDACTED**.copyright/com.example.**REDACTED**.copyright.MainActivity}: java.lang.NullPointerException
07-21 22:26:18.499 E/AndroidRuntime(15933):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2128)
07-21 22:26:18.499 E/AndroidRuntime(15933):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2252)
07-21 22:26:18.499 E/AndroidRuntime(15933):     at android.app.ActivityThread.access$800(ActivityThread.java:139)
07-21 22:26:18.499 E/AndroidRuntime(15933):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1200)
07-21 22:26:18.499 E/AndroidRuntime(15933):     at android.os.Handler.dispatchMessage(Handler.java:102)
07-21 22:26:18.499 E/AndroidRuntime(15933):     at android.os.Looper.loop(Looper.java:136)
07-21 22:26:18.499 E/AndroidRuntime(15933):     at android.app.ActivityThread.main(ActivityThread.java:5103)
07-21 22:26:18.499 E/AndroidRuntime(15933):     at java.lang.reflect.Method.invokeNative(Native Method)
07-21 22:26:18.499 E/AndroidRuntime(15933):     at java.lang.reflect.Method.invoke(Method.java:515)
07-21 22:26:18.499 E/AndroidRuntime(15933):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:790)
07-21 22:26:18.499 E/AndroidRuntime(15933):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:606)
07-21 22:26:18.499 E/AndroidRuntime(15933):     at dalvik.system.NativeStart.main(Native Method)
07-21 22:26:18.499 E/AndroidRuntime(15933): Caused by: java.lang.NullPointerException
07-21 22:26:18.499 E/AndroidRuntime(15933):     at android.content.ContextWrapper.getPackageName(ContextWrapper.java:135)
07-21 22:26:18.499 E/AndroidRuntime(15933):     at android.content.ComponentName.<init>(ComponentName.java:77)
07-21 22:26:18.499 E/AndroidRuntime(15933):     at android.content.Intent.<init>(Intent.java:3834)
07-21 22:26:18.499 E/AndroidRuntime(15933):     at com.example.**REDACTED**.copyright.MainActivity.<init>(MainActivity.java:31)
07-21 22:26:18.499 E/AndroidRuntime(15933):     at java.lang.Class.newInstanceImpl(Native Method)
07-21 22:26:18.499 E/AndroidRuntime(15933):     at java.lang.Class.newInstance(Class.java:1208)
07-21 22:26:18.499 E/AndroidRuntime(15933):     at android.app.Instrumentation.newActivity(Instrumentation.java:1061)
07-21 22:26:18.499 E/AndroidRuntime(15933):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2119)

据我所知，错误发生在 MainActivity 类第 31 行的第 31 行，周围的一些行如下所示:

public class MainActivity extends Activity {
Intent mServiceIntent = new Intent(this, Scan.class);

mServiceIntent 不会再次被引用，直到下面显示的行也位于 MainActivity 中:

 @Override
protected void onStop() {
    super.onStop();

    this.startService(mServiceIntent);
    Log.v("MainActivity", "onStop called");

如果您需要更多信息，我很乐意添加。预先感谢您的耐心和帮助。

Answer 1

您应该在 onStop() 方法中实例化 Intent (这在运行时发生)，而不是在编译时定义它。例如:

public class MainActivity extends Activity {
    Intent mServiceIntent;
...
@Override
protected void onStop() {
    super.onStop();
    mServiceIntent = new Intent(this, Scan.class);
    this.startService(mServiceIntent);
    Log.v("MainActivity", "onStop called");