java - Android Activity 上下文 NullPointerException

Question

我在 Google Play 开发者控制台上收到一些关于 Activity 上下文或应用程序上下文中的 NPE 的奇怪错误报告。我无法在我的设备上重现此错误，并且我知道它在大多数设备上运行良好。

这是我的主要 Activity，是应用程序启动时首先打开的一个:

public class ActSplash extends Activity {
  private Sync sync;
  private Context ctx;

@Override
public void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.act_splash);

  do {
      ctx = this.getApplicationContext(); //Using just "this" doesn't work either.
  } while (ctx==null);

  sync = Sync.getInstance(ctx); //Apparently ctx is null at this point in some devices
}

这是我的同步类(单例)

public class Sync {
   private Context ctx;
   private static Sync INSTANCE = null;

   public static Sync getInstance(Context ctx) {
      if (INSTANCE == null) createInstance(ctx);
      return INSTANCE;
   }

   private synchronized static void createInstance(Context ctx) {
      if (INSTANCE == null) { 
        INSTANCE = new Sync(ctx);
      }
   }

   private Sync(Context _ctx)
   {
      ctx = _ctx;
      //NPE on following line
      WifiManager wifiMan = (WifiManager) ctx.getSystemService(Context.WIFI_SERVICE);
      ...
   }

这是堆栈跟踪:

java.lang.RuntimeException: Unable to start activity
ComponentInfo{com.xxx.xxx/com.xxx.xxx.ActSplash}: java.lang.NullPointerException
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2140)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2178)
at android.app.ActivityThread.access$700(ActivityThread.java:141)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1271)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:5118)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:792)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:555)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
at com.xxx.xxx.Sync.<init>(SourceFile:82)
at com.xxx.xxx.Sync.void createInstance(android.content.Context)(SourceFile:67)
at com.xxx.xxx.Sync.com.xxx.xxx.Sync getInstance(android.content.Context)(SourceFile:72)
at com.xxx.xxx.ActSplash.void onCreate(android.os.Bundle)(SourceFile:40)
at android.app.Activity.performCreate(Activity.java:5058)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2104)
... 11 more

在调用之前我该怎么做才能确保我有 ctx!=null:

sync = Util_Sync.getInstance(ctx);

Answer 1

因为你的代码不会改变

do {
  ctx = this.getApplicationContext(); //Using just "this" doesn't work either.
} while (ctx==null);

如果无法获取ctx，这将导致无限循环。

Activity 是一个上下文!请参阅http://developer.android.com/reference/android/content/Context.html

Known Indirect Subclasses AbstractInputMethodService, AccessibilityService, AccountAuthenticatorActivity, ActionBarActivity, Activity, ActivityGroup, AliasActivity, Application, BackupAgent, BackupAgentHelper, ContextThemeWrapper, and 23 others.

尝试

ctx = this;

更新

在发布您的同步代码之后

private Context ctx;

从未使用过，因此将其删除。