我正在尝试在我的 Android 应用程序中建立一些网络。
导致我的错误的事件序列如下:
在我的 fragment 中，如果我的 VolleySingleton 类不存在，我会得到一个新实例。
public static VolleySingleton getInstance() {
if (vInstance == null) {
vInstance = new VolleySingleton();
}
return vInstance;
}
在创建 VolleySingleton 的实例时，我创建了一个请求队列并将其设置为等于 Volley.newRequestQueue(myApplication.getContext());
public class VolleySingleton {
private static VolleySingleton sInstance = null;
private ImageLoader imageLoader;
private RequestQueue mRequestQueue;
MyApplication myApplication = new MyApplication();
private VolleySingleton() {
mRequestQueue = Volley.newRequestQueue(myApplication.getAppContext());
imageLoader = new ImageLoader(mRequestQueue, new ImageLoader.ImageCache() {
private LruCache<String, Bitmap> cache = new LruCache<>((int)(Runtime.getRuntime().maxMemory()/1024/8));
@Override
public Bitmap getBitmap(String url) {
return cache.get(url);
}
@Override
public void putBitmap(String url, Bitmap bitmap) {
cache.put(url, bitmap);
}
});
}
public static VolleySingleton getInstance() {
if (sInstance == null) {
sInstance = new VolleySingleton();
}
return sInstance;
}
public RequestQueue getRequestQueue() {
return mRequestQueue;
}
public ImageLoader getImageLoader() {
return imageLoader;
}
}
现在，myApplication 只是 MyApplication 类的一个实例，它扩展了 Application，getContext() 只是在 MyApplication 的实例上调用 getApplicationContext。这是 MyApplication 类的代码:
public class MyApplication extends Application {
private static MyApplication sInstance;
public static final String API_KEY = "xxxxxxxx";
@Override
public void onCreate() {
super.onCreate();
sInstance=this;
}
public static MyApplication getInstance() {
return sInstance;
}
public Context getAppContext() {
return sInstance.getApplicationContext();
}
}
当我尝试
return sInstance.getApplicationContext(); 时似乎发生了错误我已经设法让相同的代码在一个单独的测试项目中工作，所以我在这里不知所措。
这是我的日志:
java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context com.xxxxxxxxx.chessgame.Models.MyApplication.getApplicationContext()' on a null object reference
at com.xxxxxxxxx.chessgame.Models.MyApplication.getAppContext(MyApplication.java:28)
at com.xxxxxxxxxx.chessgame.Networking.VolleySingleton.<init>(VolleySingleton.java:25)
at com.xxxxxxxxx.chessgame.Networking.VolleySingleton.getInstance(VolleySingleton.java:42)
at com.xxxxxxxx.chessgame.RecyclerViews.FragmentViewOne.onCreate(FragmentViewOne.java:95)
at android.support.v4.app.Fragment.performCreate(Fragment.java:1763)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:913)
at android.support.v4.app.FragmentManagerImpl.performPendingDeferredStart(FragmentManager.java:842)
at android.support.v4.app.Fragment.setUserVisibleHint(Fragment.java:859)
at android.support.v4.app.FragmentStatePagerAdapter.setPrimaryItem(FragmentStatePagerAdapter.java:152)
at android.support.v4.view.ViewPager.populate(ViewPager.java:1071)
at android.support.v4.view.ViewPager.populate(ViewPager.java:919)
at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1441)
at android.view.View.measure(View.java:17430)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:875)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:613)
at android.view.View.measure(View.java:17430)
at android.support.v4.widget.DrawerLayout.onMeasure(DrawerLayout.java:851)
at android.view.View.measure(View.java:17430)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:722)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:613)
at android.view.View.measure(View.java:17430)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
at android.view.View.measure(View.java:17430)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:722)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:613)
at android.view.View.measure(View.java:17430)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
at android.view.View.measure(View.java:17430)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:722)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:613)
at android.view.View.measure(View.java:17430)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2560)
at android.view.View.measure(View.java:17430)
at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2001)
at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1166)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1372)
at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1054)
at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5779)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java:767)
at android.view.Choreographer.doCallbacks(Choreographer.java:580)
at android.view.Choreographer.doFrame(Choreographer.java:550)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:753)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:135)
at android.app.Act
最佳答案
请确保您已在 list 中将 MyApplication 类声明为 android:name="packagename.MyApplication" in application 标签
关于android - 尝试在 Application 类的实例上调用 getApplicationContext() 导致空对象引用错误，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/30419969/